Symposium on Energy Geotechnics 2023

Cutter Soil Mix-Energywall full-scale test experimental setup

Vincent Leclercq — 2023-10-02

The demand for sustainable heat production is growing worldwide [1]. Classical geothermal borehole installations are costly due to the required boring depth and space demands in the urban environment. Shallow geothermal heat exchangers comprised of thermally-activated geostructures in combination with heat pumps (also known as energy geostructures, EGS) offer an interesting alternative due to a combination of functions [2].

This paper focuses on the thermal properties of the CSM (Cutter-Soil-Mix [3]) Energywall; a novel EGS type in which heat exchanger pipes are installed in a cutter-soil-mix wall to thermally activate it. Therefore, the CSM Energywall serves the dual function of supporting the soil/overlying structure and providing geothermal energy for space heating/cooling. A test site has been setup in Amstelveen, the Netherlands, where this system will be tested for a 1 year period (starting March 2023). A test site picture is shown in Figure 1.The goals of the experiment are to evaluate the thermal efficiency of the system and to assess its thermo-mechanical behaviour by means of numerical models valiated against the data collected from site monitoring. In the first part of the forthcoming paper, test setup and experimental results (both from the field and laboratory tests of the CSM material) will be presented. In the second part, the implementation and validation of the numerical models will be described. Lastly, the paper will focus on the suitability of the CSM-Energywall as a sustainable urban heat source, with particular reference to the Netherlands context.

The following research questions have been defined for the CSM-Energywall experiments:

What is the heat extraction potential of a CSM-Energywall in sandy and in cohesive soils?
Does thermal activation affect the mechanical properties of the CSM material?
What is the long term heat production potential of the CSM-Energywall?
How accurately can the heat extraction potential be predicted using numerical models?

A test setup at Amstelveen (NL) has been built to answer the above research questions. The test site is located in a building pit of 22 x 13m. CSM-walls were built along the pit perimeter, serving both as a soil retaining structure and as a foundation for the superstructure. The CSM wall has a thickness of 0.55m and is reinforced with IPE steel beams. The bottom of the CSM wall has a level of NAP-18.0m (NAP= Normaal Amsterdamse Peil, vertical reference system of the Netherlands). The bottom of the steel IPE beams is at a level of NAP-15.6m. The ground level at the test site is located at NAP-0.7m and the groundwater table at NAP-1.5m. The building pit has an excavation level of NAP-4.70m. Heat exchanger pipe loops with a diameter of 20mm have been connected to the steel beams representing the CSM wall reinforcement (Figure 1). The loops have a depth of NAP-15.0m or NAP-11.0m. Five main groups of loops are connected to the flow distributer. Each group is then split into six subgroups. Each subgroup consists of two thermoactive steel rebars. The test setup will run for the duration of the building construction. Being able to select the geothermal activation of the shallowest or deepest loops, it will be possible to measure the energy output of both the fine-grained layers (with a bottom depth of NAP-11.0m) and the underlying sandy layers.

During the tests the following aspects will be monitored:

Flowrate of fluid through the main loops (FTB4700 Flowmeter)
Temperature of each main group of loops (PT100 thermowell)
Flowrate and temperature of heat pump inlet/outlet fluid (Kramstrup Energymeter)
Temperature of the soil (thermistor strings)

CSM material core samples have been taken for laboratory testing. Cyclic thermal loading, unconfined compression strength (UCS) tests, computerize tomography (CT) scan tests and hot disk tests will be conducted on samples taken from different depths. The goal of the laboratory testing is to evaluate:

The thermal properties of CSM material depending on the original insitu soil composition, needed to assess the thermal efficiency of the system.
The UCS of the CSM material, and its possible dependency on temperature.
The macroporisy and density distribution of the CSM material, to gain insight into the material composition.
The influence of thermal cycling on the material’s mesostructure (e.g. porosity and density distribution).

The full-scale experiment is going to be numerically modelled in 3D using the COMSOL Multiphysics Finite Element software. The model will be validated based on the experimental results of the test after 1 year and calibrated using laboratory measured thermal properties of the CSM material. The model will then be extended to predict the long term efficiency of the CSM- Energywall and its thermo-mechanincal behaviour.

Heat pump efficiency with energy geostructure: Numerical long term modelling

Thibault Badinier — 2023-09-28

Energy geostructure are a group of technical methods aiming to use the geotechnical structures as heat source exchangers for a geothermal Heat Pump (HPg) [1]. As with any heat pump system, the goal is usually to extract soil energy in order to heat buildings. HPg system efficiency is described by the COP factor (Coefficient of Performance) defined as the ratio between the quantity of useful energy for building heating and consumed electric power . Its value has a great influence on the economic sustainability of geothermal systems. This efficiency is strongly correlated to the temperature lift between the cold and hot sources of HPg. However, while operating, HPg will drain energy from the soil, decreasing soil temperature, increasing the temperature gap, and decreasing the COP factor. Such thermal anomalies are supposedly dissipated through seasonal operations of the system. However, inappropriate conditions might result in a multiyear soil temperature shift [2] and durable degradation of HPg efficiency [3]. Specifically, energy geostructure are often installed as dense networks, such as pile groups. Such proximity might generate thermal interaction and affect the surrounding soil temperature [4, 5]. In the case of an underground water table, seepage might help to dissipate the thermal anomaly [2], but it will also generate stronger and more directional thermal interaction [5, 6].

Based on numerical modeling [5], we were able to predict the COP evolution for a small energy pile. We propose now to use this modeling process to predict the COP evolution of a more realistic geothermal installation. The study case model is based on a housing building constructed in Gonesse France, in 2012 [7]. The model represents a group of 20 energy piles, each 12 m long, used to heat 1000 m² of housing with a peak heating demand of 15,5 kW. The system is supposed to be used at peak power supply for 12 hours out of 24 hours during four consecutive weeks. The final resulting COP is considered an indicator of installation performance. We studied the effects of pile proximity (inter-pile distances from 1.2 m to 6 m) and seepage velocity (from 0 m/day to 2 m/day). We expect to observe a beneficial effect of the water movement as it renews the soil energy around the pile. Also, we expect to observe a negative effect of the pile proximity, as the thermal anomalies will have a cumulative effect on the soil temperature drop.

All the numerical modeling is realized with CESAR-LCPC software, which is a FEM software specialized for civil engineering problems [8]. It has the possibility of treating diffusive problems such as thermal problems or hydraulic diffusion in porous media. Also, CESAR-LCPC could be used with a Python script interface, which is useful to describe the needed iterative process and conduct the parametric studies. The numerical modeling processes principally aim to solve the thermal problem in the soil and foundation systems. It takes into account heat transfers from conduction defined by Fourier’s law: ( has the material thermal conductivity), and the heat transfer due to underground water displacement (advection phenomenon) defined has (with the volume thermal capacity and the underground water flow velocity). The seepage velocity field is then computed preliminarily in order to take account of the disturbance effect due to the structure. The thermal load applied to the model results from the heating demand from the building, as the thermodynamic law states energy conservation: , knowing the definition of the COP factor. Staffell and al. [3] proposes an empirical relationship between the COP factor and the temperature lift between the hot and cold sources of HPg. Completing the problem by adding a

few simple thermal resistive equations and assuming a hot source at 40°C, we can solve the overall equation in order to compute the COP factor depending on the soil temperature.

The results show, as expected, a progressive COP decrease during the 4 weeks as long as soil temperatures decrease. Moreover, in the case of a seepage velocity below 0.2 m/day, the temperature and COP factor tend to decrease without a steady state. Figure 1 summarizes the numerical results, showing the final COP value depending on seepage velocity and interpile distance. It shows that the absence of seepage will have a negative impact on system efficiency as long as the pile proximity. Logically, in a smaller group, a cluster effect will intensely affect soil temperature, while the absence of seepage won’t renew soil energy. We can conclude that in cases of slow seepage (below 0.2 m/day), it is possible to improve system efficiency by opting for more distant piles. Also, for seepage over 0.5 m/day, the distance between piles in the same group will have a limited impact on the heat pump's efficiency.

In an extended study, this modeling process has been complemented with summer mode ability and used for longer simulations, allowing the study of a multi-year thermal shift and various seasonal activation patterns (balanced and unbalanced, peak use, etc.).

Improving thermal storage of energy screw pile groups with phase change materials

Luis A. Bandeira Neto — 2023-09-29

To achieve affordable housing in a carbon-neutral society, new buildings require a dual-purpose approach that comprises efficient construction and a green energy supply. Energy screw piles [1, 7] meet this demand as they combine the agility of screw pile drilling with the capability of extracting clean shallow geothermal energy. Moreover, the screw piles can be filled with phase change materials (PCM) to provide latent thermal storage. Studies regarding the use of PCMs as borehole backfill in a ground source heat pump system (GSHP) conclude that PCM implementation can improve GSHP performance. However, most commercially available PCMs have low thermal conductivities (λ) which undermine heat exchange rates [5, 10]. Besides, using PCMs as a composition of a concrete energy pile can reduce the pile structural performance since PCMs usually have a low mechanical capacity [2, 8]. The authors recently introduced PCM-sand mixtures as a core in the central hollow part of an energy screw pile, which does not impact the pile structural capacity [6]. The mixtures with a higher PCM content benefit the pile heat exchange through its latent heat, but only when the PCM does not reduce the mixture’s λ. To overcome this problem, this work tests a new underground heat exchange system where instead of mixing the PCM in the energy screw pile filling material, regular screw piles (i.e., without the heat exchange tubes) are filled with pure PCM, acting as thermal storage piles. A numerical model built via COMSOL [4] is used to evaluate how this combination of screw piles performs thermally when supplying/rejecting heat for a GSHP system operating for a whole year.

This work uses a validated numerical model [3, 9] to simulate a grid of evenly distributed screw piles, where Energy Piles (EP) and Thermal Storage Piles (TSP) are positioned interspersed, evenly spaced 0.7 m apart. Inside the EPs, an U-loop pipe is inserted in the pile steel case and the remaining is filled with grout. In contrast, the steel case of the TSPs is filled with only PCM [11]. All other material properties and the screw pile geometry are based on [1]. The thermal load is based on the design of a building located in Melbourne, Australia (Figure 1(a)). The model considers the hourly operation of a GSHP for one year, calculating the GSHP Coefficient of performance (COP) at every time step and adjusting the ground thermal load. For comparison, two simulations (one where the TSPs are filled with PCM and a reference where they are filled with only air) are undertaken. Besides the energy stored, the COPs of both cases are compared as a relative increase/decrease percentage (COP_change = (COP_PCM / COP_Reference) – 1).

Figure 1(b) presents the energy stored by the PCM and its corresponding percentage in a solid state over the simulation. As the EPs reject heat underground due to GSHP summer operation for cooling, the TSPs temperature rise and store sensible heat, until they reach their phase change temperature (T_pc), when the PCM melts to store significantly more heat energy in a shorter time window due to latent heat. Conversely, the PCM solidifies when EPs extract more heat than reject due to GSHP winter operation (heating) for a certain period. The PCM melts again on the following summer, restarting the whole process. The peak energy stored in the TSPs is 190.3 MJ/m³. Even though the thermal load varies hourly, the PCM phase change process happens without immediate responses to the load variation. Figure 1(c) presents the resulting change in the hourly COP considering cooling (CCOP) and heating (HCOP) from implementing PCM in the TSPs, compared to the reference scenario (empty TSPs). The extra heat stored by the PCM lowers the circulating fluid temperature while the latent heat is engaged (months 2 to 7 and 12), which results in a higher CCOP and a lower HCOP. By plotting a monthly average of the COP_change (grey markers), it is clear that the impact of the PCM on the COP is positive when cooling is dominant and slightly negative when heating becomes dominant while the latent heat is engaged. When the PCM is implemented in the EP [6, 10], the heat exchanged by the EP drops once the phase change process is over due to the low λ of the PCM, but by positioning it outside the EP the higher CCOP is sustained even after the phase change ends. However, lowering the fluid temperature increases CCOP while lowering HCOP at the same time, which can harm thermal performance if not designed properly. The results underlie the thermal storage potential available not only in screw piles, but also in any hollow pile foundation, by simply implementing PCM in the hollow case.

The impact of heated basements on the performance of borehole GHEs

Nikolas Makasis — 2023-10-02

Increasing urban development is leading to a growing demand for subsurface utilisation. As more infrastructure is built into the subsurface, heat from tunnels, sewers, and basements, among others, alter the thermal state of the ground, acting as sources and sinks of heat, leading to a net-increase of underground temperatures, a phenomenon known as the Subsurface Urban Heat Island (SUHI) [7-8]. This additional heat can have impacts on, for example, health and maintenance of underground structures, increased ventilation costs for underground spaces, and quality and quantity of groundwater flow [1,2]. However, this additional heat in the subsurface can also be harvested by ground-source heat pump (GSHP) systems to provide heating to buildings [3-6], operating more efficiently due to the higher ground temperatures and reducing these temperatures through operation, thus mitigating the risks and impacts of SUHI.

This work demonstrates how heated basements can contribute to the operation of GSHP systems to provide heating. The area of Downing College, located in central Cambridge, UK, is used as a case study site, investigating how much of the college’s heating demand a number of geothermal boreholes could provide, when the heat from the building basements is taken into account and when it is not. Measured gas consumption data from the college are used to estimate the heat demand. The geology of the site is obtained by importing historical borehole records for the wider Cambridge area into the British Geological Survey (BGS) Groundhog® Desktop Geoscientific Information System and constraining the domain using BGS generated superficial deposit and bedrock geology maps, thus producing a 3D lithological profile for the region*, while the hydrological conditions were obtained using measured water level time-series data, from water wells in the area, curtesy of the Environment Agency. A total of 88 boreholes are considered, placed symmetrically in the college courtyard, between the main buildings. Typical single U-loop borehole configurations are used with pipes of 32 mm outer diameter. Acknowledging that the effect of heated basements is greater in shallower regions of the subsurface, two typical borehole length values are considered: 50 m, providing about 40% of the required heating load, and 100 m, providing 100%. The operation is simulated over 50 years, using a full 3D numerical model coupling heat transfer, groundwater flow, and pipe flow governing equations, created in COMSOL Multiphysics®. The temperature of the heated basements is assumed to be maintained at 18 °C throughout the simulation. During the first 10 years, the GHEs are not operating, to allow heat accumulation in the ground to occur from the heated basements, following which, 40 years of operating GHEs are simulated.

The results of the simulations suggest that an increase in performance occurs when heated basements are present. For all scenarios, the fluid temperatures keep decreasing over time, as the ground temperature around the boreholes keeps decreasing, making it more difficult to extract heat. Over the 50 years of simulation, the fluid temperature reaches a minimum of -3.20 °C and -2.13 °C for the 50 m GHEs, and 4.52 °C and 3.88 °C for 100 m GHEs, in both cases the first value being the simulation with heated basements and the second without. The COP values over time are presented in Figure 1-right, showing the difference in COP between the cases with and without heated basements increasing over time for both 50 m and 100 m GHEs, for the former at a higher rate.

This increase corresponds to the increase in the average fluid temperature between the cases with and without heated basements, also shown in the figure, and reaches a maximum difference of about 0.08 for the 50 m GHEs and 0.05 for the 100 m GHEs, corresponding to a fluid temperature difference of about 1.10 °C and 0.65 °C, respectively. These figures correspond to a reduction in operating costs (assuming an electricity rate of £0.34 per kWh) of £17,352 for 50 m GHEs and £18,197 for 100 m GHEs. While for this case these values are relatively small considering 40 years of operation, the fluid temperature difference suggests that more energy can be extracted by the GHE borheoles when heated basements are present, especially for shorter boreholes, and thus potential savings could be obtained by reducing the number of GHEs.

Investigating the use of hydro-geothermal energy from tunnel drainage system for de-icing roads: results from a pilot study

Mustafa Mustafa — 2023-09-28

De-icing roads is a complex process that requires careful consideration of multiple factors, as traditional de-icing methods can have significant environmental and economic impacts. Therefore, the use of sustainable methods such as renewable energy, can potentially help to mitigate some of these issues. In order to investigate this concept, a thorough investigation into the use of hydro-geothermal energy from a tunnel drainage system for preventing the accumulation of ice and snow on traffic areas at the tunnel portals was conducted.

As part of this investigation, a pilot system consisting of nine fields with underlying heat pipes was constructed being monitored by several temperature sensors and further equipment. The fields utilise the readily available mountain water from the 1.284 m long road tunnel ´Füssen´ at the German/Austrian border as a natural heat source [1], eliminating the need for a heat pump. The sensor readings were then compared to the results obtained from a coupled hydro-thermal numerical model (Figure 1). The model considers all heat fluxes acting on the free surfaces (e.g. short and long-wave radiation), and determines the heat flux density required to keep the ice and snow free by means of the energy balance formed at the surface of the field. The validation of the simulation is carried out on the basis of the measurement data which were recorded in periods in which the external atmospheric boundary conditions can be determined as precisely as possible.A parameter study was also conducted that included different weather conditions, variable field geometry, pipe materials, and surface materials such as concrete, or asphalt, as well as the minimum field activation times, the results of these studies confirmed, among other things, that the utilization of copper pipes results in higher temperatures on the surface of the road [2]. Furthermore, it was determined that an activation time of 9 hours prior to the forecast of a weather event is sufficient to raise the temperature of the road surface to remain ice-free.

The results of this study provide compelling evidence for the efficacy of the direct, passive outdoor temperature control concept in mitigating snow and ice accumulation on traffic areas adjacent to tunnel portals. This pioneering approach not only safeguards winter safety by maintaining hazard-free surfaces but also presents the opportunity to extend the lifespan of the traffic areas through the implementation of cooling strategies during the summer season. Notably, upon activation, the road surface consistently achieves the necessary temperature to effectively melt snow and ice during moderately intense snowfall within a relatively short timeframe of approximately 5 to 9 hours. These results highlight the practical viability and efficiency of the direct, passive outdoor temperature control method in combatting snow and ice-related challenges.

It was concluded that the findings of this study have yielded positive results and have served as proof of concept for the utilization of hydro-geothermal energy from tunnel drainage systems but also from other sources as e.g. BHE as a viable means of preventing ice and snow accumulation on roads. The data obtained from numerical simulations and measurements were found to be in agreement. This method holds the potential to greatly enhance the safety and accessibility of roads during the winter months, while also reducing the dependence on traditional de-icing methods that can have detrimental environmental impacts, on top of that, the method has an added benefit of being used in the summer to cool down the roads to prevent heat deformations. The results of this pilot project provide a foundation for further research and development in this field, with the ultimate goal of implementing this technology on a larger scale.

Finding common ground: identifying shallow geothermal potential for the city of Cambridge, UK

Nikolas Makasis — 2023-09-28

Urban expansion and extensive anthropogenic utilisation of the subsurface can lead to thermal changes in the ground, as structures such as basements, sewage systems, and tunnels reject or absorb heat to/from the ground. This phenomenon, known as Subsurface Urban Heat Island, has been widely documented and studied in recent years [1,7]. Investigations have shown that significant soil and groundwater temperature anomalies can be caused, with local hotspots and temperature differences up to 20 °C [6]. These ground temperature anomalies can affect, for example, ground- and drinking water quality, ecosystem biodiversity, and geothermal energy utilisation, with the latter being the focus of this work.

The city of Cambridge, UK, shown in Figure 1-left, is adopted as a case study site, and a novel scalable large-scale subsurface modelling methodology [4] is used to obtain an understanding of the ground thermal state, accounting for natural and anthropogenic influences. The geology for the region is obtained by importing historical borehole records for the wider Cambridge area into the British Geological Survey (BGS) Groundhog® Desktop Geoscientific Information System and constraining the lithologies using BGS generated superficial deposit and bedrock geology maps, producing a 3D lithological profile. Water table readings from borehole wells supplied by the Environment Agency are used to create hydraulic head and water table maps for the region. Hydraulic and thermal properties for the materials in the domain were obtained from available literature^* [2]. The main anthropogenic features are basements, assumed to be heated at 18 °C, and sewers, assumed linked with building density and at 15 °C. Following the methodology, the domain is separated into 1096 blocks, each 200m by 200 m laterally and 100 m in depth, clustered into 10 archetypes. Each archetype comprises a set of features resulting in a ground thermal state common across all blocks within an archetype [4].

Having thus obtained the spatially varying ground temperature, the performance of typical shallow geothermal systems throughout the domain is assessed, initially investigating the theoretical geothermal potential. Figure 1-middle, shows the amount of heating power a typical 100 m double U-loop borehole can supply, providing a constant ground load from 1^st of October to 31^st May over a 50-year operation period. The power is computed using the Finite Line Source model and g-functions [5], setting a lower limit of -2 °C for the ground loop circulating fluid temperature. The results show that hydro-geological features and anthropogenic thermal influences in the region can result in spatial variation of geothermal potential of up to 0.3 kW, or about 1746 kWh per year. A sensitivity analysis indicates that no single feature dominates in the contribution to the magnitude of geothermal potential, suggesting that both natural and anthropogenic sources are important influences on how much energy the ground can provide.

Extending the analysis by incorporating estimated residential heating demand data [3], Figure 1-right shows the percentage of residential demand that can be fulfilled using geothermal boreholes, assuming these are drilled in suitable parking and non-major road areas for each block, at a minimum spacing of 6 m to avoid thermal interference. The calculations use g-functions to compute how much of the estimated heating demand a single borehole can supply, using half-hour demand distributions for 50 years (repeated annually), and multiplied by the estimated number of boreholes in each block to determine the total geothermal energy that can be supplied. For a large portion of the modelled domain, the entirety of the residential heat demand is expected to be feasibly fulfilled using shallow geothermal energy. Certain areas, mostly agricultural and green spaces with no to low demand, contain no suitable borehole drilling locations, i.e., parking or road areas (a conservative assumption adopted in this study), resulting in no energy being supplied geothermally (light gray). Average demand supplied within the remaining region is 91%, with a standard deviation of 21%.

As the world urgently seeks to transition to a more sustainable energy infrastructure, utilising different clean energy technologies in a more extensive and organised way becomes increasingly necessary. Geothermal energy technologies can be particularly suited for coordinated large-scale utilisation, due to the significant capital costs and the continuous nature of the ground, acting as a shared resource for large communities. This work briefly demonstrates the capacity that geothermal technologies have to fulfil a significant portion of the residential heat demand at large scales, using the city of Cambridge as an example, and that organisations or governments can take advantage of the potential that exists in finding common ground.

Geothermal gradients in a high-organic content landfill

Sara Otero Manrique — 2023-10-02

INTRODUCTION

The biodegradation processes that take place in municipal solid waste (MSW) landfills deliver not only leachate and biogas as byproducts but also heat due to the exothermic nature of the biochemical reactions that decompose organic matter [1,2]. This heat is a potentially significant source of shallow geothermal energy. Furthermore, in low-income municipalities across the world, the MSW organic load is typically much higher than that of wealthy nations.

The study of the geothermal potential available in sanitary MSW landfills is a relatively recent field of research [3–7]. Evidence indicates that, in North America and Europe, MSW landfill temperatures can be about 35°C higher than that of typical shallow soil profiles [5,6], and may rise above 65°C as a result of the anaerobic MSW biodegradation [1]. A few studies have even assessed the implementation of geothermal technology for harvesting shallow geothermal energy at landfills [8,9].

Research on geothermal energy potential and its possible recovery from high-organic content (HOC) landfills is still scarce. This may be explained due to the tropical climates where many HOC landfills exist, where no meteorological seasons take place. Nonetheless, HOC landfill geothermal energy may benefit operational, agricultural, or farming activities within and around the facility.

Based on in-situ temperature measurements and computational simulations, the present study estimates the geothermal gradients and recoverable energy potential available in a HOC MSW landfill cell after 11 years of closure.

METHODS

The Doña Juana landfill (DJL) is the main MSW engineered disposal facility in the city of Bogotá, Colombia. DJL spans across 626 hectares and is divided into 11 disposal zones. At least 61.5% of the MSW at DJL is oxidizable organic matter [10], which is significantly higher than the organic fraction in landfills of high-income countries, i.e., 12% to 30% [11]. Some official measurements of organic content at DJL have reported values as high as 74.5% [12]. The DJL has an extensive network of vibrating wire piezometers (Geokon model GK 4500), which include thermistors that allow temperature measurements. In August 2021, the DJL operator kindly measured temperatures in all their piezometers for the present project. Overall, 190 measurements were recorded with temperatures of up to 52.9°C. Additional data including landfill topography, cell age, and closure date was made available. The DJL’s “Zone 8” was selected as the landfill cell for modeling. This zone (41 Ha) operated from March 2022 through September 2010. In 2021 (11 years after closure), Zone 8 showed up to 50.5°C at 23.4 m of depth inside piezometer PZ-13 (see Figure 1b).

A two-dimensional finite-element model was constructed to simulate a representative section of the DJL’s Zone 8. Modeling of heat transfer in porous media was implemented in Comsol Multiphysics® assuming no unsaturated flow across the landfill volume during up to 100 years of heat production. A published heat generation function [13,14] was used, which is surprisingly independent of MSW organic content. A conservative value of DJL’s organic fraction (61.5%) [10] was adopted, as well as a compacted density (1150 kg/m³). Because direct measurements of MSW thermal properties at DJL are not available yet, these were estimated by adjusting published values [15] by proportion with respect to organic content, thus yielding thermal conductivity of 1.15 W/m.K and heat capacity of 2280 J/kg.K. In addition, permeability and porosity values were adopted from the MSW literature.

Model calibrations to fit the field-measured temperatures were undertaken by varying only the independent parameter (dubbed “composite climatic-operational condition factor”) of the heat generation function [14]. This parameter depends on mean annual ambient temperature and rainfall, in-landfill MSW density, and annual rise of the landfill during disposal (respectively, 12.3°C, 700 mm/year, 1150 kg/m³, and 7.82 m/year for DJL). Finally, as a first approach to estimate recoverable heat, the “volumetric method” was used assuming a typical recovery for fractured aquifers of 2.4% across the entire extraction volume. For the calculation, a extraction step of one year was adopted, and the DJL cross-section was assumed to have a thickness of 200 m.

RESULTS

The calibration of the numerical model with in-situ data yield a peak heat generation rate of about 3 W/m³, which is between 1.3 and 15.5 higher than that of North American landfills [14]. Figure 1 displays an animation of the calculated temperature evolution during 50 years, as well as the measured and calculated geothermal gradients at piezometer PZ-13. The volumetric method predicts a peak heat recovery of 2.4 kW on year three, determined by calculation of the heat generated annually during 50 years. These results likely underestimate the actual recoverable energy at the landfill; follow-up studies, currently underway, are modeling the three-dimensional implementation of ground-source heat pump exchangers.

Figure 1. DJL’s Zone 8 heat transfer simulation results. a) Temperature distribution and evolution through 50 and 100 years. b) Measured and calculated geothermal gradients inside PZ-13

CONCLUSIONS

This study estimates the spatial and temporal distributions of temperature, as well as the geothermal gradients, within a high-organic content landfill. The two-dimensional heat-transfer finite-element model developed for the present research is calibrated against actual temperature measurements recorded 11 years after closure of a landfill cell. Because previously published MSW heat-generation functions depend only on operational and climatic variables, future research should pursue a modified heat generation function that accounts for MSW organic content.

The present study has limitations, including: 1) the computational simulations were not three-dimensional; 2) actual MSW thermal properties evolve with time; and 3) fully coupled THBCM may be required to capture the complex behavior of a HOC landfill. Nonetheless, the authors hope that this study helps bring awareness to landfill operators and government decision-makers about the feasibility of HOC landfills as sources of shallow geothermal energy.

Effect of pile spacing on the thermal performance of thermo-active pile groups

Ryan Yin Wai Liu — 2023-09-28

Thermo-active piles differ from conventional piles in a way that they have pipes embedded within them, which allow a carrier fluid to circulate through and exchange heat with the ground, in order to provide low carbon heating and cooling. Sustainability targets, such as the Merton Rule, which requires a proportion of the energy demand of the building to be generated on site using renewable sources [1, 2], have facilitated the growing popularity of designing piles to be thermo-active in the United Kingdom [3, 4, 5]. In order to fulfil these sustainability targets and to determine accurately the energy savings by designing piles to be thermo-active, the thermal performance of thermo-active piles has to be determined.

Thermo-active piles are often deployed as a group, with values of spacing considerably smaller than those adopted in fields of borehole heat exchangers. As a result, thermal interference occurs between them, penalising their thermal performance [6], compared to the case of a single standalone thermo-active pile, as the piles are heated or cooled by adjacent piles within the pile group. Considering thermo-active piles with identical geometry, pipe arrangement and heating/cooling conditions, it is not difficult to establish that the upper bound of thermal performance corresponds to piles operating as single standalone thermo-active piles, without the effects of thermal interference. When thermo-active piles operate as a group, the spacing between the piles and the pile group size govern the effects of thermal interference, hence the penalty in thermal performance compared to the upper bound case. In order to quantify the penalty in thermal performance when thermo-active piles operate as a group, this paper considers infinitely-large thermo-active pile groups, where piles are separated by 3, 4, 5 and 6 times the pile diameter, respectively. This approach establishes lower bounds of thermal performance, which are dependent on the pile spacing for the thermo-active pile considered. The realistic thermal performance of the thermo-active piles within a finite-sized pile group with a given pile spacing would therefore lie between the lower and upper bounds as explained above.

The thermo-active piles considered in this research are in diameter, in length with double U-loop pipe arrangement. They are heated up with a constant inlet temperature of above the initial ground temperature, which is assumed to be , for a duration of one year. A flow rate of per U-loop is adopted and water is used as the carrier fluid, which has a volumetric heat capacity of . The thermal conductivities of the thermo-active pile and soil are and , respectively, and the volumetric heat capacities of the thermo-active pile and soil are and , respectively.

Three-dimensional thermal numerical analyses are conducted using COMSOL Multiphysics^® [7]. To establish the upper bound of thermal performance, a single thermo-active pile is placed at the centre of a domain which is by and is deep, where all the domain boundaries are prescribed with a no change in temperature thermal boundary condition. To establish the lower bounds of thermal performance, which are solely dependent on the pile spacing as infinitely-large thermo-active pile groups are considered, the thermo-active pile is placed at the centre of a domain where the length of the sides is given by the pile spacing (therefore, pile spacings of 3, 4, 5 and 6 times the pile diameter correspond to domain sizes in plan of , , and , respectively). Due to symmetry, the four sides of the domain are modelled as adiabatic to

account for the presence of the adjacent piles. The domain is deep and the top and bottom boundaries are prescribed with a no change in temperature thermal boundary condition.

The evolutions with time of thermal performance (in terms of power per unit pile length) are presented in Figure 1(a), while Figure 1(b) plots the thermal performance of an infinitely-large pile group as a percentage of that of the single pile. Note that the power of the thermo-active piles is evaluated based on the temperature differentials between the pipe inlets and outlets. Clearly, the power per unit pile length (see Figure 1(a)) reduces with time due to the reduction in the thermal gradient between the hot pipe and the surrounding concrete, eventually reaching a relatively constant value after a year of operation. It can also be seen that, when the pile spacing reduces, effects of thermal interference (i.e. where the limited volume of soil between piles is heated up by several piles) increase significantly, resulting in considerably lower powers. Referring to Figure 1(b), it can be observed that when piles are separated by 3 times the pile diameter, the lower bound of power is only of a single pile (i.e. the upper bound) after around 120 days of operation, whereas when the piles are separated by 6 times the pile diameter, the lower bound of power still retains of the upper bound after a year of operation.

Performance evaluation of a new type of horizontal ground heat ex-changer: Coil-Column System (CCS)

Huu-Ba Dinh — 2023-09-28

Ground source heat pump system is well-known as an environmentally friendly and high-efficiency technology for heating and cooling the building. The horizontal ground heat exchanger (GHE) has been paid more attention to in recent years as it has a low construction cost and easy installation [1]. However, the drawback of horizontal GHE is the requirement of a large installation area [2]. In this study, the performance of a new GHE type, named Coil-Column System (CCS), is evaluated. CCS is expected to increase the total heat exchange rate of the GHE, which will result in reducing the required installation area.

Firstly, the numerical analysis was conducted using COMSOL Multiphysics to investigate the heat exchange performance of the CCS, and previous exchanger types (e.g., spiral-coil type and straight-line ground heat exchanger (Figure 1(a)). The numerical model was validated using an in-door thermal response test (TRT) in the mock-up steel box with a dimension of 5m × 1m × 1m (Figure (1(b)) [2]. Afterward, the parametric study was conducted to evaluate the effect of the pitch of the coil and installation depth on the heat transfer efficiency of the CCS. Finally, the feasibility of the CCS was comprehensively accessed through the heat transfer performance and economic parameters (internal rate of return, payback period).

Strong agreement between the outlet fluid temperature demonstrates that the proposed numerical model is suitable to simulate the heat exchange in the ground heat exchanger (Figure 1(c)). The comparison results of different GHE types indicate that the heat exchange rate of CCS is double that of the spiral-coil and even three times higher than that of the straight-line type (U-type, conventional type) (Figure 1(d)). The parametric study shows that a higher heat exchange rate is observed at a shorter pitch of CCS. However, the heat exchange rate of CCS is almost independent on the pitch after 168 h. This is attributed to the reduction in the temperature gradient of soil temperature and the inlet fluid temperature and thermal interference at the narrow space between pipes. In addition, an increase in installation depth results in an increase in the performance of CCS for both the short term and long term because the deeper CCS has a higher difference between fluid temperature and soil temperature.

It should be noted that using the shorter pitch (longer pipe length) or increasing the installation depth causes a significant increase in the material cost, and excavation cost, respectively. The economic analysis results of CCS indicate that the installation depth of 4 m and the pitch of 0.15 m has the highest internal rate of return and shortest payback period since they can compromise between the material cost and heat transfer performance. Furthermore, although the investment cost (construction cost) of a CCS is significantly higher than that of the straight-line type and spiral coil type, its high heat exchange performance results in a shorter payback period (5.2 years compared with 12.0 years for straight-line and 7.0 year for spiral-coil) and higher internal rate of return (15.6% compared with 7.3% for straight-line and 13.9% for spiral-coil [2]) (Table 1). Based on the heat exchange efficiency and economic aspects, it is feasible to use the CCS as a new type of horizontal GHE to increase the total heat exchange capacity or reduce the area required for the installation and expect to spread the use of geothermal energy. In further study, the factors that may influence the performance of the CCS (i.e., weather change, rainfall infiltration, soil-atmosphere interaction) will be evaluated.

Design and Reliability analysis of energy pile using soft computing technique and a comparative study between the developed soft compu-ting models

Ashutosh Kumar — 2023-10-02

Geothermal or energy piles, are environmentally friendly piles that extract heat energy from shallow depths of the earth surface to heat or cool the structures constructed over them, such as multi-storey residential buildings, industrial complexes, and shopping malls [1]. Through the energy piles, the heat is injected into the ground or extracted from the ground by the fluid circulating mechanism inside the heat exchanger pipes (HEP). The fluid is passed through the (HEP), which are attached to the reinforcement cage of the pile foundation element [2]. The use of energy piles to meet the thermal needs of the built structures has proven to be both environmentally and economically viable, as well as having significant social benefits [3]. A lot of uncertainties are associated with geotechnical engineering applications, which are unavoidable as they deal with natural materials. The reliability analysis of geotechnical structures has gained much attention in the last few decades [4]. Therefore, this paper aim is to study the reliability analysis of the ultimate group capacity) of energy piles along with the comparative study of the models developed using soft computing techniques. In the current study, cone penetration test (CPT) was carried out at Manali, Chennai (India), which falls in earthquake zone 3. Furthermore, () of piles was determined using CPT data by considering various parameters (such as pile length (), pile diameter (), cone resistance, average cone penetration resistance, an average of minimum cone penetration resistance ), Young’s modulus (), temperature change (), and coefficient of thermal expansion ()) and then the reliability analysis was performed. IS 2911 (Part 1): 2010 was followed to find out the . The total load (mechanical load and thermal load) coming on a single pile was computed by assuming mechanical load as 100 kN and thermal load as stated in [5]. The average annual temperature variation of the ground surface was considered as 21 and the average yearly ground temperature as 13 to determine the thermal load applied to piles through the heat-circulating fluid (HCF). A cast-in-situ concrete energy pile of M35 grade was considered in this work. Based on the total applied load and of the pile, the various parameters of the bored cast-in-situ concrete energy piles (9.0 m × 0.7 m) were determined. Two U shape copper pipes having a diameter of 40 mm, and a thickness of 3mm, were embedded and placed 250 mm apart in the energy pile to carry fluid, i.e., water [5] as shown in Figure 1. A laminar flow having a heat-carrying flow inlet velocity of 0.183 m/s and a flow rate of 0.326 m³/h was considered for this work[6]. The thermal conductivity and heat capacity of the steel and concrete materials used in this work are {44.4 (W/m K) and 475 (J/kg K) } and {1.8 (W/m K) and 880 (J/kg K)}, and Young’s modulus of steel and concrete are 200× (MPa) and 32× (MPa), respectively [7]. The reliability index (β) was calculated using equation 1 for reliability analysis.

β = ( 1 )

where and are the standard deviation of capacity (C) and demand (D). After designing and selecting various parameters of the energy pile, 100 sets of data were generated randomly for,,

Thermal performance of energy barrettes in an urban environment

E. Ravera — 2023-09-28

Energy geostructures are renewable heating and cooling technologies that promise to massively decarbonize buildings with other shallow geothermal heat exchangers. However, the performance of these systems in urban environments can be affected by the presence of waste heat underground – a phenomenon associated with so-called subsurface urban heat islands. This study presents field experiments and 3-D time-dependent numerical simulations presented as a part of a broader study [1] to investigate the performance of energy barrettes: novel energy geostructures specifically targeting the structural support and renewable energy supply of tall buildings. This work particularly explores the performance of several energy barrettes included in a complex foundation located underneath the Testimonio II building ensemble in the Principality of Monaco – an extremely dense urban area. Testimonio II includes 348 housing units in two residential towers, a 50-place nursery, and a new site for the 700-student Monaco International School. The building has underground parking garages on multiple levels, supported by a complex foundation consisting of barrettes, piles, diaphragm walls, and slabs. Some of these foundation elements were thermally activated together with a vertical geothermal borehole field drilled underneath the building to contribute to its power supply.

Field experiments and numerical simulations were performed to investigate the behaviour of the considered foundation. The field experiments on energy barrettes consisted of thermal performance tests, which involve the continuous monitoring of key variables for the operation of geothermal heat exchangers and allow determining their energy performance over time. The developed tests involved the application of a constant heating power of approximately 3 kW to individual barrettes with different pipe configurations and/or geometries over timeframes ranging from 170 to 230 hours. Water with a flow rate of 0.2 kg/s was used as the heat carrier fluid. The equipment used for the tests made it possible to measure the inflow and outflow temperatures and the flow rate of the heat transfer fluid circulating in the pipes of the energy barrettes, as well as the outside air temperature. The resulting tests aimed at unravelling the thermal behaviour of energy barrettes as a function of the pipe configuration and the location of such heat exchangers at the site. Numerical simulations made it possible to obtain complementary information on the performance of the tested barrettes based on material parameters gathered through laboratory tests and the literature. The simulations were run with the software COMSOL Multiphysics (v. 5.5). Such simulations reproduced the energy barrettes, the ground, and the pipes embedded within them. The mathematical formulation employed for these analyses is presented elsewhere [e.g., 1] and thoroughly simulate mass transfer, heat transfer, and deformation phenomena within and around energy geostructures, such as barrettes.

Figure 1 presents the results of the developed experiments on two energy barrettes constructed at the site (called C07 and C17). A different pipe configuration was mounted on each of the wider edges of these barrettes, yielding three types of U-shaped pipe configurations (called hereafter U1, U2, and U3, with the U1 configuration involving a shorter U-shaped pipe mounted on one side of one barrette, whereas U2 and U3 involving a longer yet equal U-shaped pipe mounted on one side of two barrettes installed in different locations of the site) and a W-shaped pipe configuration (called W hereafter). For all four tests, the measured initial ground temperatures are relatively high for a temperate climate like the one characteristic of Monaco. This result is attributed to the presence of waste heat in the underground of the considered urban area, which features one of the highest population and construction densities across the world.The influence of the pipe configuration can be analysed by comparing the temperature trend characterising the energy barrettes with the U1 and U2 or U3 configurations. Accordingly, it is possible to remark that the energy barrette with the U1 configuration achieves greater temperatures compared to the barrette equipped with the U2 configuration. This result is due to the different pipe lengths in play, which involve a smaller heat transfer for the barrette characterised by a shorter U-shaped pipe. By comparing the temperature trend characterising the energy barrettes with the W configuration with the others, it can be assessed that the considered pipe configuration is the one leading to the highest heat transfer with the ground because of the longest pipe length into play. The influence of the site location can finally be appreciated by considering the thermal response of the energy barrettes equipped with U2 and U3 configurations. Despite some differences in the initial temperature, the thermal response of the considered barrettes is approximately the same when superimposed in a unique graph. This result indicates relatively uniform thermal properties at the considered site. Differences between experimental and simulation results range up to 17% for all pipe configurations, primarily concentrated in the first 4 hours. Afterward, the disparities decrease significantly, limited to a maximum of 2%. The differences observed are due to potential discrepancies between the actual and modeled initial conditions (such as non-uniform ground temperature) and variations in the features of the pipes in the barrettes (such as heat exchanger positions and lengths). However, the results obtained demonstrate the effectiveness of the modeling approach in simulating energy barrettes in real-world scenarios.

This study concisely investigated the thermal performance of rectangular energy barrettes using field experiments and 3-D finite element simulations. Conclusions of generalized relevance that can be drawn from this study are as follows:

Significant subsurface temperatures are observed in the Principality of Monaco due to a subsurface urban heat island. This peculiarity does not change the thermal response of energy barrettes as compared to situations exhibiting lower ground temperatures, but it does reduce their cooling thermal potential – an aspect to be considered in design.
For given thermo-hydraulic operational parameters, the thermal performance of energy barrettes equipped with W-shaped pipes is better than the performance of energy barrettes equipped with U-shaped pipes.

Numerical modelling of Energy Quay Walls to assess their thermal be-haviour

Marco Gerola — 2023-09-28

The use of the subsurface as a heating and cooling source through shallow geothermal installations has increased in the last decades [10]. So-called Energy Geostructures (EGs) represent a more innovative technology, serving the dual purpose of providing structural support to the ground/building and exchanging heat with the ground [2]. EGs can serve the purposes of several energy applications: they can be coupled with ground source heat pumps to provide space heating and cooling as well as domestic hot water.

Energy sheet pile walls (ESPs) are a novel type of EG consisting of sheet pile walls equipped with steel pipe heat exchangers. When they are used to support the banks of canals or in port docks, they allow extraction of thermal energy from both soil and water [12]. The latter are called Energy quay walls (EQW) and are currently experimented at selected sites along canals in the Netherlands [7]. The energy efficiency of EQWs is expected to be influenced by the undisturbed ground temperature profile, ground thermal conductivity and thermal capacity, and operational and construction parameters [8] [9]. However, due to the lack of standard methods for the design of EQWs, further studies are needed to fully understand both their thermal and thermo-mechanical behaviour [1]. Based on the data collected from an EQW test field installed in Delft (NL) (Figure 1(a)), a Finite Element (FE) numerical model was developed for the accurate 3D analysis of the heat exchange phenomena taking place in the EQW.

The EQW test site is characterized by two different heat exchanger types: the first one reaches 3 m depth and is aimed at exchanging heat mostly with the canal water. The second one has a depth of 15 m and exchanges heat with both water and soil. A monitoring system was installed composed by a total of 56 thermistors, 20 thermowells and 5 flowmeters to measure respectively the soil and canal water temperature, the heat exchanger fluid temperature and the flow velocity of both heat exchanger fluid and canal water. Different thermal activation combinations of deep and shallow loops were tested to assess the EQW thermal behaviour and the induced temperature changes into the soil [5].

To analyse in detail the EQW thermal performance, the FE numerical model was built with the COMSOL Multiphysics software. The considered heat exchange processes are convection within the heat-carrier fluid and between the canal and the soil, and conduction in the soil and other solid domains. Radiation phenomena were considered negligible in the model. The FE model was set up using the built-in “heat transfer in pipes” module allowing quicker computations by representing heat exchanger pipes in a simplified 1D fashion. The domain dimension was chosen in order to keep the lateral and bottom surfaces far enough from the EQW, to avoid boundary effects. For the boundary conditions, a fixed temperature of 12°C [3] was assigned to the bottom domain boundary while the detected air temperature time history was assigned to the top of the domain. Thermal insulation was assigned to the lateral boundary surfaces. The water canal was directly simulated as part of the domain. The heat convective flow of water was taken into account by using inflow and outflow boundary conditions and by assigning to the canal the detected water velocity. A thermal initialization calculation was performed to start the calculation with accurate initial conditions in terms of temperature distribution within the domain [6]. The thermal conductivity, specific heat capacity and density of the different soil layers were determined through empirical correlations based on cone penetration tests results carried out near the EQW [11], corroborated by literature datasets based on the geological characterization of the soil [4]. Heat exchange between the EQW and the surrounding soil was simulated by activating the heat transfer flow within the pipes, assigning the measured inlet temperature and fluid velocity.

An example comparison between the measured gained thermal power and the simulated results for the first 200 days of thermal activation of the EQW is shown in Figure 1(b). Except for the first 20 days of thermal activation, characterised by unreliable measurements because both the heat pump and the monitoring system were being tested to check the integrity of the system, it can be observed that the numerical simulation effectively reproduces the gained thermal power, as well as the induced temperature changes into the soil (not shown for the sake of brevity).

After model validation against the data collected from the test field research activities continue. Further numerical parametric analyses are performed to identify the best combination of design and site parameters to maximize the energy efficiency. Additionally thermo-mechanical analyses are used to assess the magnitude of thermally induced displacements and stresses.

An example of thermal retrofitting for the Piedicastello tunnel

Simone De Feudis — 2023-10-01

Over the last decades, global warming has become one of the major issues to cope with: as it stands, human activities are responsible for a global surface temperature increase of approximately 1.1°C since the pre-industrial age. Climate change is directly linked to the level of global warming, thus affecting different kind of regions around the world in multiple ways, leading not only to increments of heat extremes, but also to changes in rainfall patterns, sea level rise, amplification of the permafrost thawing, ocean acidification and much more [1].

In this context, carbon dioxide emissions due to heating systems are one of the main enemies in the pathway for reaching the 1.5°C Paris climate goal and energy geostructures could play a relevant role. Among them, the thermal activation of tunnels has raised interest [2, 3, 4] and recent full scale projects have demonstrated the feasibility of such technology [5, 6]. However, so far applications have been related to new tunneling projects, whereas possibilities of implementation in the existing heritage of tunnels have not yet been investigated.

The Authors are developing investigations for the thermal retrofitting feasibility of existing tunnels. In this abstract the attention is posed on the case study of the Piedicastello tunnel, located in the city of Trento, crossing the city centre through a 100 m high spur of limestone, called Doss Trento. This tunnel used to be part of the Italian roadway network but, following the construction of a new tunnel in 2007, it fell into disuse, being partly transformed into a museum one year later, i.e. the tunnel is now a closed environment.

One of the design hypotheses identified for thermal retrofitting of the Piedicastello tunnel consists in taking advantage of its 100 m overburden by drilling radial borehole heat exchangers (rBHEs), arranged along several tunnel cross sections. In Figure 1a, a 3D view of the rBHEs solution is shown. This solution leaves the lining intrados visible for future inspection and is essentially unaffected by the internal aerothermal condition of the tunnel.

In order to assess its thermal performance, some 3D numerical models were built with the Finite Element software Feflow [7]. The above-mentioned models are 200.0 m high and wide, with a thickness that varies as a function of the rBHEs cross section interaxis distance (Figure 1-b). The two horse-shoe shaped tubes of the Piedicastello tunnel have an external equivalent diameter of 12.7 m, with an 80 ÷ 130 cm thick concrete lining. Heat exchanger pipes, instead, were modelled through one-dimensional elements (“discrete features”), with a cross section of 201 mm², corresponding to an external diameter of 20 mm and a thickness of 2 mm.

Both thermal and hydraulic boundary conditions were set considering the lack of information about the groundwater regime of the Doss Trento and the internal aerothermal conditions of the Piedicastello tunnel. Nonetheless, given its unusual conditions (absence of moving vehicles and closed environment), the latter would likely have a minor role in the heat exchange process. Accordingly, setting adiabatic boundary conditions at the tunnel contour and considering a dry rock mass, a preliminary sensitivity analysis was performed with the aim of drawing some meaningful conclusions about two key design aspects of the rBHEs solution. To this purpose, three different borehole length setups (short – 12.50 m, mid – 18.75 m and long – 25.00 m) and three cross section interaxis distances (3.0 m, 5.0 m and 7.5 m) were investigated. The results of the preliminary numerical simulations are shown in Table 1.

From the sensitivity analysis, it seems evident that, in the considered cases and ranges: (i) as expected, the longer the rBHEs, the higher the heat that the system can exploit, accordingly the rBHEs optimal length should be designed considering also the pumping and drilling costs, (ii) two consecutive instrumented cross sections have virtually no interaction if the interaxis distance is set higher than 5.0 m.

Starting from these valuable evidences, future research will investigate: (i) the influence of the groundwater regime, as well as the internal aerothermal conditions of the tunnel, (ii) new alternatives for the thermal retrofitting of existing tunnels and (iii) the optimal rBHEs length with respect to a cost-benefit analysis that will consider both drilling and pumping costs.

Unveiling an underground climate change in the Chicago Loop with a district-wide sensing network

Anjali Naidu Thota — 2023-09-28

The subsurface temperatures of many urban areas are significantly rising, causing an emerging underground climate change, also known as subsurface urban heat islands (SUHIs) [3,11]. SUHIs result from two types of heat sources in the underground: large-scale drivers at the surface and localized drivers in the subsurface. Large-scale drivers consist of infrastructure at the surface that generate heat in the atmosphere, which eventually diffuses into the subsurface. Localized drivers consist of underground infrastructures that directly reject heat in the subsurface [8, 11]. The impacts of SUHIs lead to globally concerning issues that have detrimental effects on the biodiversity of subsurface ecosystems, public health, and subsurface transportation infrastructure [2, 5, 9]. Considering these impacts, it is crucial to understand the key variables and fundamental mechanisms that govern this silent hazard. The current literature mostly focuses on the intensity and effects of SUHIs that show highly heterogeneous temperatures around localized SUHI drivers [1, 4, 6-8]. However, limited information is available about the intensity and features of the sources (i.e., the localized drivers) of SUHIs. To explore this problem, this study summarizes the features and measurements of a unique subsurface sensing network deployed in the Chicago Loop district by Rotta Loria et al. [10] to monitor the temperature across a myriad of underground built environments and the ground. This facility enables to understand the inherent characteristics of the sources of SUHIs and underpin future studies devoted to the spatial and temporal evolution of SUHIs.

The sensing network includes >150 HOBO temperature sensors deployed in various underground structures (e.g., building basements, parking garages, train lines, pedways, tunnels, underground streets) as well as surface parks and streets. Figure 1 shows the relationship between the daily average subsurface and surface air temperature for the monitored parking garages, building basements, and metro tunnels. An analysis of the monitoring data reveals that the temperature in underground built environments is generally warmer than the surface air temperature during winter and cooler during summer. Furthermore, the temperature in such environments is markedly heterogeneous, with maximum values of up to 36°C. Temperatures within the same level of a considered environment (e.g., lower level x of environment X) can vary up to 15°C, and temperatures across different levels of the same environment (e.g., lower levels x and y of environment X) can vary up to 10.8 °C. The differences in air temperatures among the monitored environments can be attributed to the influence of different architectural and operational features, such as the materials constituting the envelope, the number of distribution channels and apertures, and the presence of ventilation systems and sources of waste heat, including human activity, underground transport, and operating utility equipment. An analysis of the hourly average temperatures in parking garages and building basements further reveals a surge in temperature during the working hours of the day. This rise in temperature during daytime hours was observed to be more prominent starting from March 2021, when COVID-19 restrictions started to become less stringent in Chicago, and people transitioned to a new normality. Such a surge in temperature was more evident in parking garages indicating a correlation between increased air temperatures and human and vehicular activity. Monitoring data referring to a depth of 4 m in the subsurface underneath Grant Park in the Loop and a depth of about 12 m in the heart of such a district underneath its buildings reveal that the ground temperature at such locations reads 11°C and 18°C, respectively. The significant difference between such results is attributed to the fact that the ground in the monitored park does not appear to be affected by sources of waste heat, whereas the ground in the heart of the Loop is indeed influenced by sources of waste heat. Specifically, with a temperature differential that can be as high as 25 °C compared to the ground temperature in Grant Park, the monitored underground environments in the Loop appear to be the key cause for the observed underground climate change.

This study yields two significant outcomes: on the one hand, a severe SUHI for the Chicago Loop district; on the other hand, an inherently heterogenous nature of temperatures within underground built environments, which arguably characterize the Loop and many other cities worldwide. Waste heat is continuously rejected into the ground as the temperatures of the localized drivers significantly exceed the undisturbed ground temperatures. A temperature variability characterizes localized drivers belonging not only to different categories (e.g., building basements and parking garages), but also to the same category (e.g., parking garages), with temperatures varying across the same level or different levels of a given environment. Therefore, the heterogeneous nature of localized drivers in SUHIs warrants consideration in robust modeling efforts and the data presented in this study serve as a resource for future simulations. Specifically, this study can foster a better understanding, utilization and management of subsurface energy resources. Understanding these aspects is crucial for the assessment of the geothermal energy potential of urban areas, the study of the variation in the thermal properties of geomaterials, and the optimization of the design and performance of geotechnical systems

Thermomechanical behaviour of soils and soil-structure interfaces in thermo-active geo-structures

Ali Golchin — 2023-10-02

In recent years, the demand for sustainable, low carbon-emission energy solutions has risen, leading to an increased interest in shallow geothermal energy systems. These systems, such as ground source heat pump (GSHP) systems, harness the Earth's stable temperature at shallow depths as a heat sink and source of energy. Energy piles are a type of GSHP system that performs both structural and thermal energy-exchange functions [e.g. 1]. Despite the technological advantages of these systems, understanding the thermomechanical behavior of soils and soil-structure interfaces remains a challenge, limiting safe, efficient, and cost-effective designs. This research aimed to address these challenges by developing thermomechanical constitutive models for soils, implementing these models in boundary-value solvers, and analyzing the experimental results from direct shear tests performed on soils to understand their thermomechanical behavior at the soil-structure interface. By comprehensively examining the complex interactions between energy piles and surrounding soils, the research results offer a foundation for improved design standards and implementation.

The methodology followed the following three main directions:

Development of thermomechanical constitutive models for soils: A thermomechanical constitutive model based on the thermodynamic framework of Hyperelasticity-Hyperplasticity was developed to capture the major thermomechanical behaviors of fine-grained soils when subjected to temperature variations [2]. By utilizing a flexible yield surface within the framework, a more precise representation of soil behavior can be simulated. This framework was extended to develop a dual-surface thermomechanical model with an additional yield surface and a temperature-dependent kinematic rule to capture the shakedown behavior of soils subjected to heating-cooling cycles more accurately [3], as illustrated in Figure 1 (left) where two triaxial tests are simulated. The enhanced model addresses the need for improved representation of diverse soil behaviors and their implications for the performance of energy pile systems.

Numerical aspects to improve simulation capability: A new flexible yield function was introduced that addresses the common issue of undesired elastic domains and erratic or divergent gradients in the numerical implementation of constitutive models with flexible yield surfaces and plastic potentials for use in implicit stress integration schemes [4]. By providing a robust and efficient framework for return mapping algorithms, this new yield function supports more precise simulations of soil behavior. An advanced numerical algorithm was devised to implement the thermomechanical constitutive model within a boundary-value solver such as the finite-element method [5]. The algorithm incorporates Gibbs energy potential, Lode angle dependency, and additional features to ensure accuracy, robustness, effectiveness, and convergence during simulations.

Experimental analysis of the soil-structure interface including the impact of initial shear stress and thermal cycles: Laboratory-scale direct shear tests were conducted on fine- and coarse-grained soils to investigate their thermomechanical behavior at the interface with a concrete structure [6, 7]. A focus here was on the impact of thermal cycles; in particular, under realistic working stress conditions. Figure 1 (right) presents example results, where thermal cycles are applied in a temperature controlled shear box, with a clay-concrete interface which had a constant shear stress. Thermal creep is observed, which also has a hardening effect on future shear loading. By exploring the variations in soil behavior under differing temperature and loading conditions, the research deepens the understanding of the complexity inherent in soil-pile interactions for energy pile applications.

This comprehensive investigation of the thermomechanical behavior of soils and soil-structure interfaces in the context of thermo-active geo-structures represents an essential step forward in optimizing energy pile systems for diverse applications. The developed constitutive models, numerical algorithms, and experimental analyses contribute valuable insights for improving energy pile design, ultimately leading to more efficient, safe, and cost-effective solutions.

Future research should continue refining our understanding of the complex processes governing soil behavior in energy pile systems, with emphasis on consolidating the developed models and their implementation for a range of soil types and conditions. Additionally, further experimental studies should focus on elucidating the interactions between soils, piles, and the broader environment to identify potential opportunities for maximizing the performance of energy pile systems through innovative design and implementation.

Analysis of the subsurface and fluid temperature monitoring by long-term operation of borehole heat exchangers

Makarakreasey King — 2023-09-28

Owing to the prevailing escalation in energy costs, escalating energy demands, depletion of conventional resources, and escalating greenhouse gas emissions, there has been an expeditious upsurge in the worldwide proliferation of alternative energy sources. Geothermal systems, in particular, are highly coveted as environmentally friendly and sustainable renewable energy reservoirs, offering efficacious cooling and heating capabilities for various structures (Xu et al., 2020; Ghasemi-Fare & Basu, 2013). The energy exchange efficiency of geothermal systems is significantly contingent upon the ground temperature, enabling the discharge of energy into the ground for cooling purposes in summer and its retrieval for heating requirements during winter. However, long-term operation of geothermal energy systems can engender an imbalance in thermal recovery through the heating and cooling processes, consequently influencing the thermomechanical behavior of the subsurface over time (Baek et al., 2017). Geothermal systems, also known as ground source heat pumps (GSHPs), are designed to operate for around 20 years, while the heat source component can be utilized for an extended duration ranging from 30 to 50 years. Understanding heat transfer characteristics and long-term performance of geothermal energy systems is crucial for optimal utilization and efficiency. However, research often lacks field investigation data, specifically for long-term monitoring and validation of fluid temperatures and subsurface temperature profiles. This research conducted comprehensive long-term measurements to assess fluid and subsurface temperature behavior in a borehole heat exchanger (BHE) system. The study examined the impact of sustained BHE performance on surface and subsurface temperature dynamics. A sophisticated three-dimensional numerical model was developed based on empirical insights from the field study and validated using monitored data. The analysis included inlet/outlet fluid temperature, spatial heterogeneity of ground temperature profiles, thermal recovery processes, and alterations caused by residual thermal energy.

Furthermore, additional temperature sensors were deployed at various depths to monitor temperature changes at distinct levels within the subground.

In this study, an advanced three-dimensional (3D) numerical model was developed utilizing the Finite Element Method (FEM) to effectively capture the intricate heat transfer processes occurring around BHEs. The geometry mesh was generated using a specialized triangular mesh generator, enabling spatial variation of element sizes to accurately represent the geometry and temperature distribution patterns. To address the specific requirements of U-tube heat exchanger loops, where the steepest temperature gradient is anticipated, a meticulously chosen element size of 0.7 mm, was employed. As the model extends towards the lateral boundaries, particularly in the radial direction, the element size gradually increases to ensure faithful representation of the system's characteristics. To properly account for diverse vertical temperature gradients, the number of elements and the vertical distance between slices were systematically adjusted within a range of 0.01 to 5 m. This meticulous variation facilitated accurate consideration of the distinct vertical temperature profiles. Notably, the finest discretization was applied near the bottom of the BHE (approximately z = -155 m) and at the top surface, characterized by a significant temperature gradient.

The depicted Figure 2 showcases the model's performance in analyzing the fluid profile in comparison to the monitored data over a duration of 600 hours. This comparison reveals that the maximum relative difference between the monitored and simulated values for the inlet and outlet fluid temperature does not exceed 2.649 and 2.296 degrees Celsius, respectively. These findings offer compelling evidence of the exceptional accuracy of the proposed numerical modeling approach, thus affirming its suitability for examining the long-term sustainability of BHE systems.

Shear behavior of the sand-concrete interface under cyclic thermal cycles

Kexin Yin — 2023-09-29

Energy piles - foundation elements also used as heat exchangers - are subjected to daily and seasonal heating and cooling cycles, which might result in the modification of thermo-mechanical behavior of the soil-pile interface. In order to evaluate the effect of the pile’s geothermal activation on a sand-concrete interface, at laboratory scale, a series of direct shear tests were performed using a device adapted for thermomechanical loading. The constant normal load (CNL) direct shear tests were carried out for three different thermal loading scenarios, i.e., at a constant temperature of 13°C, and for two different cooling-heating (8-18°C) cycles. The volumetric deformation after 10 thermal cycles is evaluated, and the shear results are compared.

A lot of studies [1, 2, 3, 4, 5, 6, 7, 8] have been performed to investigate the thermal effects on the shear behavior of soil-structure interface through direct shear tests. However, in previous studies all the thermal cycles were performed before the shearing phase. Hence, the effect of temperature cycles on the sand-concrete interface after the shear strength has already been mobilized (i.e., interface shear test) has not been well investigated so far. In fact, in real exploitation conditions, the thermal loading is usually added after a part of the mechanical loading or deformation occurred at the soil-pile interface, given the fact that the geothermal exploitation starts only once the building is in operation. To better understand the effects of thermal cycles on the shear properties of sand-concrete energy piles interface after a prior shear phase, a series of interface direct shear tests were performed in laboratory: (1) Reference Test (RT), shearing under constant temperature of 13°C (undisturbed soil temperature below 5 m in the Parisian region, France); (2) Cyclic Test 1 (CT-1), displacement controlled shearing under constant temperature (13°C), 10 thermal cycles (8 to 18°C), finally displacement-controlled shearing until the constant volume state was generated; (3) Cyclic Test 2 (CT-2), shearing to 1/2 of the residual shear strength, 10 thermal cycles (8 to 18°C), finally displacement-controlled shearing until the constant volume state was generated (CT-2 test). The monotonic displacement-controlled direct shear tests were conducted under constant normal stresses equal to 50 kPa, 100kPa and 150kPa that correspond to the typical normal effective stresses acting on the soil-pile interface at different depths.

Figure 1 presents the volumetric strain (ε_v) during the thermal cycles of CT-1 and CT-2. The sand samples are characterized by a contractive behavior during each heating phase and followed by an expansive behavior at the high temperature of 18°C. The samples continue to expand in the next cooling phase, then they start to contract when the low temperature of 8°C is constant. The volumetric strain changes are attributed to the thermal-induced particle rearrangement during the heating-cooling cycles. After the 10 thermal cycles, the sand samples show an overall contractive response. The thermal cycles induce more vertical strain on the samples of CT-2 than CT-1 (see Figure 1). The possible reason is that shearing to 1/2 residual shear strength develops more unstable particle rearrangement in CT-2 samples than shearing to the residual shear strength in CT-1 samples.

The peak and residual interface friction angles of CT-1 and CT-2 after the 10 cyclic thermal cycles are compared with RT in Table 1. The peak interface friction angle of CT-1 is 28.9°, 0.6° lower than the one of RT (29.5°). Whereas a higher peak interface friction angle is mobilized in CT-2 (31.7°). CT-1 is characterized with a residual interface friction angle of 27.9°, which is 0.5° lower than the 28.4° of RT. However, the difference of residual interface friction angles between CT-2 (28.5°) and RT (28.4°) is only 0.1°. To conclude, the effect of the 10 cyclic thermal cycles on the sand-concrete interface is quite limited, it slightly decreases the peak as well as residual interface friction angles of CT-1 and increases these of CT-2.

Casting and installation of segmental precast quadratic concrete driven geothermal energy piles

Habibollah Sadeghi — 2023-09-26

Geothermal energy pile foundations are used both for structural purposes and to provide sustainable, clean, and cost-effective ground energy for heating and cooling buildings [1]. The majority of energy piles are cast-in-place concrete piles, which require extensive and expensive drilling during construction. A better alternative is to use precast concrete segmental driven energy piles, which can be cast in large quantities with high-quality assurance at a concrete factory, after which they are transported and installed into the ground [2]. This study aims to present a novel method of casting and installing segmental energy piles that utilize an innovative steel joint [3] for connecting the precast concrete segments on-site. In addition to structural integrity, the steel joint provides the possibility of a leak-proof coupling and continuity for heat-exchanging pipes embedded in concrete segments. The precast-driven pile foundations are made of high-strength concrete with a 14-day compressive strength of almost 80 MPa and have a quadratic shape with a maximum length of 12 m and a square width of 270 mm or 350 mm. In the 270-mm piles, a single U-loop can be used, while in the 350-mm piles, two U-loops can be used as presented in Figure 1.

The heat-exchanging pipes are coupled inside the sidewall channels of the steel joint which are shielded using a steel cover plate, riveted to the joint. At the tip of the bottom segment of the piles, there is a strong steel rock shoe with a hardened steel point that allows the piles to penetrate the bedrock [4]. At a construction site, the joint and pipes can be connected quickly without any welding or electric equipment that expedites the installation work and obsoletes the health and safety risk associated with electricity. Utilizing the precast energy piles presented in this study increases the quality, and speed of installation while decreasing the overall cost of the project compared to cast-in-place energy piles.

To prove the proper performance of the piles under real construction conditions, structural integrity tests and hydraulic pressure tests were performed according to BS EN 12794:2005 [5] and ASTM F2164–21 [6], and the results are briefly presented and discussed in the present study. Structural integrity tests consisted of impact tests in which 1000 blows imposing a minimum stress level of 28 MPa were applied on the pile segments connected using the new driven energy pile joint. During the impact tests, the pile structure and the joints remained undamaged. After the impact tests, the pipes were pressurized up to 100 psi (690 kPa) and no leakage or pressure drop was observed. Then the pile segments were cut according to the standard into shorter segments and subsequent bending tests were performed to measure the bending capacity. The bending tests show that the driven energy pile joints have a similar bending capacity as conventional normal joints. The results of this study show that segmental precast concrete energy piles using the new steel driven energy pile joints, are a perfect alternative for cast-in-place energy piles, which can provide sufficient structural and bearing capacity and can be used both for heating and cooling buildings.

Design a geothermal bridge deicing system for an in-service bridge

Gang Lei — 2023-10-02

Deicing on bridge deck surfaces is essential to ensure the safety and mobility of motorists during severe winter weather. Conventional snow/ice removal chemicals such as salt cause bridge corrosion and provoke environmental issues. Therefore, a new external geothermal heating system for bridge deck deicing and snow melting has been proposed, which can be installed onto in-service bridges. Conventional hydronic heating systems are developed based on the hydronic loops embedded inside bridge decks in the construction phase and have been studied by different research groups [1–3]. However, there is a greater need for deicing the in-service bridges. Therefore, an external heating system has been developed by attaching the hydronic loops to the bridge deck's bottom surface, which is usually covered completely with insulation materials such as geofoam, spray foam, and fiberglass [4]. To provide access to the bottom surface of the bridge deck for bridge inspection, a new external-geothermal hydronic heating panel system has been developed in this study.

Studies on geothermal bridge heating systems mainly focused on energy analysis of the existing systems, thermal effects of control logic, and the development of models to simulate bridge heating systems neglecting the piping network. This study presents the design of a bridge external heating system for an in-service bridge located in the Dallas/Fort Worth Metroplex in North Texas, USA, to maintain high heating efficiency. The geothermal deicing system includes geothermal heat pumps, ground loops, and bridge loops. The required heat flux on the bridge deck surface to prevent ice formation is calculated based on the local weather data to determine the number of heat pumps. According to the total heating load determined from the design heat flux, geothermal water-to-water heat pumps are sized given the ground temperature profile and thermal conductivity test results. The pipe network is designed based on the target flow rate and standard parameters for pipe materials using Taco Hydronic System Solutions software. Figure 1 illustrates the heating system's schematics. Only one bridge span is shown in the figure. One bridge span includes four heating panels with a 6-inch gap between each panel as the access for bridge inspection. The control room hosts heat pumps, flow centers, manifolds, control, and monitoring instrumentation. The ground loop arrays are not shown in the figure.

The deicing system design procedure consists of three phases: (1) selection of design weather condtions based on historical weather data, (2) calculation of the required heat flux on the bridge surface, and (3) sizing of ground loop heat exchangers (GHEs). The final design decisions stem from the integrations of the three phases. First, design weather conditions are selected based on realistic winter storm scenarios. Second, the required heat flux, 80 Btu/hr.ft2, on the bridge deck surface to prevent ice/snow formation is determined by considering previous design values, historical winter storms, and heat flux calculations by ASHRAE [5]. Lastly, based on the required heat flux, the GLHEs are sized using the design software GLHEPRo developed by Spitler et al. [6]. Inputs to the model, including the heating loads, the thermal recharge rates, a description of the ground thermal properties, a description of the borehole geometry, the fluid physical and thermal properties, and a description of the heat pump, are summarized in Table 1. The ground thermal properties are determined based on field thermal response tests. A 30% propylene glycol solution is specified as the heat transfer fluid. A borehole diameter of 15.24 cm, an HDPE single U-tube with a nominal diameter of 3.18 cm, and thermally enhanced grout are also specified.

Aside from heating system design, pipe network design is critical to maintaining the target flow rate for every pipe branch. Therefore, pipe hydraulics is analyzed for pressure loss to ensure sufficient head pressure for the entire pipe network. This study evaluated piping network design based on target pressure loss and common design parameters such as pipe materials and installation types. The pipe network design results show that the final system consists of 4 heat pumps and 10 water pumps. Combinations of pipes in parallel and series are arranged for efficiency and operational safety. The heat pumps are designed to supply fluid to the bridge deck at approximately 38 ℃ with a total flow rate of 302.8 l/min. The borehole field configuration selected consists of 16 boreholes, each 91.5 m deep and a design flow rate of 18.92 l/min. The borehole field will supply fluid to the heat pumps at a minimum temperature of 18 ℃. The geothermal bridge deicing system is currently being installed onto the bridge and is expected to be completed by the summer of 2023.

Thermal energy storage with tunnels in different subsurface conditions

Annik Schaufelberger — 2023-09-28

The widespread use of the underground and global climate change impact the urban subsurface temperature. Changes in the subsurface environment can affect the performance of underground thermal energy storage systems, especially when convection may characterize such systems in view of augmented thermal losses. This work focuses on tunnels equipped with ground heat exchangers, typically called energy tunnels, to serve as seasonal, medium-temperature underground thermal energy storage systems (UTES). Besides their structural purpose, energy tunnels can be used to inject, store and extract heat from the ground by means of a heat carrier fluid circulating through an integrated pipe system embedded within them. By storing thermal energy during periods of overproduction and utilizing it during high-demand periods, energy tunnels help reduce reliance on non-renewable energy sources without the need of additional infrastructure.

Specifically, this work addresses the storage performance of energy tunnels in different subsurface environmental conditions influenced by convection through 3-D thermo-hydraulic finite element simulations validated against full-scale experimental data. The results of this study are described in detail by Schaufelberger et al. [4].

The focus of the study lies on the influence of convection heat transfer driven by groundwater flows and airflows, whose properties may be altered by rising temperature in urban areas due to subsurface urban heat islands. The rationale behind this work is that Rotta Loria [3] recently highlighted promising storage efficiencies of up to 70% for energy tunnels characterized by favourable subsurface conditions for storage applications (i.e., lacking convection heat transfer). However, knowledge about the performance of energy tunnels in the presence of convection heat transfer has remained limited to date, representing a barrier for the practical implementation of such systems.

Simulations are run with the software COMSOL Multiphysics® [2]. 30 cycles of a seasonal, medium-temperature storage system are simulated by considering an infinitely long tunnel, composed of 1.4-m-wide lining rings (thermally activated over their complete length), and the surrounding ground, drawing from the geometry of the case study reported by Barla et al. [1]. One cycle corresponds to a 6 months heat injection interval with a fluid inlet temperature of 57.3°C, followed by another 6 months of energy extraction with a fluid inlet temperature of 17.3°C, which corresponds to a flow characterized by a temperature equal to the initial ground temperature. A Cauchy boundary condition applied on the interior lining represents heat exchange with tunnel air, with the convective heat transfer coefficient (positively correlated with airflow velocity) and the air temperature as input parameters. Three distinct parametric studies investigate the influence of: (1) groundwater velocity , (2) air convection heat transfer coefficient and temperature , and (3) thickness and thermal conductivity of an insulation layer on the storage performance. Thermal losses and storage efficiency are computed to characterize and compare the resulting performance for each set of parameters.

When considering a subsurface environment without convection heat transfer, a storage efficiency of about is reached for the considered case study. This evidence results from slightly different geothermal parameters between the present work and the previous case study considered by Rotta Loria [3].

Meanwhile, complementary results obtained in this work show that energy storage via tunnels is advisable only when limited groundwater flow and airflows are present at sites, otherwise pronounced heat losses are induced by convection heat transfer.

Thermal losses are increasing nonlinearly with the applied convection heat transfer coefficient , and thus with increasing airflow velocity (Figure 1(a)). Moreover, an increase in underground temperatures, which may result from subsurface heat islands, has a positive impact on the storage performance of tunnels. Higher average tunnel air temperatures (, , ) allow an augmentation in extractable thermal energy thanks to direct harvesting of heat from the tunnel air (Figure 1(a)). At the same time, a lower amount of injected heat can be achieved during the storage interval due to a lower temperature gradient between carrier fluid and tunnel environment. As a result, thermal losses are reduced in case of warmer tunnel air, leading to a significant increase in efficiency (Figure 1(b)). In case of higher air temperatures, the increase in storage efficiency is more significant over the first couple of storage cycles compared to lower temperatures. Nonetheless, even for high air temperatures, the efficiency drops when it comes to an increase in airflow velocity that may be induced by ventilation and train movements (Figure 1(b)). This effect can be reduced with help of an insulation on the inside of the tunnel lining, increasing the overall thermal resistance.

This study aimed to identify impacts of changes in subsurface environments on the thermal energy storage performance of underground tunnels used as heat exchangers. The findings indicate a positive influence of subsurface temperature rises on the thermal energy storage performance of underground tunnels. Meanwhile, the findings indicate a generally detrimental role played by convection heat transfer for the performance of such systems. Based on the result of this work, it is concluded that underground environments with limited convection or significant thermal insulation are necessary to ensure satisfactory thermal energy storage performance of energy tunnel systems.

3D FE modeling of a real-world energy piled foundation

Arianna Lupattelli — 2023-09-29

Nowadays the growth in the consumption of fossil fuels, especially in the building sector, has become increasingly impacting, being closely related to the worsening of atmospheric pollution. Numerous efforts are therefore being made to resort to the use of technologies that allow the use of clean energy [1, 4]. The heat contained in the most superficial layers of the Earth's crust is called Low Enthalpy Geothermal Energy (LEGE) and represents one of the most common forms of renewable energy [3]. This resource can be effectively exploited in various applications: among these, Energy Piles (EPs) associate the traditional mechanical function with the energetical one by exchanging the heat with the ground [2, 6, 7]. The heat exchange is obtained by means of a fluid that circulates in pipes anchored to the reinforcement cages of the piles. This system allows for satisfying a large part of the energy needs of a building, providing for heating and cooling according to seasonal needs.

In this work, a case study of a private building located in the area of Perugia City (central Italy) is presented. The building is founded on n.55 bored and cast-in-place piles, with a length of 10 m. To supply the heating and cooling needs of the building for about 290 m² living area, all the piles have been instrumented to operate as heat exchangers with the ground. Thermo-energy monitoring was designed for n.4 foundation piles: for each one, n.10 K/PT100 thermocouples were installed in 5 cross-sections; other n.12 thermocouples were installed in the ground along 4 vertical metal pipes, placed symmetrically at distances of 0.5 and 1.5 m from the pile axis, and at 3 depths.

To experimentally study the Thermo-Mechanical (TM) behavior of the soil involved in the heat exchange process during the EPs operation, an initial TM characterization has been performed by means of laboratory tests. For the mechanical characterization of the soil under thermal load, thermally-controlled direct shear (DS) tests have been performed; while for defining the soil thermal properties, a Hot Disk TPS 2500S apparatus and an ATT DM 340 SR climatic chamber have been used. As for the DS tests, soil samples have been obtained from the cores extracted on-site at a depth of 10 m. The aim of the experimentation was to evaluate the effect of temperature on soil strength and at the soil-concrete interface, by using a modified direct shear test equipment described in [5]. Specifically, in the traditional shear box, a silicon heating mat featuring a nominal power of 2.5 W has been installed at the bottom of an R_ck350concrete plate to heat the sample. A temperature probe has been inserted into the plate, and the device has been connected to an external control box, with temperature regulation. Two different cement plates were used: a first one of 9 mm thick, to test the influence of temperature in the soil sample; and a second one of 19 mm thick, equal to the upper edge of the lower half-box, to allow the execution of the test at the interface between the two materials. The Hot Disk TPS 2500S apparatus has been used to estimate thermal conductivity, thermal diffusivity, and volumetric specific heat (with a nominal error of 3%, 5%, and 7%, respectively) of the samples, according to the ISO 22007-2 standard, which describes the Transient Plane Source (TPS) method. The experimental methodology was the following: i) drying of all the samples at 100°C, up to weight variation lower than 0.1%, and partial saturation of some, with a water content within the range 19–36%; ii) samples placement in the controlled environment of the climatic chamber, which can be artificially controlled in terms of temperature (within the range -40–80°C ±1°C) and relative humidity RH (within the range 10% ± 3%). The thermal properties have been evaluated after exposing the dry and partially-saturated samples to the following climatic conditions for at least 3 days and upon weight stabilization (0.1% variation).

Referring to the available data of the building design and to the experimental results obtained, a 3D Finite Element (FE) model of the EP foundation has been reconstructed to analyze its thermo-hydro-mechanical behavior during normal operating conditions, using the FE code Comsol Multiphysics. The geometry of the entire foundation has been modeled in the central part of a volume of homogeneous soil. The domain has a dimension of 200 x 200 m, and the bottom boundary has been placed at a depth of 50 m from the ground surface. All the problem domain has been modeled and discretized, adopting a user-defined mesh with tetrahedral elements (Figure 1). Piles have been modelled with a linear-elastic material, the soil has been modelled as linear elastic–perfectly plastic with a non-associated Mohr-Coulomb failure criterium, while the pile-soil interface has been considered as perfectly rough. The mechanical load has been modelled as a uniform pressure acting on the top surface of the raft. An initial constant temperature of T₀=20°C has been assumed throughout the domain, and the temperature of each single EP has been increased/decreased linearly to simulate the normal operation of the EPs system, for a total duration of 1 year. The results of TM interaction between the EPs, considering different possible configurations of thermal activation/inactivation of EPs groups, have been analyzed. The axial load and the thermally induced vertical displacements in the EPs were found to strongly depend on pile spacing. Significant thermal interaction effects between EPs may occur for closely spaced thermally activated piles, in the case of cyclic heating and cooling. A sensitivity study performed on the thermal properties of soil, depending on the considered water content, confirmed that proper soil thermal characterization is of great importance for an accurate prediction of the THM effects. Since the site is still under construction, and the monitoring system is not operating yet, further analyses will be devoted to comparing the numerical results with the temperature distribution field obtained from the monitoring system.

In-situ experimental study on heat exchange capacity of long-span energy tunnel exchangers

Wen Guan — 2023-10-02

Energy tunnel is a new type of geothermal exchanger in ground source heat pump systems due to their technical and cost improvements over traditional borehole geothermal exchangers. The main reason that the factors affecting the heat exchange capacity of energy tunnel are unclear makes its development be restricted. An in-situ full-scale study using the thermal response tests (TRT) and thermal performance tests (TPT) is performed to investigate the factors affecting the heat exchange capacity in Badaling Energy Tunnel of Beijing-Zhangjiakou High-speed Railway. This paper analyzes the influences of constant heating power, inlet water temperature, air temperature in tunnel, circulating water flow velocity, operation mode, and pipe connection on the heat exchange capacity of the energy tunnel. The test results reveal that (1) the heat exchange rate of the energy tunnel decreases with the increasing constant heating power, (2) the heat exchange rate of the energy tunnel is positively proportional to the inlet water temperature, (3) the heat exchange rate of the energy tunnel at a circulating water flow velocity of 1.1 m³/h is larger than that at 0.5 m³/h, it is smaller than that at 0.8 m³/h, (4) the air temperature in tunnel has a great effect on the heat exchange rate of the energy tunnel, and the pipes connected in parallel or connected in series have a slight effect on the heat exchange rate of the energy tunnel, (5) the average heat exchange rate in an intermittent operation mode is approximately 18.5% larger than that in a continuous operation mode. The test results can be used for the thermal design of energy tunnels.

In-situ investigation on the effects of groundwater flows around bore-hole heat exchangers

Valériane Gigot — 2023-09-28

Although vertical Borehole Heat Exchangers (BHE) is a booming technology for both cooling and heating buildings, several improvements could still be proposed in the dimensioning of such systems. Nowadays, most of the dimensioning methods consider only radial conductive heat flux around BHE using a homogeneous ground thermal conductivity determined from thermal response tests (TRT) or tables. Impacts of groundwater flows on the heat refurbishment around BHE are generally not explicitely considered.

Many numerical or analytical studies have investigated and quantified the positive impact of groundwater flows on the performance of BHE [4]. However, those results are rarely compared and validated with in-situ temperature measurements around BHE. Such measurements require the installation of temperature sensors in the ground around BHE.

In this work, an experimental platform composed of 4 vertical BHE drilled at depths of 85 m has been exploited. The 4 vertical BHE cross a succession of horizontal geological layers (Fig. 1). The study focuses on the heat transfers in a 30-m thick sand unconfined aquifer layer, whose 17 m are saturated. Each BHE is equipped with PT100 (installed at the extremities of the unconfined aquifer and just below the groundwater table level). Based on the expected direction of groundwater fluxes, the upstream BHE is thermally activated with a pre-determined heat injection and duration. The temperature evolution is recorded by means of PT100 sensors in the activated BHE and in the three non-activated BHEs. Groudwater velocity in the upper part of the aquifer is characterized through a non-convential tracer test [1] performed in a piezometer drilled at the center of the 4 BHE (v= 7 10^-7 m/s).

A clear impact on the groundwater flows on the temperature field in the aquifer around the activated BHE is observed. To quantify the heat transfers in the ground around the activated BHE, a methodology was developed to infer the hydro-geothermal parameters of the ground, namely the intrinsic thermal conductivity, the volumetric heat capacity and groundwater velocity and direction. From an analytical solution considering conductive, advective and dispersive heat transfers [2], the hydro-geothermal parameters of the ground are obtained by fitting the measured to the predicted temperatures evolution (Fig. 2 – Table 1). The obtained hydro-geothermal parameters demonstrate (i) a groundwater velocity in the upper part consistent with the value measured in-situ, (ii) the important role of the saturated aquifer that significantly enhances the apparent thermal conductivity of the ground and, in case of groundwater flows, induces an anisotropic propagation of the temperature plume [3](Fig.3), as well as (iii) the non-uniform groundwater flows along the saturated part of the aquifer (Table 1).

A case study of thermal interaction at urban scale

Maria Romana Alvi — 2023-09-29

Due to the rise of awareness regarding the problem of climate change and greenhouse gas emissions, the European Union has increased its involvement in climate change mitigation policies. For this reason, investing in renewable energy sources has become urgent and mandatory.

Geothermal energy represents a large source of environmentally friendly energy with a low carbon footprint [1] and, thus, shows a high potential to help supply the thermal demands of buildings in urban areas which are usually characterized by a high-density population. However, especially in the shallow depths of urban areas, it is difficult to find natural undisturbed underground thermal conditions because of anthropic interventions. Moreover, these areas are being increasingly used for energy purposes, for example implementing shallow geothermal systems, as a proficient technology to provide clean thermal energy and supply thermal demands of buildings in both winter and summer seasons [2].

This abstract will focus on thermal interactions in urban areas with reference to the case of the city of Turin where, within the project of Metro Line 2 (ML2) currently at its outlined design stage, the thermal activation of both Cut&Cover and TBM tunnels and of metropolitan stations is envisaged, with the creation of an urban energy geostructure. This infrastructure will extend, in a first stage, in the central districts of the city for about 10 km with 1 depot, 13 stations and 14 shafts and will represent an essential milestone for the city transportation system, reaching decentralized areas and revolutionizing the surface space [3].

The subsurface in the central area of the city is characterized by sand and gravel deposits overlying, at approximately 40 m depth, clay layers, resulting in a very productive aquifer. Consequently, the geothermal and industrial use of water is of extreme importance [4] and more than 30 geothermal open loop systems are present, based on the public authorities databases. Together with a smaller number of geothermal closed loop systems, they exchange heat according to the user's seasonal cooling and heating demands, exploiting the subsurface resource. Moreover, anthropic entities such as underground car parks, urban tunnels and building basements exchange heat fluxes with the ground as well, affecting indirectly the aquifer and subsoil both hydraulically and thermally.

With the construction of the ML2 and the thermal activation of its structures, the actual conditions may be affected. It is indeed of interest to understand and evaluate the interaction between the ML2 and the other existing systems. To this extent, a 3D Finite Element numerical model with thermo-hydraulic coupling of the city area crossed by the first two ML2 lots was built by using the FEFLOW software [4]. Thanks to the interoperability of the numerical code and the Geographic Information System (GIS), it was possible to include in the model all the relevant information collected about hydrogeology, existing and planned geothermal systems and anthropic structures. Adequate material properties and boundary conditions were imposed (Figure 1(a)) on the basis of the data collected during the geognostic and hydrogeological survey campaigns of the outlined design. To reproduce the current thermally and hydraulically disturbed subsurface environment, the model was run to simulate a 5 years lifespan where all the existing systems are active and validated against available monitoring data. Subsequently the thermal activation of the ML2 was introduced and its effect on the subsurface was predicted for a period of 4 years.

Finally, through GIS processing, the results allowed to extrapolate thermal maps of the study area (Figure 1(b)). The modelling showed that the thermal alteration due to the ML2 is limited to the corresponding depths of its structures and downstream of the infrastructure. The central district of the city, where the largest number of existing geothermal systems are present, is the area that is shown to be most influenced and where the thermal plume of existing open loop systems may locally affect the thermal exploitation from the ML2 structures. It is also shown that open loop systems in the city impact massively the ground temperature more than the energy geostructures of the ML2 do.

Heat transfer performance of large energy pile group in a hybrid system subjected to imbalanced thermal load - A numerical investigation

Wanqi Tian — 2023-10-02

Introduction

When a energy pile system is coupled with other heating or/and cooling systems, determining the share of thermal demand that energy piles can meet is crucial in designing the size of each system. In the long term, imbalance in thermal load can affect the heat transfer performance of energy piles, and may also lead to annual accumulation of ground temperature over the years, ultimately causing system failure. Furthermore, the mechanical responses of energy piles are also affected by the temperature changes in the pile body and surrounding soil during operation. While Current researches have predominantly focused on mechanical response under temperature load, understanding the heat transfer performance of energy pile groups is equally necessary. Existing studies on heat transfer largely draw from ground source heat pump (GSHP) systems, despite differences in depth and diameter between energy piles and borehole heat exchangers (BHEs), particularly for closely spaced piles. To address this gap, we conducted a series of numerical experiments based on the Tsinghua Science Museum project to investigate the thermal behavior of energy pile group under annual, imbalanced load and the influence of pile spacing. Based on simulation results, we provide design recommendations on how to determine the proportion of heat load that can be carried by energy piles in hybrid systems.

Method

We utilized COMSOL software to conduct three-dimensional modeling of a single pile, and developed a two-dimensional model of a pile group using a self-programmed Matlab program. The accuracy of the model was evaluated by comparing it with the results obtained from energy pile field tests (thermal response tests and thermal performance tests) conducted in Shunyi, Beijing. Subsequently, we established two-dimensional and three-dimensional numerical models for the Tsinghua Science Museum project in Beijing. We performed a series of numerical experiments to investigate the behavior of energy piles under balanced and imbalanced temperature loads for one year. Furthermore, we simulated the thermal responses of energy pile groups with different pile spacings of 2m, 2.85m, and 4m.

Results

The cooling load surpasses the heating load causing an imbalanced load. The soil temperature rises noticeably by 2.6 °C compared to the initial state after an operation period of 1 year, indicating an undesirable phenomenon of soil heat accumulation. With continuous running over years, the system will fail due to the excessive soil temperature. Moreover, the heat exchange liquid temperature ranges from -4.35 ℃ to +39.65 ℃, which exceeds the acceptable range of 2 ℃ to 33 ℃. To address this concern, a hybrid system can be utilized where the energy pile system bears a balanced portion of the thermal load, and the auxiliary system bears the remaining load. Simulation under various thermal load proportions reveals a linear relationship between the maximum/minimum outlet temperature and the total heat exchange volume. An energy pile system undertaking 60% heating load and 40% cooling load can run stably for long periods. The soil temperature varies in a range of 8.5 ℃ to 17.9 ℃, while the pile body temperature has a variation range of 4.0 ℃ to 22.8 ℃ (Figure 1).

The study results for various pile spacings are presented in Table 1. Pile spacing plays a significant role in soil heat accumulation and heat exchange fluid temperature. Despite an invariable pattern in the inflow and outflow temperature curves for varied pile

spacing, the flow temperature range is significantly influenced. The smaller the spacing, the more prominent is the thermal accumulation in the soil, leading to larger changes in soil temperature and the flow temperature. During design simulation, if the range of flow temperature exceeds the limits of the energy pile system, it may present a safety risk during system operation. In such cases, increasing the pile spacing can render the system more reliable and safe.

Conclusions

(1) Whether the system can operate normally can be judged by whether the simulation result of the heat exchange liquid temperature exceeds the limit value. There is a linear relationship between the flow temperature and the total heat exchange. Therefore, the proportion of the load that the energy pile system can bear in the hybrid system can be determined. For the Tsinghua project, the energy pile system can bear 60% of the heating load and 40% of the cooling load.

(2) The pile spacing affects the magnitude of variation in heat exchange fluid temperature by influencing the degree of heat accumulation in soil. If the range of flow temperature exceeds acceptable limits, it is advised to increase the pile spacing during the design phase to ensure the energy pile system can operate effectively.

Interpretation of Thermal Response Test (TRT) in energy piles using Bayesian Inference

Norma Patricia López-Acosta — 2023-09-28

An efficient design of energy geo-structure systems requires an accurate characterization of the structure and ground thermal parameters (e.g., thermal resistance of the ground heat exchanger

Numerical investigation on the thermal performance of energy tunnels under groundwater flow and tunnel airflow

Xiangdong Dai — 2023-09-29

The integration of underground tunnels with ground heat exchangers (GHEs), also known as energy tunnels, is a promising technology that has gained attention in energy geotechnics research. This study investigates the coupled effects of groundwater and tunnel-air flows on the energy tunnel system via 3D thermo-hydraulic numerical modelling. It is found that the combination of groundwater flow parallel to the tunnel and limited airflow velocity results in reduced operational efficiency of ground source heat pump (GSHP) due to the strong thermal interference along the tunnel. The study also highlights the importance of real-scale modelling to evaluate the thermal yield of long energy tunnels, particularly when dealing with parallel groundwater flow in geothermal tunnel design.

The use of shallow geothermal energy is a renewable solution towards net-zero carbon emissions. In recent years, a variety of conventional geotechnical structures, such as piles, retaining walls and tunnels, have been transformed into energy geostructures to also serve as GHEs to harness renewable energy for space heating and cooling purposes [7]. The thermal performance of energy tunnels is significantly affected by the presence of groundwater flow [1, 6] and the airflow in the tunnel [2, 4]. The groundwater flow and tunnel airflow can both be present in the field, however, their coupled effects on the energy tunnel system are hardly explored in the literature. This study aims to bridge this gap via real-scale numerical investigations.

This study uses 3D thermo-hydraulic numerical modelling developed with the finite element package COMSOL Multiphysics [3]. The numerical model was successfully validated against an energy tunnel model-scale laboratory experiment [5]. Figure 1 shows the overall geometry and boundary conditions of the numerical model. A 48 m long tunnel section is thermally activated with the embedment of HDPE pipes (outer diameter of 25 mm and 2.3 mm thickness) placed in the lining segments at a spacing of 200 mm. A single tunnel ring consists of six segments with absorber pipes joined between them. Every four adjacent rings are connected in series to form a complete loop (with a single inlet and outlet), which is then connected in parallel to a flow and return header pipe(s). The entire thermally activated section comprises six loops, with identical fluid inlet temperature and flow rates, as they are fed by the same flow header pipe. The outlet fluid from each loop is mixed along the return header pipe to the GSHP. A constant 36 kW cooling thermal load is applied, and the simulation is run for ten years to allow the system to reach a steady state condition considering the minimum groundwater flow and airflow. The thermal properties of the ground are prescribed as 2 W/(m·K) and 1100 J/(kg∙K) for thermal conductivity and specific heat capacity, respectively.

Figure 2 shows the entering water temperature (EWT, header outlet) and the coefficient of performance (COP) of the GSHP after 10 years of operation for different groundwater flow velocities and directions for tunnel air velocities = 0.05 m/s and 2 m/s.

Results show that an increase in and leads to a decrease in EWT and an increase in COP. However, the effect of groundwater direction and velocity is more significant when airflow is limited ( = 0.05 m/s). The parallel groundwater flow leads to a higher EWT and lower COP than when the flow is inclined and perpendicular to the longitudinal tunnel axis. Consequently, the thermal performance of the system under parallel flow with a higher can be even worse than the perpendicular and inclined flow with a lower . When = 0.05 m/s, the EWT of parallel flow increases by 15% to 20% compared to the perpendicular flow, resulting in a maximum decrease of 15% in the COP. The effect of groundwater flow is much less significant when airflow is fast ( = 2 m/s): for increases from 0.05 to 2 m/d, EWT and COP change by no more than 13% and 5%, respectively, and the impact of groundwater flow direction is minor.

Thermally induced shear displacement of sand-concrete interface under different stress levels

Mouadh Rafai — 2023-10-02

Energy geostructures are modern technologies developed to exploit shallow geothermal energy. Their installation has recently increased rapidly throughout the world. Their use is devoted to the heating and cooling of buildings, and their operation can induce thermal cycles at the soil-structure interface in the range of 5–40 °C [1]. In addition, their activation would produce thermally-induced stress and strain in structure. Over the last few decades, several full-scale tests have been performed to study the geotechnical aspects of energy piles [2,3]. The main results show that the thermal operations may slightly influence the mechanical performance of the energy pile by increasing the additional thermal compressive stresses and changing the pile axial strain. Numerical studies [4,5] concluded that the temperature increment can lead to thermal expansion of the pile, causing its uplift and the generation of negative shear forces at the pile–soil interface near the pile head. Considering that the thermal operation of energy geostructures leads to temperature change along the piles as well as in the neighbouring soil due to the heat exchange phenomenon, many researchers have investigated the temperature effects on the behaviour of soil and soil–structure interface [6,7,8,9]. Others investigated thermally induced soil deformation using modified triaxial or oedometric apparatuses allowing temperature control [10,11,12]. These results show expansive deformation of coarse-grained soils upon heating and contractive deformation upon cooling for all relative densities. A modified geothermal direct shear box (MGSB) developed at Gustave Eiffel University [13] was used to investigate the effects of temperature cycles on soil–concrete interface. The preliminary results showed that the sand expands upon heating and contract upon cooling. An accumulated contraction and shear displacement were observed after each thermal cycle. However, these preliminary results should be carefully re-analysed since the device was not fully calibrated under various thermo-mechanical loads.

In this study, the MGSB was utilized after its calibration [13], to investigate the thermally induced vertical and lateral displacements of sand–structure interface. It consists of two half-shear boxes with dimensions of 200 mm x 200 mm x 80 mm. A Peltier module was used to heat and cool water connected to a pipe embedded in the concrete plate, which was used to simulate the pile surface. Normal and shear stresses were applied to represent the geostatic stress of soil and pile load, respectively. Four linear variable differential transformers (LVDTs) were placed in contact with the lower and the upper boxes of the device to measure the shear displacement. One LVDT was placed at the top of the box to measure the vertical displacement of soil. To prevent heat losses, the system was insulated using an insulating foam. Two series of test were performed. The four tests of the first series were conducted on dense (D) sand and the three tests of the second series were on loose (L) sand, with relative densities of about 90% and 30%, respectively. For each density, three normal stress levels were tested: 25, 50 and 100 kPa. For each normal stress, a shear stress equal to 40% of normal stress (45% of the shear strength) was applied. Test D_50 was repeated because of technical problems that happened during the first test. After samples preparation, two phases were carried out. The first phase corresponds to the

application of normal and shear stresses. Regarding the second phase, 20 thermal cycles were applied, varying the soil temperature between 14 °C and 32 °C.

The mechanical results showed a good agreement compared to standard direct shear tests. The thermo-mechanical results showed that the succession of soil expansion and contraction upon heating and cooling respectively lead to an accumulated irreversible contraction and continuous shearing displacement after each thermal cycle with different magnitudes, depending on the soil density and stress level. In addition, the rate of these incremental irreversible displacements decreases when the cycle number increases. Moreover, the increase in the stress levels will lead to an increase in the accumulated lateral displacement after 20 cycles, and this increment is significant in loose sand compared to dense sand under the same mechanical conditions. Overall, the thermally induced sand-concrete interface displacements are affected by stress level and strongly depend on the sand density. The thermally induced settlement may be critical in the case of floating energy piles under high mechanical load, and greater than 45% may lead to greater in the case of loose soil would be greater than 0.3 mm. Finally, this study suggests that using a homogeneous stress design method for energy geostructures may adversely affect the design.

A full-scale investigation of the short-term continuous and intermit-tent operation of an energy piled wall section

Luis Villegas — 2023-09-28

The growing demand for highly efficient renewable energy technologies has positioned shallow geothermal energy as an attractive alternative. The initial high capital cost of traditional ground heat exchangers (GHEs), mainly associated with drilling, has extensively hindered their implementation. Conventional geostructures overcome this drawback, and their use as GHEs is getting more attention. Piles [4, 8, 10], pavements [7], and walls [1, 5] are some of the structures that are being used. Energy piles have been the most popular among them, and their implementation has increased steadily since the 1980's [5]. After more than two decades of intensive research, a large dataset of full-scale energy pile foundation observations is available [6], and a thermo-mechanical conceptual framework [2, 3] has been formulated.

Piles are not limited to foundations; they are used in retaining walls. In contrast to foundations, retaining walls mainly work under lateral stress conditions instead of axial loads. In principle, energy piled retaining wall's initial state, circuits configuration and boundary conditions control their response. Nevertheless, in the context of (energy) piles, observations appear limited to a few full-scale thermo-mechanical studies to date [1, 5]. More attention to this energy geostructure from the scientific and engineering practice communities is required, and the documentation of additional case studies is needed to promote the energy piled walls implementation more broadly.

In this work, a series of full-scale thermo-mechanical field testing has been performed in an energy piled wall section. The retaining wall system section consists of three piles equipped with pipes to work as heat exchangers. Each houses 4 U-loops in series made of high-density polyethylene (HDPE) pipe with an outer diameter of 25 mm and HDR of 11. The pipe circuits are connected in parallel through a common header manifold operated on the surface. Two of these piles have been instrumented with 10 pairs of vibrating wire rebar strainmeters with temperature sensors located on diametrically opposing sides of the pile. One is close to the excavation, whereas the other is to the ground side - see more details in [10].

The thermal activation of the wall section occurred at 34.7 m depth and was achieved through an in-situ Thermal Response Test (TRT) unit. The unit heats a carrier fluid through constantly powered electrical heating components and pumps the fluid in the pipe loops within the GHEs. To emulate a conventional thermal load (e.g., energy piles 30-70 W/m, [9]), the entire wall section is supplied with a heating power of 5.5 kW over 5 days active period, and the thermal load is provided under either continuous or intermittent conditions (i.e., 5 thermal cycles, 12 hours on/off). Five recovery days allowed the ground to return partially to its initial thermal equilibrium. The testing period lasted 26 days of constant monitoring.

The obtained measurements in one pile are presented in Figure 1. It compares the measured strains at different construction stages (i.e., excavation to 32.5, 36.4, and 34.7 m depths) against those observed at the end of both operation modes (i.e., 5 days, 5th cycle). It is noticed that the temperature-induced strains recorded by the available sensors (i.e., 11/20 in this case) are, in most cases, within the axial strain's envelope derived from previous excavation and construction stages. Only one sensor at 33 m indicates tensile axial strains larger than those observed at different excavation depths; nevertheless, it represents a minor deviation compared with the increment recorded at the same depth during the last excavation and anchors removal. Based on these observations and assuming a thermoelastic response, thermally induced axial stresses remain within the design ranges delimited by design envelopes.

Thermally induced long-term behavior of energy piles under vertical-horizontal combined mechanical loads

Huaibo Song — 2023-09-29

At present, energy piles have been gradually applied to support the foundation of bridge abutments, high-rise buildings, earth retaining structures, seafront structures, or buildings located on the sloping ground [1, 2, 3]. During the long-term service, energy piles are subjected to vertical-horizontal combined mechanical loads while under thermal cycles. Nevertheless, most of the research presented in the literature focused on the effects of thermal cycles on the axial mechanical responses of energy piles [4, 5, 6]. Studies evaluating the effects of thermal cycles on the long-term behavior of energy piles under vertical-horizontal combined mechanical loads are scarce.

To this end, this study presents an experimental method based on a small-scale pile model installed in saturated kaolin clay to study the long-term thermomechanical performance of energy piles under thermal cycles and inclined mechanical loads. As shown in Figure 1, the experimental setup consists of a cylindrical steel soil tank, a temperature control system, and a data acquisition system. The soil tank has a diameter of 548 mm and a height of 980 mm. A two-layer geotextile filter and an insulation PVC material covered the internal and external surfaces, respectively. The model pile is made of an aluminum tube with a length of 600 mm, an outer diameter of 20 mm, and an inner diameter of 18 mm. The outer surface of the pile was coated with sand to maximize the roughness.

During the tests, the model pile was placed at the center of the soil tank. The distance between the pile tip and the bottom of the soil tank was 340 mm, which equals 17 D (i.e., the outer diameter of the pile), and the distance between the outer surface of the pile and the inner wall of the soil tank was 264 mm, corresponding 13.2 D. The above values are all greater than 10 D and the influence of boundary effect on the test results can be considered to be insignificant [7]. Besides, the particle-size scaling effect can be negligible because the ratio of the pile diameter to the mean particle size of the kaolin clay is larger than the threshold value of 40 suggested by Fioravante [8].

The testing protocole can be divided into four steps. (i) First, a sand layer of 40 mm was placed on the bottom of the soil tank to ensure that water could flow from the bottom to the top more quickly. Then the kaolin clay with an optimum water content of 29% was compacted by 50 mm thick layers until the soil tank was full of kaolin clay with a maximum dry density of 1450 kg/m³. (ii) Second, a water reservoir was used to inject de-aired water from the bottom of the soil tank for two months. (iii) Third, the vertical mechanical load of 100 N, corresponding to 20% of the pile resistance, was applied to the pile head for two days, and the settlement of the pile head was observed to be stable. Then, three levels of horizontal loading of 35, 71, and 109 N were selected to apply to the pile head and maintained for one month at each level to consolidate the soil before the coming of thermal cycles. (iv) At each horizontal load level, fifteen thermal cycles with an amplitude of ±4.5 ℃ were applied to the pile by the temperature-controlled bath and a peristaltic pump. Each thermal cycle was completed within 24 hours, which consisted of a heating duration of 3 hours, a cooling duration of 3 hours, and a recovery duration of 18 hours for active heating to return to the initial temperature.

The results show that irreversible settlement and horizontal displacement of the pile head are induced by the thermal cycles. The effect of thermal cycles on the mechanical response of the model energy pile is more pronounced in the horizontal direction compared to the axial direction. The most significant increment of irreversible pile head settlement and horizontal displacement is caused by the first heating-cooling cycle; then, as the number of thermal cycles increases, the subsequent increments decrease and tend to stabilize. Furthermore, the finite element method was employed to simulate the model test and compare experimental and numerical results to discuss the mechanisms at the origin of the irreversible displacement to gain further insight into the experimental data. The results highlight the critical role of inclined mechanical loads in the pile-soil interaction subjected to long-term thermomechanical loading.

Long-term numerical investigation of Northern Gateway Heat Network

Taha Sezer — 2023-10-02

Considering climate change and countries’ measures towards greenhouse gas emissions (e.g., European energy and climate targets of achieving net-zero greenhouse gas emissions by 2050) [1], it is crucial to implement renewable energy sources and district-level heating and cooling systems. Many district-level heating and cooling systems have been established in Europe. However, just a limited number of them are renewable energy source based. One of the most efficient ways of heating and cooling supply is to use geothermal energy as it is renewable, sustainable, and environmentally friendly. Groundwater heat pumps (GWHPs) utilise low-temperature sources to provide heating and cooling to buildings, which makes them a highly efficient, environmentally friendly, and low-carbon technology suitable for both small and large-scale applications.

Northern Gateway Heat Network project, which is a groundwater-based heat pump system aiming to provide heating and domestic hot water to around 300 dwellings, offices, and healthcare buildings, is considered as a case study in this research. It was designed to supply 75% of the total heating demand with an 800-kW output heat pump benefitting from the groundwater extracted from the depths between 135m and 200m at around 12.5°C. BH1, BH2 and BH3 are the injection wells, whilst BH4 and BH5 are the abstraction wells (see Figure 1) [2, 3].

A 3D finite element modelling was carried out in FEFLOW [4] to investigate the thermally affected zone and the possibility of thermal feedback resulting from water injection temperature affecting abstraction temperature. The 3D modelling aims at investigating the impact of the system in the long term (100 years) and how to improve the system’s efficiency and sustainability by considering four different scenarios: (1) continuous space heating (12 months), (2) space heating (10 months) and recovery (2

months), (3) space heating (10 months) and cooling (2 months) and (4) aquifer thermal energy storage (swap injection and abstraction wells in 2 months cooling period) (ATES).

The results suggest that thermal feedback occurs after almost 60 years for the actual case (continuous space heating), which decreases the system’s efficiency. Figure 1 illustrates the thermal plume development after 100 years of space heating operation (cold water injection at 5°C). Although the spacing between the production and injection wells is around 530 m, the abstraction temperature was affected by the cold-water injection, and it decreased by almost 10% at the end of 100 years of simulation at BH4.

The space heating and recovery, and space heating and cooling also witnessed a decrease in water extraction temperature at BH4. The injection temperature was set to 35°C for the space cooling period. Adding two months of recovery or two months of space cooling delayed the thermal feedback time by 17% and 8%. The results show that the system can supply two months of space cooling without a need for an additional supply from the heat pump. Furthermore, when two months of space cooling is added, the thermal energy gain increases by 20% compared to space heating-only operation. The ATES operation also increased the thermal energy gain by 173% thanks to the stored energy in the ground and higher volume of water extraction during the space cooling period. Table 1 shows the thermal energy gain from each well in each scenario, calculated considering the thermal feedback impact.

This paper highlights the long-term numerical investigation of the Northern Gateway Heat Network, considering different scenarios, including continuous space heating, space heating and recovery, space heating and cooling, and ATES. It also discusses the thermal impact and thermal plume development in different scenarios and how the thermal plume affected the system efficiency due to thermal feedback. The results indicate that the GWHP system can provide direct space cooling, thereby significantly increasing the sustainability and efficiency of the system. Moreover, the application of the ATES system further improves the system’s sustainability and efficiency.

Thermal activation of sewers and embedding in a heating-cooling network

Till Kugler — 2023-09-28

The energy district approach is a crucial aspect of the transition to a more sustainable energy system in heating. This approach views individual users not as separate entities for energy optimization, but as a network of users within a specific district to leverage synergies. This requires an energy generation system that both utilises and distributes the energy resources available in the district. In the future, as a result of climate change, residential buildings as well as workplaces in Central Europe will have to be cooled more frequently. The energy supply system must must have the capability to transfer thermal energy of different levels through the network.

For this purpose, the ´IWAES´ project aims to use the per se necessary infrastructure of urban wastewater management, which is supplemented by thermal absorbers as well as transport pipelines, to meet its objectives.

The overarching goal of the heating network is to make an urban district completely energy self-sufficient. To achieve this, waste heat from individual users is balanced out, thermal surpluses are stored using phase change storage and all available heat and cooling sources in the quarter (borehole heat exchangers, photovoltaics, thermally activated foundation elements) are integrated into the network. In the ´IWAES´ project, the primary thermal energy source and sink is the thermal activation of the wastewater system, which is also used as thermal infrastructure. The thermal utilisation of the sewage network has the advantage that the temperature level of the sewage and the surrounding soil can be used for both heating and cooling.

Common geothermal energy sources (borehole heat exchangers etc.) use the thermal energy available in the ground, which is replenished by solar inputs and the deep geothermal flow. The energy sources can therefore only be utilised efficiently for a certain number of hours per year and must then be regenerated either by time-intensive natural heat flows or energy-intensively by extra energy sources (solar thermal, etc.).Wastewater thermal plants regenerate themselves by the constant outflow of wastewater as well

as through the flowing duct air. Investigations carried out proofed that the air in the sewer flows independently of the direction of the wastewater flow and consequently induces a convective heat transfer between the inner wall of the sewer and the sewer air. The thermally activated sewer, called "hybrid sewer", as designed in the project consists of helical absorber pipes installed outside and optionally additional absorber pipes installed inside. Additional transport pipes attached above the sewer allows a transportation and distribution of heat energy enabling a thermal balance between heat supply and demand and therefore between the individual users of the district (Figure 1).

On the basis of real hybrid sewers installed and equipped with measurement technology, the performance of the hybrid sewers can be further investigated by means of three-dimensional hydrothermally coupled simulations and transferred into a simplified design approach. Furthermore, the material and operating parameters that have a decisive influence on the thermal performance of the activated sewer are investigated and the extent to which the geometry can be optimised with regard to performance and economic, manufacturing and ecological issues.In the ´IWAES´ project, the hybrid sewer is integrated into a specially developed multi-stage balancing concept, which is based on the principle of energetic subsidiarity and thus represents an innovative possibility for local heat and cold generation [1]. The concept represents a fifth-generation heat network [2], so that every consumer can also be a producer of thermal energy at the same time and is connected to the thermal network via a bidirectional acting house connection.

The focus of this contribution is on the presentation of the developed technical components and the thermal activation of the sewers as well as their performance.

Initial studies showed that up to 15 % of the thermal energy demand of an urban district can be obtained from the thermal energy of the wastewater alone. Furthermore, it has been shown that the thermal energy generated by the hybrid canal can be used efficiently for both heating and cooling, and that higher extraction rates can be achieved over a longer time than with conventional geothermal extraction systems. It was also found that the efficiency of the heating network increases significantly with the number of different types of use (server rooms, swimming pools, etc.) and differential energy sources. Furthermore economic and ecological studies have also shown that the developed concept is clearly advantageous compared to conventional thermal supply concepts.

The research that is now pending represents the validation of the developed concept on the basis of real laid and measurement-equipped hybrid ducts. In addition, a functional, economic and legally permissible operating model will be investigated.

A neural network approach for quick dimensioning of energy walls

Alessandro Poveromo — 2023-09-28

Shallow geothermal energy can play a major role in the perspective of the decarbonization of heating and cooling demands in the building sector. However, the high realization cost and the large land consumption of these systems reduced the diffusion and the positive support provided to the achievement of EU climate goals. A novel system called GeothermSkin [1] fits into this context: it is a very shallow energy wall provided with a net of pipes able to transform the underground portions of buildings into geothermal collectors, thus conferring to the structural element an additional energy function. The hydraulic circuit comprises reticulated polyethylene Pe-Xa pipes fixed to the wall's external surface in contact with the ground and crossed by a heat transfer fluid. It enables to overcome the limits of traditional geothermal applications because it is characterized by low investment cost, ease of installation and applicability to existing buildings subjected to retrofitting. A prototype of such a system was installed in Torino (Italy) and coupled with an electric heat pump which, by providing hot or cold heat transfer fluid to a fan coil, allows to heat and cool a test environment.

In this study, the data collected during heating tests carried out between October 2019 and February 2020 have been processed to evaluate the heat pump’s performance and the heating power supplied per unit area of GeothermSkin. The aim is to make available to the designer a tool able to ease the sizing of GeothermSkin providing an indicative estimate of the surface to be equipped with such a system to fulfil, partially or completely, the building's heating demand. By doing so, the diffusion of shallow geothermal energy would be improved and the carbon footprint of the residential sector minimized.

The data processing's results conceal the relationship between the thermal power and climate variables such as solar radiation, external temperature, temperature and moisture of the ground. The knowledge of the correlation between the performance and the variables would allow to forecast the heat power supplied by the geothermal system in different environmental and ground conditions and to evaluate the surface of GeothermSkin required to meet the heat load of the building.

To discover such correlation and maximise knowledge, a data-driven approach can be followed. In this study, an artificial neural network multilayer perceptron (ANN), belonging to supervised machine learning algorithms, has been trained for this purpose. The ANN is designed with 4 neurons for the input layer, 2 hidden layers each consisting of 64 neurons and 1 neuron for the output layer which applies a rectified linear unit (RELU) activation function to ensure a positive predicted power. During the training process, the training set originating from the performed tests on the prototype facility is completely analyzed 100 times (this hyperparameter is called epoch) in each of which the training set is splitted into 64 batches randomly sampled and analyzed with a learning rate specifically optimized. The training data, composed of environmental and ground condition inputs (i.e. external temperature, ground temperature, solar radiation and relative humidity) paired with the heat power produced during the real operation of the geothermal system, are provided to the ANN which looks for the pattern with backpropagation to minimize an error function evaluated between the forecast thermal power and the real one with gradient descent algorithms.

After training, the artificial neural network can take in new unseen inputs represented by the annual average environmental and ground condition for each Italian province (same as the ones already mentioned for the training) and estimate, as an output, the annual average heat power provided by the geothermal system in that specific location with a mean absolute percentage

error of 7%. Average environmental and ground data of each province are available from the ERA5-Land dataset, a long-term record of our climate history generated to track the climate change within the project Copernicus launched by the European Center Medium Weather Forecast (ECMWF) [2]. The dataset is obtained by combining a weather model with observational data from satellites and ground sensors and includes several climate variables with high spatial and temporal resolution.

Knowing the average thermal power forecasted for each province normalized by the area of the GeothermSkin ( ) it is referred to (i.e. 34.5 m², corresponding to the surface equipped with pipes during the tests carried out), in watt per square meter of equipped wall, and the maximum annual average power required by a building according to its latitude and the Italian legislation , in watt per square meter of building, the surface of the energy wall required per unit area of building is obtained as: The results are shown in Figure 1 and suggest that the area of GeothermSkin per unit area of building able to provide an annual average thermal power sufficient to meet the annual average heat load in Italy varies between 0.12-0.43 m²/m². Table 1 shows the results concernings 8 provinces. It is possible to observe that, overall, the required area decreases from North to South because the maximum annual average power required by a building is lower and the average thermal power forecasted is higher. Indeed the warmer, sunnier and more stable weather in Southern region allows GeothermSkin to extract a higher thermal power and operate with a great coefficient of performance thanks to the smaller difference between the ground and the internal buildings' temperatures.

The effect of climate change on the behaviour of thermo-active diaphragm walls

Andrew Minto — 2023-10-02

Energy geo-structures are becoming more common as a renewable energy solution which utilises shallow geothermal energy to provide heating and cooling to buildings and civil infrastructure projects. There has been an upward trend in the use of thermo-active geo-structures in the United Kingdom, from 150 in 2005 to over 5000 in 2012 [2]. Studies so far have shown that diaphragm walls subjected to combined thermo-mechanical loading show overall increases in lateral displacements, bending moments, shear forces, axial forces, and settlements on the retained side with thermal cycles [6, 7, 8].

This study uses a variation of a validated numerical model [4] to predict the behaviour of thermo-active diaphragm walls in the longer-term including accounting for the influence of climate change. This numerical model also assesses the impact of different modelling assumptions on the model output by comparing a simplified model with a more complex model where atmospheric temperatures affecting ground surface temperatures are included. Research from the IPCC shows increases in ambient temperatures and in maximum extreme temperatures as a result of climate change. Two models are compared in the IPCC report [5], Representative Concentration Pathway (RCP) 2.6 where governments across the world work together to meet the recommendations in the report to reduce overall warming, and RCP8.5 where the current trends of heating continue without any governmental interventions. According to climate model RCP2.6, global mean surface air temperatures will increase by around 1.5°C and extreme daily temperatures (i.e. days where temperatures exceed expected temperatures for the time of year) will increase by around 2°C over the 50-year period chosen for this study, whilst using RCP8.5 global mean surface air temperature increases by approximately 3°C and extreme daily temperatures increase by approximately 5°C.

This study uses the same model geometry as a model validated through centrifuge testing [4] which is a 5m deep cantilever diaphragm wall, like those that would be used in deep excavations for open-topped tunnels in large civil infrastructure projects. The numerical model was developed using the semi-coupled thermo-hydro-mechanical (THM) thermal module available with PLAXIS 2D and uses the HS-Small soil model [3] to allow for variation in shear modulus at small strains (i.e. those associated with retaining walls) to be captured more accurately. Thermal loads were applied by applying heat flows through a plate element with the same mechanical properties of the model diaphragm wall used in centrifuge testing. This model attempts to capture the influence of atmospheric temperatures by applying thermal boundary conditions to the ground surface as well as to the thermo-active diaphragm wall. Studies have demonstrated that a more complex model capturing atmospheric temperatures increases the impact of thermal loading [4], whilst also capturing a more realistic temperature profile throughout the entire depth of the soil body both adjacent to the wall and in the free field condition. In this study, atmospheric temperatures were captured in RCP2.6 by cycling between 30°C and -5°C over the course of the cycle to represent a typical ‘hottest’ and ‘coldest’ day of the year in cycle 1, increasing to 32°C and -5°C in line with the temperature increases described in the IPCC report [5]. Ambient mean temperatures between heating and cooling cycles increased from 20°C to 21.5°C over the 50-year period. In the same model, the temperature within the diaphragm

wall cycled between 35°C and 10°C in cycle 1, representing the typical operation of a ground source heat pump (GSHP) in heating and cooling. These temperature inputs increased to 37°C and 10°C over the 50-year period. The RCP8.5 atmospheric temperatures cycled between 30°C and -5°C to represent summer and winter in cycle 1 and increase to 35°C and -5°C over the 50-year period. Thermal flows in the diaphragm wall cycled between 35°C and 10°C in cycle 1 and increased to 40°C and 10°C over the 50-year period. The results from this study show increases in lateral displacement, maximum bending moments, positive and negative shear forces and axial forces (compressive (-) and tensile (+)). These results are shown in Figure 1.

Significantly, the RCP2.6 model shows that these increases begin to stabilise over the 50-year period, with the increases in the final cycles becoming negligible. With the RCP8.5 model these increases are increasing linearly at the end of the modelling period which can potentially have significant implications not only in terms of serviceability of the structure, but potentially on ultimate limit state as linear increases suggest that design limits will eventually be reached. The findings from this study generally agree with other similar studies, such as a study that analysed the geotechnical performance of energy piles with additional environmental loading as a result of climate change which found that additional compressive and tensile stresses are generated during heating and cooling phases, respectively [1].

A direct shear setup for studying the THM shear response of the soil-structure interface in the context of energy geostructures

Amirhossein Hashemi — 2023-09-28

Developing pathways for climate-resilient development involves integrating mitigation and adaptation actions, ensuring sustainable development [1]. A climate-resilient development can be achieved through the inclusion of effective mitigation approaches into development planning, reducing vulnerability, conserving ecosystems, and restoring ecosystems [1]. In this regard, energy geostructures are introduced as an effective mitigating approach, providing renewable energy while limiting the emission of greenhouse gases [2-4]. The operation of the system is closely linked to daily and seasonal cycles, which leads to cyclic temperature and water content fluctuations at the soil and the soil-structure interface [4, 5]. Thus, the study of the shear response of the soil–structure interface subjected to different thermo-hydro-mechanical (THM) conditions is of importance. Testing techniques used to study the THM behaviour of unsaturated soils require advanced laboratory equipment, as well as protocols for correcting measured data due to errors in the test conditions and apparatus calibration. This paper presents the development of a new direct shear setup to measure the non-isothermal shear strength of the partially saturated soils and soil-structure interface. The modified setup, a unique one to the authors' knowledge, enables simultaneous control of temperature, matric suction, and mechanical stress state within the soil specimens. The operational temperature range of energy geostructures (i.e., 5°C to 50) is applied through a thermal plate, developed from corrosion-resistant stainless steel with high thermal conductivity, placed at the base of the soil specimen. Matric suction (i.e., in the range of 0 to 100 kPa) is controlled using the axis-translation technique and measured using a pressure transducer connected to the back of the top cap, incorporating the HAE disk, facilitating the measurement (Figure 1) [2, 4-6]. The direct shear setup is modified to accurately measure the shear strength and deformation characteristics of soil samples under controlled laboratory conditions. The design of the device is based on previous direct shear devices but includes several improvements to enhance its accuracy and ease of use [4, 5]. The device has been tested to measure the shear response of both soils and soil-structure interfaces, and the results were compared to those obtained using conventional direct shear devices [2, 4, 7-10]. The results indicate that the new device is accurate and reliable and represents a significant advancement in the field of soil testing.

In this study, the interface is formed by kaolin clay, a temperature-sensitive clay, and concrete, a structural material widely used for energy geostructures [4, 11]. The soil samples were prepared by static compaction at 30% initial water content () and an initial void ratio () of 1.2. Initially, all samples were inundated with distilled water at room temperature (24°C) [4, 12], followed by sequential hydraulic (i.e., matric suction, = 0 or 70 kPa), mechanical (i.e., net normal load, =100 or 300 kPa), and thermal (i.e., temperature, =24°C or 45°C) loading of the interface. The shearing was initiated after ensuring equilibrium criteria were met. It was necessary to limit the shearing rate to 0.005mm/min and the thermal loading rate to 3°C/hr to maintain the drained condition [4, 13]. Additionally, the vertical displacements associated with hydraulic load were subjected to an equilibrium criterion of 0.025%/day strain rate [14]. As presented in Table 1, the results revealed that the apparent interface friction angle was not significantly affected by matric suction at varying temperatures, but a slight decrease has been observed upon heating at all matric suctions. The apparent adhesion increased in response to temperature increase/decrease, with a decreasing rate as suction increased, while the interface desaturation led to higher apparent adhesion at all temperatures, corresponding to peak and residual values. In interfaces subjected to identical normal stress and temperature but different matric suction values, the shear stress-shear displacement curves showed greater peak stress with increasing matric suction. Furthermore, despite the same net normal stress and matric suction, higher temperatures resulted in slightly lower peak shear stress and less contractive volume change behaviour at the interface. Non-isothermal volumetric behaviour and soil dilatancy play a significant role in governing the THM shear response of the interface [15].

A novel experimental technique to evaluate soil thermal conductivity in a transient state

Ali Pirjalili — 2023-09-28

As a result of climate change, heat island effects, and an increase in geothermal applications, soil deposits are being subjected to increasingly variable thermal stress, resulting in changes in soil properties and behaviour. Analysing the variations in soil behaviour caused by thermal loads is crucial due to the substantial impact these changes have on the state of soil stress. A critical step towards this goal is to thoroughly understand the heat transmission process in a soil deposit. Thermal loads are transported to the soil primarily by heat conduction and convection. When there is no water seepage, heat transmission is dominated by conduction [1]. The heat conduction is governed by the soil thermal conductivity, k, which is affected by the thermal conductivities of the soil components (i.e., air, water, and solid particles) [2], as well as other physical parameters such as moisture content and density [1, 2]. Investigating the thermal conductivity in different soils has resulted in the development of various predictive models, including theoretical, empirical, and mathematical models [4]. Also, two main techniques are typically used to assess thermal conductivity in experimental tests: needle probes and thermal cells [3].

This study introduces a novel approach to understanding the influence of affecting parameters on thermal properties in a transient state of a cylindrical soil sample. The proposed method enables the comprehension of spatio-temporal variability of thermal conductivity in soils subjected to temperature fluctuations. Most studies have relied on steady-state assumptions to measure or estimate thermal conductivity contradicting the variable nature of soil thermal properties. However, the existing transient tools and methods suffer from limitations such as a small sample size and very short testing durations [3]. Consequently, these constraints prevent an accurate capture of the extent to which thermal conductivity would vary due to thermally induced variations in physical properties. As a result, reliable thermo-mechanical analysis of soil subjected to heat sources is hindered. To achieve this goal, a state-of-the-art thermal cell was meticulously designed and constructed at the UNSW Geo-Energy Laboratory. This innovative thermal cell enables the monitoring of thermal and physical properties variations in a cylindrical soil sample with a height of 500 mm and a diameter of 20 mm, facilitating the assessment of temporal and spatial variability in thermal properties. The application of thermal loads to the thermal cell is accomplished by utilising a spiral copper pipe wrapped around the outer walls of the cell. To ensure efficient heat transfer, a layer of graphite sheet is applied to the cylinder wall, promoting uniform heat transmission between the cylinder wall and the pipe. The thermal cell and a schematic representation of the sample set-up are illustrated in Figure 1. When the soil sample is prepared within the thermal cell, the thermal loading mechanism is activated to quickly elevate the temperature of the cylinder's outer wall to a target level. Once the thermal load is applied, the temperature gradient between the soil sample (with an initial temperature of T₀) and the applied temperature (i.e., T_out) produces a transient, one-directional radial heat flow in the sample resulting in variations in the soil temperature which begin at the cylinder’s wall and progress towards the centre until the system reaches a steady-state condition. The resultant temperature distribution through the sample, however, depends on the soil moisture content, initial void ratio, density, etc., and as the heat transfer is governed by thermal conductivity, it could be concluded that thermal conductivity would change spatially and temporally as a result of variations in the mentioned parameters. Thus, to monitor temperature changes, a series of thermal sensors (8 sensors) are located at different distances from the origin of the cylinder (to observe temperature distribution from the source of the thermal load) and also at different elevations (to check the uniformity of the thermal load through the cylinder height). Finally, an analytical solution is employed to analyse the obtained data and estimate the thermal conductivity of the soil, which would be different for various experimental scenarios (i.e., initial void ratios, density, moisture content, etc.). The analytical solution is developed by extending the solution proposed by [5] to capture changes in thermal conductivity in a soil cylinder exposed to temperature increase. Figure 1a shows typical test results in terms of the variations of the temperature through the test which are obtained under the constant applied temperature of 55 C for a dry soil sample with an initial void ratio of 0.4. For a constant initial void ratio and density, when the results of various applied dimensionless temperatures (Q) are plotted against dimensionless time (Fourier number, F_o) (Figure 1b), it is observed that the temperature profile at a specific radial distance is independent of the applied target temperature and the resultant temperature gradient while the soil density is constant and the soil fully dry.

Finally, to test the validity of this methodology, the induced temperature variability measured at various depths and radial distances in the soil is compared with the results from a FE heat transfer model, showing good agreement (the comparison is not shown here due to space limitation). It is concluded that the proposed experimental approach is reliable for investigating the spatio-temporal variability of thermal conductivity for a cylindrical soil sample subjected to temperature elevation hence physical parameters variability.

Conduction Shape Factors for Thermally Active Retaining Walls

Aakash Gupta — 2023-10-01

The walls of a structure below the ground level can also be utilised for exchanging heat energy from shallow depths, thereby accessing shallow geothermal energy for indoor heating and cooling, in addition to their primary role of resisting structural loads. This thermal activation of walls is achieved by installing the heat exchanger pipes at the time of construction of the wall, and these are commonly termed energy walls. These energy walls are used for underground parking spaces, basements and underground stations. For the efficient thermal design of these structures, simple analytical solutions are required to characterise their heat transfer. In this extended abstract, we compare the temperature change computed across energy walls for different calculation methods, including steady-state shape factors and numerical approaches.

Energy walls follow a repetitive pipe arrangement which can be installed horizontally and vertically; however, vertical arrangements are more common due to ease of installation [1]. This leads to a simplified repetitive geometry shown in Figure 1(a), where the pipe is placed closest to the ground, whether we assume a constant temperature, and the other side of the wall is assumed insulated as per [2]. These pipe arrangements within walls can be compared with pipes buried in the ground, e.g. for district heating or other applications (Figure 1(b), (c)). These buried pipes, mostly large-diameter pipes, carry fluids at higher temperatures for large distances, making understanding heat transfer and heat losses extremely important. Therefore, heat conduction in this field has been explored in more detail with the result that shape factors for steady-state heat transfer are readily available [3].

Conduction shape factors (S) are easy to apply and can be used to assess the temperature difference between the heat transfer fluid and the back of the energy wall (T_C) due to the thermal resistance (R) of the wall concrete via equation (2). There are two shape factors which are tested in the work; S_B1=single buried pipe (Figure 1(c), equation (3) [4] and S_B2=equally spaced buried pipes (Figure 1(b), equation (4)) [4], and compared to steady state shape factor (S_N) calculated numerically. The reason for testing them both is because the pipes in the wall are of minimal diameter, 20mm to 28mm, compared with wall dimensions of 0.4m to 1.2m or pipe spacing of 0.3m to 0.8m. T_C calculated from the two shape factors is compared with results from two numerical models for a range of geometries. At first, the geometry shown in Figure 1(a) is solved for the steady state, after which a ground length is added to it, the transient state is analysed, and T_Cis calculated via equation (1).

…(1) …(2)

…(3) …(4)

where Q (W/m) is the heat flux, S is the shape factor, s is spacing between the consecutive pipes (m), D is the pipe diameter(m), and K is the thermal conductivity of concrete (W/mK). T_P and T_W are the temperature changes at the temperature changes The results are presented in Figure 1 for varying pipe spacing and wall thicknesses: wall thickness W=0.6m, 0.8m, 1m, and 1.2m; 3 pipe spacing s=0.3m, 0.5m, and 0.8m. In all cases, the pipes are 25mm in diameter and offset 75mm from the back of the wall. Constant heat flux per unit area, q=12*s W/m^2, is applied for 1 year in the case of the transient analysis.

he buried pipes in the ground form a good analogy with the embedded pipes of energy walls for the calculation of temperature changes due to thermal resistivity. Compared to the results from the steady state numerical analysis, T_SN, T_C from the equally spaced pipes model has an accuracy between 99.1-99.7%, while that from the single buried pipe model has an accuracy between 81.5-97.3%. This shows that for shape factors in energy walls, pipe spacing is essential, but wall thickness does not play such a significant role in the general range. Therefore, in Figure 1(e), the steady-state responses from different thicknesses overlaps. However, when the effect of the ground behind the wall is included for the transient simulation, the results at a pseudo-steady state are not identical and depend on the pipe spacing and wall thickness. Models with smaller wall thicknesses reach a steady-state-like condition quicker, whereas models with higher wall thicknesses may need almost a year to reach a steady state. Through this analysis, it can be concluded that the shape factors used for the buried pipe can also be used for the energy walls. Of the two studied shape factors, the formulation S_B2 shows values very similar to the numerically calculated values.

Deformations caused by subsurface heat islands: a study on the Chicago Loop

Alessandro F. Rotta Loria — 2023-09-28

The ground beneath urban areas is warming up due to anthropogenic activity, leading to subsurface urban heat islands [1]. A recent review of the literature suggests that subsurface heat islands are causing an increase in average ground temperature between 0.1 to 2.5°C per decade down to 100 m of depth in various cities across the world [2]. Studies highlight multiple impacts of subsurface temperature rises on urban areas. Subsurface temperature rises can affect the biochemical state [e.g., 3] and hydrogeological state [e.g., 4] of the urban underground, leading to shifts in plant growth and thermal pollution of groundwaters, among other issues. Subsurface temperature rises can also cause transportation infrastructure and public health issues, such as overheated subway rails that force trains to slow down or stop to avoid incidents, and extreme air temperatures underground that cause heat induced diseases [e.g., 5,6]. Meanwhile, subsurface temperature rises represent an opportunity, as geothermal technologies can harness and reutilize additional heat from the ground [e.g., 7,8].

Motivated by the lack of a fundamental understanding of the impacts of subsurface heat islands on the performance of civil infrastructure, this study addresses such knowledge gap and validates its underlying hypothesis with reference to a real case study: the Chicago Loop district – the densest district in the US after Manhattan, which suffers from an urban heat island [2]. Two facilities are used to explore this complex problem: a 3-D computer model of the Chicago Loop and a wireless temperature sensing network installed in surface and subsurface environments across such district.

The developed computer model reproduces the urban morphology of the Loop with due account of the building basements, underground parking garages, subway tunnels, and train stations that characterize such a district. Based on a substantial amount of temperature data gathered from these underground built environments and the ground surface, the model allows for the simulation of the waste heat continuously injected into the ground of the studied area. The employed simulation approach consists of 3-D, time-dependent, thermo-hydro-mechanical finite element modeling, which not only allows to quantify the temperature variations that characterize the subsurface of the Loop in space and time but also their effects on its deformation and the groundwater flow.

Simulations are performed over 100 years: from 1951, when the subway tunnels in the Loop were completed and the morphology of its underground built environments approached the current state, till 2051. The simulation results provide ground temperature values that match with recent data collected from the heart of the Loop’s subsurface (Figure 1). On the one hand, this evidence allows retrieving the evolution of the temperature field across Loop from the 1950s to date. On the other hand, this result allows for the prediction of temperature rises that are likely to develop over the next thirty years in the subsurface of the Loop (Figure 2(a)). Jointly, the results provide a first quantification of the thermally induced ground deformations and displacements resulting from subsurface urban heat islands considering the Loop (Figure 2(b)).

This work indicates that temperature variations of several degrees Celsius and thermally induced displacements of several millimeters have affected the subsurface of the Loop over the past 70 years due to waste heat deriving from underground built environments and the ground surface. Vertical ground displacements of the order of millimeters can affect the operational performance of foundations and earth retaining structures, as they can fully mobilize the shaft capacity of piles or induce excessive deflections for

walls and slabs [9]. Therefore, the underground climate change that has affected the Loop over the past 70 years might have silently contributed to the documented operational issues for buildings and infrastructures in such district [10,11], such as excessive settlements of foundations and cracking of structural members. Currently, the ground underneath the Loop is in a thermal quasi-steady state. Accordingly, the obtained results indicate limited average temperature rises and thermally induced displacements for the subsurface in the years to come. However, the ongoing underground climate change should be mitigated to avoid future unwanted impacts on civil structures and infrastructures.

This paper reveals a silent yet threatening impact of subsurface urban heat islands on the operational performance of civil structures and infrastructures (e.g., building foundations, earth-retaining structures, and other underground facilities). The root of this issue lies in thermally induced ground deformations and displacements caused by subsurface urban heat islands, which develop slowly but continuously in the urban underground and can become an issue with time. In this study, a 3-D computer model of a city district, informed and validated by temperature data gathered in the field, has been used to explore the impacts of subsurface heat islands on the performance of civil infrastructures. The results of this work lay the groundwork for future investigations referring to other city districts and specific building foundation types that may be affected by subsurface urban heat islands.

Lessons learnt from a full-scale installation of energy walls

Marco Barla — 2023-09-28

The possibility of equipping diaphragm walls as ground heat exchangers to meet the full or partial heating and cooling demand of overlying or adjacent buildings has been explored in recent years [1, 2, 3, 4]. In this extended abstract a specific installation of energy diaphragm walls is described and the lessons learnt during the implementation discussed.

The construction site is located in the city of Torino, in the campus of the Politecnico di Torino, where a new underground car park is under construction. The structure consists of four underground floors to be used as a car park for the staff at the academic site. The area of the excavation corresponds to a rectangle 173 m long and 35 m wide. The excavation is supported by more than 170 diaphragm walls. The maximum depth reached by the bulkheads is 17,65 m, while the excavation reaches approximately 15 m below the current ground level and above the water table. The ground at the site is an alluvial deposit consisting of sand and gravel locally cemented to form layers of conglomerate [5]. The construction process involves excavation by means of clamshell buckets, filling by bentonite slurry, lowering of the steel cage in two sections and bottom up filling by concrete.

To boost the sustainability of its campuses, the Politecnico decided to make effective use of energy geostructures and to equip all single diaphragm walls with heat exchangers. A series of PE-Xa (cross-linked polyethylene) pipes of reduced diameter (32 mm) was introduced to allow the circulation of a heat transfer fluid.

Considering that the steel cage of the diaphragm walls is constituted of two elements, the pipes were binded to the lower section of the rebar cages while laying in the construction site, before being lowered into the excavation. Two U-shaped pipes are installed for each diaphragm wall so that a distance of 60 cm separates each pipe. The inner pipes will be connected to the inlet, while the external pipes will be connected to the outlet. Pipes were placed on site with spacers to ensure the adequate passage of the concrete aggregates and avoid problems due to contacts with the ground at the excavation contour (Figure 1(a)). A distance of about 10 cm is achieved thanks to the spacers. An extra length of pipes is fixed to the rebar cage for later use.

The steel cage is lifted by the crane and then lowered into the excavation. Once in place, it is hung to be connected with the upper part of the cage. Then lowering starts again. In this case, the extra pipe, previously hung onto the lower steel cage are unrolled and connected to the upper part of the steel cage by hand by the workers. This process requires the lowering of the cage to be interrupted systematically to allow for the workers to quickly make fast connections (Figure 1(b)). Once the whole steel cage is within the ground, the pipes are placed at rest with a protective coating. All pipes will be connected to inlet and outlet header pipes running outside the car park excavating area and directed to the central thermal power plant of the campus.

The pipes’ condition was tested several times during the installation. First, a visual check, a pressurization and an emptying were performed before connecting the pipes to the lower part of the steel cage. Then, the pipes were filled with water before concrete casting and kept under pressure at 4 bar for 12 hours minimun during curing. The installation was considered successful if the pressure didn’t reduce by more than 0.1 bar. Despite the design considered the risk of losing up to 10% of the pipes due to unpredictable failures (e.g. crush, failures, contact, etc.), the final result, due to the careful workmanship, was that none of them had to be abandoned.

The excavation of the area started in 2022, while completion and commissioning of the underground car park are expected for 2024. Connection of the heat exchangers to the heating system will take place after completion resulting in the largest application of energy walls in Italy.

Application of a soil temperature model for simulating the effect of dis-trict heating pipes on drinking water pipelines

John van Esch — 2023-09-29

According to Dutch legislation the drinking water temperature is required to be below 25°C at the tap to meet legionella prevention [1]. This requirement possibly limits the storage of heat in the subsoil and the transport of heat through pipelines. In order to asses the impact of heat transport pipelines on the drinking water temperature in adjacent drinking water pipelines, a soil temperature model was developed. This coupled micro-climate [2] and heat transport model relates atmospheric conditions to the temperature distribution in the sub-surface [3]. The subsurface part of the model is based on a finite element discretization of the transient heat conduction equation in a single phase material. The coupling of the subsurface part of the model with atmospherical part of the model requires that the sensible heat flux, the latent heat flux, net radiation and the subsurface heat flux are in equilibrium at the land surface. Simulation periods of a number of years are discritized in time steps of one hour. The model is validated by comparing the model results with a large number of field measurements in the Dutch city Rotterdam-Kralingen [3].

The validated model is used to predict the drinking water temperature in transport, primary and secondary pipeline networks. The transport networks supply heat and water to the primary networks. Primary networks distribute their heat and water to secondary networks that transport it to buildings.

with and without heating pipelines parallel to drinking water pipelines, with a limited flow of drinking water. The inner diameter of the heat supply and return pipelines, which are usually located close to each other, is 160.3 mm. The heat supply temperature is 90°C and the return temperature is 70°C. The inner diameter of the drinking water pipelines is 152 mm. The heat transport pipelines consist of an inner steel pipe with a wall thickness of 4 mm, a PUR isolation of 56 mm and a PE sleeve of 4 mm. The finite element discretization captures the geometry of the insulated pipeline and associated effective thermal properties. Drinking water pipelines have a PVC wall with a thickness of 4 mm. The drinking water pipeline cover depth is 1.00 m, the distance in between the drinking water pipeline and heat return pipeline is 0.75 m and the soil cover of the heat supply pipelines is 1.00 m. Tiles cover a dry sandy soil for which the specific heat capacity is 800 J/kgK, the thermal conductivity is 1.6 W/mK and the density of the soil is 1600 kg/m³. The figure shows a seasonal variation of the drinking water temperature of 13.8°C and an increase of the drinking water temperature due to the presence of the heat transport pipelines of 2.0°C, which is almost constant throughout the year. The maximum drinking water temperature is found in August and amounts to 23.8°C if the heat pipelines are absent and 25.8°C if they are present.

Table 1 presents the results of a study with variation of the drinking water pipeline soil cover, the distance in between drinking water pipeline and heat return pipeline and the soil cover of the heat supply pipelines. Based on this table decisions can be made on the type of construction and the effect of the heat transport pipelines on adjacent drinking water pipelines.

The table shows that the cover depth of the drinking water pipeline strongly influences the increase of the drinking water temperature due to atmospheric conditions. A decrease of the soil cover from 2.00 m to 0.75 m raises the maximal temperature by 4.2°C. The presence of the heat transport pipelines increases the temperature of the drinking water by 1.7°C for the case of a small cover depth and 2.1°C for a large cover depth. A decrease of the distance in between the drinking water pipeline and the heat return pipeline from 2.5 m to 0.25 m raises the maximal drinking water temperature by 2.6 °C. The calculation results show that the drinking water temperature is not sensitive to the proposed cover depth variation of the heat supply pipelines. The computational results for all networks considered, are collected in an expert tool that can be accessed via: https://www.tkiwatertechnologie.nl/projecten/opwarming-van-drinkwater/.

The use of geothermal energy to prevent road pavement icing and damage in cold climate areas

Alice Di Donna — 2023-09-28

In cold climate or alpine regions, freezing temperatures and snow can result into road icing, with consequent threat of the circulation security (Figure 1(a)). When the road surface temperature goes below zero, not only the road surface ices, but also the soil below the pavement can undergo freezing. The phenomenon of soil freezing is complex due to the multi-phase and porous nature of the soil itself. Generally speaking, as ice expands in the soil pores, the soil volume increases (of about 9%, depending on the soil porosity and water saturation). This can induce frost heaving, with consequent damage of the road pavement. In this framework, the possibility to use geothermal energy systems embedded in tunnel linings for road pavement de-icing has rarely been investigated. In such systems, the concrete tunnel lining is equipped with a circuit of pipes, in which a heat carrier fluid circulates and exchanges heat with the ground. As the rock or soil around the tunnel is generally hotter than the external air temperature, heat is extracted from the rock or soil to the fluid, transported through the pipes circuit outside the tunnel, and used to de-ice the pavement road.

The objective of this study is to go a step forward in the investigation of this innovative application, for both road de-icing and prevention of frost heave. A portion of a geothermally activated tunnel and its connection to a road pipe circuit was simulated numerically (Figure 1 (b)). Appropriate climatic conditions were considered as boundary conditions at the surface level and inside the tunnel, according to available data. The efficiency of the technology was investigated under different climatic and exploitation scenarios. Figure 1 (c) shows as an example the thermal field obtained after the activation of the geothermal plant. The evolution of temperature on the road pavement surface and at the road pipes depth (30 cm from the road pavement surface) are presented on Figure 2 (a), together with the evolution of the external air and tunnel temperature, imposed as boundary conditions according to the monitored climatic conditions in the city of Oslo [1].

The geothermal system was activated after two years of simulation, by changing the fluid pipes velocity from 0 (non active system) to 0.9 m/s (purple curve on the right axis on Figure 2 (a)). The results show that the road temperature increases thanks to the activation of the system (i.e. when the fluid pipes velocity is imposed to 0.9 m/s).

Furthermore, a recently developed thermo-hydro-mechanical constitutive model for frozen soils [2] was used to quantify the soil heave due to freezing, with and without the activation of the geothermal system. For this purpose, a similar numerical model has been developped where a column of saturated soil below the road and the tunnel were modelled. The phreatic water table is at the ground surface level and gravity as well as the geostatic stress conditions are considered. The soil is a clay with a low hydraulic conductivity of K_w=4.1×10^-10 m/s and a thermal conductivity of Γ=2.35 W/m/K. The other soil properties are: soil stiffness E=100 MPa, Poisson’s ratio ν=0.3, porosity n=0.25, solid grain thermal volumetric expansion coefficient α_s=3.4×10^-4 K^-1. The same soil surface climatic conditions used in the first part were applied. The temperature evolution obtained in the road from the previously mentioned analyses (Figure 2 (a)) was imposed to reproduce the effect of the geothermal activation on the road behaviour. Furthermore, the soil surface is free of stress to study the soil swelling and settlement. The bottom limit of the soil column is hydraulically drained and allows heat flux (at constant liquid pressure and temperature). The lateral far field boundaries are thermally adiabatic, hydraulically impervious, and with oedometric conditions. The results were firstly compared with the case without geothermal activation to quantify the effects in terms of road heave and settlement (Figure 2 (b)). Swelling and settlement are due to thermal expansion and compression of the soil under positive temperature (T>0°C) and to frost heave and thaw settlement under negative temperature (T<0°C). The liquid pressure at the road surface is also depicted (Figure 2 (b)). These results and comparisions allowed understanding the performance of the technology in reducing road mechanical damage induced by freezing.

Thermo-mechanical behavior of energy micropiles

Melissa Fabiola Yozy Kedib — 2023-10-02

Since the first application of piles as energy geostructure in Austria in the 1980s [1], the use of geostructures has steadily increased worldwide. Energy geostructures are complex systems that combine structural, geotechnical, and thermal performance. They are based on the exploitation of the soil thermal energy at shallow depths to provide heating and cooling to buildings [1], allowing to reduce both the consumption of non-renewable energy and the harmful carbon emissions. Pile foundations are currently among the most used geostructures. They are particularly suitable for exploiting the ground thermal energy, as they reach depths where soil temperature is quite constant and independent from daily or seasonal variations [2–4] .

Micropiles are small, drilled, and grouted-in-place piles having a diameter between 90-300 mm and a length up to 20 m [5–7]. Generally, they are constructed by drilling a borehole, placing reinforcement, and grouting the hole [6]. Micropiles can be loaded directly to transfer structural loads, both axial (compression and tension) and lateral, to a deeper stable stratum like in new foundations or they can reinforce the soil to theoretically make a reinforced soil composite (reticulated piles) that resists applied loads like in underpinning of existing foundations [6]. Due to the relatively small cross-sectional area of micropiles, load carrying capacity resulting from end bearing is generally considered to be negligible in soil or weak rock, thus micropiles mainly rely on shaft resistance for load bearing [6,7]. Innovative and vigorous drilling and grouting permit high grout-bond values to be generated along the micropile’s periphery. High-capacity steel elements, occupying upto 50% of the hole, can be used as the principal load bearing element with the surrounding grout serving only to transfer by friction the applied load between the soil and the steel [6]. The axial capacities of micropiles are in the range of 50–500 kN and up to 1000 kN when installed using pressure-grouting techniques [7]. In the Titan system, the hollow bar simultaneously acts as the drilling rod, injection tube and reinforcement for the micropile [8]. This system is fast, simple and flexible, can be used in restricted sites when access is difficult, can be installed with low vibration and noise in any type of soils including unstable soils. In the context of building rehabilitation, retrofitting, and underpinning of existing structures, the energy micropiles stand frequently as the best solution over energy piles [6]. The Titan 73/53 Energy micropile is a dual used micropile: as a geothermal energy pile with an equal extraction capacity as double U-pipe and as a foundation pile with characteristic load capacity of R_M,K= 900KN [9]. Although energy micropile (EMP) systems are very similar to energy pile (EP) systems, their behavior cannot be assumed to be similar both from the mechanical point of view (axial load supported mainly by lateral resistance with almost null end bearing capacity) and the thermal point of view (potentially lower energetic efficiency, due to shorter primary circuit and smaller contact surface with the soil) [10]. For example, pile diameter and pipe diameter have been found to play respectively an important role and an insignificant role in energy piles [11] while the contrary was observed for energy micropiles [12]. Nevertheless, the thermal performance of micropiles in terms of specific heat flux (about 50W/m in fine grained soils) has been found encouraging to predict their use in heating, ventilation and air-conditioning systems[10]. Thermal response tests conducted on TITAN 73/53 energy micropiles show that their specific extraction capacity can be 111W/m in wet sediments, and between 60-80W/m depending on the soil’s properties[9]. To date, several studies are dealing with the thermo-mechanical behaviour of energy piles [13] and also the thermal performance of energy micropiles has been looked at to an extent by various

researchers [10,12,14]. However, the coupled thermo-mechanical behaviour of energy micropiles is not well understood. In fact, the short and long terms effects of combined thermal and mechanical cyclic loading, the “group effects” of energy micropiles, among others, still need to be investigated. This study investigates the thermo-mechanical behavior of a TITAN 73/53 energy micropile, with the aim of identifying suitable evaluation criteria and design parameter for optimizing the structural, geotechnical, and thermal performance of energy micropiles. To achieve this objective, thermal response tests on instrumented TITAN 73/53 energy micropiles, and 3D Finite element analysis will be carried out. The results of the in-situ tests will be used to calibrate the numerical model of our simulation. Then the short and long terms performances of the system in various thermal, geotechnical, and structural conditions will be evaluated.

The use of a residential building’s foundation as Energy Geo-Structures: A case study in the Mediterranean environment

Lazaros Aresti — 2023-09-28

Geothermal energy, a type of renewable energy obtained from the Earth, can be used by Ground Source Heat Pumps (GSHPs) for space heating and cooling. GSHP systems extract/reject heat from/into the earth via a network of pipes or tubes known as Ground Heat Exchangers (GHEs),. GHEs can be horizontal or vertical and come in a variety of forms, including slinky loops and vertical spiral loops for horizontal configurations, and single U-tube, double U-tube in series or parallel, coaxial pipe, and spiral/helical pipe for vertical configurations. Although GSHPs exhibit a higher performance than conventional Air Source Heat Pumps (ASHPs), the systems are less appealing as investments due to their longer payback times and greater upfront costs. A competent design of the GSHP system with the appropriate GHE sizing, including the case of employing the GHEs in the structures foundations, can minimize the initial cost. Such structures are referred to as Thermo-Active Structure systems or Energy Geo-Structures (EGS), which have already found applications as energy piles, barrette piles, diaphragm walls, shallow foundations, retaining walls, embankments, and tunnel linings [1, 2, 3].

Residential buildings in Cyprus have a higher cooling than heating demand, with an extended payback period for the investment, especially if the system is solely utilized for heating [4]. EGS coupled with a GSHP could offer a substitute to reduce the system’s initial expenses and make it more attractive as an investment. EGS have not yet seen applications in Cyprus, therefore an initial computational analysis utilizing the COMSOL software and relevant data is taken into consideration. To determine the heating and cooling loads, a three-bedroom, two-storey house with a total floor space of 190 m², with nearly Zero Energy Building (nZEB) characteristics, is used as the typical case for a single detached residential building in Cyprus. The TRNSYS software was used to calculate the heating and cooling loads. In order to analyze the rejected/absorbed energy from the earth, typical foundations were constructed as full sized models in COMSOL (Figure 1). With the exception of pipes, where a 1D solution is employed, the convection diffusion equation for heat transfer is utilized with 3D conservation of heat transfer for an incompressible fluid. A detailed methodology can be found in Aresti et al. [5].

For the COMSOL compuations, the site ground temperature characteristics and the local ambient temperature were taken into account. Based on the provided loads, the inlet and outlet temperatures were hourly calculated for the highest demand months: February for winter – heating, and July for summer – cooling.

To assess the potential of using the foundation bed as an alternative to energy piles, a comparison on the performance of the two cases is presented in Figure 2 for both summer and winter conditions. The foundation bed is seen to work in a steady manner, which is highly desired in a system. However, it performs less well, with a lower COP than the energy piles system, which exhibits greater COP values than the foundation bed system both in winter and summer. It remains that both systems exhibit high enough COPs and nearly steady conditions. The COPs vary between 4.4 and 4.8. A yearly simulation (not shown here) yielded a lowest COP value of 4.5.

The systems were also assessed economically by being compared to a convectional ASHP system. The economic study demonstrated that the suggested foundation systems had relatively short payback times, making them a plausible alternative. More detailed discussion on the results can be found in Aresti et al. [5]. As a future objective, an environmental and Life Cycle Analysis investigation, similar to the one performed by Aresti et al. [6], could provide a deeper insight of the plausibility of using the building foundation as EGS.

The potenential for thermal energy from Tunnels beneath Manchester and Crewe: a case study

David Barns — 2023-09-29

The first section of high speed railway in the UK was built between London and the Channel Tunnel (High Speed 1 or HS1) and has been operating since 2007. Current construction of High Speed 2 (HS2) Phase One will take the network from London to Birmingham with Phase Two sections between Birmingham and Manchester due to be built by 2035. The University of Leeds have been working with HS2 Limited to identify the most suitable locations for use of tunnel sourced thermal energy, to gain an initial idea of likely scale of the thermal resource, as well as consider financial viability. Attempts to incorporate heat transfer pipes for thermal activation of tunnel linings in the UK [1] had previously not been successful due to a combination of economics and insufficient programme time to permit necessary stakeholder engagement and accommodate the design and construction changes. With awareness and policy support for green energy growing in the last decade, HS2 Phase 2b represents a significant opportunity to (i) commence the design process sufficiently early to encourage adoption of an energy tunnel solution; (ii) develop a novel case study of energy geostructures used with district heating networks. A summary of the key findings of this study are given below.

Tunnels at Crewe and Manchester will comprise twin bored running tunnels, 8.8m and 7.5m in diameter, supported by precast segmental linings, and 6.8km and 12.8km in length, respectively. The ground conditions vary along the length of the proposed route, including sequences of glacio-fluvial deposits and/or glacial till overlying Triassic bedrock of the Mercia Mudstone or Sherwood Sandstone Groups. The glacial tills and Mercia Mudstone can be assumed to have a thermal conductivity of approximately 2 W/mK, but much higher values may be appropriate for the Sherwood Sandstone depending on the horizon, fracture characteristics and groundwater flow conditions. As well as the ground thermal and hydrogeological conditions, the size of the thermal resource within and around the tunnels will depend significantly on the internal air flow and temperature conditions. Using design charts [2, 3], heat transfer rates of between 12 W/m² and 25 W/m² were chosen for sensitivity analysis.

These outline heat transfer rates were applied to 500m long lengths of tunnel that could be accessed from the four tunnel portals and from either side of six ventilation shafts that will be constructed along the route. This suggested that each portal could provide sufficient thermal energy to supply up to 500 homes, with each ventilation shaft having double that capacity. Final resource size would depend on the detailed tunnel ventilation design, as well as outcomes of ground investigation which are currently in progress.

Early consultation with stakeholders along the route suggested a genuine enthusiasm and appetite for future consumption of tunnel sourced thermal energy. However, a route to implementation including outside stakeholders remains challenging and requires new approaches. In the UK, the government is in the process of designating heat network zones [4], and implementation of these zones in the future will make it easier to supply properties adjacent to the tunnel route with thermal energy. Heat networks already being developed in Crewe and in Manchester could also be potential future customers for tunnel energy. In addition, anchor loads could be developed related to hospitals, university buildings and social housing estates that have been identified close to the route of tunnels. However, careful planning is required to ensure alignment of construction and public sector decarbonisation plans.

While the greatest environmental and social benefit could be drawn from using the tunnel heat beyond the boundary of the HS2 project, without detailed information about future customers and their energy demands it is hard to determine the financial viability

for this scenario. Therefore costs and benefits of adopting energy tunnels connected to ground source heat pumps were determined for the case of providing heating and cooling for self consumption within station buildings, over station development and other structures related to railway operation. These were compared with adoption of air source heat pumps (ASHPs) as a suitable counterfactual given operation of the scheme will not take place for a further ten years or more, when use of gas boilers is not expected to be permitted. For these conditions, the geothermal activation of the tunnel linings was shown to generate positive financial returns, subject to economic uncertainties such as interest rates, inflation and energy prices. Energy tunnels were able to deliver heat at lower costs compared with ASHPs when real interest rates (interest rates minus inflation, a measure of the real cost of borrowing) are below 4%. For context real interest rates in the UK are currently negative as inflation is greater than the Bank of England interest rate. Assuming interest rates at 3.5%, Figure 1 shows the sensitivity of payback time for energy tunnel construction to key input parameters. Additionally, a Monte Carlo analysis for 20,000 simulations found that 80% of cases would return a positive net present value, 75% of cases would deliver an internal rate of return over 3.5% and payback in under 30 years.

Taken together, the economic indicators, the size of thermal resource and positive stakeholder engagement suggests that the option to deploy energy tunnels should be developed further as the scheme design proceeds. Technically this would include specific ground investigation for thermal and hydrogeological parameters, tunnel ventilation design, and determination of energy demands. These steps also need to be accompanied by careful alignment of the design and construction programme with extensive liaison and coordination with offtakers, and parallel heat network development, substantially increasing the challenge of implementation.

Numerical modelling of energy piles under combined loading

Mirna Doghman — 2023-10-02

The growing use of fossil fuels and other non-renewable energy sources has made climate change a critical global issue. In order to counter this threat, several countries are engaged in an ecological transition, and are looking for technologies using renewable energy sources. In this context, energy geostructures, such as thermo-active (or energy) piles, have been developed, consisting in fixing heat exchanger pipes to the reinforcement cages of foundation piles to extract/inject the heat from/into the ground with the purpose of meeting the building heating and cooling demands. Their specificity is their dual function: structural support and energy exchanger.

In the case of energy piles, two aspects can be critical and should be considered in their design. The first is the nature of the cyclic thermal loading, which can affect the mechanical response of the energy pile. In fact, during temperature variation along the pile, stresses change and pile head movements are induced (Figure 1), due to the thermal dilatancy/contraction of the pile and the behaviour of the soil-pile interface [1, 2, 3, 4, 5, 6]. Consequently, cyclic thermal loading can induce a deterioration of the shear stresses at the soil-pile interface and hence a deterioration of the pile's bearing capacity [7]. The second aspect concerns the adaptation of design under combined lateral and axial loads. Indeed, the co-existence of a lateral loading can affect the axial response of the pile and vice-versa [8]. These configurations are the most favourable for installing heat exchanger pipes since, mechanically, they require reinforcement cages all along the pile height. Studies on energy piles have mainly investigated their behaviour under axial loading. Energy piles under lateral loading have hardly been considered [9].

The aim of this paper is to present a practical calculation tool for modelling energy piles that takes into account the combined loading on the pile response. An original 1D finite element approach is developed for engineering practice, taking into account the rheology of the problem. The pile is discretised in beam finite elements with three degrees of freedom at each node (vertical displacement, horizontal displacement and rotation). The soil is modelled by surface shear and normal springs. This tool is based on the solution of the equilibrium equation of the global system by an iterative plastic correction procedure. This correction is based on the yield criterion defined in the code.

The main strength of this approach lies in its capability to consider a 3D failure envelope for an energy pile, capturing its behaviour under combined axial, lateral and cyclic thermal loading. It can clearly represent the critical effects of these loads by adopting an appropriate behaviour law for the soil-pile interface. In addition to being practical and easy to use, this tool has the advantage of reducing calculation time compared to more complex 3D numerical methods, especially in the case of cyclic thermal loading.

Improving data accessibility and application for underground climate re-search at the British Geological Survey

Steve Thorpe — 2023-09-28

The British Geological Survey (BGS) is a world leader in urban 3D geological modelling. This reputation was largely developed through the Glasgow and London cross cutting projects and led to follow-up high profile international 3D modelling-focused pro-jects in Europe (Horizon 2020 COST SubUrban Action), the Middle East (Abu Dhabi) [1] and Asia (Singapore) [2]. BGS has a collection of over a million UK borehole records held in its National Geoscience Data Centre, and thousands of new ground inves-tigation records are added each year. This data provides vital geological, geotechnical and geoenvironmental information that is essential for societal challenges such as climate change and sustainable energy production. BGS allows access to this data via web portals, allowing the public, academia and private-sector industry access to this vast wealth of subsurface information in a spatially-delivered platform.

However, in recent studies BGS estimates that 80% of borehole data is not shared centrally and openly, resulting in an esti-mated loss of data and knowledge to the UK economy valued in the region of £150-200 million per year. BGS are leading initia-tives to help address this problem such as the ‘Dig to Share’ and ‘Big Borehole Dig’ projects which work with the construction and engineering industries to change the culture around data sharing [10]. The BGS has also put in place data sharing agreements with regional and national organisations, like Network Rail, Environment Agency, Highways England and the Welsh Government, committing them and their contractors to provide their ground investigation records to the BGS. In the last two years, the BGS has been working with the UK government to develop standardised clauses around ground investigation data that can be used by all public sector organisations. These clauses are now in the latest version of the UK “Construction Playbook”, which sets out key policies and guidance for how publicly funded projects and programmes are assessed, procured and delivered [11]. The Playbook should be adopted by central government and arm’s length bodies on a ‘Comply or Explain’ basis. All these initiatives and policy augmentations are slowly changing the mindset of how subsurface data is stored, managed and made available for others to use in order to make dramatic savings in cost, time and reduction in risks.

Borehole data is pivotal for understanding geology in urbanised areas, particularly in the shallow subsurface (top 100 m), as anthropogenic deposits often mask and obscure geological outcrop. Borehole data is often interpreted into 3D geological models, and in turn this has led to improved knowledge of subsurface geology and processes in urban areas. 3D geological data and in-formation has been leveraged for use across many types of projects, such as tunnelling and construction, for example the Far-ringdon Tunnel in London 2012 [3], water resource management [4] and geohazards, particularly the shrink-swell clays that cause subsidence[5]. In 2022, the BGS published the London, Cardiff, and Glasgow 3D geological models onto the BGS GeoIn-dex web viewer portal where users can view synthetic boreholes, cross-section and horizontal slices through the 3D geological models to improve subsurface knowledge in these areas. These 3D geological models underpin three studies into the geothermal energy potential under these cities. Two of which belong to the UK Geoenergy Observatory programme in Cardiff and Glasgow, and the other a collaboration between the BGS and the university of Cambridge in the borough of Kensington and Chelsea in London.

The Cardiff Urban Geo-Observatory is a collaboration between British Geological Survey and City of Cardiff Council, and comprises several years of baseline temperature data, an operational shallow open-loop ground source heat pump, and a 3D geo

logical model of the superficial geology focused on the target unconsolidated sand and gravel aquifer [6]. The observatory is the largest of its kind in the UK, providing open access data though a bespoke web-portal (www.ukgeos.ac.uk/observatories/cardiff). The data from which has been utilised in further research showing how this geothermal energy resource could be optimised in ur-ban environments [7].

The Glasgow observatory is a research facility designed for investigating the shallow, low-temperature mine-water heat energy and potential heat storage resources. The observatory site is typical of towns and cities with a post-industrial urban and coalfield legacy. Towns and cities are where the greatest demand for decarbonising heat lies. The Glasgow Observatory is a result of a four-year period of borehole planning, drilling, logging and testing, including 3D geological modelling. A mine water reservoir classifica-tion established from the observatory boreholes highlights the resource potential in areas of total extraction, stowage, and stoop and room workings. Since their spatial extent is more extensive across the UK than shafts or roadways, increasing the mine water energy evidence base and reducing exploration risk in these types of legacy workings is important. Mine-water heat abstraction is a technology that is proven but not widely realised. The observatory enables research into the questions that remain about this heat source, from size and sustainability to environmental impacts [8].

The BGS collaborated with the University of Cambridge on the Translucent Cities project from 2018-2019, the objective of which was understand the impact of existing basement structures and potential new structures on the use of the geothermal prop-erties/energy use in closed loop heating and cooling systems. This study demonstrates that the amount of heat from basements rejected to the ground constitutes a significant percentage of the total heat loss from buildings, particularly in the presence of groundwater flow. Understanding the relationship and geometries in 3D of the geological strata, and the thicknesses these, com-bining this with groundwater flow was fundamental in identifying factors to improves the sustainable utilisation of geothermal energy sources [9].

Cooling underground substations worldwide using heat pumps

Nikolas Makasis — 2023-09-29

The use of underground space in urban environments has increased at an accelerated rate over the past few years, particularly to satisfy transport infrastructure needs. Transport tunnels are being constructed at increasingly greater depths in the already congested subsurface. While transport tunnels have demonstrated significant potential as sustainable heat sources for heating under - and above-ground spaces, due to the large area in contact with the ground [4,5], cooling the substations that serve the tunnels, containing plants and machinery, is a task that poses a significant economic and environmental challenge. To tackle this, a new method that takes advantage of airflow in the tunnels and the thermal mass of the ground surrounding the tunnels has been recently introduced [2]. Similar to energy tunnels [1], these systems use water-filled high-density polyethylene (HDPE) pipes integrated into the tunnel space to exchange heat, however using the pipes to reject heat from the substations to the tunnels. This work adopts four cities, namely Sydney in Australia, Guangzhou in China, London in the UK, and Stuttgart in Germany, and investigates the suitability of this approach to cool substations for different meteorological and geological conditions around the world. Moreover, varying heat exchanger lengths and tunnel air temperatures are incorporated in the analysis, to assess the impact of these key parameters on system efficiency and performance.

A detailed 3D finite element heat and mass transport model is utilised in this work, simulating the tunnel lining, surrounding soil, the airflow within the tunnel, the pipes attached to the tunnel linings (tunnel air side), and the circulating fluid within the pipes [2]. The location farfield temperature and material properties are shown in Table 1, for the soil conditions in the four locations and for the common materials (shaded in grey). A parametric analysis is undertaken to investigate the performance of these systems under varying conditions. For each of the locations, five different pipe leg length values are used: 50 m, 100 m, 150 m, 200 m, and 300 m and the air velocity is assumed as 0.5 m/s, typical of these tunnels. In addition, two different distributions for the temperature of the air entering the tunnel are used: the estimated tunnel air temperature () distributions based on available literature and measurements [3, 6, 7], as well as the surface air temperatures (), both shown in Figure 1-left. The latter is used to understand the impact of natural ventilation of tunnels on the system, which could inform the placement of the pipes with respect to the positions of ventilating shafts along the tunnel length.

The results are shown in Figure 1-right, in terms of the cooling thermal power that a single pipe loop can provide (total pipe loop length being ), when operating constantly over 20 years – to replicate the real conditions in cooling substations. These values are obtained such that the temperature of the fluid does not reduce below 5 °C, such that the heat pump efficiencies remain high. The locations are shown based on the line colour and the air temperature distribution used for each location is represented by the line type (solid:, dashed:). The results show that the cooling provided can vary significantly based on the investigated parameters and suggest that the temperature of the air flowing in the tunnel is very influential to the system performance. The most favourable outcome is achieved for Stuttgart (green), which has the lowest tunnel air and farfield temperature, and the least favourable for Guangzhou (red), which has the higher air and farfield temperature. Comparing the two air temperature distributions used (solid vs dashed), using instead of , which is lower in all cases, increases the performance of the system. This is most obvious for the case of London, with an increase between 2.5 kW and 5 kW, likely due to the low value of the farfield temperature as opposed to the significantly high tunnel air temperatures (). The length of the pipes used in all cases increases the amount of energy, however that increase is logarithmic in nature and therefore after about 150 m to 200 m the investment might not be justified. Overall, this work shows that there is significant potential to utilise heat pump technologies to cool tunnel substations, especially in cooler climates such as central-northern Europe.

Influence of parallel and series U-loop configurations on the thermal behaviour of energy piles

Mohammed Faizal — 2023-10-02

Energy piles are foundation piles that have heat exchanger pipes installed in them for exchanging geothermal energy between buildings and the ground through the use of a ground source heat pump (GSHP). This is a cost-effective and environmentally friendly method to supplement heating and cooling of buildings. Heat transfer occurs between the piles and the ground through a heat transfer fluid circulating in the pipes. The heat exchanger pipes in the piles are commonly made from polyethylene pipes formed into U-loops [e.g., 1, 2, 3, 4, 5, 6, 7]. The U-loops are configured into series or parallel configurations to circulate water in the pipes. A number of studies have investigated the thermal and thermo-mechanical behaviour of energy piles with U-loop heat exchangers [e.g., 1, 2, 3, 4, 5, 6, 7]. Despite this widespread research, the fluid flow and temperature variations in the individual U-loops are not well understood for parallel and series U-loops in the piles. Given that the fluid flow behaviour varies between the two configurations [8], it can be hypothesised that the thermal behaviour of the energy piles may vary as well.

This study investigates the influence of series and parallel U-loop configurations on the variations in fluid temperatures and flowrates in the individual U-loops of energy piles, and the effects of these variations on the geothermal energy extracted by the piles. Heating experiments were conducted on a set of four field-scale energy piles installed below a 5-storey building in Brighton group sandy soils. The energy piles have a length of 15 m and diameter of 0.9 m, but different numbers of U-loops (1, 2, 3, and 4 U-loops in Piles 1, 2, 3, and 4, respectively). The fluid temperatures and flowrates were monitored in the individual U-loops of the piles that were connected to a plumbing manifold located in the monitoring room (Figure 1). A comparative thermal performance analysis of the two configurations was conducted to derive conclusions on the preferred configuration for improved thermal performance of the piles.

The flowrate variations and change in fluid temperatures in the individual U-loops at Day 15 of group tests are shown in Figure 2. The flowrates were inconsistent between the U-loops in the parallel configuration but remained constant in the series U-loops. Heat exchange occurred in all the U-loops in parallel, whereas only the first few U-loops in the series configuration exchanged heat with the ground. The results suggest that all the energy piles in the group with parallel U-loops were thermally active and could improve the performance of energy pile systems compared to energy piles with U-loops in series.

Thermal and structural response of a pavement solar collector prototype

Taher Ghalandari — 2023-09-28

The last two decades have seen a growing trend toward energy harvesting technologies from asphalt pavements, mainly to discover a suitable replacement for fossil fuels to tackle the high demand for energy due to global population rise, urbanization, and environmental problems. Energy extraction from asphalt pavements, using Pavement Solar Collector (PSC) systems, is one of the most highly promising technologies. This is due to their extensive availability including in roads, cycle lanes, parking lots, airports, in addition to their great potential to absorb solar radiation. PSCs (also called hydronic asphalt pavement) circulate water or other liquid, through the pipe network that is embedded in the asphalt pavement. The primary aim of PSCs is to extract heat from the asphalt pavement in the summertime and use the harvested heat to provide snow/ice-free asphalt surface and prevent black ice formation. The PSC systems could potentially improve road safety, reduce the need for de-icing chemicals, and provide energy-efficient outdoor heating [2]. The heat source for PSCs can be from a central boiler or a renewable energy source such as solar thermal or geothermal. As a result, the system can be designed to be energy-efficient, reducing the overall energy consumption and carbon footprint of the roads.

The large-scale research prototype of the Heat Exchanging Asphalt Layer (HEAL) was designed and constructed on a bicycle path at the University of Antwerp’s Groenenborger Campus to investigate the thermal and structural performance of the PSC systems. The total area of HEAL is nearly 65 m² (14 m x 4.6 m) with four heat exchange sections (8.5 m x 1 m each) and two reference sections of 30 m² (i.e. without the heat exchange layer). The HEAL system has four main parts: the heat exchanger section, technical unit, borehole thermal energy storage, and control system (see Figure 1). The heat exchanger section was designed and configured into four interconnected sections in order to provide different scenarios, including series, parallel, full power, partially activated, depending on the project settings and purpose (e.g. harsh snow or freezing temperatures) [1].

The energy harvesting efficiency of PSCs has been estimated to be mainly between 20%-30%, reaching to a maximum of 50% [3; 4]. The results of the experimental tests on the HEAL prototype indicated that the series configuration achieved around 20% efficiency, while it was 25% for the parallel configuration, and could theoretically reach a maximum of 34%. With respect to annual energy gain, recent studies reported that the PSC efficiency has a wide range between 0.6 and 1.21 GJ/m²/year [5; 7]. Finally, it was concluded that the energy harvesting capability of large-scale PSCs is not only determined by the geometrical properties and geographical location of the installed systems, but also by the operational conditions such as fluid flow and weather parameters [1]. The required heat energy for snow-melting asphalt surfaces strongly depends on the weather parameters of the cold season. Hence, PSC systems use 100 to 900 W/m² of collected heat to provide ice/snow-free surfaces. In a recent study, a set of systematic experiments were designed and performed in the HEAL prototype to assess its seasonal energy balance. The output results demonstrated that the maximum hourly heat extraction rate was 91 W/m², compared to the average hourly power consumption of 15.2 W/m² to provide ice-/snow free road surface. Although the experiments of the study took place over a limited number of days, a comparison between average heat gain and power consumption showed that applying a low-temperature supply in wintertime could save above 80% of the collected heat in the summertime for the same number of operational days [1]. As a result, the remaining excess low-temperature heat in the storage can be used for various applications, such as providing (preheated) domestic hot water and heating to nearby buildings.

In terms of the structural performance, the application of PSCs can reduce the temperature gradient of the asphalt pavement, thus increasing the service life of the pavement. Mallick et al. [6] claimed that the service life of the pavement could be extended between 3-5 years by using PSC systems. Furthermore, controlling the temperature profile of the asphalt pavement could potentially reduce pavement distresses, such as top-down cracking, rutting and fatigue cracking [1]. One of the main challenges in the design of PSC systems is related to the appropriate structural and geometrical designs to ensure that the potential structural damages are in an acceptable range for roads. The pipe depth is a key design parameter to fulfill a balanced trade-off between harvesting maximum heat (pipes closer to the surface) and minimum structural damage (pipes deeper in the asphalt layer). The ongoing and future research on the structural performance of the HEAL system includes: i) comprehensive assessment of the asphalt pavement service life, using field data and multi-layer elastic models ii) evaluation of the asphalt pavement’s rutting and shear failure in the laboratory for samples with and without HEAL.

Thermo-mechanical analysis of energy tunnels accounting for thermo-plasticity

Maria Julieta Rottemberg — 2023-09-29

Energy tunnels are typically comprised of tunnel linings that are equipped with heat transfer pipes, thereby converting them into tunnel ground heat exchangers (tunnel GHEs). This innovative technology offers renewable methods of space heating, cooling and hot water production. While still relatively uncommon, it is becoming increasingly popular due to the growing interest in sustainability and innovation, particularly in large infrastructure projects [8, 7, 10]. Additionally, the increasing need for clean and renewable sources of energy has made energy tunnels an attractive solution for addressing the global climate challenge [7, 11, 1].

The geotechnical behaviour of energy tunnels that subject the soil to thermal loads is still not fully understood. Numerical analyses conducted in the past use simple soil constitutive models, such as the linear elastic model [2, 3, 9] and elastic-perfectly plastic Mohr-Coulomb model [6, 4, 5]. Using simple constitutive models for tunnel design may not accurately depict the complex behaviour of soil materials under different conditions, and the complex interplay among the deformation, flow and thermal models governing the soil behaviour. This can lead to calculated stresses and strains that differ from actual values, resulting in potentially unsafe designs [12, 13].

Due to their multi-phase nature, saturated soils exhibit both reversible and irreversible thermal volumetric changes. The reversible volumetric change, resulting from thermal expansion/contraction, is typically assumed to occur within the elastic thermo-mechanical response of the soil. Conversely, the irreversible change is more appropriately treated within the elasto-plastic response regime.

In addition to volumetric changes, thermal effects can impact the stiffness and strength of soil, ultimately influencing the load transfer mechanisms and stress distributions within the system. These changes, in turn, can lead to additional deformations in both the tunnel and the surrounding soil. It should be noted that simply applying proportional stress paths to any of the Mohr-Coulomb models will result in solely elastic responses and fail to reproduce the irrecoverability of the volumetric response. Therefore, the incorporation of an elasto-plastic framework for analysing thermo-mechanical coupling is required to obtain more accurate predictions with respect to changes in effective stress and ground displacement.

In this study, the coupled thermo-mechanical behaviour of energy tunnels is investigated employing a thermo-elasto-plastic constitutive model developed in the framework of the critical state soil mechanics and considering incorporating thermally induced volumetric change in the model that involves adding a thermal strain component to the model. This is achieved by including a temperature-dependent parameter that accounts for the volumetric change due to change in temperature. The model is implemented in the finite element package COMSOL Multiphysics and used to study the thermo-mechanical behaviour of an energy tunnel in 2D plane strain condition (see Figure 1). The in-situ state of stress is reproduced, and the construction sequence is simulated (soil-structure convergence). The analysis focuses on the stresses and deformations that develop within and around energy tunnels due to the influence of geothermal operation.

The proposed model is validated following several steps. First, the mechanical model is calibrated against field data from tunnels constructed in soft soils [10]. Then, thermo-mechanical effects are validated using existing monitoring data from real-scale tunnels available in literature [3,4,10,14]. For further validation, model estimations are compared with results from small-scale thermal-mechanical tests, such as thermal-triaxial tests. This comprehensive validation strategy thus ensures a rigorous and reliable assessment of our model’s performance in replicating soil behaviour near energy tunnels.

Geomechanical Response in Energy Transition Applications: Assessing its Role with Data Assimilation

Ilshat Saifullin — 2023-10-02

The subsurface is characterized by significant uncertainties that pose challenges for geothermal energy production, CO₂ sequestration, and hydrocarbon field development. To handle this uncertainty, engineers often use ensembles of models with different porosity and permeability distributions. In addition, history-matching procedures can further reduce uncertainty. However, traditional methods using well bottom hole pressures as observations can be challenging for geothermal or CO₂ sequestration projects with a limited number of wells. To overcome this limitation, additional observation data can be used. In this study, we propose using vertical displacements to evaluate the subsidence/uplift of the model. Pressure depletion, for example, leads to surface subsidence due to rock compaction. We suggest measuring subsidence during field development and using it as an objective function to minimize the difference from measured data. Our approach provides a more reliable and accurate way to evaluate the model's subsurface behavior, helping to reduce uncertainties in geothermal energy production, CO₂ sequestration, or hydrocarbon field development.

We used a state-of-the-art simulator called DARTS [1] to simulate fluid flow in the subsurface, and computed geomechanics response using a physics-based proxy. We then performed history matching using ensemble smoother with multiple data assimilation (ES-MDA) of the reservoir subsidence to better understand the reservoir properties, such as permeability [2]. Our workflow involved generating an ensemble of models with varying permeability and computing vertical displacements using the physics-based proxy model for geomechanics [3]. We then applied ES-MDA to find the model with permeability that fits best to the reference subsidence surface. Flow pattern in heterogeneous reservoir determines in-situ stress changes, which could induce seismicity in a presence of natural faults. We used fault slippage criteria for each scenario. Data assimilation procedure provide us history matched model, and, therefore, more reliable assessment of induced seismicity risk. We used 2.5D extruded mesh with heterogeneous permeability and uniform geomechanical properties.

Drivers to allow widespread adoption of ATES systems: a reflection on 40 years experience in The Netherlands

Martin Bloemendal — 2023-10-02

Heating and cooling of buildings accounts for ~25% of the primary energy end use, hence is critical to decarbonize. In many climatic conditions heating and cooling systems can be decarbonized using seasonal thermal energy storage to overcome the mismatch in availability and demand [1], with Aquifer Thermal Energy Storage (ATES) being an example system (see Figure 1). ATES systems are relatively cheap, require limited above ground space, and can reduce primary energy use by ~50% and gas by 80-100%. In the Netherlands, adoption of ATES systems is high [2], with over 3000 systems in place. As an early adoptor, the Netherlands has around 40 years of experience. Since suitable conditions are present across the world [1], many other countries are making plans for large-scale adoption. ATES adoption in the Netherlands has been a great success story, which has developed due to key enabling policies. Depending on local conditions these policies could be simply adopted, but could also require adaptations. This paper provides an overview of key drivers for high adoption rate and successful exploitation of ATES in the Netherlands.

Building energy performance standard sparked market demand, certification ensured quality: The Energy Performance Coefficient (EPC) regulation required building owners to make energy efficient buildings. From the early ‘90s, required EPC values became gradually stricter, until around 2022 where the net energy use of a new building needed to be 0 or smaller (i.e. energy positive building) [4]. This rule created a large demand for ATES. However, in the ‘90s and early ‘00s building owners simply applied ATES to get a building permit, resulting in many poorly performing or idle ATES systems [5]. An installation certification [6] created quality standards and banned “cowboys” from the market. Key elements are market standards for design, installation and operation, for both the wells and the surface plant, including communication protocols, as very few companies offered the whole market chain [7-9]. It was also defined that provinces can enforce optimal energy performance from building owners.

Planning of ATES wells: ATES development began with a first come–first served permitting principle. New systems required a minimal distance between new and already existing wells of at least 3 times the maximal thermal radius (Figure 1), leading to sub-optimal subsurface use. Studies showed that smaller distances between wells should be applied [10-12], and that similar well types

can be placed together [10, 12] with negligible effects on individual performance [10]. Planning and coordination by the authorities helps optimization, where so called “areas of interference” are designated to ensure the integration of high density ATES systems.

Standardisation and simplification of surface plant design: Unlike for gas fired boilers, not only installed capacity matters when installing an ATES system. The total thermal energy stored and produced from an aquifer has a great impact on long term performance. Restoring energy balance in the Aquifer is a key element in this, making ATES also suitable for either heating or cooling dominated buildings/climates. Hence, the aquifer temperature follows from the building heating and cooling use, and inherent uncertain heating and cooling lifetime loads led to complex surface plant designs allowing flexibility to add or dump heat to/from the system in many ways. However, such designs were in practice too complex to build and operate. When the certification scheme was put in place in 2013 [6], basic designs were included in the market standards to ensure manageable systems [13]. The basic rule is to keep design simple.

Well standards: ATES specific well design standards were developed to ensure high quality wells, as ATES systems and wells need to perform for decades following buildings lifespans [14]. Key is to have the contractor responsible for maintenance, this provides an incentive to construct high quality and robust wells. Experience from drinking water wells in anoxic unconsolidated formations showed that wells were prone to mechanical clogging [15], hence limits to near well flow velocity to limit the transport of particles were part of the design standards. Alternating extraction and injection in ATES wells means they are less prone to mechanical clogging than water extraction wells [16, 17]. This resulted in balancing high production rates with a planned well rehabilitation every 10 years [9]. Wells in unconfined aquifers are prone to chemical clogging due to dissolved groundwater components, in particular from mixing the groundwater from reduced (anoxic) and oxic zones. In ATES systems, groundwater is mixed and are therefore more prone to chemical clogging than dedicated extraction wells. Therefore, the ATES design standards prohibit installation of ATES well screens across redox zones. Drinking water industry standards for sizing of gravel pack grain size and sealing of wells were adopted to prevent well intrusion of sand and aquifer short circuit flow. Since wells that are heavily clogged are difficult to rehabilitate, the standards also include timely maintenance. The standards have contributed to a longer lifetime of wells and pumps, with limited reported problems with wells.

On the long-term behaviour of a deep underground gravity energy stor-age system: A numerical approach using the HCA model.

Luis Mugele — 2023-10-02

Worldwide, the energy transition is reflected in the constant development of renewable energy sources such as wind power or photovoltaics. These energy sources meanwhile provide cost-effective and mass-available energy. However, renewable energy sources are subjected to natural fluctuations. This can lead to intermittent energy supply and grid instability, highlighting the urgent need for massive energy storage systems to ensure a reliable and resilient energy supply [1].

Gravity energy storage (GES) is a concept for large-scale energy storage [1, 10]. One approach is to store energy in a subsurface cavity. Through this cavity filled with water and covered by soil, potential energy can be stored through a volume and pressure increase. In a practical application, this process can be achieved by pumping water into the cavity. By discharging the water through a turbine, the stored energy can be recovered later. The earliest concepts of this innovative energy storage system proposed a geomembrane-lined bag filled with water and covered with several metres of soil as a ballast [7, 8]. However, to achieve a remarkable energy storage capacity with this concept, large volume fluxes are required. Numerical investigations [6, 9] in addition to large and small-scale laboratory tests [2, 8] show that the resulting large deformations of the soil can cause a stability problem in the soil and can result in large energy losses and soil collapse. To overcome these drawbacks, the concept of a deep cavity was presented in [3], which is schematically shown in Figure 1. The advantage of this new concept, which is the subject of the present work, is that the energy storage is carried out with significantly lower volume fluxes and, in contrast, with increased pressure.

This concept leads to a significant reduction of the displacements and strains of the overlying soil, which allows investigations of the long-term stability of the overall system. Using the HCA model [5] in combination with the Hypoplasticity with intergranular strain [4], the cumulative soil behaviour can be modelled and cycles of energy storage can be simulated numerically. This corresponds to a lifetime of approx. 100 years for daily charging and discharging. Based on the investigations from [3], an extended numerical model is presented in this work and a comprehensive parameter study is carried out. In the finite element calculations, the impact of the groundwater is analysed for the first time in a quasi-static, coupled stress-pore fluid diffusion analysis both on the soil behaviour and on the energy storage. The axisymmetric numerical model and the spatial distribution of strain amplitude after cycles for saturated sand are shown in Figure 1.

In addition to the energy capacity, energy efficiency and its degradation due to the cumulative soil behaviour as a result of cycles, the effects on the settlements and inclinations of structures on the ground surface are also investigated. Preliminary results are presented in Figure 1. The results of this extended numerical investigation show that the GES in the presented configurations leads to stable soil behaviour with simultaneously high energy efficiency and acceptable energy capacity. Stability problems were not detected even after storage cycles. The cumulative effects caused by the cyclic deformation are controllable. This study clearly shows that GES can contribute to energy storage and energy transition in the future.

Upscaling rocks mechanical properties to study Underground Hydrogen Storage feasibility

Martin Lesueur — 2023-10-02

Underground Hydrogen Storage (UHS) is a feasible option for large-scale energy storage considering the advancements of the large-scale production of green hydrogen. One of the main engineering objective is to ensure the continuous safety of the storage, such that subsurface operations can be carried under safe stress regimes. Despite obvious similarities to Carbon Capture and Storage, a few differences make the task a new research challenge.

The first one relates to the cyclic nature of UHS, which is expected to be carried at variable frequency and injection/production loads. Current models are not adequate for the lower frequency range considered in this application. In that case, the visco-plastic nature of rocks becomes non-negligible and needs to be taken into account.

As a second observation, UHS revolves around a new gas, much lighter than CH4 and super critical CO2. Hydrogen’s atoms are so small they can diffuse even inside rock and this absorption causes rock matrix mechanical properties to weaken. This process is know as Hydrogen Embrittlement. When unaccounted for, such physical phenomenon could lead to catastrophic failure of the caprock, which is supposed to maintain stability to ensure safe storage. The caprock being responsible for the confinement of the hydrogen in the reservoir, development of cracks would enhance greatly permeability of an otherwise impermeable medium, resulting in an environmental disaster as the hydrogen suddenly leaks towards the subsurface and through groundwater aquifers.

No empirical model is able to capture those two behaviours at the macro-scale since they are both phenomena principally related to grain-scale physics. As such, this contribution presents a Digital Rock Physics framework to upscale rock mechanical properties from the grain-scale. Rocks of interest are microCT-scanned to extract the digitized microstructure. Direct numerical simulations of elasto-plasticity are performed for different stress paths in order to compute the full yield surface (see Figure 1) instead of just the Uniaxial Compressive Strength. While most studies use Discrete Element Modelling to consider grain contacts explicitly, our simulator uses Finite Element Modelling which allows more flexibility in the approach to model multiphysics processes present during UHS. The contacts are modelled instead as an upscaled plastic law. Details of the numerical algorithms are presented in [1].

As a first case study for this framework, we present a comprehensive parametric study on the impact of cementation on rock strength for real microstructures of granular materials. The framework is then coupled with a numerical erosion algorithm that simulates homogeneous precipitation of mineral matter to represent cementation. New results on the influence of cement property namely Young’s modulus, friction and cohesion on the rock’s yield surface are explored. This study contributes to preliminary results on Hydrogen Embrittlement which directly influences those same mechanical properties. However more work is needed to model realistically the Hydrogen Embrittlement, which is the aim of our new PhD project OCEAN. The process will be observed experimentally at the micro-scale in order to calibrate the simulator. MicroCT-scan images will determine the spatial distribution of the phenomenon. Visco-plasticity will be implemented from [2] to go one step further and determine the effect of Hydrogen Embrittlement during cyclic injection/production of hydrogen.

The creation of expanded diameter gravel wells in unconsolidated for-mations for High-Temperature Aquifer Thermal Energy Storage Sys-tems: Theoretical and numerical evaluation of borehole stability

Tessel M. Grubben — 2023-10-02

High-Temperature Aquifer Thermal Energy Storage (HT-ATES) systems have the potential to cost-effectively store large volumes of thermal energy, bridging the supply-demand gap for variable renewable heat sources, such as solar thermal or power-2-heat conversion [3, 4]. These systems involve the injection and extraction of heated and cooled groundwater in aquifers via tube wells [11]. A HT-ATES system will be showcased at TU Delft, which involves the use of an Expanded Diameter Gravel Well (EDGW) to increase well capacity and reduce mechanical clogging compared to conventional wells [9]. This has the potential to reduce the number of wells needed and lower the costs of the HT-ATES system.

An EDGW has previously been constructed at depth in unconsolidated formations using a jetting technique for borehole expansion [8]. This well (expanded 2.6 fold from 600 mm to 1570 mm diameter) was taken into routine operation for drinking water production. A second expanded borehole (expanded 4.1 fold from 600 to 2460 mm diameter) collapsed upon testing the enhanced removal of the filter cake before it could be completed as EDGW. The missing explanation for the collapse of the second well highlights a knowledge gap regarding the stability of an expanded diameter borehole in unconsolidated formations. To prevent collapse of future expanded boreholes and to better manage the drilling process, this study aims to investigate the effects of an enlarged diameter on well stability through a theoretical analysis.

The stability of the EDGW borehole is evaluated in two ways, see Figure 1 for a schematic of the workflow. Firstly, the effects of an enlarged diameter on the stability of the well are evaluated analytically using a poroelastic framework [1, 2, 7]. Different conditions are taken into account regarding the stress state, mud pressure, and hydraulic conductivity of the aquifer. Secondly, field test conditions for the anticipated EDGW in the HT-ATES system are simulated numerically using the two and three-dimensional finite element software. The EDGW to be built in Delft, will be constructed within the Maassluis formation. The target aquifer is located at a depth of 120 – 182 m and is mainly composed of sandy units [5]. The simulation is divided into an initial, drilling, and open borehole (seepage) stage [6, 10]:

During the initial stage, in-situ stresses and pore pressures are applied.
The well bore volume will be removed during the drilling stage, resulting in a stress state that is no longer in equilibrium, and a mud pressure is applied to the borehole wall.
For the open borehole prior to backfilling stage, the effect of time-dependent fluid flow due to mud losses is evaluated on well stability.

During the open borehole stage, both situations with and without the presence of a filter cake (varying in thickness and hydraulic permeability) will be evaluated. Furthermore, the effect of the presence of thin clay/silt layers on the stability problem will also be taken into account, where special attention is paid to the roof of the well.

The final results of this study are presented in the form of critical conditions regarding stress state, required mud pressure, and hydraulic conductivity for enlarged diameter boreholes in unconsolidated formations. Additionally, a design for the EDGW field test as part of the HT-ATES system in Delft is proposed, taking into account uncertainties such as the in-situ stress state and strength parameters of the formation.

CO2 storage: threshold capillary pressure estimate in a remoulded caprock specimen

Vincenzo Sergio Vespo — 2023-10-02

The underground permanent storage of CO₂ is one of the actions promoted by the European Union to achieve neutrality in terms of carbon emissions by 2050.It is the only readily available option for so-called hard-to-abate sectors such as cement, steel, chemical plants, where a significant proportion of carbon dioxide emissions are linked to the industrial process and therefore cannot be avoided by means of electrification or renewables [1].

Ideal geological storage should be able to store CO₂ for millions of years and maintain 99% of stored CO₂ for at least a thousand years. A necessary condition for not having leakage is that the caprock has an adequate sealing capacity, ensured only if the threshold capillary pressure of caprock p^*_c is larger than the capillary pressure p_c due to the CO₂ injection. The threshold capillary pressure is experimentally determined through tests in which the non-wetting fluid is forced to penetrate into water-saturated specimen [2].

This work reports the results of two threshold tests performed on remoulded caprock specimens using CO₂ in supercritical conditions (scCO₂) and air as non-wetting fluid.

The tested material is a non-plastic silty clay with liquid limit w_L= 50% and plastic limit w_P= 25%. It comes from a disturbed core extracted from a deep formation in the Po Valley (Italy). The material was mixed with distilled water at w = 1.2 w_L and then consolidated in oedometric conditions up to a vertical effective stress of 1.1 MPa. The porosity at this stress state is equal to 0.43.

Direct determination of the threshold capillary pressure in the laboratory is achieved by a step-by-step increase of the non-wetting fluid pressure at the inlet of a water-saturated sample, while the water pressure is kept constant at the outlet. When the difference between the non-wetting fluid pressure and the water one is higher than the threshold capillary pressure, water drainage occurs, and the non-wetting fluid can break through the sample [3]. Depending on the permeability, several days might be required for one measurement of p^*_c for a tight rock. The magnitude of non-wetting pressure steps plays a key role in this methodology, since small pressure increments result in long test times, while large pressure increments result in lower accuracy of the threshold capillary pressure estimation. To be able to perform the test using scCO₂ as a non-wetting fluid, it is necessary to set the CO₂ pressure greater than 7.4 MPa and maintain the temperature T of the entire experimental apparatus greater than 31.1 °C.

A high-pressure oedometric cell [4] is used to perform the step-by-step tests. This device allows the study of specimens with a diameter of 35 mm and a height of 12.5 mm. Water can be injected at both sides of the sample, two different pumps are used to control pressure up to 16 MPa. A third pump connected to the upstream side is used to inject the non-wetting fluid with a range up to 25 MPa. The axial stress is applied with a hydraulic jack that allows the application of a maximum total vertical stress equal to 100 MPa. The vertical displacements are measured by three LVDTs, which measure the relative displacement of the cell with respect to the piston.

Two tests were performed to determine the threshold capillary pressure of the remoulded caprock specimen in relation to the following fluid systems: scCO₂/H₂O and air/H₂O. Each threshold test consisted of the following stages: at the beginning the sample was saturated with water (under constant volume) after which a total axial load was applied. After the consolidation was completed, the intrinsic water permeability was assessed (k = 5.2∙10 ^-18 m²). Then followed the injection stage of non-wetting fluid. In both tests the effective vertical stress was set equal to 1.1 MPa. The water backpressure and the initial value of non-wetting pressure were set equal to 10 MPa in the scCO₂test and 1 MPa in the air test. The former test was conducted at T = 35 °C, the latter at T = 25 °C.

Figure 1(a) and Figure 1(b) show the trend of capillary pressure p_c and outgoing volume V_out during the scCO₂ and air injection stage, respectively. The capillary pressure range for which there was a net increase in the outgoing volume was 500 ÷ 600 kPa in the scCO₂ test and 570 ÷ 670 kPa in the air one. The magnitude of outgoing volume implies the overcoming of the threshold capillary pressure p^*_c and the establishment of a convective flow of non-wetting fluid through the sample. In both tests, no deformation of the specimens occurred during the injection stage.

Therefore the difference between the threshold capillary pressure values determined for the two tests, carried out at an effective vertical stress of 1.1 MPa, is about 70 kPa. Instead, using the tangent method to indirectly estimate the threshold capillary pressure, through the pore size distribution of the remoulded caprock (Figure 1 (c)) and the Washburn-Laplace equation, the difference for the two fluid systems would be equal to about 170 kPa (≈ 2.5 times the real experimental value). This would suggest that the tangent method can only be used for a rough estimate and the threshold capillary pressure is not only a function of the pore size distribution, but also largely depends on the actual pore interconnectivity. For a deeper analysis, further experimental tests are in progress.

Evaluation of the reliability of buried gas pipelines exposed to ground movement in the perspective of their use for hydrogen transportation: A state-of-art review

Mariam Joundi — 2023-10-02

Hydrogen (H2) has been recognized as having the potential to become the cornerstone of a low-carbon energy system. As a result, it has garnered much interest from policymakers and industry as a focus for future infrastructure.

In its race towards climate neutrality by 2050, the European Union seems to be betting on green hydrogen as a high-potential energy source. Europe envisages to develop more than 11,000 km of pipelines transporting hydrogen in 2030 [2]. The French government seems to prioritize the reuse of gas networks for the transport and storage of H2 [4]. On the other hand, the substitution of gas by hydrogen leads to an evolution of the mechanical characteristics of steel pipelines, and consequently, increases their vulnerability vis-à-vis external stresses (loads, ground movements, natural hazards, etc.). It has been shown that pipeline steel could lose up to 40% of its ductility after exposure to 100 bar H2 [3]. Thus, the reuse of the existing gas networks for the hydrogen transport raises today a set of scientific and technical questions. The problematic of pipelines vulnerability is getting even more serious by both the climate change and the evolution of human activities associated with the ecological transition.

In the framework of a thesis that we recently started (October 2022), we aim to understand the interaction between buried networks (gas networks reused for hydrogen transport) and ground movements, with a purpose to enhance risk prevention and to reduce the associated consequences. The aim is to establish a reliable analytical model enabling sensitivity studies to be performed and the influence of the uncertainties on the results to be assessed. The outcomes will be also valued by developing fragility curves that constitute operational approaches when assessing damages in an uncertain context.

The aim of this study is to synthesize the existing research work available in the literature addressing the behaviour of buried pipelines subjected to various ground motion phenomena. This review covers the various modelling approaches and simulation techniques for the soil-pipe system found in the literature. The main limitations of the existing methods are also stated.These studies encompass a diversity of ground motion sources including, but not limited to: Fault movements (strike-slip, normal-slip), soil settlements due to tunnelling, differential ground settlements related to groundwater lowering, and soil liquefaction. The main research methodologies employed are analytical analysis, numerical simulation and experimental testing.

One way to study the behaviour of the buried pipelines is the analytical modelling based on theoretical analysis [6, 7, 8]. For this approach, reasonable assumptions and simplification are necessary when simulating the pipe-soil system but also the ground movement. Soil-pipe interaction models found in previous study are mainly based on Winkler model, Pasternak model and others. The pipeline material behaviour was considered to have either elastic, bilinear or tri-linear stress-strain response. The adopted simplifying assumptions in the abovementioned studies can be prominent in certain cases of practical interest.

Analytical models based upon empirical curve fits have also been employed in some studies. The fitted functions can be derived from a numerical parametric analysis, by performing regression analyses on data obtained from a large number of configurations covering different values of the input parameters, such as pipe diameter, burial depth, soil subgrade modulus, pipe elasticity, and maximum displacement of the soil [11]. The derived empirical expressions can be used to directly estimate pipe response induced by a soil settlement.

Another major way to investigate this topic is the numerical simulation. Numerous studies were carried out using numerical finite element models [10, 12]. The results of these computational methods are becoming increasingly accurate as currently available numerical analysis techniques allow this problem to be solved in a rigorous manner, minimizing the number of required approximations. However, the non-linear behaviour of the pipeline steel, the soil-pipeline interaction and the second-order effects induced by large displacements make these analyses quite challenging.

Apart from the above analytical and numerical methodologies, there have been a growing number of papers involving the experimental simulation of buried pipes under ground motion. These investigations were based on centrifuge tests [1, 5] and other designed testing apparatus [9].

However, despite the advancement of modelling techniques, there are still some limitations when examining the behaviour of buried pipelines subjected to ground settlement such as the need for accurate soil and pipe parameters and the lack of consideration of external loads.

Coupld thermo-hydraulic modelling of heat storage in an embankment

Ahmed Boukelia — 2023-10-02

Under temperate climates, the storage of excess heat collected in summer to compensate for insufficient heat supply during winter corresponds to the concept of long-term heat storage [1]. Fisch et al. [2] analyzed the cost-performance ratios of twenty-seven existing or planned large-scale storage systems in Europe. Their results showed that the seasonal storage model could meet 50-70% of the annual heat demand, while the diurnal model could meet only 10-20%. The storage of thermal energy is possible in different modalities among which heat storage in the form of sensible heat consists of raising the temperature of a material without changing its initial state [3]. Giordano et al. [4] estimated that a volume of 34 m³ of water could store 10 GJ of sensible heat, whereas 43 m³ of saturated soil or 62 m³ of unsaturated soil (S= 50%) would be required to store the same amount of energy.

To store sensible heat in the ground, heat exchanger loops could be buried in the soil [5]. In an embankment, heat exchanger tubes could be installed during construction with no additional drilling or excavation [6]. This is a major advantage making geothermally-equipped embankments quite promising as eco-friendly devices for energy storage. However seasonal energy storage/retrieval from earth structures arises many challenges with respect to: (i) controlling heat loss during the intermediate periods (spring/autumn) [7]; (ii) managing the moisture transfer within the structure to ensure energetic efficiency and mechanical stability; (iii) evaluating the impact of cyclic temperature variations on the thermo-hydro-mechanical properties (THM) of the soil. Therefore, a coupled thermo-hydro-mechanical model would be necessary to correctly predict the long-term behaviour of these systems and ensure their sustainability.

In this study, the possibility of heat storage in backfill structures was investigated from a geotechnical point of view. It is worth mentioning that neither the technical aspects of heat pump design, nor the economic efficiency of the system will be addressed. The thermal efficiency of horizontal heat exchangers in an embankment was numerically investigated using coupled finite element analysis. The thermo-hydro-mechanical properties of the compacted soil, typically used for building embankments, were first investigated through laboratory testing. The tested soil was sampled near Paris, France [8], with an optimum water content of 16% and a maximum dry density of 1.81 Mg.m^-3. This material was classified as sandy lean clay, CL, according to the Unified Soil Classification System [9].

The finite element software Code_Bright was used to investigate first the TH behaviour of an embankment, equipped with heat exchangers, during energy storage/retrieval phases through thermo-hydraulic numerical simulations. The analysis considered the heat conduction as well as the vapour and liquid water transfer mechanisms in the unsaturated porous medium. The van Genuchten Model for the water retention curve and a power law was selected for the unsaturated hydraulic conductivity. The parameters of these hydraulic constitutive models were fitted from laboratory tests except the power parameter which was estimated based on [10]. The default procedure was used to handle the temperature-dependency of the thermal conductivity of the soil’s solid phase with parameters fitted on laboratory measurements while the thermal parameters of the water and air phases were assumed to be constant.

The parallel horizontal heat exchangers composed of tubes of 0.06 m in diameter were modelled in a 10 m height embankment built on a subgrade soil supposed to have similar thermal and hydraulic properties (Figure 1). Based on the literature [3], at summer time the temperature in the heat exchangers (T_exchangers) was imposed in the range from 25 to 50 °C during the injection phase, and to 5°C during the retrieval period in winter and was maintained constant during each season.

Autumn and spring seasons were considered as relaxation periods during which no temperature variations were applied in the exchangers. Various aspects were investigated such as the exchanger tubes locations and spacing, the heat injection-extraction scenarios, with and without insulation cap, and long-term behaviour over several years [8].

These simulations demonstrated the feasibility of seasonal heat storage in an embankment in temperate climates. Long-term simulations for a 10-year period showed that the heat loss during intermediate seasons tends to decrease reaching a plateau after 7 complete thermal cycles.

For example, in one of the studied cases, 3 staggered rows of exchangers were modelled [Figure 1]. The horizontal distance between the exchangers was considered to be 3 m. To limit the temperature loss, an insulation sheet of 0.05 m thick with a low thermal conductivity of 0.03 W/m.K. [4]. The modelling results showed that the embankment core temperature initially at 12°C, increased evenly up to 34.4°C during summer and then decreased down to 14°C by the end of winter.

Currently the improvement of this approach is under process to consider more realistic climatic conditions (for both temperature and humidity) based on measured meteorological data. The effect of the continuity of the exchangers’ loop inducing temperature variations along the path will also be taken into account through a 3D calculation, and would enable the thermal efficiency of the storage to be assessed more accurately. Finally, the consideration of mechanical behaviour should provide an insight on how the stability of the embankment could be affected by the thermal cycles.

Characterizing water transfer in compacted soils in the context of energy storage, contribution of magnetic resonance imaging (MRI)

Rosin-Paumier Sandrine — 2023-10-02

The concept of energy geostructures consists in enhancing existing civil engineering structures from an energy point of view thanks to the insertion of heat exchanger tubes. In order to consider the use of compacted soil embankments for heat storage [1], a good knowledge of water movement is required. Indeed, the increase in temperature of the exchangers leads to a migration of liquid/vapour water to the surface layers [2]. A loss of moisture may alter the thermal capacity of the soil [1] and therefore the system energy efficiency. To determine unsaturated hydraulic properties, the Instantaneous Profile Method (IPM) consists in inducing transient flow in a long cylindrical sample of soil and then measuring the resulting water content and/or pore water pressure profiles at various time intervals [3]. The unsaturated hydraulic conductivity (k_u) is then computed using Darcy’s law.

The direct evaporation technique allows the measurement of the suction profile in a soil column submitted to drying [4] using filter papers whose water retention curve is known. This method is quite reliable but requires long equilibrium time. Thus, the tests need several weeks and do not give accurate results at the early stages of drying, as the soil state is close to saturation. The analysis of the results when the soil retention capacity is low is therefore complicated, especially when the flow is coupled with temperature variations.

In this study, this classical method was compared to a faster alternate method using nuclear magnetic resonance imaging (MRI) that allows for non-intrusive, continuous and small-scale monitoring of the effect of thermo-hydraulic solicitations on water transfer within the sample. The use of MRI for monitoring the drying/wetting front progression appeared in 1970 [5]. Simpson et al. [6] presented the successive studies that have made it possible to develop a method for monitoring the movement of water within a sample. These measurements can be 1D, 2D and 3D.

The tests were performed on a sandy lean clay sampled in the Paris region in France. The soil was dried, pulverized, and sieved through a 2 mm sieve before experiments. The compaction state was selected to optimize the heat storage capacity [4] with a water content of 16.3% and a dry density of 1.79 Mg.m^-3. Ten cylindrical samples of 70 mm in diameter and 20 mm in height were compacted and superimposed with introducing filter paper disks between them. The regular weighting of each sample and each filter paper during three months allowed the derivation of the k_u curve by the IPM method.

For the MRI method, 9 samples of 36 mm in diameter and 10 mm in height were compacted at the initial state then brought to different degrees of saturation. The measurement of the water proton relaxation time using a Bruker Biospec 24/40 superconducting magnet (100 MHz), allowed the definition of a calibration curve which connects MRI signal to the water content of the samples (Figure 1b). A Single Point Imaging pulse sequence [7] was used to obtain the signal intensity (encoding time 75 µs < t < 200 µs). In an image acquired with the SPI method, the intensity of each voxel (S) is :

(1) Whereis the magnetization in a pixel and is the transverse relaxation time.

The calibration curve is shown in Figure 1b. Then, a larger sample of 30mm in height was compacted and exposed to an air flow at 20°C and 30% relative humidity through using a remote humid air generation system. 27 measurements points were monitored with an interval of 1.1 mm along the sample. Four examples of the recorded profiles are shown in Figure 1c. The evolution of the water content inside the sample is then calculated using the calibration curve to interpret the water migration in the samples submitted to humidity- and temperature-controlled air (Figure 1a). The pre-established retention curve of the soil will be used to calculate its matric suction. At each depth, the flow velocity and the hydraulic gradient will be calculated from the water content profiles and suction profiles respectively in order to determine the unsaturated hydraulic conductivity.

These experimental developments allow the non-intrusive and continuous quantification of water movement in unsaturated soils during the application of coupled thermo-hydraulic solicitations. The understanding of these fundamental parameters is a key step for studying thermo-hydromechanical behavior of compacted soils considered for heat storage. The results provide insight into the feasibility of these structures in terms of their long-term energy efficiency and stability.

The development of testing apparatus to measure carbon losses in soils

Andrew Minto — 2023-10-02

Soil is the largest terrestrial store of carbon in the world. It is estimated that soil contains approximately 2344 Gt of organic carbon globally [6]. Organic carbon is mainly sequestered in soils through plants [4], however, carbon can also be lost as a result of inappropriate land management, although other factors such as temperature and precipitation are also influential [7]. There have been efforts to monitor carbon losses from soil using soil reflectance [3], spatial and temporal data, [7], as well as more traditional method like sampling from controlled plots [1] although there is still a lack of understanding of the magnitude of carbon losses in soils and the mechanisms which drive organic carbon losses. This study will attempt to quantify organic carbon losses through dissolution and local erosion with the use of a newly developed testing apparatus. The schematic of the testing apparatus is shown in Figure 1.

The testing apparatus will be stored in an outdoor location which ensure field conditions are captured and prevents any influence of laboratory atmospheric conditions. This project will develop a sampling strategy which will include regular testing to monitor overall changes on soil organic carbon using a number of plots with different plant species since plant species play a significant role through their root activity which determines the elevation of CO₂ in soil [5]. The sampling strategy includes samples taken from each location shown in Section A-A in Figure 1. Soil samples are tested for bulk density, pH and total organic carbon through loss on ignition testing as well as other soil physical properties such as particle size distribution and permeability. Regular sampling and testing for soil physical properties will allow for correlations between a soil’s physical state and organic carbon losses to be determined. There is also some evidence that levels of organic carbon (and hence organic matter) can influence geotechnical properties of soils [2]. The sampling strategy also includes water samples taken from each run-off collection point as shown in Figure 1. Additional sampling will be undertaken following any periods of significant rainfall.

The results from this study will be used in conjunction with field work in order to better understand the mechanisms which drive organic carbon loss in soil and hence provide recommendations on approaches to prevent carbon loss. Monitoring of chosen location over time to quantify carbon losses using a combination of sampling and testing and using Earth Observation data, similar to previous approaches taken in previous studies [8].

Geochemical Interaction between CO2 and caprock for safe carbon se-questration

Mohammed Elnur — 2023-10-02

Carbon dioxide (CO₂) emission into the atmosphere from human activities and industrial processes continues to pose a major environmental and health threat to public safety worldwide with many governments launching initiatives to reduce the impact of CO₂emission. Carbon dioxide capture and storage (CCS) is a process of separating CO₂ from industrial facilities and other point sources and injecting it in a deep geological formation such as depleted oil and gas reservoirs for long-term storage [1]. Usually, CO₂ is injected into a deep formation at a depth more than 1000 m where in-situ pressure and temperature is above the critical point for CO₂ (31.1°

Incorporating phase change materials in geothermal energy piles for thermal energy storage

Abubakar Kawuwa Sani — 2023-10-02

Introduction

Geothermal energy piles (GEPs) are foundation elements that are installed in the ground to support the weight of the building to a competent strata. Energy loops are installed into the piles to allow heat rejection or extraction via the circulation of fluid through the loops. In winter, low-grade heat is extracted from the ground (source) and transferred to the building (sink) to achieve space heating. Conversely, in summer, heat is removed from the building and rejected into the ground to achieve space cooling. The system relies on the temperature gradient between the ground and the ambient air temperature; where in winter, the ground temperature is higher than the air temperature, while in summer, the opposite is true. Thus, the system can sustainably and continuously supply the heating and/or cooling demand of a building once the temperature gradient between the ambient air and the ground is maintained. To improve the sustainability and performance of the system, the heat energy rejected into the ground can be stored and used at a later time when space heating is required [1]. Energy storage substances such as phase change materials (PCMs) can be incorporated into energy piles to store the heat that is rejected into the ground to improve the performance of the GEP system [2]. PCMs are materials that stores or releases heat energy during phase transition. Many researchers including [3,4] have reported using PCMs in buildings to improve thermal comfort and minimise the energy demand during heating and cooling periods, however, not much work has been done with regard to incorporating them into pile foundations to improve the performance of GEP systems.

Thus, this study carried out a laboratory experiment to investigate the effect of the addition salt hydrate PCM on the energy performance of geothermal energy piles.

Laboratory test and procedure

In this study, lab-scaled models of energy piles were constructed, with and without PCM. The piles are characterised by a length and a diameter of 300 mm and 100 mm, respectively. In order to add PCM into the piles, the PCM was encased and sealed in a PVC tube with an inner and an outer diameter of 6mm and 8mm, respectively. The same size of PVC tube was used for the energy loops. The energy loop (U-shape), and the encased PCM incorporated tubes are attached together and placed in the mould prior to concreting, for the pile model with PCM (see Figure 1a). Whereas only the energy loop was installed in the control sample.

In the test setup, a layer of sand (220mm thick) was placed and compacted in an insulated wooden box (520mm × 520mm × 520mm). Afterwards, the pile was then installed at the center of the box and then filled with sand, in layers and compacted. An insulation sheet was used to cover the top of the box to prevent heat loss and the ambient air temperature influencing the tests’ results. Water at a temperature of 32°C, was circulated through the pile continuously for a duration of 4 days (96 hours). Instrumentations were installed in the pile and soil to monitor and record the changes in temperature during the tests (see Figure 1b).

Results and discussion

Figure 1c shows the results comparing the average energy rejected, and stored, in the ground for the piles with and without PCM. The maximum heat energy stored at the beginning of the test was found to be about 460W and 360W for the control and PCM piles. However, at the end of the tests, the energy stored was 230W and 430W for the control and PCM pile, with an average energy stored as shown in Table 1. The addition of salt hydrate PCM absorbs the heat rejected into the pile, consequently improving its thermal performance by about 22% in comparison to the control pile. Similarly, the magnitude of temperature developed in the pile and surrounding soil were observed to be lower in the pile with PCM. This can ultimately reduce the induced stresses and strains induced in a pile due to heat energy extraction or rejection.

Conclusion

This paper investigated the feasibility of using salt hydrate PCM in energy piles for heat storage. It was found that the energy rejected in a pile with PCM increases by up to 22% compared to a non-PCM pile.

Reactive Transport Modelling of carbon mineralization – a machine learning-based approach

Ersan Demirer — 2023-10-02

Carbon mineralisation is currently one of the most promising Carbon Capture and Storage (CSS) options for permanent gas sequestration since it ensures a rapid conversion of carbon from the gas phase to a carbonate mineral. Mafic and ultramafic rocks are the best geological options for mineral carbonation due to the relatively fast dissolution rates of the mineral components that can release carbonatable divalent cations such as Mg²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Ca²⁺ and Fe²⁺. Once in solution, these cations react with dissolved carbonate ions to form, under convenient pH and temperature conditions, carbonate minerals like calcite, magnesite, siderite, rhodochrosite, or dolomite.

The viability of carbon mineralization with injection of freshwater has been proven in the CarbFix project [1]. The main feature of the CarbFix method is that CO₂ (and other minor gases) is not injected under its gaseous form but already dissolved in an aqueous fluid, which greatly enhances the mineralisation rates. The methodology, however, requires vast amounts of water that can be a limited resource in many regions. Seawater may be the most viable water source for underground carbonation in many areas, but its use raises questions on the efficiency of the carbonation process. The high ionic strength of seawater makes the geochemical interactions between CO₂-rock and seawater more complex and, so far, more unpredictable. Reactive transport modelling (RTM) based on experimental thermodynamic and kinetic data of seawater-basalt interaction [2, 3] can shed some light on the expected mineralisation progress in sites with different mineralogy and variable temperature conditions. Figure 1 shows the results obtained with a reactive transport model that simulates the injection of CO₂-charged seawater in basaltic rock.

geochemical reaction calculations in particular might be considerably slow and are sometimes redundant as they might be based on a very similar set of input values. These challenges are usually faced by introducing a number of simplifications in the models to meet the objectives of the simulation studies. Thus, alternative ways to solving the full system are a priori a potential way to reduce the computational burden of numerical models.

One of such alternatives is Machine Learning (ML) and, more specifically, Artificial Neural Networks (ANN), which are inspired on biological neural networks as the ones found in the human brain. They contain several computational “neurons” arranged in different layers. Each of these neurons can be activated based on the response of the other connected neurons. The training process consists of feeding data into the ANN and fitting the connection weights between the neurons to reproduce and learn the hidden relationships of the data. Recently, the authors have coupled ANNs of a geochemical system into a reactive transport simulator [5]. In this approach, Supervised Machine Learning with ANNs is used to accurately predict the geochemical evolution in a reactive transport framework without the need of actually performing the (expensive) chemical equilibrium calculations in the reactive transport model.

The goal of this work is to apply and further develop the technology based on Supervised Machine Learning algorithms to drastically boost the efficiency of reactive transport models of carbon mineralization in the Carbfix system [6]. Firstly, a set of geochemical models of CCS are developed in batch systems (0D) as well as reactive transport models in 1D axisymmetric, 2D, and 3D RTM modelling with different degrees of complexity. Modelling focuses on the injection of CO₂-charged seawater into basaltic rocks. In this system, CO₂ is consumed via a two-step reaction: (1) Dissolution of Basaltic Glass and release of cations (Ca and Mg), and (2) precipitation of Ca-Mg carbonates and other secondary minerals. The RTM models consider a 1 km domain and a characteristic mineralization of 3-4 years. As a first step, these models are solved with the convential (non ML-based) reactive transport simulator iCP [7]. Then, ANNs are trained and validated with the latest developments in setting up neural networks and used to solve these RTM systems with a Comsol-ML library. The resulting ANNs present good accuracies with coefficients of determination (R²) above 0.95 for all outputs, and a batch calculation speedup of 30 as compared to traditional chemical equilibration with PhreeqC. Preliminary results of RTM using ML to predict the chemical step indicate a speedup of on order of magnitude and reasonable accuracy as compared to conventional RTM. Our results demonstrate that it is conceivable to replace a geochemical solver in a RTM environment with significantly faster surrogate-based models powered by a machine learning algorithm

Underground hydrogen storage in Germany: Geological and infrastructural requirements

Katharina Alms — 2023-10-02

Hydrogen storage is crucial for the success of the hydrogen economy. In addition to storage tanks and pipes the geological subsurface could also offer cost-effective solutions for storing large quantities of hydrogen in salt caverns, aquifers, and depleted hydrocarbon fields. However, experience with underground hydrogen storage is limited to salt caverns, which have size and space limitations. In this contribution we therefore define positive indicators for pore storage systems and estimate storage capacities based on national CO₂ and natural gas storage assessments in Germany.

With a focus on the geological assessment of potential storage horizons, we first define positive and cautionary indicators for safe storage operations on the basis of a thorough literature review, including theoretical and experimental studies. For example, we find that optimal storage conditions in terms of energy content and hydrogen quality are found in sandstone reservoirs in the absence of carbonate and iron-bearing accessory minerals at a depth of approximately 1100 m and a temperature of at least 40 °C. Porosity and permeability of the reservoir formation should be at least 20 % and 5x10^-13 m² (~500 mD), respectively. The pH of the brine should moreover fall below 6 and the salinity should exceed 100 mg/L in order to limit microbial activities and hydrogen solubility in brine water.

Second, we estimate hydrogen storage capacities based on published natural gas and CO₂ storage volumes [1,2] and their respective physical properties. These estimates provide an upper bound that is independent of the positive and cautionary indicators defined in here. Nevertheless, we show that up to 8 billion cubic metres, or 29 TWh energy equivalent of hydrogen could be stored in underground gas storage facilities if all natural gas were to be replaced by hydrogen (Figure 1). In addition, saline aquifers could offer storage capacities of 81.6 to 691.8 Mt of hydrogen, based on CO₂ storage assesments [2]. This corresponds to 3.2 to 27.3 PWh of hydrogen energy equivalent. The majority of which (~95 %) is located in the North German Basin. Fig. 3 shows the distribution of all pore storages (active and inactive; red and grey dots) and saline aquifers (green shaded areas) comparing it to the planned grid expansion initiative IPCEI (important projects of common European interests) [3]. These capacities would meet predicted storage requirements in Germany considering industrial, transport, and heating demands of 34 to 667 TWh (final hydrogen demand in 2050) many times over [4].

We conclude that pore storage systems could play a crucial role in the future German hydrogen infrastructure, especially in regions with large industrial hydrogen demand and likely hydrogen imports via pipelines and ships. We therefore recommend that future research focus on assessing the technical storage potential of these sites and their compatibility with planned hydrogen infrastructures and industrial demand.

Geomechanical reservoir modelling with Thermodynamics-based Artificial Neural Networks (TANNs)

Farah Rabie — 2023-10-02

Geological subsurface storage is a promising strategy for large-scale, cost-efficient energy storage systems. Stress sensitivity has notably influence on the long-term stability and serviceability of subsurface reservoirs set for energy storage use. The evolution of the field stress of stress-sensitive reservoirs and the associated structural deformation, often a consequence of fluid production and injection, can cause significant changes to the reservoir stress-dependent properties [1]. These properties include pore and void compressibility, and porosity. For fractured and faulted reservoirs, field stress variation may also impact fracture conductivity, alter pre-existing fractures, and reactivate faults [4].

Current geomechanical reservoir simulators incorporate the evolution of the reservoir mechanical state by means of material constitutive modelling, typically within a Finite Element Modelling (FEM) framework. In efforts to honour the heterogeneity and multi-scale nature of subsurface reservoirs, multi-scale solution methods are commonly adopted. For a reservoir multi-scale simulation, the micro-scale problem must be iteratively solved for different input parameters, render-ing the solution method computationally exhaustive, particularly for two-way coupled problems.

The combination of machine learning-based solution methods and FEM frameworks can address the computational inefficiency of conventional multi-scale reservoir modelling schemes. In principle, a machine learning algorithm trained to learn the constitutive behaviour of a material can replace in-built material constitutive models in a FEM framework. We resort to Thermodynamics-based Artificial Neural Networks (TANNs), a physics-based data-driven machine learning algorithm introduced in [5] for material constitutive modelling. TANNs have been shown to guarantee the thermodynamic consistency of learnt material constitutive models. The first and second laws of thermodynamics are directly encoded in the architecture of TANNs through the definition of two scalar functions, an energy potential and a dissipation function, and the computation of their differentials [3].

TANNs can be incorporated in FEM tools, in what is referred to as TANNxFEM in [6]. We present the application of the TANNxFEM framework to geomechanical reservoir modelling. This work aims to introduce a computationally efficient reservoir modelling framework, for which uncertainty quantification is possible. The proposed framework can maximise the use of information inherently contained in high-dimensional data, and significantly reduce computation demands necessary for accurate statistical evaluation, while warranting physical and geological realism.

TANNs are first trained on analytical material data, computed by numerical integration of an incremental, thermodynamically consistent material model presented in [2]. The trained TANNs are then imported into a user material subroutine for the finite element package Abaqus. Through an input file, the parameters of the trained neural network are read as a set of material properties. The material subroutine then evaluates the incorporated trained neural network to construct the stress and elasticity tensors necessary for large-scale finite element reservoir simulation. The above proposed framework is validated against a large-scale finite element simulation with a user material subroutine implementing the constitutive model used to first generate the training data for TANNs. The free-energy, dissipation, and the stress-strain response of the two finite element simulations are compared for model verification.

A reactive transport model for calcite-rich caprocks in the context of ge-ological carbon storage

Liliana Gramegna — 2023-10-02

Increased concentrations of greenhouse gases in the atmosphere are known to be the primary cause of the increase in global surface temperature and, consequently, climate change [6]. In order to limit global warming, several decarbonisation strategies have been proposed: among them, geological CO₂ storage represents very likely the only short- to medium-term option for substantially improving CO₂ sinks and reducing net carbon emissions into the atmosphere.

To achieve secure and lasting storage of CO₂ in underground spaces, it is necessary to have both a reservoir and a low-permeability caprock (seal), that maintains its integrity. Deep saline aquifers, depleted oil and gas fields and unminable coal seams are thus the primary targets for the underground storage of supercritical CO₂ [2]. The occurrence of natural reservoirs implies that certain lithotypes have a certain sealing capacity, which can prevent leakage of gas to the atmosphere over long geological time periods (i.e. million of years); however, in order to assess the risk of CO₂ leakage through caprock above storage sites, the potential caprock alterations due to the contact with CO₂ must be considered. In fact, although certain caprocks can be suitable for hydrocarbons, CO₂ in contact with the seal may pose additional risks. As for natural hydrocarbon accumulation, shales and evaporites are potential seals also for carbon storage. With particular reference to shales, their mechanical and transport properties are controlled by the behaviour of clay minerals. The sealing efficiency of intact shale caprocks is in fact dominated by the high specific surface clays, which are characterized by high capillary entry pressures, but are also susceptible to electro-chemical interaction. From the engineering perspective, capillary and electrical phenomena also have relevant effects at the Representative Elementary Volume scale, causing mechanical couplings which could affect porosity, clay fabric, hydraulic conductivity, compressibility, and shear strength [3]. Besides these electro-chemical effects, CO₂ dissolution and diffusion in water also result in acidification of the in situ brine, causing chemical reactions with some caprock minerals and potentially affecting the mechanical and transport properties [4].

This work presents a reactive transport model to assess the effects of pore water acidification on caprock materials. The model includes the water mass balance equation for the saturated porous medium and the mass balance equation for all the primary species dissolved in water, according to the theoretical approach presented in [7]. The modelling approach proposed accounts for both the aqueous (homogeneous) reactions of the CO₂ dissolved in water (assumed to be in equilibrium) and the dissolution kinetics of calcite in the acidic environment induced by CO₂ injection (see [1] for details). Calcite dissolution is finally linked to porosity changes via the reactive surface area of the mineral and the reaction rate. Chemo-hydraulic coupling is addressed by considering porosity changes in the storage term of the balance equations and by introducing a suitable link between hydraulic conductivity and current porosity. The model requires the solution of an initial chemical speciation problem, which has been performed with the software PHREEQ-C, and then the integration of a set of partial differential equations (the mass balance equations of water and of the master primary species dissolved in water) and non-linear algebraic equations (to obtain the concentration of all the chemical species involved), which in this study has been performed via the Finite Element software Comsol Multiphysics ®, according to the approach presented in [8].

The numerical model has been validated according to the numerical benchmark proposed in [5], developed to reproduce a geo-chemical scenario where mineral dissolution causes permanent alterations in the transport properties of a porous medium. In partic-ular, the benchmark reproduces a flow-through columns experiment, with the porosity and permeability of a calcite-rich interlayer changing under sulfuric acid attack. Figure 1 shows a comparison between the predictions of the current model and the model proposed in [5] in terms of the spatial evolution of porosity, permeability and dissolved calcite, after 400 hours since the injection of sulfuric acid. The model is able to reproduce that, as the acidic front proceeds into the sample from left to right, calcite is progressively dissolved causing porosity changes and the consequent permeability increase. The agreement between the two models is satisfactory, despite some slight differences that may be attributed to the different numerical mesh and the different numerical solver scheme.

Experimental investigation of the effects of fatigue on caprock materials in H2 storage projects

Andrea Ciancimino — 2023-10-02

The intermittent nature of renewable energy sources unavoidably limits their application as primary power supplies. In this sense, balancing the production/demand cycles over seasonal and annual timescales is essential for decarbonizing our economy. An effective long-term solution is represented by hydrogen underground storage in depleted hydrocarbon fields [1]. During overproduction periods, extra power is stored in the form of hydrogen in a porous reservoir formation, surrounded by a low-permeability caprock. Ensuring the integrity and the sealing efficiency of such material over the fatigue loading induced by storage/withdrawal cycles is thus fundamental for preventing hydrogen from escaping into the surrounding formations.

A substantial amount of research has been conducted in the past to define the impact of fatigue on the response of either hard or soft rocks (e.g. [2]). Nevertheless, very few studies focused on the cyclic behaviour of caprock-like materials, i.e. structured stiff clays subjected to large confining pressures. Those are characterized by the distinctive stress-strain behaviour of clays together with a significantly larger undisturbed shear strength. When subjected to cyclic loading, progressive destructuration takes place, leading to softening of the response and eventually to fragile failure. This research fits within this context, focusing on the development of an effective methodology to assess fatigue-related risks in H₂ storage projects. To this end, a series of monotonic and cyclic triaxial tests were performed to investigate the influence of loading frequency f (or, equivalently, the period T = 1/f ), maximum deviator stress q_max and loading amplitude A on the response of the material. The latter is a stiff clay with silt (clay fraction, d < 2mm, is equal to 50% and the silt fraction, d < 60mm, is 49%) with an average carbonate content of 40%. The liquid limit is w_L = 43% and the plastic limit is w_P = 25%. Specimens were consolidated under K₀ conditions to an effective stress state consistent with its lithostatic value (namely, q = 6.3MPa and p’ = 8.1MPa). Monotonic and cyclic shearing were applied through loading-compression paths under undrained conditions.

A preliminary summary of the results of the cyclic tests is presented in Figure 1a, where the fatigue life N_f is plotted against the corresponding q_max. Only two tests (represented as triangles in the plot) were interrupted before failure, due to their excessive duration. Most of the tests were conducted employing A ≈ 5.3MPa, except for two tests imposing A = 3.25MPa. The tests were carried out either with T = 5min (f = 3.3∙10^-3Hz) or T = 250min (f = 6.7∙10^-5Hz). Consistently with previous findings (e.g. [2]), N_f decreases with increasing q_max. In particular, N_f seems to be well predicted by a linear regression (in logarithmic scale) when the tests are conducted at consistent A and f values. The influence of A is instead somehow counterintuitive as decreasing A implies a reduction of N_f. Such a result is in contrast to what has been observed for other materials (e.g. [3]), for which a larger amplitude A causes a faster degradation. Viscous effects might explain the observed behaviour. For a given q_max, adopting a smaller A implies oscillating around a larger mean deviator stress q_mean. Assuming that the creep strain rate increases as the stress state approaches the limit state surface [e.g. 4], smaller A values would also imply summing larger creep strains to those induced by cyclic degradation.

The dependency of the mechanical response on the strain-rate applied at shearing provides further evidence of the relevance of viscous effects. Figure 1b reports the comparison between two cyclic tests (A and B in Figure 1a) conducted applying the same stress history, but different f (and, thus, average ). The specimen sheared with a lower f (test A) presents a significantly larger accumulation of with increasing loading cycles N_cyc, leading to failure after few cycles. Conversely, slower destructuration takes place in test B, which requires a larger number of cycles to fail. Figure 1c-d compare the cyclic stress-strain responses observed for tests A and B with the monotonic curves obtained employing strain-rates which are consistent with the equivalent average ones implicitly adopted in the cyclic tests ( equal to 0.01%/min and 0.5%/min). The cyclic response observed in test A is well-enveloped by the monotonic curve obtained for = 0.01%/min. Destructuration takes place with cyclic loading, inducing progressive softening of the material response, so that, during the 7^th cycle, the strength reduces below q_max, leading to sudden failure. Conversely, about 60 cycles are needed in test B before approaching the monotonic resistance characteristic of = 0.5%/min.

The relevance of the strain rate on the material response suggests that laboratory tests should be used with care for boundary value problems having very low loading frequencies, such as hydrogen storage. As those frequencies are not compatible with feasible experimental times, a viable testing procedure might require monotonic and cyclic tests at different strain rates. The cyclic tests shall be done at frequencies larger than the field ones. The monotonic tests shall instead be run at both the strain-rate of the field and the one of the cyclic tests. This would allow rescaling the cyclic response of laboratory tests to the one of the field frequency.

Carbon sequestration reservoir at Rock Springs Uplift, Wyoming, USA

Kam Ng — 2023-10-02

The Rock Springs Uplift (RSU) located in southwest Wyoming USA as indicated in Figure 1 is one of several sites across the country to be characterized for suitability for CO₂ sequestration. The RSU was identified for several reasons: proximity to a large CO₂ emissions point source, the overall geometry of the RSU structure, and geologic formations recognized to have properties required for long term storage of CO₂. Original rock cores of the Weber Sandstone formation and the dolomite facies of the Madison Limestone formation from the RSU were prepared into 25-mm diameter rock samples. These samples were vacuum-saturated to 100% saturation with synthetic brine, aged with brine for 800 hours at in-situ temperatures and initial confining pressure of 3.5 MPa, and aged with CO₂-rich brine (400 hours first with brine and another 400 hours with compressed CO₂-saturated brine) at in-situ temperatures and initial confining pressure of 3.5 MPa. During the aging process, the pore and confining pressure were increased in small steps, typically 0.69 to 1.38 MPa at a time using high precision syringe pumps (Teledyne ISCO 260D, Lincoln, NE, USA) until the test conditions were met without fluid flow. Laboratory hydrostatic and triaxial experiments were performed at in-situ reservoir conditions with the temperatures of 90°C for Weber Sandstone and and 93°C for Madison Limestone under three differential pressures of 6.9 MPa, 34.5 MPa and 55.2 MPa.

Geomechanical results of the aged rock samples are presented and discussed. Under the triaxial compression, Weber Sandstone exhibited brittle failure strengths while Madison Limestone exhibited a brittle-ductile transition behavior. For the sandstone samples with similar initial porosity, the Young’s moduli increased and Poisson’s ratios decreased as the result of CO₂ under the linear elastic regime. No relationship between stress (or strain) data and CO₂ in the nonlinear plastic regime was observed. However, the confining pressure has a greater effect than CO₂ on geomechanical behaviors. For the limestone samples, the change of elastic constants due to CO₂ is more significant than that of sandstone samples. However, no consistent trend was observed on the limestone samples,

and the effect of CO₂ is not the dominant factor influencing the plastic properties of rocks. DH1c with the lowest initial porosity and differential pressure exhibits the highest peak strength, Young’s modulus and Poisson’s ratio, indicating the effect of initial porosity. Considering the effect of CO₂ on Mohr failure envelopes, the decreasing trend of cohesion and increasing friction angle was observed in both sandstone and limestone samples. Table 1 summarizes the maximum deviatoric stress, elastic properties, and shear strength parameters of Weber Sandstone and Madison Limestone samples.

BCH modelling studies on biocementation process in mitigating leaks from a CO2 sequestrated aquifer

Pavan Kumar Bhukya — 2023-10-02

Global warming is having a severe impact on the climate of the earth. Minimising global warming is a significant challenge [1]. Geological sequestration is one of the ways to capture the most liberated greenhouse gas, such as carbon dioxide (CO₂), in deep saline aquifers. Due to the high heterogeneity in the caprock adjacent to aquifers, they often have discontinuities (i.e., fractures, faults) responsible for the leaks from the CO₂-sequestrated aquifer [2]. This scenario demands the sealing of discontinuities close to the CO₂-sequestrated aquifer. In recent times, Microbially Induced Calcite Precipitation (MICP), also known as the biocementation process, has proven its potential in cementing the pore spaces in soil and rock mass [3, 4]. The MICP process utilises bacteria to release enzymes and drives the precipitation of calcium carbonate with the hydrolysis of the urea. The biocement produced through this sustainable process has a binding ability and cements the soil/ rock mass. Thus, biocementation can be employed in plugging the leakage paths of the carbon storage aquifer. However, various scenarios must be tested before implementing the MICP technique in the field. Generally, Bio-Chemo-Hydraulic (BCH) domains influence the MICP process and significantly affect the calcium carbonate content with their coupled interdependency [4]. In addition, geosequestration also needs hydraulic modelling studies to better understand the transport of CO₂ plume migration in an aquifer [2]_.Being a complex process, implementing MICP for sealing the leakage paths during carbon geosequestration becomes intricate. Therefore, coupled BCH modelling studies are conducted in the present work to evaluate the sealing ability of the MICP process to plug the leakage paths in the caprock.

A deep saline aquifer with high permeability was considered for carbon geosequestration and subsequent leakage reduction through the MICP process. The mathematical framework for the present work was acquired from Landa-Marbán et al. [2]. The framed problem comprises the fluid flow of two phases (i.e., water and CO₂) and the transport of biochemical species (i.e., microbes, oxygen and urea). The Darcian advection and Fickian diffusion were considered for advective and diffusive transport, respectively. The reaction of biochemical species was accommodated with a reactive term in the transport equation. The microbial processes considered during the transport of microbes are the attachment, growth and decay of the microbes. The biochemical reaction rate for urea hydrolysis was based on a first-order kinetic reaction rate using Michealis-Menten kinetics. The growth rate of microbes with oxygen consumption was followed as per the Monod kinetics. The detachment of microbes due to fluid flow in the pores occurs according to a power law related to Darcy's flow velocity. The calcite precipitation rate is considered to be the same as the urea hydrolysis rate, assuming a calcium-rich environment. A porosity-permeability relation with critical porosity and minimum permeability parameters was adopted for evaluating the changes in pore spaces over the biocementation process.

The current work considers two aquifers connected through a vertical leakage path in the caprock (Figure 1). The primary objective of the work is to plug the leakage path between the aquifers for long-term storage of carbon dioxide. The model geometry of the considered problem is detailed in Figure 1. The top and the bottom aquifers have the same hydraulic properties. However, the leakage path has a higher permeability. The CO₂injection was performed before and after the treatment of the aquifer domains with the MICP process. The CO₂ injection was performed for 500 d to study the influence of the sealing capacity of the MICP process on the CO₂ front migration. For the MICP treatment, the left boundary of the bottom aquifer was subjected to biochemical injections. Initially, the microbes were injected into the aquifer, and the transport of microbes was further accommodated by water injection and no flow conditions. The oxygen and urea were injected consecutively with the same injection strategy to employ the MICP process. After completion of the biocementation process, the CO₂ was injected to observe the leakage rate.

The carbon sequestration into an aquifer with a preexisting leakage path resulted in the migration of CO₂ into the top aquifer (Figure 1). The modelling studies showed that the leakage fraction was 17% when normalised with the injection flow rate (q) at the end of 500 d of CO₂ injection. Later, the transport of biochemical species from the MICP process resulted in the precipitation of calcium carbonate near the leakage path. The MICP has proven effective in reducing the porosity and permeability of the leakage path, which is 100 m far from the injection point. Ultimately, the MICP treatment for sealing the leakage path reduced the leakage fraction to 0% by the end of the third treatment cycle.

Overall, the current study conducted the field scale modelling scenario on the biocementation process for leak mitigation from the CO₂-sequestrated aquifer. The study ascertained the influence of calcite precipitation on the leakage rate from a CO₂-sequestrated aquifer. The MICP process significantly influenced the carbon storage capacity of the aquifer by closing the discontinuity pores inside the caprock, similar to Landa-Marbán et al. [2]. The alterations in the porosity and permeability of the leakage path due to biocementation indulged in improving the carbon storage capacity of an aquifer. The study also recommends injection strategies for the biocementation process to accommodate more significant precipitation of calcium carbonate near the leakage zone. Further modelling studies are needed to understand the plugging mechanisms of the leakage paths for the long-term storage of carbon dioxide.

Integration of subsurface dynamic coupled modelling and monitoring technologies for CCS: examples from existing projects

Vincenzo De Gennaro — 2023-10-02

Introduction

Host rock injectivity and storage complex integrity (containment) for CO₂ sequestration are critical factors to ensure that capacity targets are met while preventing CO₂ leakage over time and confirm permanent storage.

Figure 1 sketches the typical CO₂ injectivity problem. Evaluating injectivity, capacity and containment requires assessing key phenomena such as: seal and faults capillary controls, non-isothermal reactive flow, geomechanical failure induced in the seal, along faults and/or fractures and in the vicinity of the wells. Additional analyses will also be needed to evaluate the completion integrity of the wells (injectors, monitoring and existing wells) under variable load conditions in terms of pressure, temperature and chemistry. The listed phenomena, and their likely effects, are analysed by means of dynamic coupled modelling, considering co-existing (geo)mechanical and dynamic flow simulations. Therefore, coupled modelling improves the risk analysis allowing to evaluate the Thermo-Hydro-Mechanical-Chemical (THMC) processes associated to CO₂ injection.

Analyses

In this presentation we will provide some examples of integration of subsurface dynamic coupled modelling and monitoring technologies for CCS. Some of these examples are derived from existing projects.

Injectivity impairment during CO₂ injection is a well-known issue often observed at well and field scale (e.g. Snøhvit, Ketzin, Decatur projects [1], [2]). Loss of injectivity is governed by the interactions between CO₂-brine phase behavior, CO₂-solid phase (rock) behavior, salt precipitation, multi-phase flow. Characterizing zonal injectivity and developing mitigation strategies will be therefore important for project success. Figure 2 (left and center) shows an example of time-lapse CO₂ saturation evolution during injection and consecutive non-isothermal processes associated to Joule-Thompson effect (H₂O vaporization, cooling and induced salt precipitation due to dry-out). Temperature and pressure changes can lead to near wellbore failure (thermal and pressure induced fracturing) impacting injection performance due to near wellbore fracturing and consecutive permeability changes (Figure 2 right).

To close the evaluation loop, the integrated subsurface characterization workflow is completed by measurements to image the subsurface before and after CO₂ injection. This provides essential insights into the storage complex quality before injection and effective monitoring during injection and permanent storage, tracking the position of the CO₂ pressure and concentration plumes, assessing surface heave, and detecting any risks or potential leakage paths. There is a tight link between subsurface dynamic coupled modelling and existing monitoring technologies. This link expands the well-known MMV (Measurement, Monitoring and Verification) process into a more integrated 3MV process (Modelling, Measurement, Monitoring and Verification), opening to the definition of a subsurface digital twin supporting successful CCS operations. We will briefly introduce methods and technologies enabling the subsurface dynamic modelling and monitoring of CO₂ injection and storage. Seismic and borehole petrophysical and geophysical measurements for initial subsurface characterization and consecutive monitoring will be described.

p-y curves from in-situ ROBOCONE tests: a similarity approach for laterally loaded piles in clay

Abigail H. Bateman — 2023-10-03

The design of offshore piles requires assessment of the lateral load-displacement response, achieved using lateral nonlinear springs (p-y curves) distributed along the pile [1]. Specifically, a p-y curve describes the horizontal displacement, y, resulting from a distributed lateral load, p_, at the pile-soil interface, at a specific depth. Numerous methods are available to obtain p-y curves, either relating them to soil properties [2, 3, 4], or using empirical relationships with cone penetration test (CPT) data [5]. An advantage of the latter method is that the p-y curves are obtained from in-situ testing, which offers undisturbed, continuous measurements and does not require additional laboratory test data. However, the CPT approach relies heavily on empirical correlations and the loading applied to the soil by the CPT is notably different to that of a laterally loaded pile. An alternative solution is being pursued within the EPSRC-SFI funded ROBOCONE project, which involves the development of a horizontal loading module that can be included in a standard cone penetrometer [6]. The aim of this proposed module is to directly obtain lateral load-displacement curves, referred to herein as a ROBOCONE P_R-y_R curve, by moving part of the module horizontally, perpendicular to the long axis of the cone penetrometer (Figure 1(a)), closely replicating the soil loading in the pile problem under consideration.

The soil resistance from the horizontal loading of the ROBOCONE module, , is achieved by applying a constant displacement, , over a short length of pile, , at a specific depth; while for a pile, a force-displacement relationship at the pile head arises due to the effect of individual p-y relationships applied over a much greater portion (“active length”) of the pile, which is closer to a plane-strain “horizontal soil slice” problem. Therefore, while the horizontal loading applied to the soil by the ROBOCONE module is similar to that applied by a pile under lateral load, the ROBOCONE - curve may not be directly substituted for a p-y curve for any soil-pile configuration. Instead, a correction factor in some form must be employed to transform between the two curves, without the intermediate stage of recovering soil properties from the ROBOCONE data and then evaluating the resulting p-y curve.

A simple analytical method to obtain a correction factor for piles in clay is the similarity approach, which takes the form of a linear-transformation of axes. This method was first introduced by Skempton [7] who suggested that a stress-strain curve of

a representative clay soil sample can be considered similar in shape to a load-settlement curve of a vertically loaded foundation. It has since been employed to relate a stress-strain curve from an element test with a p-y curve [1, 2] or a t-z curve (describing the vertical pile displacement as a result of a distributed axial force) [8], the former of which is still used in current design codes [3]. Additionally, the similarity approach has not only been used to obtain curves transformed from element test results, but also to obtain an m-θ curve (describing the pile rotation as a result of a distributed moment) directly from a t-z curve [9, 10].

To employ the similarity approach to this scenario, it must be assumed that a ROBOCONE P_R-y_R curve can be considered similar in shape to a p-y curve, at a specific depth. Following this assumption, two steps are required to obtain a p-y curve, illustrated in Figure 1. Firstly, the y-axis of each curve is normalised by a bearing capacity factor, , the soil undrained shear strength, , and their respective dimensions, which naturally bound the ordinates of the two curves between 0 and 1. Secondly, the x-axis of each curve is normalised by a characteristic length, the pile diameter , and a factored ROBOCONE diameter, , respectively; where is a linear-transformation factor intended to collapse the two curves into a single curve.

To employ this method, a value of is needed. In the original work, Skempton [7] determined this factor by matching the dimensionless initial stiffnesses of the two curves he considered; this approach is followed here with the ROBOCONE P_R-y_R and p-y curves (see Figure 1). Firstly, from dimensional arguments, the initial stiffness of the ROBOCONE P_R-y_R curve, , is a function of the soil shear modulus and Poisson’s ratio, and , and the dimensions and , multiplied by a constant . can be estimated from elastic solutions for rectangular footings or embedded or buried cylinders (e.g. [11]). Secondly, the initial stiffness of the p-y curve is the Winkler spring stiffness, , for which many solutions are available (see a recent summary [12]). The resulting expression for is shown in Figure 1(c), which can be used to directly relate a ROBOCONE P_R-y_R curve with a pile p-y curve in practice. By taking as double the stiffness of a rigid rectangular footing on an elastic half-space [11] and for a fixed-head slender pile [12], to was obtained. This suggests that (1) the ROBOCONE P_R-y_R curve should be “stretched” to obtain a p-y curve, (2) the multiplier is independent of and , (3) tends to increase with increasing (e.g. free-head over fixed-head pile [12]) and ROBOCONE slenderness . This initial result indicates the magnitude of the scaling factor for estimating a p-y curve at a specific depth. However, this method requires numerical verification and/or field or physical modelling observations to be undertaken prior to application in design practice. This will be undertaken as part of the ROBOCONE project.

Permeability evaluation based on Nuclear Magnetic Resonance analysis for gas hydrate reservoir

Jun Yoneda — 2023-10-03

Permeability (or hydraulic conductivity) is one of the most important parameters for analyzing hydraulic behavior of underground. For example, in the geotechnical engineering, permeability is necessary to calculate water inflow and draw down of water level during excavation, to predict the deformation time due to the consolidation, and to evaluate the barrier of the ground for radioactive waste disposal. In the petroleum industry, permeability is considered an essential parameter which is directly linked to the productivity of oil and natural gas. Therefore, predicting permeability using wireline logging or logging while drilling (LWD) is required during the assessment of the reservoir. Nuclear Magnetic Resonance (NMR) logging is performed at this point. Low-field NMR measurements are sensitive to the hydrogen proton, 1H, in liquid water but not to the 1 H bond in solid form, making it possible to obtain the bulk water volume. In addition, the hydrogen proton NMR- T₂ (transverse or spin-spin) relaxation time distribution is used to infer pore-size information and to estimate permeability [1, 2]. The major methods used for estimating permeability, k, are the Schlumberger Doll Research (SDR) model and Timur-Coates (TC) model [3, 4]:

Where f_NMR is the NMR-estimated (effective) porosity. SDR model uses the averaged value of the T₂ distribution plotted on log-scale of time and matches it to the experimental reference permeability with the fitting parameter a [mD/(ms)²]. The TC model seeks to differentiate between the fraction of free (mobile) ﬂuid volume (FFV) and bound ﬂuid volume (BFV) and estimates permeability with FFV/BFV ratio and the material constant c [m²/s²]. The permeability in the research and development of methane hydrate, which is expected to be a next-generation resource, has been estimated using above methods. Methane hydrate is an ice-like solid crystalline structure of water and methane molecules. Because hydrate is solid, NMR is not sensitive, the initial (effective) permeability can be estimated by NMR analyzer. On the other hand, A strong disconnect (up to 2 orders of magnitude) between NMR-based estimates of permeability with those determined from a laboratory study of pressure cores from the same reservoir has further cast uncertainty in the application of borehole NMR data in hydrate evaluation [5, 6]. Recently, authors provided comparisons between NMR-derived values and direct ﬂuid ﬂow estimates of permeability in naturally occurring hydrate-bearing sediment [7, 8]. On the basis of the NMR measurement for hydrate-bearing pressure core sediments, which were recovered from the Gulf of Mexico and Alaska North Slope, the following new hydraulic radius model was proposed [7].

This equation estimates the specific surface based on the NMR signal and has been shown to successfully estimate the initial permeability of a hydrate reservoir by expressing a hydraulic radius model based on Kozeny's equation [9]. However, the proposed equation is not universal and has been found to overestimate for clayey and silty hydrate free sediment. This study summarizes the methods and parameters used to estimate permeability using NMR signals from hydrate-bearing sediment obtained from logging and laboratory tests for the Gulf of Mexico and Alaska North Slope. In addition, we reports parameters for predicting model for muddy sediments which was recovered from overburden of hydrate-reservoir in the Hyuganada sea in 2022 Japan [10, 11]. When using the hydraulic radius model to predict permeability of hydrate-bearing sediment, reasonable estimates can be obtained by setting the shape factor for grains C_s = 1/2 (for a circular grain) and the shape factor for the flow path c_n = 4 (for the cylinder network), respectively. Although, for hydrate free muddy sediment, C_s = 1/3 for parallel plate grain shape and c_n = 2 for the fracture network model matched its permeability. Furthermore, the conversion parameter from T₂ relaxation time to the pore size for hydrate free muddy sediments r₂ = 0.009 (mm/s) was confirmed based on the result of mercury injection capillary pressure analysis, which is lower than r₂ = 0.0325 (mm/s) of hydrate-bearing sediments.

Offshore geotechnical challenges of the energy transition

Susan Gourvenec — 2023-10-03

Offshore wind is the most mature of the offshore renewable energy technologies and has a significant role to play in the energy transition. 2000 GW of offshore wind capacity is anticipated globally by 2050 in order meet the targets of the Paris Agreement [1]. These ambitions correspond to a 35-fold increase compared to current installed offshore wind capacity, which has taken three decades to achieve, thus requiring an installation rate between now and the mid-century far out-stripping that currently achieved [2]. The pace and scale of offshore wind ambitions to support the energy transition present a range of challenges for the offshore geotechnical sector and the broader offshore wind sector, with impacts across the supply chain regarding availability of raw materials, vessels, equipment and personnel, and across each stage of the life-cycle of the projects from marine spatial planning, site investigation, design, manufacturing, installation, operation and decommissioning. To address these challenges, the sector faces key imperatives, including (i) to determine where best to place future offshore windfarms to meet technical, social and environmental requirements and identify where technology and knowledge gaps exist, (ii) to improve efficiency in deriving geotechnical design parameters in order to alleviate pressures on people, vessels, equipment and labs for site investigation and interpretation of test results; (iii) to improve efficiency of design outcomes, i.e. to achieve greater capacity:weight efficiency for foundations, anchors and mooring systems to ease supply chain, manufacturing and installation pressures, while maintaining necessary reliability; (iv) to improve time efficiency to complete designs in order to make design calculations and processes less time- and personnel-intensive to carry out; and (v) to improve the life-cycle cost of delivering the necessary offshore wind to enable a net zero future through considering the end of engineered life at the design stage.

Figure 1 illustrates the magnitude of the projected and necessary growth in offshore wind globally to 2050 in terms of installed offshore wind capacity and number of turbines, with an indication of the associated geotechnical implications for this pace and scale of growth.

Potential solutions to address the identified challenges and associated imperatives include those to address specifically geotechnical challenges, but also geotechnical solutions to offshore wind challenges beyond the seabed. For example, (i) use of geospatial mapping and analysis to determine the available seabed areas for future offshore windfarms and identify suitable foundations or anchors for particular regions, which can in turn inform on supply chain or critical technology and knowledge gaps [4]; (ii) innovations to increase efficiency of offshore site investigation, including software [5] [6] and hardware approaches [7] to facilitate seabed characterization at meaningful spatial and temporal scales; (iii) innovations in design philosophy, concepts and methods to improve design outcomes for foundations and anchoring systems, including whole life geotechnical design [8], novel moorings and anchors [9] [10] [11] and integrated design [12]; (iv) creation of optimization design tools that can directly address the design question, avoiding numerous ad hoc individual analyses to hone in to a workable solution [13] [14]; and (v) methods to inform on requirements for end of engineered life, e.g. retrieval or stability if left in situ beyond the design life [15] [16], and geospatial mapping to understand the distribution of assets at the end of engineered life in order to plan for recycling or disposal [17].

Lessons learned from large-scale physical modeling of tapered jacking piles: HSDT

Lars Bo Ibsen — 2023-10-03

To provide an improved understanding of the behavior of screw piles in disturbed soil, as a series of companion papers, laboratory tests with full scale tapered jacking piles were performed in the current study. The designated tapered pile has the similar dimensions, material properties and cone-shaped bottom, with the exception of threads eliminated.

The main objectives of this paper are to: (1) establish the robust and efficient simple high strain dynamic testing (HSDT) requirements for validating displacement tapered piles capacities while exploring the installation effect; (2) determine a correlation between static and dynamic bearing capacity in compression for steel tapered piles installed by jacking in sand with various relative densities.

Out of nine large-scale laboratory expriments at Aalborg University Offshore Geotechnics Laboratory [1-3] with the tank filled with Aalborg University Sand No.1, this research exclusively presents some lessons learned from an exemplar tapered jacking pile installation (Test 4) in dry sand (relative density and its response to monotonic and impact loadings. The selected piles have diameter of D=76 mm and 89 mm with length of 2.07 m. The layout of the miniature CPTs carried out to measure soil state is shown in Fig. 1, with four CPTs performed following each jacking installation campaign to study the soil state after the installation of the piles (Fig. 1). In order to fulfill the objectives, the piles are installed by a static compression load that includes an unloading/reloading step, followed by dynamic testing in order to facilitate the determination of the bearing capacity (Fig. 2). The hammer test was carried out on both piles, by a hammer from increasing drop heights of 218, 418, 618, 818 and 1018 mm (Fig.3). The hammer consists of a sleigh with multiple attached steel plates weighting 19.31 kg on average and the sleigh itself weighs 18.55 kg. Tapered pile jacking resulted in dilation extending to a depth of 2.8when diameter exceeds from 76 mm to 89 mm. By contrast, in the region around pile P76, the soil was entirely densified. The tendency to densify the soil near tip area due to jacking installation is observed in both tests, where the initial viod ratio varied from 0.67 to 0.59 and from 0.63 to 0.57 corresponding to piles P76 and P89, respectively (Fig.4).

Subseuqntluy, Danish Pile Driving Formulas (DDR) introduced by [4] was ustilised which is according to the required potential energy from a hammer stroke to exceed the pile penetration resistance. The mass of the hammer G is defined by:

where elastic settlement =, A is cross-sectional area, E is the Young’s modulus, l is the installed length of pile which is approximately 1.9 m, and is the maximum static force required to install the pile. The efficiency factor is set as 1. During the impact loading tests, the settlement of the pile is measured with a 1 mm accuracy for each hammer stroke, and the limiting settlement, s was set to yield a certain level (0.1D) when the drop height is 618 mm. Thus, the dynamic bearing capacity = , and ratio= are shown in Table 1.

Reducing Uncertainty in Site-specific Stress Prediction for CO2 Storage in the North Sea, Norway

Yu Feng — 2023-10-03

Two CO₂ storage sites located in the western Norwegian North Sea (NNS), called Aurora and Smeaheia, are currently under construction and assessment respectively. In geological storage of CO₂, the in situ minimum horizontal stress is an essential input parameter for assessment of both containment and induced seismic risks [1]. To infer the stress states at certain depths at a site where no data is available, the standard approach is to perform a classical linear regression on stress data versus depth and treat the fitted trend line as the best site-specific stress predictions along depth [2]. However, stress data are often highly limited at CO₂ storage sites; for example, Aurora and Smeaheia have only five in situ stress measurements available at best respectively. Such limited data may severely underrepresent the true stress distribution at one site. Data scarcity coupled with measurement error and spatial variability poses a challenge to reliable stress prediction, and hence it is crucial to quantify and reduce uncertainty in site-specific stress prediction for CO₂ storage. Stress uncertainty is actually the required input information in the more rational probabilistic risk assessment framework. A natural solution to reducing uncertainty is to integrate stress information from other sources.

On the Norwegian continental shelf, extensive data has been accumulated from previous petroleum projects. Of the publicly available NPD stress database, Figure 1a shows the distribution of versus depth (< 3,000 m) for each site within the study area containing Aurora and Smeaheia, and reveals a certain degree of similarity between the stress trends at the 11 sites. Such similarity, aligned with other published results [2], may be attributed to the relaxed sedimentary basins where gravitational loading dominates the lateral stress distribution rather than tectonic components, with the between-site variation arising from differences in the geological conditions and pore pressures [3]. When facing limited data for a site like Aurora and Smeaheia, the current approach is often to either directly use the stress trend from other sites having richer data or expand the coverage area to include more data. Such semi-subjective information borrowing approach, although effective in many cases, may lead to overly confident stress predictions as it fails to account for possible between-site heterogeneity in stress trends.

Bayesian inference has been widely used as a rigorous and powerful statistical approach for quantifying uncertainty, as well as combining information from different sources via informative prior distributions. Hence, historical stress data may be integrated into Bayesian analysis of site-specific data in the form of prior distributions, with stress uncertainties being quantified and updated as the posterior distributions [4, 5]. When developing prior distributions for site-specific stress prediction, it may be tempting to combine all historical stress data for a holistic Bayesian analysis, yet such complete pooling approach may give an overconfident summary of prior information in that it ignores the possible stress heterogeneity between sites.

This paper presents a Bayesian hierarchical (i.e., partial pooling) model (BHM) that explicitly accounts for between-site heterogeneity/similarity when constructing prior distributions from historical stress data, and demonstrates how the proposed model effectively borrows historical information to reduce uncertainty in site-specific stress prediction for CO₂ storage in the NNS study area. Figures 1b illustrates the prior predictions of versus depth at the Aurora site from the Bayesian complete and partial pooling models. Although the complete pooling model gives less uncertain stress predictions than the partial pooling model as indicated by the narrower 90% prediction intervals (PIs), it does not well capture the five unseen stress measurements at Aurora in that two out of five stress values fall outside the 90% PIs. This suggests that complete pooling analysis indeed gives overconfident prior distributions out of the NPD database, and is thus not suitable for integrating historical data into site-specific stress prediction for CO₂ storage in the NNS. On the other hand, the partial pooling model gives fairly good prior predictions of the five unseen stress values at Aurora, albeit with larger uncertainties. This result demonstrates the effectiveness of BHM as a framework for formulating proper informative priors from historical data, and an encouraging implication is that probabilistic risk assessment is allowed even with no site-specific stress data at this storage site, which is not possible if external information is not integrated properly. Figure 1c shows the posterior stress predictions updated with the five site-specific stress values from the two Bayesian models in question. After incorporating the site-specific data, the complete pooling model still over-predicts the two stress values at depths with barely noticeable updating, while partial pooling gives considerably more accurate stress predictions with reduced uncertainty.

On the installation effects of open ended piles in chalk

Matteo Oryem Ciantia — 2023-10-03

Chalk is a highly porous rock formed by cemented calcite grains. It covers areas of the UK and is widespread under the North Sea where offshore wind turbines (OWT) are currently being installed and where future offshore expansion will be sited (Figure 1(a)) [1]. Large piles are often driven in chalk to support OWT. The installation process causes the intact rock below the pile tip to crush into a putty characterised by a mechanical behaviour very different from the intact chalk. The difficulty to predict the final state of the putty and the stress around the pile after installation is the underlying reason for inadequate current design guidance for piles in chalk. Considering that for OWT, foundations account for 20-25% of the total development cost , pile design improvements in chalk, would be extremely beneficial from an economical and environmental perspective.

Current guidelines for the design of piles in chalk (CIRIA C574) originate from the analysis of a limited number of pile tests [2]. These guidelines suggest crude average ultimate unit shaft friction (t_sf) design values of between 20 and 120 kPa for low-medium density and high/very-high density chalk, respectively. t_sf estimates are thought to be conservative hence introducing significant increases in cost and carbon footprint (Figure 1(b)). Reducing the level of conservatism (e.g. enabling more confident use of higher t_sf) would reflect in significant savings of steel and consequent reduction of the cost and embodied carbon. Such design considerations are possible but require a better understanding of the long-term mechanical behaviour of the damage processes intact chalk experiences during dynamic installation in hydro-mechanical (HM) coupled conditions.

In this work the coupled HM effects developing during pile installation in chalk are investigated numerically using a robust and mesh-independent implementation of an elasto-plastic constitutive model at large strains. The model, implemented into an open-source Geotechnical Particle Finite Element (G-PFEM) code [3], is shown to be able to capture the damage of the rock until the formation of a chalk putty layer around the shaft of a model piles jacked in chalk. In particular the complex flow processes occurring in the soil around both open and closed ended piles of variable shape jacked into saturated chalk are investigated. A fully coupled hydro-mechanical formulation, based on regularized, mixed low-order linear strain triangles is used [5]. To capture the relevant features of the mechanical response of chalk, a finite deformation, non-associative structured modified cam clay is used [6]. The model formulation is based on a multiplicative decomposition of the deformation gradient and on the adoption of an elastic response based on the existence of a suitable free energy function. Bonding-related internal variables, quantifying the effects of structure on the yield locus, are used to provide a macroscopic description of mechanical destructuration effects. To deal with strain localization phenomena, the model is equipped with a non-local version of the hardening laws [7]. The G-PFEM model is shown to be capable of capturing the destructuration associated with plastic deformations below and around the pile shoulder; the space and time evolution of pore water pressure as the pile advances; the effect of soil permeability on predicted excess pore water pressures, and the effect of chalk putty formation on predicted values of the load displacement curve. Installation effects are highlighted by comparing the axial performance between wished in place piles and piles which considered the full installation process.

A centrifuge study into the installation response of steel, open-ended, tubular piles, dynamically driven using prolonged impulses

Tristan Quinten — 2023-10-03

Fuelled by technological innovations and the growing commitment of countries to reduce their carbon footprint, offshore wind has steadily been gaining ground on non-sustainable sources of energy. According to the International Energy Agency (IEA) [2], it is foreseen that wind will be the principal source of energy in Europe by 2027. However, in an effort to protect the marine environment from sound pollution [1], regulations applicable to offshore wind endeavors have become more stringent. To sustain the growth of the offshore wind sector, more durable installation methods are required. Prolongation of the impact duration has been identified as a suitable method to protect the marine environment from sound pollution [7]. However, the market introduction of this technology is hampered by a lack of understanding of its effects on soil-structure interaction. Therefore, concerns exist on (mono)pile drivability, as well as the performance of the foundation under axial and lateral loads. To address this issue, a series of centrifuge experiments is performed in the centrifuge facility of Delft University of Technology (DUT). The experiments are conducted at 50g acceleration and show the effect of blow prolongation on the drivability of miniature steel, tubular pile (outer diameter, D = 42 mm; wall thickness, t = 2 mm) in dry GEBA sand at 80% relative density. The blow-prolongation technology that is assessed is IQIP’s BLUE Piling (BP) Technology [4]. Details on the actuator used to simulate BP technology in the centrifuge are provided by the work of Quinten et al. (2022) [6]. Prior to dynamic installation, the pile was allowed to settle in into the sample under 1g conditions. Subsequently, an second self-weight penetration phase is initiated by increasing the centrifuge acceleration to 50g, while the ram acts as dead-weight on top of the pile. Following the self-weight penetration phase, the centrifuge is intermittently stopped and reinitiated to (re)set the actuator. The BP ram has a mass of 1.889 kg ram and stroke of 40 mm. The results of the BP experiment are compared against those from centrifuge experiments involving impact hammering (IH), the most widespread method of installation for monopiles in the offshore sector. A detailed description of the actuator, the miniature impact hammer of DUT, is provided by Quinten et al. (2022) [5]. The model pile was pre-embedded at a depth of 50 mm under 1g conditions prior to the dynamic installation phase. The hammer operates at a driving frequency of 10 Hz and is equipped with a ram of 0.140 kg, which is released from a height of 40 mm. Comparison of the results of both experiments, reveals striking differences in the cumulative settlement behavior of the pile as well as the pile stresses. The cumulative pile displacement charts, as shown in Figure 1, show significant differences between the two installation methods. For BP (Figure 1a), a series of 3 single blow experiments was conducted. For IH (Figure 1b), the experiment lasted for a total of 37 consecutive blows. The realized pile set during the experiment is comparable between the two tests. The average normalized displacement equates to 0.5D and 0.03D per blow for BP and IH respectively. The soil level inside the pile cavity was measured following the execution of both experiments. The associated measurements indicated that both piles were driven in a fully coring mode. When considering the prototype pile dimensions and soil conditions, this finding corresponds well with the work of Jardine et al. (2005) [3]. Further differences between IH and BP were observed in terms of (peak) pile stress. The driving forces are reduced from 25 kN for IH to 6 kN for BP, respectively. The driving factor behind this reduction is the decrease in interface stiffness between the ram and the anvil. The latter completely offsets the effects associated with the use of a significantly heavier ram mass in the case of BLUE Piling. When the driving forces are expressed as a percentage of the pile yield limit, aforementioned figures respectively equate to 42% and 10%. Extrapolated over the full installation sequence, the latter would contribute to a reduction in the fatigue accumulated during installation.

The results presented here form the first step towards understanding the effect of blow duration soil-structure interaction for blow prolongation technology. For the set of installation parameters and boundary conditions considered in this study, it is shown that the differences in pile installation behavior can be captured using centrifuge modeling. The prolongation of blow duration results in a significantly different overall installation behavior. When looking at the driving forces, the decrease of the interface stiffness between the ram and anvil produces the anticipated decrease in peak driving force. A sustained physical modeling effort is required to ultimately lay the basis for a predictive installation framework for blow-prolonging technology, which would arguably accelerate its adoption by the industry. The latter should help the reek the associated benefits, particularly in terms of fatigue reduction and sound remediation in the near future.

Development of a new T-bar for the geotechnical centrifuge at TU Delft

Yuen Zhang — 2023-10-03

An accurate estimation of undrained shear strength of clay seabed is important for interpreting lateral pile-soil interaction response. The cylindrical T-bar is a widely used site investigation tool for profiling the undrained strength (su) of soft soils. As such, a new miniature T-bar penetrometer is designed and fabricated at TU Delft for characterization of the undrained shear strength profile of clay layer in centrifuge models, and OCR profile can be then derived from undrained shear strength profile with known pre-consolidation stress level. The miniature T-bar penetrometer head is a cylinder of 5 mm in diameter and 20 mm in length and the miniature T-bar penetrometer head is connected with a rigid shaft (Figure 1(a)). The T-bar penetration resistance along the depth of clay sample can be obtained and further interpreted into the undrained shear strength profile.

Rate of penetration is one of the key parameters that govern the drainage behaviour of soil response around the T-bar and the resulting penetration resistance. A very low rate of penetration leads to partial pore water dissipation and thus partial drainage condition. When a very large rate of penetration is applied, the fully undrained condition is achieved. However, the effect of T-bar penetration rate has not previously been fully examined for normally consolidated and over consolicated clay in the centrifuge. In this paper, the tip resistance profile of T-bar penetration tests under different rates of penetration is analyzed to obtain undrained shear strength profile of clay soils.

A series of T-bar tests are conducted in both normally consolidated and slightly over consolidated clay samples at 100 g. The sample drainage state is tested by varying the rate of penetration from 0.01 mm/s to 5 mm/s. The results are interpreted by two methods: (i) the conventional method by converting the measured penetration resistance to soil strength using a single bearing factor, indicating a full-flow mechanism at failure [1]; and (ii) an approach considering soil buoyancy and a reduced bearing factor arising from the shallow failure mechanism, indicating the shallow correction procedure has a significant influence on the soil strength profile inferred from a T-bar penetrometer test [2]. The interpreted undrained shear strength profile provides soil property and OCR information for further monopile tests in the centrifuge, allowing a comprehensive study of the soil-structure interaction on soft soils.

Silent piling for offshore jacket foundations in sand: DEM and centrifuge modelling

Benjamin Cerfontaine — 2023-10-03

The race to decarbonise the economy has led to an exponential growth of offshore wind farm developments across the globe. While monopiles are the dominating installed foundations, jacket structures are expected to be more and more common, as wind farms extend to deeper waters [1] and innovative technologies are required to alleviate some important challenges. Firstly, straight shafted piles are not particularly efficient to sustain large tensile demand induced by the push-pull axial loading of the foundations. Secondly, stricter regulations on underwater noise make mitigation methods for pile driving more expensive and silent piling techniques could be used as an alternative [2]. Figure 1(a) describes a new screw pile foundation that meets those two challenges [3]. The foundation is installed by rotary jacking, which avoids any impact related noise. The pile is composed of a large upper shaft, that is designed to resist the lateral load applied by the jacket structure, and a smaller lower shaft which reduces the torque demand during installation. A helix is attached close to the pile tip, to provide an enhanced axial resistance and facilitates the installation of the pile [4]. The main challenge during the pile installation is the very low reaction force that may be available offshore at the pile head, which consists only of the pile self-weight and the installation tool. The aim of this work is to demonstrate that screw piles can be installed for offshore applications by rotary jacking at low reaction force, via (geotechnical centrifuge, [2]) and numerical (DEM, [5]) modelling.

Figure 1(b) compares the vertical installation force measured in the centrifuge with a DEM simulation, as a function of the advancement ratio (AR), defined as the vertical displacement of the pile during one rotation normalised by the helix pitch. Figure 1(b) shows that at constant installation rate (fixed AR), the lower ARs lead to a tensile force measured at the pile head, i.e. the pile pulls on the installation actuator. The DEM simulation of is in good agreement with the centrifuge result at the same AR and shows that the vertical force trend reverts to a compressive value once the upper part of the shaft enters the ground.

Figure 1(c) shows the vertical force associated with each pile component during the DEM simulation (AR = 0.5). The greatest penetration resistance (lowest negative value) is the transition piece, reaching -4MN, when the pile tip reaches a depth of 25m. The shaft penetration resistance is small and probably reduced by the rotary movement that changes the inclination of the shear stress along the shaft, as explained in previous publications [6]. The base and helix contributions are largely positive (in tension), as described in Figure 1(c). Consequently, the tension (“thrust”) created by the helix compensates the penetration resistance of the rest of the pile and enables its penetration with a very low reaction force applied at the pile head.

The effect of the AR on the helix behaviour was previously explained as follows [7, 8]. Helix overflighting (AR< 1) forces a displacement of the sand particles upwards. This forced displacement is opposed by the existing soil, which acts as a non-linear spring, which is progressively compressed. The soil above the helix is then compressed, as depicted in Figure 1(d), which represents the vertical stress in the sand bed and around the pile. Another effect of the pile overflighting is the considerable reduction of the vertical stress under the helix, which facilitates the pile penetration. On the contrary, a large compressive stress under the transition piece is at the origin of its large penetration resistance and its geometry may need to be further optimised.

In the field, where the pile head condition is a constant reaction force, the AR will simply vary such that the thurst provided by the helix will compensate the pile penetration resistance. Results shown in Figure 1(b) prove that there exists a set of AR between 0.10 and 1.00 such that the necessary force is equal to zero at any depth, thus ensuring the fasibility of its installation at low reaction force, providing a torque of 23MNm can be applied.

The physical and numerical modelling of a new type of pile have shown that it is a viable option to support jacket structures, as it can be installed without driving and with a very limited reaction force. The DEM simulations have revealed the underlying mechanisms explaining the centrifuge results, while centrifuge results were used to validate the DEM simulations. This project showed that the combination of the two techniques is very powerful to speed up technology development.

Understanding the behaviour of a new robotic device for next-generation site investigation: A 3D DEM Exploration

Sathwik Kasyap Sarvadevabhatla — 2023-10-03

Accelerating advancements in robotic engineering provide an opportunity for developing economical and efficient site characterisation devices, particularly for critical offshore applications. A new intelligent robotic device, ROBOCONE (Figure 1), is being developed for site characterisation and design of offshore structures, which would be capable of mimicking the load histories of the soil inside the ground at a desired depth [1]. The obtained soil constitutive properties would support an efficient design of offshore structures, considering “whole life” stress-strain histories. ROBOCONE could apply lateral, shear, and torsional modes (Figure 1(a)) of loading on the surrounding soil, operated remotely from the ground. In the pursuit of developing such a device, a prior understanding of its behaviour is essential. The lateral loading static and cyclic responses of offshore foundations are critical aspects of their design. The present work highlights the lateral module (P-Y Module) behaviour of the ROBOCONE device (Figure 1(a)) under various stresses, using 3D Discrete Element Modelling (DEM). The P-Y module moves horizontally at a desired depth in the ground and this lateral motion is numerically simulated using commercial DEM software, Particle Flow Code (PFC3D) [2].

The proposed dimensions of the P-Y module are 200mm long and 54mm diameter (D_RC). Present DEM model uses rigid walls as P-Y module placed in a rigid monodisperse 3D granular assembly with grain diameter of around 1/3^rd D_RC [3]. The Hertz contact model micro-parameters for grain-grain (E₅₀ value of 27.4±2.2MPa) and grain-wall contacts are shown in Table 1. A 10 times higher shear modulus of grain-wall contact than the grain-grain contacts is sufficient to have stiffer response. The servo mechanism in PFC enables the model to reach desired average stresses. The primary objective of this work is to understand the ROBOCONE P-Y module behaviour and thus the lateral soil reaction curves without the influence of boundary. For the current testing mode, where the module moves laterally in the soil, the ratio of the moving object to soil bed dimensions for no boundary effects is unprecedented. A simple and innovative method of using periodic boundaries is proposed in the present study. Conventionally, periodic boundaries replicate the representative volume element infinitely at the boundaries. Due to the presence of ROBOCONE in the centre of the model, the required extent reduces to half in X and Y directions, as schematically shown in Figure 1b. This paper discusses the model side to ROBOCONE diameter, L/D_RC, (e.g., 9, 18, 30) influence on the P-Y curves (at a given stress level). The effects of random particle generations for different models in PFC software can be minimized using same central core zone of contacts with ROBOCONE for each simulation, while the extent of model side is increased. ROBOCONE wished-in-place rather than driving/jacking in the model reduces the DEM computation costs. Limited amount of lateral displacement of the ROBOCONE (around 2.5mm) is sufficient to establish the effectiveness of the proposed modelling. Optimum loading rate (the lateral velocity of P-Y module) of around 0.8mm/s, which can maintain the inertial number lower than the threshold value of 7.9x10^-5[4] further helps the computational efficiency.

Once the optimum L/D_RC value is obtained, the quantitative outcomes from these simulations are the lateral soil reaction curves at different stress levels and also the bending moments on the ROBOCONE P-Y Module. Though the ROBOCONE model in these simulations is a rigid wall and doesn’t mobilize in bending, the moments calculated from the magnitude and locations of each contact force on the P-Y module provide an estimate of bending stresses generated on the module. These results provide a basis for developing an understanding of the physical capacity of the robotic device in sustaining the lateral forces. Figure 1(c) shows preliminary results of soil reaction curve from the P-Y module at 500kPa stress level for L/D_RC value of 9. The changes in the contact forces during the P-Y module motion can be qualitatively assessed through the changes in the force chain network (Figure 1(d)). Figure 1(e) shows a representative plot of bending moments calculated at different lateral movement stages of the P-Y module, and the maximum value at a desired displacement of the P-Y module suggests the physical design requirements of the ROBOCONE. The results from these DEM simulations would directly inform the design of the robotic device for ground investigation and hence improve the economical offshore foundation designs.

A simplified method for calculating the accumulation of irreversible ro-tations of wind turbine shallow foundations

Luca Flessati — 2023-10-03

Wind turbines represent a convenient source of renewable energy and their diffusion is a fundamental step for reducing carbon dioxide emissions, main responsible for global warming. Turbine towers are characterized by large heights, therefore the wind load induces considerable bending actions at the base of the structure. Although, especially offshore, deep foundations are usually adopted, shallow foundations are still an option in case of onshore installations [1]. These circular shallow foundations are massive concrete structures (Figure 1) that counteract the bending moment with their own weight. The optimization of the design and the extension of the service life are fundamental from both economic and environmental perspectives. These will increase the competitivity of energy production from renewable wind sources over fossil ones, thus boosting the transition toward sustainable energy.

From a geotechnical perspective, the isolated foundations of wind turbines are a peculiar case since the accumulation of vertical settlements and horizontal displacements are not a main concern. On the contrary, the accumulation of excessive rotations due to the wind cyclic loads may reduce the efficiency of the turbine and, in particularly critical cases, define the end of service of the structure.

The accumulation of irreversible rotation during the exercise cyclic loads may be induced by both the development of irreversible strain in the soil and the formation of cracks in the concrete foundation. This second aspect can however be neglected, since, in the current practice, very over-conservative approaches are used to design the reinforcements. Recent experimental small scale tests (Figure 1) proved that, even when stirrups are completely removed, exercise loads do not induce the formation of cracks [2]. Therefore, the accumulation of cyclic irreversible rotation is only due to the soil.

Recently, numerous constitutive relationships accounting for the cyclic behaviour of the soil were proposed [3,4]. These in principle allow to perform finite element analyses capable of reproducing the soil-structure interaction. However, during the foundation design life a very large number of load cycles (up to 10⁸) is expected, leading to unacceptable computational times.

A way to circumvent this problem is the employment of models conceived in the framework of the macroelement approach [5]. This is based on the assumption of reproducing the response of a complex system by using a very small number of degrees of freedom and by introducing an upscaled constitutive law relating generalized stresses (forces and moments) and strains (displacements and rotations). The abrupt reduction in degrees of freedom significantly reduces the computational times. However, the definition of the upscaled constitutive laws and the calibration of its parameters are particularly critical since they will be affected not only by the soil behaviour, but also by the foundation geometry and mechanical properties.

In this work, the authors intend to propose a new one-dimensional macroelement to reproduce the moment-rotation response of shallow foundations for wind turbines. This generalized constitutive law, inspired to the one proposed in [6] for reproducing the lateral response of piles, is conceived in the framework of the bounding surface plasticity theory. The main ingredients of this model are a bounding surface and a mixed isotropic-kinematic generalized strain hardening controlling the ratcheting.

The model is capable of qualitatively reproduce (Fig. 2) the experimental results of [2] and seems to be a very promising approach for design purposes. In addition, the model can also simply be introduced in structural finite element codes allowing to solve soil-structure interaction problems also accounting the turbine tower. This can be used to adequately reproduce the whole system response and to also optimize the tower design to dynamic loads.

Development of the heat flow cone penetration test (HF-CPT)

Leon Vrielink — 2023-10-03

The heat flow cone penetration test (HF-CPT) provides in-situ values of thermal conductivity (patent pending). The HF-CPT test records include (1) heat flow (HF) measurements acquired by a CPT add-on module, i.e. heating power and temperature versus time and (2) cone penetration test (CPT) measurements, i.e. cone resistance, sleeve friction and pore pressure versus depth and time. The test method requires a short interruption of the continuous CPT penetration phase, to allow stationary HF heating and cooling cycles.

Values of thermal conductivity are derived similarly to the principles for laboratory thermal needle probes described in common ASTM standards, particularly [1].

Data processing makes use of an advanced interpretation method that accounts for the short-cylinder effects of the HF module and short timeframe heat fluxes. The novel interpretation method includes inversion of a numerical forward model of the interaction between the heat flow module and the surrounding soil. The interpretation method also integrates standard CPT results, such that both a semi-continuous thermal conductivity profile and a continuous standard CPT profile are obtained.

Validation of the interpretation method included comparison of thermal conductivity values derived from other test methods, notably laboratory transient plane source tests [1], in-situ thermal needle probe tests (based on [2]) and thermal cone penetration tests [3]. Figure 1 presents an example of validation results for predominantly clay soil. The results are seen to closely match those derived from an in-situ needle probe. As the HF-CPT measurements were taken two metres apart horizontally from the in-situ needle probe measurements, some deviation between the results is to be expected due to heterogeneity of the soil.

The post-installation performance of piles installed with a novel driving method: field tests and numerical modelling

Evangelos Kementzetzidis — 2023-10-03

Since the industrial revolution, humanity's impact on the planet has increased significantly. The growth of global economies and population size in the 20th century was fueled by the combustion of fossil fuels. In an effort to reduce the impact of human kind on the environment, governments ratified landmark agreements such as the Montreal Protocol in 1987, the UNFCCC in 1992, the Kyoto Protocol in 2005, and the Paris Agreement in 2015.

To support this effort, the European implemented the European Green Deal (2019), which aims to achieve no-net greenhouse gas emissions by 2050. Offshore wind energy, particularly large-diameter monopiles, is expected to play a substantial role in this transition. Europe has already developed over 28 GW of offshore wind power, with a global capacity of 37 GW as of 2021 [8]. However, to meet the goals of the European Green Deal, offshore wind capacity needs to scale up significantly in the next 28 years.

The installation of monopiles, the most selected foundation option for offshore wind turbines, has traditionally relied on impact hammering. This method has drawbacks such as lengthy installation times, and noise emissions harmful to marine life. An alternative approach is axial vibratory pile driving, which offers faster and quieter installation. However, certification bodies have yet to endorse its use for offshore wind farm construction owing to uncertainties relating to the post-installation monopile performance. Research efforts are being devoted to understanding the dynamic behaviour of the soil during vibro-driving and the effects of vibro-installation on pile performance. Several geotechnical research teams are investigating the post-installation lateral behaviour of monopiles and comparing the performance of impact piling and vibratory piling [1-7,9-13].

To complement the effort towards noiseless pile driving researchers from TU Delft proposed the novel Gentle Driving of Piles (GDP) method which aims to enhance traditional axial vibro-pile driving by incorporating high-frequency torsional vibrations [9]. The addition of torsional vibrations is expected to consume/redirect soil frictional resistance and limit radial expansion during pile driving, resulting in faster and quieter installation – the GDP shaker is presented in Figure 1.a.

To demonstrate the GDP technology and compare its performance with traditional pile-driving methods (impact pilling and axial vibratory driving), comprehensive medium-scale field tests were conducted in an inhomogeneous sand deposit at the Port of Rotterdam. Eight identical test piles with a diameter of 0.762 m and an embedded length of 8 m were installed using impact hammering, traditional axial vibratory piling, and the GDP method. Out of the eight test piles, four were heavily instrumented out of which two were GDP-driven, and the remaining two were installed with impact pilling (IH) and axial vibro-driving (VH).

The tests performed on the four main test piles consisted of two stages: the first stage focused on driving performance, while the second stage examined the cyclic lateral behaviour of the piles under repeated loading for different installation methods -cyclic loading programme in Figure 1.b. The test results highlighted the promise of the method towards faster (Figure 1.c.) and less perturbing pile driving [1,9,13] but also enhanced post-installation lateral response [5,6,7] compared to the traditional alternative installation methods (Figure 1.d.).

Numerical and experimental study on the effect of the installation meth-od on the lateral response of offshore monopiles

Tamara Wehbe — 2023-10-03

The successful deployment of offshore wind turbines requires the utilization of foundation installation methods that are fast, low-cost, and reliable. Offshore wind turbines foundations are commonly installed by impact driving. The noise generated during the driving process is a major constraint as it impacts marine life [1] and induces high stresses which reduce the fatigue lifetime of the foundation. Vibratory pile driving offers a promising alternative to traditional impact driving for the installation of monopiles [2], the limited data on its effects on the mechanical properties of the soil during installation in offshore conditions and consequently the mechanical response of the structure throughout its lifetime has prevented its adoption. During vibratory driving, the repeated and high intensity vibration can cause an increase in pore pressures and a decrease in both the inter-particle soil friction and the effective stress, which can eventually lead to liquefaction [3, 4].

This research aims at investigating the influence of the installation method on the soil properties around monopiles and how it affects the lateral response of such structures. In order to comprehensively understand the different phenomena involved, this problem is investigated through both numerical modeling and laboratory testing.

A numerical axisymmetric model based on the finite elements method that reflects on an offshore monopile was developed to describe the installation process using fixed boundaries or Perfectly Matched Layers (PML) [5] to realistically simulate the dynamic soil-pile interaction and ensure radiation of waves towards infinity. This model takes into account the reduction in soil strength around the pile shaft during the installation by impact or vibratory driving, along with the dynamic force applied by the vibro-hammer and the pore water evolution. Cyclic triaxial tests have been conducted on a sand extracted from the North Sea. They are used to calibrate the constitutive equations of the model. The results obtained provide insight into the onset and progression of liquefaction, the distribution of pore water pressures, and the resulting deformation of the soil-pile system and how the boundary conditons and the frequency of the imposed load affect those mechanisms.

These numerical results are also compared with 1g reduced scale experiments composed of a large cylindrical box filled with dry and saturated sand, in which a fully instrumented model pile (tube 60mm in diameter) is vertically driven. The impact or vibrations are imposed at the pile head by a scaled vibratory hammer or scaled impact hammer. In addition, the relative density and the homogeneity of the sand is carefully controlled using the pluviation method [6] allowing to test the sand at different controlled densities. After the installation phase, the lateral behavior of the pile is investigated by imposing a horizontal force with a piston. The test results will also be compared to similar tests on large diameter piles (2 meters) than are planned for fall 2023.

A hardening plasticity formulation for drained behavior of circular foot-ings

Amin Barari — 2023-10-03

Upon moving the offshore wind energy sector to deeper waters, there is an increased demand towards developing more complex foundation solutions, in particular suction caisson foundations as single or jacket supported on multiple foundations. Broadly, foundations for offshore wind turbines need to be able to withstand a variety of load combinations throughlout their lifetime.

This contribution is devoted to a comprehensive review of the performance of circular surface and shallow foundations under combined loading (VHM), and how this can principlally be understood in a theoretical framework in the context of plasticity theory [1-2]. Initially, the associated and non-associated plasticity in offshore foundation design is discussed. The plastic potential function for a non-associated plasticity framework, with the aid of two association factors and is as follows:

where indicates the apex of the potential surface; is eccentricity parameter; and represent the uppermost size of the yield surface along the vertical loading coordinate; and are curvature factors.

Assuming , two potential surfaces with and > ( ) have similar shapes despite having different intersection points with the V axis, a clear indication that associated flow is the governing plastic flow mechanism in radial plane (Fig.1).

Contrarily, the plastic potential surface for surface foundations was plotted for two different association factor values ( i. e., while all else remains unchanged. The two cases shown in Fig. 2 are for two different values of . Thus, according to Eq. (1) and Fig. 2, the normality condition is no longer relevant in the radial plane. Further, The hardening law for circular surface footings can be described by the following expression [3], in which the post-peak softening behavior is modeled by a factor :

where indicates the apex of the potential surface; is eccentricity parameter; and represent the uppermost size of the yield surface along the vertical loading coordinate; and are curvature factors.

The above hardening law describes the relationship between vertical loading and plastic settlement, where = at the infinity amount of plastic settlement ( is the post-peak vertical loading and is peak bearing capacity), The denotes initial plastic stiffness, and are plastic settlement and plastic settlement at maximum load, respectively (Fig. 3).

Although the studies on surface footings yielded values lower than unity (a clear indication of the prevailing post-peak softening behavior), this key trend is not confirmed in shallow foundations according to FE push-over analyses and scaled model tests [4].

In current study, multiple FE push-over analyses were carried out to establish a new hardening law for a multi-pod system. The validation procedure was detailed in [5-6]. As a consequence, = 1.99 and 1.85 and = 12700 and 11400 corresponding to the embedment ratios 0.5 and 1 were determined, respectively. Furthermore, the initial plastic stiffness was found significantly higher than that of surface footings [7]. Finally, the hardening law and plastic potential surface turned out to be inherently complex, because both involve several parameters that need to be calibrated (i.e., depending strongly on foundation type and soil strength). Thus, special care should be taken when assessing the skirted foundation responses on different loading planes.

Dynamic seabed-anchor capacity enhancements for taut-moored floating offshore wind

Katherine Kwa — 2023-10-03

Decarbonisation of global energy supply to meet Net Zero targets by 2050 requires rapid expansion of offshore wind [4]. Much of this growth will come from floating offshore wind (FOW) technology where seabeds are less congested, high energy wind resources are located and conventional fixed offshore wind is not practical [1]. The required scale of FOW requires a step change in mooring and anchoring technology compared to existing hydrocarbon solutions. New, efficient and reliable mooring and anchoring systems are essential to economically deliver the necessary FOW [1]. Taut mooring arrangements can be attractive for FOW turbines as they require less length and lighter synthetic mooring line than traditional chain catenary mooring arrangements. However, taut moorings transmit significantly higher mean and peak tensions to the anchor compared to catenary moorings. It is therefore important to fully quantify the capacity available from anchors during typical FOW load conditions, including dynamic effects.

This study focuses on how a numerical anchor macro model, Ancmac [6], can be used to capture and quantify dynamic anchor-seabed capacity benefits such as from seabed added mass, [5] and viscous soil strength effects. These effects can enhance the dynamic anchor capacity and are not typically considered in anchor design. Typical mooring-floater fluid-structure interaction analyses also model the connection of the mooring lines to the seabed as a fixed pin connection and so seabed-anchor-mooring interactions are not typically considered. Ancmac can replace this fixed pin connection node at the seabed as it uses mechanical analogue components (e.g. spring-viscous-slider and mass) to simply and practically link forces on the anchor with anchor and chain movements in the time domain to determine the seabed response and the current available anchor capacity. This study presents an example case where Ancmac is used to model the response of an embedded plate anchor that is subjected to a high amplitude, short period (= 6 and 10 s) high mean load event (Figure 1a). The anchor loads , , are derived from applying a 1-50 year storm design loading event on a 15MW FOW turbine [3] with a taut mooring line configuration composed of high modulus synthetic polyester rope [8].

Results show that if dynamic seabed benefits are not considered (purple line in Figure 1a), then a static anchor capacity of = 4.15 MN and corresponding anchor size of = 11.13 m² is required (for an anchor buried in slightly overconsolidated soft clay =30 kPa and bearing capacity factor =12.42). If beneficial dynamic seabed enhancements are considered (shown by red lines ), then a lower initial static anchor capacity can be adopted =3.13 MN, which corresponds to a 25% decrease in the required anchor size (=8.41 m²). During the design loading event, as the applied anchor load, increases above the available static capacity, towards the maximum applied value, the anchor begins to move (Figure 1b) generating dynamic resistance from mobilising the non-linear viscous slider and added mass resistance components. The resistance in the viscous slider component is based on a model for the change in undrained strength with increasing equivalent strain rate (=, where is anchor diameter), as shown in Figure 1c. The resistance from the viscous slider component reaches a maximum at the time marked slightly after the peak of the applied mooring line loads, at time , as shown in Figure 1a-c. Reducing the period of the applied load (=10 s to 6 s) increased the added mass resistance as the anchor is subjected to higher accelerations. As a result, the anchor experienced significantly smaller (50% less) maximum displacement and reduced velocities. This comparison is also evident in Figure 1d, which compares the contributions of resistance forces from the spring-viscous-slider and added mass components.

Ancmac can also capture the long-term enhancements in seabed strength and anchor capacity as a time-varying function of the life-cycle of seasonally varying, operational applied loads. This could, for example, further reduce the required anchor sizes, as can increase from beneficial long-term seabed consolidation effects [7]-[10]. These short and long-term seabed benefits that occur over the range of different loading timescales, and result in enhanced anchor capacities for loads that are relevant to FOW taut mooring configurations, will be further discussed during the presentation.

MPM vs. CEL: Numerical modelling of penetration processes

Patrick Staubach — 2023-10-03

The simulation of large deformation problems in geotechnical engineering has gained increasing interest in recent years. This is partly due to new technologies and industry requirements (e.g. the installation of new foundations in the offshore industry), the improved availability of numerical methods that can handle large deformations and the overall improvement in computational hardware.

A frequently adopted numerical scheme is the Coupled-Eulerian-Lagrangian (CEL) method [1,2,3], which is available in the commercial software Abaqus. Originally developed for solid-fluid interactions, it is now also used in geotechnics, modelling the soil by Eulerian finite elements. These elements decouple mesh and material movement, allowing for arbitrary large deformations. However, since elements can be partially filled by material, special considerations for integration and reconstruction of material surfaces within elements are necessary. A one point integration rule is adopted and the volume-weighted fraction of the material is used in case of integration of partially filled elements.

Another approach is adoped in the Material-Point-Method (MPM) [4], where the the continuum body is represented by a cloud of points, called material points (MPs). They carry all information of the continuum body, such as density, velocity, strain, stress, material parameters and external loads. They are not physical particles, like for example single solid grains described in Discrete Element Method, but they represent a portion of continuum body. At the beginning of each timestep, the nodal forces are computed based on the information stored at the material points by means of the shape functions. The governing equations are then solved at the nodes and afterwards used to compute the strain, stress and density, and to update the position of the material points. At the end of the timestep, the mesh is reset. For penetration problems, the “moving mesh” concept is adopted, where the mesh moves together with the structural element to keep the contact surface on the same computational nodes shared between the soil and the penetrating object, ensuring higher accuracy. For the MPM formulation used in this work, the mean stress and the state variables are averaged in each element, which can be interpreted as a “stress-smoothing” algorithm.

Up to now, no comprehensive study comparing the two methods exists. In this study, both schemes are applied to the simulation of cone penetration testing (CPT). An explicit time integration scheme is adopted for both methods.

The field of deviatoric stress (stress due to self-weight of the soil not accounted for) in a simulation of a CPT using the CEL method in conjunction with the Tresca model is depicted in Fig. 1a). Note that undrained conditions are assumed and the undrained shear strength s_u is constant in the model. Note in addition that the built-in Mohr-Coulomb model with a friction angle of zero is used for the simulation with Abaqus. Undrained conditions are realized by setting the Poisson’s ratio to 0.495. Figure 1b) shows the average vertical stress at the tip of the CPT during penetration for simulations using MPM and the CEL method for perfectly smooth surface conditions between pile and soil. A spatially constant effective vertical stress of 66 kPa is included in the value of q_c. While the trend of the curves is similar for both methods, the CEL method tends to give more unstable results, manifesting in strong oscillations of the stresses. In comparison, the MPM provides a nearly constant stress curve for greater depths, which would be expected for constant values of s_u.To confirm these findings, additional simulations with variation of mesh and time increment size where performed in case of the CEL method. Previous research [5] has shown that both can have an influence on the results of CEL analyses. However, the observed oscillations could only be marginally reduced by using a finer mesh and a smaller time increment size.

The comparison of the cone factor N_c, defined as the ratio of the cone resistance (excluding initial stress due to self-weight) to s_u, for different values of shear stiffness G to s_u for simulations using MPM and the CEL method is given in Fig. 1c). Perfectly undrained conditions and a smooth contact are assumed. For the evaluation, the cone resistance at the end of the simulation, i.e. at a penetration depth of 0.4 m, is used. For the entire range of G/s_u, good agreement is found between the two methods.

Overall, it is concluded that both numerical schemes give similar results for typical benchmark problems, but the CEL method tends not to be as numerically stable as the MPM. This is somewhat surprising considering that the MPM code (a version of Anura3D developed at Deltares is used) is developed by a comparably small community of reseachers, while the CEL method is implemented in the framework of a massively commercial code. However, the superiority of the MPM is in parts due to the adoption of axialsymmetric elements, which is not possible using the CEL method implemented in Abaqus (even though one could model only a narrow slice instead of the quarter model used here). This is also why the MPM simulations are computationally more efficient in the present case. More comparisons of the two methods will follow in future work.

Combination of kinematic and inertial loads acting on monopile founda-tions for offshore wind turbines

Julia Katharina Möller — 2023-10-03

Due to a growing number of offshore wind farm projects in seismic areas, existing design procedures for slender piles under seismic loading need to be revised to allow for the design of larger and stockier monopiles. This study illustrates how advanced finite element analyses can be used to investigate the combination of seismic loads and may thus inform standard design procedures commonly adopted in practice. In standard design methods, a soil-pile foundation system is represented by a simplified beam-on-nonlinear-springs model [1]. Earthquake loads acting on this system can be decomposed into inertial forces, which originate from the acceleration of a superstructure mass, and kinematic loads due to ground displacements caused by the propagation of seismic waves. Inertial loads are typically derived from modified acceleration spectra which account for the difference between the pile head movement and the soil displacement in the free-field [2, 3, 4]. Maximum displacement profiles can be obtained from site response analyses of the free-field soil column. The distribution of pile bending moments and internal forces can thus be found by superposition of the two loads. However, unless numerical time-domain analyses are carried out, it is not evident whether these components are acting simultaneously on the pile. In the design of long and slender piles, a common assumption is that inertial and kinematic loads may be considered separately, as the former tend to affect only the near-surface zones while the latter dominate at larger depths [2]. However, their combined effect on a far less flexible monopile exhibiting rotational mechanisms is not yet understood, neither is the influence of soil liquefaction triggered by the seismic ground shaking. Experiments on pile groups presented in the literature suggest a dependence of the phase angle on the ratio between the fundamental frequency of the soil deposit and the superstructure [5]. The analogous scenarios are illustrated for a monopile-supported wind turbine in Figure 1.

The turbine is considered as a single-degree-of-freedom system with the mass off the rotor nacelle assembly lumped together and located at hub height. The time history of the inertial force acting at hub height (), as well as the horizontal ground displacement at surface level (), are schematically shown in Figure 1(b) for a very stiff soil. In this case, the fundamental period of the soil deposit () is significantly lower than that of the superstructure (), which causes the response of the turbine system to act out of phase with the ground movement by 180°. For a soft soil deposit with much higher fundamental period than the turbine (Figure 1(c)), the kinematic and inertial loads are expected to act in phase.

To investigate the combination of seismic loads acting on a monopile foundation in the time-domain, advanced numerical analyses are carried out on three-dimensional Finite Element models. Soil layers with widely different material properties are considered, including stiff marine clays and liquefiable sands with varying relative densities. The uni-directional motion records adopted as base excitations are sufficiently strong to induce large lateral ground movements and impose significant kinematic loads on the foundation. Through the of use of appropriate constitutive models in a hydro-mechanically coupled formulation, the response of the turbine sub-structure system is simulated while accounting for the degradation of stiffness in the surrounding soil and significant rises in excess pore pressures in the sand layers. An intensely nonlinear material behaviour typically leads to a shift of the fundamental frequency of the deposit [6], which can be deduced from the amplification of accelerations at various depths, with high frequencies dominating in the stiff soil layers and low frequencies in the soft liquefied sand. For a stiff soil deposit prior to liquefaction, local extrema in pile displacements at mudline level are therefore expected to act out of phase with the inertial force resulting from the acceleration of the rotor nacelle assembly at the turbine tower top (Case ). As the stiffness diminishes in a soft or fully liquefied ground, pile and free-field ground displacements are expected to move in-phase with the inertial load, as is characteristic for cases where the period of the soil deposit exceeds that of the superstructure (Case ). This change in behaviour highlights the influence of soil stiffness degradation and extensive ground liquefaction on the combination of seismic soil-structure interaction effects in the time-domain, which can be captured by advanced numerical methods.

Effects of installation advancement ratio on cyclic uplift response of a single-helix screw pile: experimental and numerical investigation in sand

Wei Wang — 2023-10-03

A screw pile consists of one or several helices connected on a central straight shaft or core. These kinds of piles are widely used onshore and typically installed by applying torque at the pile head with additional vertical compressive force (also referred to as crowd force) if required. Current standards and industrial guidelines recommend that installation follows the pitch-matched approach i.e. advancement ratio AR = 1.0 [1], which means that the pile vertical penetration for each rotation equals the helix pitch, because it is suggested that this leads to reduced installation disturbance.

where is the vertical penetration per rotation and is the helix geometric pitch.

Recently this kind of pile has been proposed for upscaling from typical onshore sizes as an alternative silent foundation/anchoring solution for future offshore renewable energy application. This may include use as foundations for jacket structures or anchors for wind turbine in deeper water. However, the increase in pile sizes may require prohibitive vertical force if pitch-matched installation is adopted [2] in sand. One of the solutions to this is over-flighting (AR < 1.0) where the pile is over rotated during installation. When the pile is over-flighted, sand below the helix can move upward through the helix opening resulting in a higher stress and potential densified zone above the helix (Figure 1(a)). The increased stress above the helix can push the pile downward and consequently reduce or even eliminate the vertical installation force. In addition, the increased stress and potential densification in the soil above the helix can result in better monotonic uplift performance [3, 4, 5, 6, 7].

As foundations for offshore renewable energy applications, the screw piles also need to be able to perform under cyclic loading from wind, wave and current for example. Investigations of uplift cyclic performance of pitch-matched screw pile installation have been reported, but the over-flighting effects have not received significant attention. To give confidence in the maintenance of the beneficial effects of over-flighting on cyclic performance, centrifuge tests and discrete element modelling (DEM) were used in this study. The centrifuge tests provide more reliable physical evidence than 1g tests but are less costly and more accessible than field test. Based on the results of centrifuge test, the DEM models are validated and used to allow separate evaluation of the mechanism of the helix and the pile shaft and assessment of soil state evolution during cycling, which are difficult to capture in field or centrifuge tests.

The centrifuge tests show that the screw piles installed at lower AR accumulate displacement at lower rates with cycling. which is also captured by the DEM (Figure 1(b)). In addition, DEM suggests that shaft behavior controls cyclic performance because the shaft friction resistance is stiffer than the helix bearing resistance at small displacements. The radial stress on the pile shaft can be enhanced by over-flighting installation. Although it degrades with cycling, the enhanced radial stress due to over-flighting remains higher than that of the pitch-matched pile and therefore the over-flighted piles have improved cyclic performance. On the basis of improved cyclic uplift performance and low vertical installation force, over-flighting installation may be utilized for offshore screw piles performing under cyclic uplift loading where installation forces are a concern or loading is predominantly one-way tensile loading.

A laboratory study of the effect of installation parameters on the lateral behaviour of monopiles in sand

Anderson Peccin da Silva — 2023-10-03

Monopiles are the predominant type of foundation used for offshore wind turbines. The increase in size of monopiles and the stricter environmental regulations in terms of underwater noise levels has motivated the development of alternatives to the conventional impact-driving method of monopile installation. One of the alternatives is the (axial) vibratory installation, which has been previously studied in field [1, 2, 4] and laboratory [3, 5] conditions. However, there is limited knowledge on the effects of vibratory installation (and how these effects differ from those caused by impact-driving) on the lateral response of monopiles.

This extended abstract presents the results of an ongoing Join Industry Project (SIMOX – Sustainable Installation of XXL Monopiles) which aims at comparing different installation methods from the point of view of driveability, noise emissions and lateral response. The present abstract particularly focuses on the lateral response of monopiles. As a first step towards the large-scale onshore field tests to be executed in 2023, a laboratory study was conducted at the Water-Soil Flume at Deltares, in Delft (NL), which consists of a tank with 9.0 m of length, 5.5 m of width and 2.5 m of depth, with a multipurpose wagon on rails above it (Figure 1). The tank was filled with saturated Sibelco S90 sand up to a height of 2.4 m in compacted layers of 50 cm.

The experimental programme consisted of 4 batches (batches 1, 2 and 3 with dense sand and batch 4 with medium-dense sand) in which piles of diameter D = 32 cm, embedded length L = 1.5 m and wall thickness t = 4 mm or 10 mm were installed and loaded laterally. While batch 1 focused on driveability aspects, in the other three batches 8 piles were installed (per batch) and subsequently subjected to lateral loading. The centre-to-centre distance between piles was 8D in the loading direction and 6.5D perpendicular to the loading direction. The distance between the piles and walls of the tank was 4.3D. All distances were larger than the minimum distances recommended in the literature [6]. The piles were installed with two different methods: vibratory-driven, using a hydraulic vibro-hammer APE-23 with an eccentric moment of 1.3 kg.m, and impact-driven, using a dropping weight impact-hammer HL750. For each method, selected installation parameters were varied, namely the vibratory frequency and lowering speed of the crane for

vibrated piles, and dropping mass and fall height for the impact-driven piles. Lateral loading was applied at the pile head by means of an in-house built lateral loading device consisting of an electric motor connected to a spindle. The lateral displacements were measured by a magnetostrictive linear position sensor, independent of the lateral loading system. The loading regime consisted of an initial monotonic loading up to 25% of H_ult (where H_ult is defined as the load at which the pile exhibits a displacement of 0.1D at ground level – obtained from 3D FE analyses), followed by cyclic loading (1000 cycles for most piles) and a final monotonic stage up to the maximum load.

The different combinations of lowering speeds of the crane (low, high) with the different frequencies of the vibro-hammer (low, high) led to different types of vibratory installation: crane controlled or free hanging. Under crane-controlled installation, the full weight of the pile-hammer system is sustained by the overhead crane, hence the crane load measured by a load cell oscillates around the static weight of the pile-hammer system. Under free-hanging conditions, on the other hand, the load taken by the crane is zero, meaning that the weight of the pile-hammer system is fully taken by the soil – both by shaft friction and tip resistance.

The results in dense sand showed that the initial monotonic response of the piles is affected by the vibration mode, and not by the installation frequency or penetration speed alone. While the crane-controlled vibrated piles exhibited lateral stiffness similar (i.e. slightly lower) to the impact-driven piles, the free-hanging vibrated piles showed markedly lower lateral stiffness compared to the other piles. The softer behaviour observed for these piles could be attributed to various reasons, such as the difficulty in controlling the pile verticality during installation. For all free hanging piles, inclination was observed approximately half-way through installation, which was corrected in the final part of installation. The exact inclination during installation was not measured, which will be done in the onshore field tests. The test results showed, however, that during cyclic loading the differences in lateral stiffness decrease, i.e. the softest piles in the initial monotonic stage were the ones that exhibited the largest gain in lateral cyclic stiffness, whereas the stiffest piles in the initial loading stage were the ones with the smallest increase in stiffness during cyclic loading.

Installation of Monopiles: Interpretation of Vibro-Installed Lab-Scale tests

Mario Martinelli — 2023-10-03

Nowadays, monopiles are the most common foundation type for offshore wind turbines, consisting of a single open steel pipe that is driven into the seabed to support the weight of the turbine and tower. Traditionally, monopiles are installed by impact driving, where a hammer repeatedly strikes the pile until it reaches the required penetration depth [1]. However, for large-diameter monopiles, impact hammering can be impractical due to significant underwater noise emissions [2] and the necessity for oversized structures to withstand the high stresses induced by the hammering. Stressing the importance of this issue, it is worth noting that as national standards become stricter in this aspect, noise mitigations face challenges in keeping up.

Furthermore, it should be emphasized that vibratory hammering, although continuous rather than impulsive, also produces significant underwater noise. Currently, noise regulations predominantly concentrate on impulsive noise. However, as more becomes known about the effects of continuous noise emissions on the environment, vibro-driving methods may also be subjected to stringent norms. In recent years, there has been a growing adoption of vibro-driving techniques for installing monopiles, which employ a hydraulic vibrator to generate vertical vibrations that reduce the soil resistance around the pile, facilitating its insertion into the ground

Accurate predictions of the installation of monopiles using the vibro-driving technique are still challenging, as open questions currently remain on the soil reaction during driving and the pile-soil-equipment interaction [3]. The effectiveness of vibro-driving depends on several factors, including soil conditions, pile diameter, and the frequency and amplitude of the induced vibration. Limited data are available for model calibration and validation, especially for offshore conditions; presently, various research works are contributing to the development of new vibratory pile driving modelling frameworks, with a view to engineering practice [4.5].

This presentation shows some of the results of an ongoing research project, namely the Sustainable Installation of XXL Monopiles (SIMOX) in the Netherlands. In particular, the focus is to illustrate the interpretation of the vibratory installation of lab-scale piles (32 cm diameter and 1.5m long) performed in saturated sand and well-controlled initial conditions. This took place at the Deltares research facilities in Delft, the Netherlands. Several properties were varied during the penetration-controlled tests, including the penetration rate, the loading frequency, and the soil relative density.

In the SIMOX laboratory experiments on vibratory pile driving, two distinct installation regimes have been identified, i.e. the ‘vibrating’ and the ‘penetrating’ regimes, governed by the interrelation between penetration rate and periodic pile velocity – see Figs. 1 and 2, respectively. For these two regimes, the soil reaction during driving differs in both quantitative and qualitative terms. Through detailed analysis of the experimental data, we have observed that the vibratory pile installation process is majorly influenced – apart from the driving input excitation itself - by the dissimilar soil reaction experienced by the pile for different installation settings. These findings highlight that the effectiveness of vibratory pile installation is not solely dependent on the method of excitation, but also on the specific installation settings employed (i.e. eccentric moment, driving frequency and crane load). By understanding this mechanism, we can comprehend and improve further the present state of drivability modelling approaches and the vibratory pile driving process.The present results serve as a basis for further numerical and experimental investigation, with the aid of field data to be collected in subsequent onshore field tests.

Effect of heating on the behaviour of bentonite in the presence of salt solution

Anil Kumar Mishra — 2023-10-04

Due to its high swelling tendency, layers of compacted bentonite is provided as a buffer materials at the nuclear waste repository site around the world. The development of the diffuse double layer (DDL) by the mineral montmorillonite, which is present in bentonite, provides the bentonite its swelling characteristics when permeated with water. The basic engineering properties, hydraulic conductivity and swelling behaviour of different bentonitehave been investigated by many of the researchers. However, when the bentonite is exposed to a higher temperature, it may undergo a transformation in its mineralogical composition and severely loses its tendency to develop the DDL. This in turn will impact the bentonite’s swelling and hydraulic characteristics. Therefore, it is essential to investigate the influence of temperature on the swelling and hydraulic characteristics of bentonite.

Most of the earlier studies were mainly conducted on oven dried soil at room temperature in the presence of deionized (DI) water as the pore fluid. Considering that the an extremely large storage period and generation of high temperature caused by the decay of radionuclide’s, the bentonite’s functionality under complex thermal and chemical conditions must be ensured.

Previous studies [1, 2, 3] have shown a reduction in the swelling and liquid limit with an increase in the temperature. Similarly, previous studies [4] have indicated a substantial impact of salt solution on swelling, hydraulic and consolidation tendency of bentonite.

From the previous sinvestigatios it can be concluded that the temperature and salt have a definite impact on the behaviour of bentonite. However, no studies have been performed to investigate the collective influence of salt solution and temperature on the bentonite. Hence, the primary objective of this present study to investigate the combined impact of temperature on swelling and hydraulic behaviour of the bentonite.

Materials and Methods

Bentonite investigated in this research is in powdered form obtained from local source. Since an initial temperature of 250⁰C was observed in the Yuca mountain repository site [5], a temperature of 300⁰C was selected for study. Furthermore at higher temperature such as 500⁰C the montmorillonite undergoes a significant change in its swelling behaviour [6], the study was also performed at 500⁰C. Since most of the waste contains ions like Na⁺ and Ca²⁺, solutions of NaCl and CaCl₂ were selected for this study.

After pre-heating the bentonite at 300 and 500⁰C for 24 hours in a muffle furnace, various experiments were performed as per relevant ASTM standards.

Results and Discussions

The data in Table 1 compares the liquid limit, plastic limit and free swelling of pre-heated bentonites in the presence of 0 N, 0.1 N NaCl and 0.1 N CaCl₂ solution. The data shows that the liquid limit, plastic limit and free swelling of bentonite decreases due to increase in the temperature. The data also shows that the heating has less effect on liquid limit, plastic limit and free swelling for the bentonite in the presence of NaCl and CaCl₂ salt solution. The data also indicates that with the rise in the temperature from 300⁰C to 500⁰C, the salt has minimal impact on the behaviour of bentonite. By pre-heating the bentonite, the plasticity behaviour alters due to reduction of interlamellar water and the influence of the DDL becomes negligible. Therefore, the variation in the liquid limit with the salt was minimal for the bentonite pre-heated at 500⁰C. However, as the DDL is dominant for the bentonite at 25⁰C, the variation in the liquid limit was higher due to a change in the salt.

The plot in Fig. 1 indicates the influence of heating on the swelling potential and swelling pressure of bentonite in the presence 0.1 N of NaCl and CaCl₂ solution. The plot also shows that the swelling potential and swelling pressure decreases with increase in the temperature and salt. However when permeated with the salt, the bentonite pre-heated at a higher temperature shows a neglible impact due to presence of salt on its swelling behaviour.

Conclusions

The study was perfomed to investigate the impact of pre-heating on the behaviour of bentonite in the presence of salt solution. The study concluded that the swelling and plasticity behaviour of bentonite decreases due to pre-heating at a higher temperature. The result also concluded that the bentonite pre-heated at a higher temperature undergoes a marginal change in its behaviour due to the exposure of salt solution.

Thermal and mechanical creep of clay in hypoplasticity

Merita Tafili — 2023-10-04

Clays and clay soils or shales have attracted a lot of interest in a variety of applications, including the development of geothermal resources [1], energy foundations [2], oil exploration [3], energy storage [4], and the storage of nuclear waste [5]. The continued operation of ground source heat pump installations can lead to considerable long-term settlements, which could negatively affect the adjacent or underlying foundations [1]. Therefore, thermal volume change has been widely experimentally investigated in clays [4, 7, 8, 9]. Especially in [4] the authors studied the thermal and mechanical consolidation of saturated marine clays through laboratory element tests, where excess pore pressures were generated by heating samples at constant water content and then allowed to dissipate. In constant stress creep experiments described in [4] it was documented that thermal creep strains typically increased linearly with log time at rates controlled by the prevailing temperature.

On the other hand, the mechanical time dependency of the stress–strain behaviour of soft soils, especially highly plastic clay, is generally too significant to be ignored [10, 11]. The constitutive modelling of the time-dependent stress–strain behaviour of soils has been an active area of research for five decades and has attracted much attention from the international geotechnical community in recent years as denoted in [12]. In [13] a visco-hypoplastic (VHP) model for normally and overconsolidated clays has been proposed. Probably the most salient feature of hypoplasticity itself is that loading and unloading can be described with only one equation as with the strain and stress rate denoted as and , respectively. The elastic stiffness tensor is represented by ; is the degree of nonlinearity and is the flow rule (direction of hypoplastic strain). The last part of the equation expresses the time-dependent strain rate (i.e. viscous) with the material parameters as the viscosity index and being the compression index. denotes the overconsolidation ratio. As may be observed, the model is not restricted solely to time-dependent clay materials, because does not represent a singularity for the constitutive equation as in other hypoplastic models. The model has been extended in [14] to account for the small-strain stiffness and the mechanical behaviour under cyclic loading. It follows the critical state theory and incorporates a loading surface for the definition of , see Fig. 1A). Time-dependent one-dimensional behaviour of clays is in most cases explained by the isotache framework, which assumes a unique relation between effective stress, strain, and strain rate in compression, shown as loci of constant strain rate in space, see Fig. 1B). The creep deformation at constant effective stress (; Fig. 1C)) corresponds to a decrease in strain rate of the soil (path A to B in Fig. 1C)). Consolidation stress history, represented by swelling along the path AC, causes a marked reduction in compressive creep rates at low (CD), while expansive/dilative creep strains occur at higher (CEE).

Due to the temperature dependency of the compressibility of clays the isotachs may be considered as temperature dependent as well. As shown in [6] the isotache loci at a given strain rate for normally consolidated states are functions of strain rate and temperature. Increases in temperature cause additional compressive thermal creep strains for NC and lightly OC states (BB and DD in Fig.1C)) and augment the swelling strains at higher (EE).

In the framework of hypoplasticity, the non-isothermal behaviour has not gained much attention [15]. This work is devoted to the extension of the VHP model formulation to present a unified model for both thermal and viscous strains in clays. The model predictions are assessed through comparisons with existing laboratory experiments from [7] on two clays with measurements of both isothermal/mechanical creep and thermally induced creep strains, providing a thorough calibration scheme as well.

Dewatering and consolidation of clay slurries

Ismail Myouri — 2023-10-04

Dewatering, which is the process of separating (colloidal) suspended particles from a solvent (usually water), is used in many engineering applications (sanitary engineering, dredging engineering…). Key questions associated with dewatering in the context of the reuse of dredged sediment are (1) what is the process kinetics, (2) how can these processes be optimized and (3) can the dewatered sludge be reused and for which application? Dewatering and consolidation are functions of the suspended particles’ size and type, and their solvent-mediated interaction. In this presentation, some examples will be given about the dewatering of suspensions and slurries as found in engineering applications [1]. The presentation will focus on the behaviour of mineral clay suspensions (kaolinite, montmorillonite, illite…) composed of particles of different particle sizes [2-5]. We will show that, depending on the particle size distribution and solvent properties, the system is either undergoing a slow sedimentation dominated by thermodynamic forces or a rapid sedimentation dominated by gravity. The sedimentation is followed in time using NMR and inferential image analysis, and the particles are characterized by size, density and electrokinetic charge. We show that the time evolution of the sedimentation behaviour can be modelled using an advection-diffusion equation. The advective term is a function of gravity, whereas the diffusion term represents either a hard-sphere repulsion or an effective stress, depending on whether thermodynamic forces dominate the system [6-8]. We show that after solving this advection-diffusion equation numerically, the results based on the theory of Gibson does match the experimental data collected from NMR in the phase of slow kinetics. For the early stages of settling and consolidation, where the system kinetics are fast, we show that the data are contaminated by artifacts due to the limited time of signal acquisition imposed by the NMR method. We show that is possible to overcome those limitations by either sacrificing some information related to particle’s sizes using the NMR or by combining the NMR method with excess pore pressure measurements along the height of the settling columns.

Temperature Effects on Atterberg Limits

Aidy Ung — 2023-10-04

This study aims to investigate the effects of temperature on Atterberg limits of fine-grained soils with different mineralogies using fall cone tests. It is found in the literature that Atterberg limits are dependent on the clay mineralogy [1, 2] and are subjected to change upon an increase in temperature [3-5]. Measurements of liquid limit at different temperatures reveal sodium smectite is more sensitive to temperature than kaolinite. This observation is justified by relating the evolution of liquid limit to the initial changes of total specific surface areas associated with the physicochemical activity and inter-particle contacts [6]. Measurement of Atterberg limits using the conventional methods of Casagrande’s cup and threading is not very practical when the impact of temperature on these parameters is investigated. The trial and error associated with this method and the time they take may hinder a robust estimation of the liquid and plastic limit. Various studies have shown that Atterberg limits measured using Cone Penetration Test are less subjective and more consistent compared to traditional ways [7-9]. For these reasons, utilizing a Fall Cone Test apparatus and a temperature control unit facilitates the investigation of the impact of temperature on Atterberg limits. In this study, first, the liquid limits measured at room temperature by Fall Cone and Casagrande’s method are compared, and then Cone Penetration is utilized to investigate the evolution of liquid limit with temperature.

Rhassoul Clay (29.4% Illite, 0.10% Kaolinite, 70.5% montmorillonite) and Illite Clay are considered in this study. The measured penetration values of the cone against moisture content at room temperature for these two clays are plotted in Figure 1. The liquid limit is the water content at which the cone penetrates 20mm into the soil in the cup [10]. Accordingly, Table 1 lists the determined liquid fall cone test and their deviation for the measured Atterberg limits using Casagrande’s cup. This comparison was performed to ensure that the difference between these two methods is not significant and the utilized device was accurate enough.

To determine the Atterberg limits at higher temperatures (40 °C and 50 °C) and lower temperatures (10 °C), a temperature control unit is used to maintain the temperature of the soil sample during fall cone testing. For this purpose, first, the cup was filled with soil and then wrapped to avoid moisture migration in and out of the sample. The sample was then placed inside the temperature controller for an hour at the desired temperature. After one hour, the sample was swiftly unwrapped, and Cone Penetration test was performed. After the penetration a thermometer was used to measure the temperature at the center of the cup. This measurement showed that the alteration of temperature during the whole process is less than 1°C. Next, more water was added to the batch and then the same steps were followed until enough data points were obtained to estimate the liquid limit. The result of these tests allows for an assessment of temperature effects on Atterberg limits, which would be helpful in the understanding of the influence of temperature on different shear strength parameters of fine-grained soil with regards to their mineralogy.

Table 2 lists the measured liquid limit values for Rhassoul Clay and Illite clay at different temperatures. The results suggest that the liquid limit of Illite is temperature independent. On the other hand, Rhassoul clay liquid limits was more sensitive to elevated temperatures and did not change with cooling. As mentioned earlier, the liquid limit is directly associated with clay mineralogy and the thermal alteration of soil characteristics [11, 12]. Therefore, the observed thermally-induced change in liquid limit should be discussed with respect to mineralogy. According to Table 1, the Illite liquid limit is temperature independent. Furthermore, Rhassoul clay comprises 70.5% montmorillonite minerals and 29.4% Illite. Providing this and the observed sensitivity of Rhassoul clay to temperature change can be attributed to the montmorillonite constituent. This argument can be further supported by testing more clays with different percentages of montmorillonite and illite.

3D microscale investigation of active deformation mechanisms of halite under conditions representative of underground hydrogen storage

Nina Du — 2023-10-04

To tackle the challenges raised by climate change, we need to rapidly switch our energy sources to low-carbon ones for all economic sectors. As renewable energies are intermittent, efficient energy carriers, such as hydrogen, will be needed to meet the energy demand. Green hydrogen is considered to be a promising energy vector for the future. However, in addition to problematics related to its production, safe and large scale storage solutions still need to be developed. The geological formations including saline aquifers and former depleted gas fields offer the largest storage capacities but are more adapted to seasonal or mid-to-long term storage. Conversely, underground salt caverns are well suited for storage/withdrawal cycles as short as daily cycling. These artificial structures, offering exceptional tightness, have already been used for decades for hydrocarbons storage but at seasonal storage/withdrawal cycles. The adaptation to short-term hydrogen storage still requires further studies to ensure the stability of the caverns under such loading conditions. Indeed, rapid cyclic loading conditions may impact the tightness and the integrity of the cavern [1, 6].

Rock salt is polycrystalline material with an essentially viscoplastic behaviour, involving different micro-mechanisms such as crystal slip plasticity and grain boundary sliding. At mechanical loading conditions representative of those operated in storage caverns, the rock salt is characterized by non-linear viscous flow. The activation of grain boundary sliding is necessary to accommodate local plastic incompatibilities between neighbouring grains. It has been shown in uniaxial loading conditions but has not been verified in triaxial conditions yet [2, 4]. The presence of brine also affects the micro-mechanisms involved, with for example phenomena like dissolution-precipitation or diffusional mass transfer along grain boundaries, and can modify the mechanical behaviour [5].

In our studies, we investigate the active micro-mechanisms in synthetic halite through in situ X-ray microcomputed tomography (XR-µCT) analysis and digital volume correlation (DVC) and damage quantification. To reproduce loading conditions representative of those in real salt caverns, we apply different confining pressures with a triaxial cell. This triaxial device, developed recently [3], is adapted to in situ XR-µCT tests. We study the development of damage networks and the evolution of pores during the deformation of rock salt under different confining pressures. Samples of halite are prepared by compaction of pure NaCl powder in dry and humid conditions. It gives samples with different brine contents and allows us to study the effect of brine on the deformation mechanims.

An effect of brine is visible, as the cracks seem to appear earlier in the dry samples. On the µCT scans, cracks start to be visible for a lower strain in the case of dry samples compared to the case of wet samples. For a dry sample in uniaxial loading conditions, the cracks were already clearly visible and well formed in most of the sample at 1.3% axial strain. For a wet sample, very few cracks only start to become slightly visible in some areas at 1.45% axial strain. This could be due to the activity of mechanisms involving brine, such as dissolution-precipitation allowing for crack healing.

The effect of the confining pressure is also noticeable. After unloading the wet sample and removing the confining pressure, we observed that the few cracks formed during the deformation of the sample opened up, indicating that the healing process was not

complete. Dissolution and precipitation are kinetically slaw processes, which are mostly active at low strain rates. Therefore, further deformation experiments must also explore the effect of strain rate.

Natural salt always contains some humidity. Therefore, the effect of brine in deformation and damage formation/healing mechanisms is important to understand, especially in the context of gas storage in salt caverns.

Compressibility behavior of colemanite added bentonite under short and long-term high temperature

Sukran Gizem Alpaydin — 2023-10-04

Introduction

The increasing energy demand and the limited fossil fuel resources make searching for new sustainable clean energy sources. In this regard, it is vital to use energy geo-structures as a renewable and clean energy source. The geo-structures are in direct contact with the soil and cause temperature changes throughout the soil mass.

Bentonite is considered suitable as an engineering barrier in deep geological disposal repositories for spent nuclear fuel, mainly because of its favorable swelling properties and extremely low permeability [1]. It was reported by many researchers that the engineering properties of clayey soils change at high temperatures [2,3]. Therefore, there is a need for soil materials that can maintain their long-term engineering properties under high temperatures. Borates are naturally occurring minerals. They can be found mainly in sediments and sedimentary rocks. The most commercially important boron minerals are tincalconite, colemanite, and ulexite [4].

In the present study, it was tried to improve the compressibility behavior of bentonite by adding colemanite under high temperature. The oedometer tests were performed under a constant temperature (80 ºC) on the bentonite-colemanite mixtures. In this context, samples exposed to short and long-term high temperature (80 ºC) were used and the results were compared with the room temperature results.

Material Characterization and Methods

The Ca-bentonite sample used which is activated with sodium bicarbonate. Colemanite was added to bentonite at a rate of 10% of the bentonite by dry weight. In this context, B10C sample represents a 10% colemanite added bentonite mixture. The liquid limits of bentonite and colemanite are 270% and 37%, respectively. The samples (smaller than 75 μm) were obtained by mixing the mixture powder with tap water at a water content of 1.5 times the pre-determined liquid limit value of the mixtures. The slurries were consolidated under a vertical pressure of 12.5 kPa for 14 days. Samples 70 mm in diameter and 19 mm in height were obtained by trimming. The samples were placed in oedometers for tests at room and high temperature (80 ºC). The experimental system was modified for high-temperature tests. The modified system consists of a conventional apparatus, a heat ring, a thermostat, and a water tank. Thus, by heating the cell water, the temperature of the sample was indirectly increased to 80 °C. For long-term experiments, samples were placed in thermal pools in clamped molds (constant volume). These molds were kept in the thermal pools under a constant temperature of 80 °C for 6 months. Then, the consolidation tests of these samples were performed at 80 °C.

Results and Discussions

The result of the consolidation tests of bentonite mixtures at 80 °C after being kept in the thermal pool for 6 months is given in Figure 1. For comparison, the test results at room temperature and 80 °C are also shown. The compression amount of bentonite increased with the increase in temperature, while the swelling amount decreased. The total compression deformation of the bentonite sample, which showed 53% compression at room temperature, reached 65% when the temperature was increased to 80 °C. However, swelling deformation decreased from 7% to 3% when the temperature was increased. When the volumetric deformation behavior of bentonite (NC) is examined, thermal contraction behavior at high temperature (90 °C) was reported [2]. In other words, the vertical compression amount of NC bentonite increases at elevated temperatures. In addition, since the structure of the clay deteriorated at high temperature and the deformations were permanent, it was an expected result that the rebound (swelling) behavior would decrease at high temperature.

There was a decrease in the amount of compression in the sample cured for 6 months, compared to 80 °C. However, although the compression amount of the 6-month cured sample decreased, the amount of compression still slightly increased compared to the room temperature. Compression-swelling deformation amounts of the samples cured for 6 months and tested at 80 °C are presented in Table 1 in detail. When the swelling behavior of additive-free bentonite was examined, increasing temperature decreased the rebound behavior of bentonite. In addition, the swelling amount of the sample cured for 6 months was determined to be almost the same as the sample at 80 °C. The reason for this was that the deformations that occurred at high temperatures were permanent.

When the temperature of the colemanite-added bentonite mixtures was increased, the amount of deformation increased. However, the amount of deformation of additive-free bentonite decreased with the colemanite addition at 80 °C, and returned to its room temperature value (approximately 53%). In other words, the addition of colemanite had a positive effect by reducing the deformation amount of bentonite at both short and long term high temperatures. The short and long term effects of 80 °C temperature on colemanite added samples were almost the same.

Thermal conductivity of dried biocemented sand at higher calcification

Shadi Zeinali — 2023-10-04

MICP-treated sand has been used for many soil stabilisation and erosion protection applications [10] with different bacteria types and paths of biocement generation. A novel application of the method is in the improvement of soil for energy geotechnics applications where a higher thermal conductivity (TC, λ) could be achieved by cementing and improving the existing contacts and developing new contacts among the grains with precipitated various calcium carbonate polymorphs formed during the process, such as calcite, vaterite and aragonite. Past studies, however, only a few, have shown a significant improvement in the TC of biocemented sand at dry and for the full range of saturation, incorporating a steady-state method [7], the transient method [5,6], and the transient plane source method (TPS) [11]. Venuleo et al. [7] studied the effect of 7.97% calcification content (CC) which led to a 250% improvement in TC in the dry state and 40% at the higher saturation range. Xiao et al. [11] presented a fitting equation to predict the TC with CC controlled by void ratio and coefficient of uniformity (C_u). Wang et al.’s theoretical model [10], developed by the soil model of Haigh [2], includes simplifications for the water content effect and geometric shape of soil phases, which has higher errors for low TC values and in lower saturation states. All the above studies are limited to lower calcite precipitation for TC measurement; however, higher calcite precipitation is reported in many studies where the mechanical properties of soil are improved [4]. Therefore, to shed light on the TC in the dried state, this study tries to measure the TC of MICP-treated sand in a dry state with a calcite content of up to 10.21%.

Medium-grained sand [1] with an average particle size of 0.79mm, specific gravity of 2.71, C_u of 1.53, and density of 1.58±0.03g/cm³ served as the reference. Bacterial and cementation solutions were mixed and injected with a continuous flow to the sand columns with a constant head and were renewed every 24 hours, 7-21 times. Post draining, the biocemented columns were accurately cut into the desired-sized discs and dried in the oven at 105°C for 24 hours. TC of the biocemented discs was measured in the dry state and at room temperature according to Hailemariam and Wuttke [3] and compared to other research [2, 5, 6, 7, 10, 11] in 3 different initial porosity groups (ɳ₀ for A=40%, B=41.3%, C=42.7%). Group A was biocemented up to a higher cementation level, as depicted in Figure 1(a, b). As the ɳ₀ in some studies [5, 7, 11] are initially more compact than the current study or have more fine materials inside (higher C_u and less D₅₀), more TC with even less CC could be achieved. Limited experimental results [9, 10, 11], comparable with groups A-2 and B, are also shown. It appears that the same CC decreased void volume in [9, 11] more, which leads to higher TC compared to the current study. This is perhaps due to different gradation and fewer fine aggregates in the

soil matrix with C_u of 1.53 compared to studies with C_u between 2-9.7 [9, 11], as mentioned in [11]. In the current investigation, the transition of the TC behaviour from part A-1 to A-2 with higher cementation can demonstrate a point (about CC of 6%) at which the CC form more effective bonds that abruptly increases the gradient of TC-CC graph from almost constant in part A-1 to 6.3% for part A-2 (up to 190% TC of reference). Finding this point for other groups needs a more comprehensive CC range than the current study. It is observed in A-2, B, and C that TC increased with increasing the CC, parallel with other studies [10]. Additionally, group C was able to offset the additional initial void available by having more CC to approximately the same TC as B. Less density increase rate (I_d) of group A-1 in Figure 1(c) suggests that the calcite condensation may have been less compact and effective bridges compared to that of groups B and C, which supports the slightly lower TC, even though it is a bit more initially compact. Figure 1(c) compares MICP-soil models available for the MICP-treated sands [8, 10, 11] and the Johansen soil model [12] for the results of the current study. Wang et al. [8] empirical model showed a scattered and overestimated result, perhaps because the model only considers the treated dried density and does not take the volume behaviour of biocementation as a bridge in the soil matrix into account. Xiao et al. [11] prediction with the closest fitting parameters to our case, given the available fitted parameters, overestimates the TC in the dry case for all samples. This can be due to the limited number of instances used for making this model, especially in the dry state and the unavailability of correct fitting parameters for C_uless than 2. The model performed well for a similar sand from Martinez et al. [5] but cannot predict the results of Wang et al. [9]. Wang et al. [10] theoretical model also appears not able to predict the exact values for TC of dry state, and, for this study, it underestimates the TC for under 4% CC and overestimates it for over 4%. Perhaps the simplifications in the model can overestimate the thermal bridge size for TC below 1.2 W/m/K and low saturation and dry state. Hence, the model may not be able to provide correct results in the dry state [10]. The Johansen soil model [12] did not match our research either since it overestimates the TC for soils with a dry density between 1.57 and 1.65 g/cm3. It also needed to fit previous MICP-treated sand investigations [8].

Overall, neither the conventional soil models nor the theoretical or empirical equations from the literature can accurately predict the results of the dry state thermal conductivity measurement on the case studies of this research. It seems that there is an optimal CC among higher cementation levels that would be sufficient for creating more efficient bonds in the soil matrix, which rapidly elevates the TC. Therefore, more investigation on thermal conductivity in the dry state, especially on the higher cementation level impact, is needed to deduct a good prediction for different cases of MICP-treated sands.

Experimental insight into the thermal nanometric response of clays

Angela Casarella — 2023-10-04

The effect of temperature on the mechanical behaviour of clay-based geomaterials is relevant in several geotechnical applications (e.g., low enthalpy geothermal systems, energy geostructures and nuclear waste disposal). The mechanical response of (saturated) normally consolidated (NC) clay to temperature variation is not intuitive as the material irreversibly contracts upon heating. Since the thermal contraction observed at the engineering scale does not correspond to the thermal expansion of the clay constituents, both in sign and amplitude, the thermo-mechanical response is usually attributed to temperature-induced changes in the arrangement of clay particles/aggregates (changes in the inter-particle/aggregate porosity) [4] or to the nano-scale thermo-mechanical behaviour of the adsorbed water between clay unit layers (changes in the intra-particle porosity) [3].

Especially for clay minerals with a large amount of adsorbed water, such as swelling clays (tens of % of the total water is absorbed in saturated swelling clay samples), the latter hypothesis has been investigated numerically by molecular dynamics modelling of a layer-water-layer system in non-isothermal conditions [3,8] and experimentally through X-ray diffraction and scattering experiments (XRD, SAXS) [5,6,7].

According to the numerical simulations in [3,8], the free energy barrier between stable system states (the number of adsorbed water layers surrounding a clay particle) decreases with temperature, inducing a possible transition between mobile and immobile water. This nanometric phenomenon may result in a macroscopic volumetric thermal contraction.

A similar picture comes from the in-situ diffraction and scattering experiments [5,6,7], where a slight decrease in clay basal spacing (distance between two consecutive clay’s aluminosilicate layers) is measured for increasing temperature. However, the experiments reported are performed in unsaturated conditions at controlled humidity and cannot be confronted with the fully saturated samples usually employed in geomechanical testing.

Measurements for monitoring nano-scale changes of fine-grained soils in their natural wet states are needed to prove the nano-scale origin of the thermo-mechanical behaviour of clays. Small-angle X-ray scattering (SAXS) has often been used to study particle orientation in compacted saturated clay [1]. Smaller features of the mineralogy and sub-particle behaviour of clays can be instead accessed by X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) [2]. In principle, SAXS/WAXS measurements capture the inter-particle and intra-particle distances by measuring the scattered intensity of an X-ray beam hitting a sample.

This research uses combined SAXS/WAXS measurements to monitor nano-scale changes induced in the clay basal distances of several fine-grained natural soils in their saturated state by temperature variations. The experiments were performed with a SAXSLAB Mat:Nordic Instrument at the Chalmers Material Analysis Laboratory on reconstituted samples of natural sensitive clay (refer to [1]) and remoulded samples of swelling (bentonite) and non-swelling (kaolin) clays.

Figure 1 shows the absolute scattering intensities I [a.U.] as functions of the scattering vector q [Å^-1] recorded at different temperatures for remoulded kaolin clay (Figure 1a) and bentonite (Figure 1b) samples (both remoulded at 0 kPa). The kaolinite (q=0.898 Å^-1) and montmorillonite (q=0.312 Å^-1) peaks are clearly visible in Figure 1a and Figure 1b, respectively. In the q-intensity plot, a change of the peak width, or a shift of the peak, indicates nano-scale strain in the material. However, no significant differences in the two peaks are recorded after a temperature increase of ΔT=+45ºC and ΔT=+95ºC, respectively; therefore, temperature variations, within the range considered for most thermal applications, do not affect both kaolinite and montmorillonite basal spacings.

The results in Figure 1a (kaolin clay) are consistent with the idea that the amount of adsorbed water in the intra-particle porosity is significantly low in non-swelling clay minerals. As a result, the kaolinite macroscopic thermomechanical response cannot be associated with nanometric changes in the intra-particle absorbed water.

On the other hand, a significantly similar scattering response is recorded for bentonite in Figure 1b. Therefore, the activity (swelling capability) of the clay mineral and, thus, the amount of the sample absorbed water seems not to affect the response of the basal spacing to temperature, which stays constant upon heating.

Differently from what has been observed by [5,6,7] in unsaturated conditions, the current SAXS/WAXS measurements show that, for modest temperature changes, intra-particle changes upon heating in saturated samples do not play a major role in the macroscopic thermo-mechanical response of clay within the temperature range considered for most thermal applications, and this is independent by clay mineralogy. Therefore, thermo-mechanical strains must be investigated at a scale beyond what is accessible for a laboratory WAXS/SAXS instrument (> 200 nm), focusing on temperature-induced particle/aggregate rearrangement.

A Phase-Field Discrete Element Method to study chemo-mechanical coupling in granular materials

Alexandre Sac-Morane — 2023-10-04

Geochemical reactions are ubiquitous for subsurface energy extraction and storage applications, like geothermal electricity production or hydrogen storage, but also for other applications aiming at reducing climate change like underground CO₂ disposals [1]. Those reactions lead to mineral dissolution/precipitation that can modify the different properties of the reservoir rock and the caprock. For example, porosity of the porous material depends strongly on the dissolution/precipitation of the solid phase. It can thus affect the evolution of its permeability, a key parameter for most of those applications that involve the transport of a fluid through the reservoir. Moreover, the mechanical behavior and rupture of geomaterials is also strongly affected by dissolution/precipitation phenomena. In the case of cemented rocks, debonding can occur during weathering and strongly weakens the material [2]. This strength degradation has also been highlighted during oedometric tests in the case of granular materials presenting no cohesion [3]. In the case of underground storage applications, if a strength reduction is induced in the caprock, cracks can be created, leading to a migration pathway and leakage. It can also affect the behavior of faults in the vicinity of the reservoir and induce earthquakes [4]

Granular material (like in fault) or sedimentary rock can be modeled by the discrete element method (DEM). This method was first developed by Cundall and Strack to model the micromechanical behavior by reproducing more accurately the interactions in an assembly of grains. In the classical approach, grains are modeled as disks (2D) and spheres (3D), however, real particles can be highly irregular. These complex shapes of the grains influence greatly the macroscopic mechanical behavior of the material [5] and accurate models should aim at capturing this complexity. To do so, different approaches have been developed in the frame of DEM like particles cluster, ellipsoids, polygonal (2D) or polyhedral (3D) particles [6]. The latter is the most accurate solution, but it tends to overestimate the roundness of the particles and show some limitations to reproduce experimental results [7]. Recently, a level-set discrete element model was developed and allowed to capture the complex shape of the grains and reproduce experimental results [8].

To model grain dissolution/precipitation, discrete elements are often considered with a homogenous decrease/increase of the particle diameter. But in some cases, like pressure-solution, the dissolution/precipitation are localized. Hence, the dissolution occurs in the high-stress area, whereas the precipitation occurs in the low-stress area with a large solute concentration. This diffusive mass transfer is done within the pore fluid. Considering the granular material as a phase, the phase-field theory (PF) is a good candidate to model with physics-based laws an addition or reduction of the quantity of material locally. The dissolution at the contact is controlled by the introduction of mechanical and chemical energy into the Allen-Cahn formulation on the phase variables, whereas the precipitation and the mass conservation are verified by a coupled diffusion formulation on the solute concentration.

In this study, an extension to the discrete element method is developed to simulate the irregular shapes of particles in a granular material and their heterogeneous change using the phase-field variable as a particle’s geometrical descriptor. This method, available on https://github.com/AlexSacMorane/PFDEM_ACS_MultiGrains, is applied to reproduce results from previous experiments on K₀evolution [9]. The influence of the grain shape has been highlighted. Then, the model is used to investigate on the pressure-solution phenomenon at several grains level [10]. For example, a so-called Andrade creep law has been reproduced.

In conclusion, this new framework enables us to model chemo-mechanical couplings, considering the true shape of the grain. It is used to investigate the influence of the different physical phenomena (dissolution, precipitation or diffusion) controlling the rate of material compaction and of the macro properties evolutions, as the porosity, permeability or strength.

Analysis of biopolymer modified oil cement under supercritical CO2

Diego Manzanal — 2023-10-04

Storing CO₂ in deep underground reservoirs is key to reducing emissions to the atmosphere and standing against climate change. However, the risk of CO₂ leakage from geological reservoirs to other rock formations requires a careful long-term analysis of the system. Mostly, oil well cement used for the operation must withstand the carbonation process that changes its poromechanical behavior over time, possibly affecting the system’s integrity.The use of nanoadditives for cement, such as bacterial nanocellulose (BNC), has been increasing in recent years. This biopolymer has particular properties that can improve cement performance, like high mechanical properties and thermal resistance. For this reason, and in light of the problems that carbonation may pose in the long term in the context of geological storage of CO₂ studies were carried out under supercritical CO₂ conditions analyzing the behavior of cement with nanocellulose additions.Rheological, mechanical, thermal, and microstructural tests were performed on samples with different percentages of BNC [1]. Subsequently, cylindrical specimens were subjected to supercritical CO₂ conditions (20 MPa and 90 °C) with different percentages of nanocellulose using two curing methods, one long-term curing at low temperature [2] and one short-term curing at high temperature [3].These results showed that BNC produces an increase in slurry viscosity but retains a greater amount of water which aids in its subsequent hydration. This could be observed in its microstructure, where a greater amount of hydration products, a higher degree of hydration, and a decrease in porosity were observed. It is likely that this increase in hydration was the reason that cements with nanocellulose had a uniaxial compressive strength up to 20% higher than neat cement. It was also observed that higher BNC contents improve the thermo-mechanical behavior under oscillating bending stress. After carbonation, the microstructure shows that the capillary porosity decreases steadily to values of 5%, which reduces the penetration of carbonic acid into the sample. All cements showed a reduction in mechanical strength, but cements with BNC had a lower degree of carbonation and better mechanical behavior, because of the lower capillary porosity prior to carbonation (Figure 1). However, these effects were not observed when the cement was subjected to a curing process under unfavorable conditions at high temperatures. In this case, the large increase in porosity dulls the short-term hydration effects and the strength of cements with nanocellulose is lower prior to the carbonation process. After carbonation, a relative increase in the strength of the samples with BNC is higher, however, it is still below the strength of neat cement [4]. These experimental studies were simulated using a coupled chemo-hydro-mechanical model. The model simulates the carbonation front advance in cement subjected to supercritical CO₂ and the changes generated by the chemical reactions using the classic balance equations of continuum mechanics relative to mass, momentum, entropy, and energy. Simultaneous dissolution of portlandite and C-S-H, dissolution of calcite, and a damage model were considered. The carbonation progress of the samples was represented and an extrapolation was made to an oil well based on the parameters obtained from the experiments and simulations.

Backfill thermal properties improvement using granular phase change materials and graphite

Tairu Chen — 2023-10-04

Phase change materials (PCMs) are ideal for thermal energy storage due to latent heat release or absorption at target temperatures. Hence, adding PCMs into backfill materials of ground heat exchangers for example, can enhance the borehole thermal energy storage capacity and the corresponding shallow geothermal system performance. However, the heat transfer efficiency between the borehole and surrounding ground might be reduced because of the low thermal conductivity of PCMs. Incorporating additives with high thermal conductivity can reduce the lowering effect of PCMs on the overall thermal conductivity of the mixture while maintaining a desirable thermal capacity. This work mixes encapsulated PCMs (EPCMs), graphite and recycled glass fines, aiming to develop green backfill materials with excellent thermal properties. The heat capacity and thermal conductivity of the mixtures with different fractions of the components are measured in a laboratory. The measured data agree with simulated results. Better thermal properties can be achieved with specific fractions of components, and the findings could help geothermal systems design for better energy utilisation.

Geothermal energy is one of the promising renewable energy sources and its utilisation can be an alternative to fossil fuels. A geothermal system requires a ground source heat exchanger with pipes installed inside a borehole, and backfill materials filling the gap between the pipe heat exchanger and the ground. Thus, the thermal properties of backfill materials are essential to the geothermal energy extraction process and influence the corresponding shallow geothermal system performance [2]. PCMs are used to improve the thermal storage capacity of backfill materials because they can absorb and release heat at a relatively constant temperature [1]. To avoid problems of leakage and volume change of PCMs during a phase change, encapsulated PCMs (EPCMs) are better suited in backfill materials [5]. However, mixtures including PCMs may hinder heat transfer because of their low thermal conductivity. Therefore, other additives with high thermal conductivity can be used to resist the low heat transfer rate [3]. Even though the thermal properties of backfill materials are fundamental to the geothermal energy extraction process, their dependence on the amount of added PCMs and other additives has not been investigated yet. This work mixes EPCMs, graphite and recycled glass fines, aiming to develop green backfill materials with excellent thermal properties. The heat capacity (), thermal conductivity () and thermal diffusivity () of the mixtures with different fractions of the components are measured in a laboratory.

SEM images of materials used, recycled glass fines, EPCMs and graphite, are shown in Figure 1(a). Cylindrical containers with a diameter of 44 mm and a height of 57 mm are used to prepare samples of mixtures. Each sample has the controlled volume fractions of components indicated by Figure 1(b) while keeping a constant overall porosity of 0.47. A decagon KD2 Pro thermal analyzer with a dual thermal needle measures the thermal properties of samples kept at 4 °C when EPCMs are in the solid phase and 45 °C when EPCMs are in the liquid phase. The geometrical mean model is used for theoretical estimations of values for comparison against experimental results [4].

The thermal conductivity () of the mixtures increases with increasing graphite volume while decreasing with increasing EPCMs volume, as shown in Figure 1(c). The difference between under solid EPCMs and liquid EPCMs is limited. The heat capacity () rises with increasing EPCMs volume but declines with the increase of graphite volume, and it is higher when EPCMs are liquid than solid. It can be observed that a trade-off between the improvement of and the reduction of exists. Hence, the thermal diffusivity (), which depends on the ratio of to , is also investigated as presented in Figure 1(d). It decreases with the increase of EPCMs volume but grows with increasing graphite volume. Besides, it is higher under solid EPCMs than that under liquid EPCMs. This indicates the backfilling materials have a faster thermal response to the change of surrounding temperature under solid EPCMs.

Nonlinear site response analyses for sands: investigating the influence of fabric anisotropy

Hilmi Bayraktaroglu — 2023-10-04

Nonlinear effective stress site response analyses (SRAs) are commonly used to estimate dynamic soil behaviour, seismic wave propagation through the soil medium, and resulting ground motions [1]. These analyses can be used to identify potential hazards (e.g., landslides, settlements, liquefaction) and to estimate dynamic loads on superstructures in areas that are prone to natural or induced earthquakes, which can help with disaster planning and risk mitigation efforts. In this study, the influence of fabric anisotropy, which is induced during the soil formation process, on the response of sand deposits has been assessed through one-dimensional site response and response spectrum analyses (RSAs). First, a novel anisotropic critical state theory (ACST) based semi-micromechanical constitutive model accounting for the effect of fabric anisotropy has been incorporated into a fully coupled dynamic code employing the u-p formulation. Then, the initial fabric anisotropy has quantitatively (both with respect to intensity and orientation ) been changed to imitate different anisotropic formations observed in natural deposits. The proposed numerical procedure shows that fabric effects stemming from the anisotropic nature of sands can significantly influence the dynamic behaviour of sand deposits, leading to significant variations in ground motions and therefore resulting in diverse spectral accelerations at the ground surface.

The loading direction dependent behaviour of sands, which can be associated with their anisotropic nature originating from the arrangement of the soil inner microstructure, is generally described/idealized using a second order fabric tensor by ACST based models. Similarly, in this study, a contact normal based second order fabric tensor together with a plastic strain driven fabric evolution formulation has been employed to link the influence of the changing inner microstructure to the relevant constitutive formulations. Further details on the fabric formulations and their multilaminate specific extension can be found in ref. [2] and [3]. Although numerous experimental studies have been conducted to investigate the influence of fabric on the undrained response of sands and advanced constitutive models have been developed to account for it, the majority of research efforts involving anisotropy have concentrated on the element test level, while practical boundary value problem (BVP) simulations are usually omitted. In order to ameliorate that trend, the practical aspects of the fabric effects in BVPs will be investigated in the next section.

To investigate the repercussions of incorporating fabric effects, two identical SRAs with different initial fabric configurations, i.e., initially isotropic and anisotropic, have been carried out and the resultant response spectrums are presented in Figure 1. These SRAs were performed for a one-dimensional column of 10 m height with a water table located at m depth, subjected to a seismic load taken from the 1987 Superstition Hills earthquake. The initial field stresses were determined assuming . Even though the different initial anisotropic configurations produce similar RSA trends over the range of the period , a significant difference in peak values is observed at sec. Whereas the initially isotropic sand returns a peak value of , its anisotropic counterpart yields (i.e., higher), in which and are the spectral and peak ground accelerations, respectively. Figure 1 illustrates two different types of structure, a one storey building and a four storey building, with approximate natural periods of and , respectively. Since most of the variation in the RSA is observed in the range , it is expected that structures residing in that period range will be affected most by the fabric anisotropy.

Analysis of soil behavior under freezing and thawing conditions

Petr Nikolaev — 2023-10-04

Understanding of the thermal, hydraulic and mechanical behavior of soils under freezing and thawing is important in solving problems of permafrost engineering and artificial ground freezing. Accurate mathematical descriptions of heat and moisture transport and the associated mechanical deformations is critical for developing such understanding. Phase change and transition during freezing and thawing are characterized by a strong non-linearity due to the rapid variations of thermo-physical properties of the material and the release of the latent heat of solidification. Such processes cannot be easily and accurately modelled via the use of local (differential) formulations [1,2]. A non-local formulation, developed within the framework of peridynamics [3–5] is proposed here for ground freezing and thawing. It expands the domain of applicability of the first work on non-local modelling of heat transfer with water flow in saturated porous media [6] into modelling the thermo-hydro-mechanical behavior of unsaturated soils.

The mathematical description of unsaturated soil behavior is based on three governing equations of conservations of water mass, energy and linear momentum, which are complemented by constitutive relationships that represent the change of soil properties.

Using the local mathematical model of unsaturated soils [7,8], a peridynamics mathematical framework is formulated. Following [6], the bond-based peridynamics considers a soil body as a collection of particles, see Fig. 1 for the 1D case. The term `bond' refers to the interactions between two points, located at depths and . The point interacts with all other points within a certain finite region , called the horizon of z.

The peridynamic form of the water mass conservation is:

where is the water pressure head; is time; is the volumetric content of a soil component; is the density of a soil component; and is the vertical coordinate. The micro-diffusivity function can be defined following [9]. Here, the subscripts i, l, a and s refer to ice, liquid water, air and soil particles respectively.

The peridynamic form of the energy conservation is:

where is temperature; is the latent heat of water solidification; is the average thermal conductivity of soil; is the vector of water flux; is the average heat capacity of unsaturated soils and is the specific heat capacity of liquid water. Here, is a micro-conductivity function and is a micro-velocity function, for more details see [6,10].

The peridynamic form of the linear momentum conservation is:

where is the acceleration of the peridynamics particle ; is a pairwise force density vector between particles and that depends on its initial and deformed positions Z and Z’; is the horizon volume associated with the particle ; is the body force on the particle . The elastic stress-strain relation for unsaturated soils can be stated as:

where is the stress increment tensor; is soil compressive modulus for unsaturated conditions, and is the strain increment tensor due to the elastic deformation of the soil matrix, that can be find from the relation for the total strain increment tensor of unsaturated soils . Here is the strain increment tensor corresponding to the change of soil volume due to the frost heave; and is the strain increment tensor corresponding to the plastic deformations of soils.

The numerical implementation of the peridynamic formulation is verified by comparing the results of simulations with analytical solutions and the formulation is validated by comparing these results with experimental data under the condition of one-dimensional heat and water transfer obtained by laboratory experiments. In the experiments, soil cylinders were cooled from one surface by a source of negative temperature. As a result, the freezing front propagated along the cylinder. The temperature profile in the column was changing, and water migration was observed. The developed model is shown to predict the results of these experiments well.

The proposed peridynamic model is the first to combine the flow of liquid water with heat transfer and mechanical deformations in the presence of a change of the water phase state in unsaturated soils. It is demonstrated that for the soils with relatively high initial water content, the developed approach describes effectively the change of liquid water content and predict soil deformation. The set of integral-differential equations are solved by a simple numerical scheme.

H2-gas diffusion in porous media as observed by NMR

Leo Pel — 2023-10-04

Not only is hydrogen (H₂) the simplest and most abundant gas in our universe, it also plays a pivotal role in the decarbonization of our energy. As such hydrogen will be one of the most important critical enablers for industry in the global energy transition. Hydrogen’s low density hinders the wide-scale deployment and therefore, it is important to come to large scale low-cost H₂ storage capacity as a source of hydrogen gas. In order, to store the energy in the TW h/GW h-range subsurface storage of hydrogen in depleted hydrocarbon reservoir and/or deep saline aquifers is needed. Concerning the hydrogen storage in depleted gas/oil reservoirs there are still many scientific questions open, which have to be addressed in order to come to reliable, efficient and cost-effective storage [1, 2]. For example, the injected hydrogen will displace the pore residual oil/gas and spread out in a reservoir; a trap structure is needed to prevent the hydrogen from escaping and allowing reproduction of the hydrogen, i.e., leakage is a major concern. Accurate predictions are needed of multi-phase fluid displacement in porous media, e.g., the interaction/displacement of cushion gas and the interactions with the solid matrix.

In this study we have used NMR to image the diffusion and interaction in porous media. NMR is one the few methods by which H_2-gas can be directly imaged in porous media. In order to do so, we have constructed a special NMR setup to measure the diffusion in porous media up to 60 bar. As the relaxation times measured for hydrogen are too short for 3D imaging this setup is limited to 1D measurement [3]. As, to be able to measure also on real porous media containing magnetic impurities, it was chosen to keep the main field at 0.7 T. The present setup is made up of an electromagnet generating a main magnetic field of 0.7 T with a gap of 70 mm and a picture is given in Fig 1. The NMR setup itself is equipped with Anderson gradient coils which can generate up to 0.3 T/m. An RF insert was made which is equipped with a special Faraday-shield, to be able to perform quantitative measurements and as such this setup is directed towards quantitative H₂profile measurements [4].

For the NMR measurements a special PEEK reactor was purchased, which can be operated on up to 60 bars. The PEEK reactor has an outer diameter of 50 mm and inner diameter of 30 mm with a length of 500 mm. The reactor can be moved through the RF-coil with the help of a stepper motor, hence giving the possibility to measure the H₂ content profile over the length of the reactor.

In the initial tests we looked at the quantitative measurement of H₂as a function of the gas pressure. These initial measurements show that the H₂ gas has a T₁ in the order of 0.5 ms. Hence, as the signal of the H₂ gas is quite low in comparison to the background noise, it was decided to measure the H₂ gas at a high repetition rate, i.e., TR=2 ms. The results as function of the gas pressure up in the initial test upto 15 bars are given in Fig 2. As can be seen a clear linear behavior is found and only a minor contribution of the background is observed. In the next experiment the reactor was filled with 0.06 mm glass beads to mimic an ideal porous material and the results are also given in Fig 2. Again, a clear linear behavior can be observed, indicating measurements can also be performed in porous media.

After this preliminary study in which we have tested the setup, we now intend to conduct measurements on porous glass beads up to 2 mm under various prewetting conditions, e.g., water, NaCl solution, oil and at various pressures. Here we want to measure the exchange of H₂ gas by other gases such as N₂ up to 60 bar and see the influence of the various prewetting conditions.

Fine particle liberation in saturated porous media under non-isothermalfluid flow

Xinle Zhai — 2023-10-04

Decline in well productivity is a widely reported phenomenon and a critical challenge negatively impacting energy and water operations in deep geological reservoirs [1, 2]. A key contributor to this problem is the detachment of in-situ fine particles present in the porous matrix, which will then migrate and travel through the porous formation until getting strained within thin pore throats resulting in pore clogging and therefore permeability damage [3]. The detachment of in-situ fine particle occurs when the mechanical equilibrium of the attaching (i.e., electrostatic and gravity forces) and the detaching forces (i.e., drag and lifting forces) exerted on the particle is disturbed. In geological reservoirs, the equilibrium of in-situ fines can be disturbed as a result of fluid flow velocities, temperature alterations in the porous formation, or reduced ionic strength of the in-situ fluids [4]. Fines migration and straining can also alter in-situ stresses through generating pore pressure changes as a result of permeability damage [5].

Multiple experimental and numerical studies have evaluated the mechanisms involved in detachment, migration, and straining of in-situ fines and the clogging of pore fluid channels [6, 7]. A number of studies have also focused on evaluating temperature-induced particle mobilization [8]. Variations of the electrostatic force with temperature is often explained through the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [9]. In a saturated porous formation containing in-situ fines, Dielectric permittivity of pore fluid decreases with an increase in temperature, weakening the repulsion between the fine clay particles and the sand surface [10]. As a result, the attaching electrostatic forces are lower under higher temperatures.

This study focuses on developing a theoretical model to evaluate the impact of the size distribution of in-situ fine particles on non-isothermal fluid flow induced fine mobilization in saturated porous media. Expressions for drag force and electrostatic force are obtained based on the DLVO theory considering coupled effects of fluid velocity, temperature, and ionic strength of in-situ fluids. The main parameters adopted in the proposed model are presented in Table 1. The proposed model predicts the maximum concentration of retained fines considering coupled effects from temperatures and pore pressures. Results are valuable for estimating permeability damage and well productivity during enhanced geothermal operations.

A sequentially coupled chemical-mechanical damage constitutive model for carbonate rocks

Hao Li — 2023-10-04

Modelling the constitutive relations of chemically corroded carbonate rocks is important for the design and stability evaluation of engineering constructions in karst areas (e.g., mines, tunnels and dams, etc.) [1-4]. This paper presents a sequentially coupled chemical-mechanical (C-M) damage constitutive model for engineering rocks in karst areas. First, the chemical damage caused by carbonate dissolution and the mechanical damage caused by external loads are investigated based on experimental results. After that, the chemical damage is expressed by the degradation ratio of Young's modulus under the effect of the chemically induced secondary pores compaction. The chemically induced secondary pore is quantified by reactive transport behavior considering the geochemical procedure of free-face dissolution and precipitation under different H⁺ concentrations, temperature and corrosion periods [5]. The mechanical damage is formulated based on a statistical theory [6,7], which underlines the strength of the mesoscopic element and considers the damage initiation threshold. To capture the nonlinear strength responses of the mesoscopic element at various chemical damage and confining pressures, a modified Mohr-Coulomb (M-C) criterion is introduced, in which the instantaneous friction angle and cohesion are expressed as functions of the confining pressure and chemical damage. The proposed model is validated and shows good agreement with experimental data.

Result of this research shows that: (1) the porosity increases with increasing corrosion period and the increase rate (i.e., the slope of the porosity curve) is steep initially and then decreases gradually (see Fig. 1 (a)). (2) As shown in Fig. 1 (b), Young's modulus decrease with porosity growth. (3) The C-M coupled damage evolution curve is S-shaped and consists of four stages (see Fig. 1(c)), i.e., a linear elastic stage, a stable damage stage, a damage acceleration stage and a post-peak stage. In damage stage 1, stress is lower than the mechanical damage initiation threshold and the stress-strain curve is in a linear elastic state. At this stage, mechanical damage doesn’t initiate (i.e., ). Therefore, the total damage (D_cm) evolution curve is horizontal and only contains chemical damage (D_c=0.165) caused by carbonate dissolution. Once the axial strain exceeds the mechanical damage initiation threshold, the damage enters stage 2. In this stage, total damage (D_cm) begins to slightly rise and steadily increases with the increasing load. In damage stage 3, the damage-strain curve rises concave upward. After the peak strength point, damage development comes to stage 4, i.e., the post-peak stage.

Testing Procedures on the Assessment of the Effects Temperature on Residual Shear Strength of Soils

Aidy Ung — 2023-10-04

Drained residual shear strength is the parameter used in the back analysis of the reactivated landslides and slip surface test [1,2]. The effects of temperature on residual shear strength were not extensively studied, and the method used to assess such effect across the literatures shows some discrepancies. Any internal or external factor impacting the applied stresses and the mobilized residual shear strength may lead to reactivating landslides. Therefore, considering the aggrevating climate change, it is essential to study the impact of temperature on residual shear strength and establish the best method for measuring this effect. The study by [3] concluded that for smectite-bearing soils, the residual shear strength decreases as temperature decreases. In the thermal ring shear tests conducted by [3], the specimen was first consolidated under the desired normal stress, and sheared in room temperature. Furthermore, the temperature was lowered while shearing and residual shear strength was measured as the specimen continued to be sheared. Changing the temperature during shearing without removing the loading arms from the top cap prohibits the specimen to experience the full thermally-induced volume changes and potentially disrupts the results . On the other hand, the study by [4] concluded that there is no significant effect of temperature on residual shear strength of soil. In [4], the specimen was cooled to 5°C at the beginning of the consolidation stage and was sheared after reaching desired normal effective stress. The main difference between these two described procedures is that [3] changed the temperature as the specimen was sheared, while [4] changed the temperature of the specimen prior to the consolidation stage.

The observed disrepencies within the literature may originate in the method of testing. Therefore, this study aims to investigate whether the instant in which the temperature changes in the testing procedure to determine the residual shear strength impacts the results. The tests are conducted in accordance with ASTM 6467 on two clays: EPK clay (99.3% Kaolinite and 0.7% Zeolite) and Rhassoul clay (70.5% montmorillonite, 29.4% Illite and 0.1% Kaolinite). Three ring shear experiments are performed on each of the selected clays. All the experiments starts with preparing the specimen at the liquid limit and place it in the container to form a specimen. In the first set of experiments, the specimen is consolidated under the first effective stress of 7kPa. Once the primary consolidation under this first load is complete, the temperature of the specimen is changed to the target value of 50°C. After the temperature and the volumetric strains stabilize, the consolidation stages proceed to a maximum vertical stress of about 300kPa and then unloaded back to the first load to initiate the preshearing stage. Preshearing is the step to develop a failure surface by shearing the sample for at least the displacement of one full revolution. After one full rotation, the reloading and subsequent shearing stages are performed and residual shear strength is recorded. The other two sets of experiments are conducted in similar fashion, with the difference in the time of the temperature change; in the second set of experiments the temperature is altered before the preshearing stage and in the third set the temperature is changed after the preshearing stage. It should be noted that during the temperature change the loading arms of the ring shear apparatus are not in contact with the top cap. These individual tests are then compared to assess whether the timing of temperature change can influence the residual shear strength of soils. It should also be noticed that the same procedure was followed without a sample in the ring shear apparatus to calibrate the impact of temperature on the system. The results, however, were not significant and thus, neglected in after the test data processing.

Figure 1 presents the obtained residual shear strength friction angles for EPK and Rhassoul Clay at room temperature and at 50°C. The preliminary results of EPK Clay shown in Figure 1 (a) suggest, although minimal, the residual friction angle of the EPK decreases as the temperature increases to 50°C after preshearing. On the other hand, the results obtained from Rhassoul Clay ring shear tests reveal that the alteration of the friction angle is more significant when changing the temperature before preshearing, see Figure 1 (b). These results suggest that the instance at which the temperature changes in a thermal ring shear test can impact the drawn conclusion. Therefore, in the absence of a global standard, it is more reasonable to choose the test method by considering real-life situations, precisely the actual temperature and its variation during the compaction or consolidation of the soil in situ. Furthermore, since clays' thermo-mechanical behavior depends on mineralogy [5], the observed trend between the residual shear strength and temperature is also consequently mineralogy dependent. Therefore, this study will be expanded further by considering various mineralogy of clays and a wide range of temperatures to provide a better understanding of both the impact of the thermal ring shear test method and the impact of temperature on residual shear strength.

A new experimental setup to investigate the cyclic response of soft soils under induced earthquakes

Ching-Yu Chao — 2023-10-04

Energy technologies, which work by extracting or injecting fluids in the ground, such as geothermal energy systems or underground liquefied gas storage, may induce seismic events, see e.g., [1]. In the Netherlands, induced earthquakes are continuously recorded from the Groningen gas field, with the largest magnitude ever recorded of M_L 3.6 at Huizinge. Even though the magnitude of these events is not high, compared to natural earthquakes, damage to the built environment is still caused because of the shallow depth of the events and site amplification, especially where soft soils are encountered [2]. Proper quantification of the induced seismic risk requires better understanding of the response of soft soils to these repeated short events, covering a range of frequencies from 1 to about 20 Hz. This motivated the development of a new advanced dynamic equipment to experimentally investigate the coupled response of soft organic clays and peats from the typical deltaic areas of the Netherlands.

Direct simple shear (DSS) apparatuses are preferred usually to investigate the soil behaviour under cyclic and dynamic loading. Among them, a number of multi-directional DSS setups have been developed to investigate the soil behaviour under multidirectional loading [3, 4, 5, 6, 7, 8]. Applying multi-directional loading to soil specimens in the laboratory is a keystone for elucidating the cyclic and dynamic soil response, as several studies have shown that the cyclic and post-cyclic response of soils is affected by multiple loading directions [6, 9, 10, 11, 12]. However, traditional DSS devices have a number of shortcomings, which are inherited by multi-directional DSS devices. The main deficiency of the DSS device is that the shear stress acting on the lateral side of the specimen cannot be controlled, and hence, a homogeneous stress state cannot be achieved, in spite of the common assumptions. Lateral stresses cannot be measured either in traditional setups, which leaves a knowledge gap on the stress state and the stress path of the sample.

In addition, the majority of laboratory element tests are performed by imposing “slow” undrained cyclic loads, to try to guarantee uniform water pressure distribution within the sample, for the sake of interpretation and modelling. However, seismic events encompass much higher loading frequencies than typically available, with loading rate effects playing a key role in the response of soft soils such as organic clays and peats. In order to fully understand the cyclic behaviour of soft soils, “fast” cyclic tests are crucial.

The innovative multidirectional shear device, developed in the section of Geoengineering at TU Delft (Cyclic-Dynamic shear simulator for Organic Soft Soils, CYC-DOSS), was designed to overcome some limitations of previous equipment. The underlying idea is to abandon the homogenous stress-strain state assumption and monitor the response with local sensors, which allows conditioning a numerical back-analysis of the test data. The new device shown in Figure 1 is characterised by (1) servo-hydraulic control; (2) multi-directional loading in 3 axes; (3) bender elements to measure both P-wave and S-wave velocity; (4) fully controlled cell pressure and back-pressure; and (5) possibility to reproduce the full acceleration time history of seismic events. The device is capable to apply loading frequencies up to 25Hz and a wide variety of multidirectional cyclic loading patterns. The apparatus is equipped with advanced sensors, also developed at TU Delft, including local pressure, displacements, and accelerations devices. The sensors are installed to reduce a priori assumptions on the soil response, better interpret the experimental results as a small-scale physical model and further investigate in depth the soil response under a variety of cyclic loading histories. The experimental information from the setup will be used to develop and calibrate an advanced bounding surface constitutive model for soft organic soils.

Effect of temperature on the compressibility behavior of glass fiber-bentonite mixture

Yusuf Batuge — 2023-10-04

Storage of nuclear wastes in deep geological disposals is an important concept in terms of energy geotechnics. With its low permeability and high swelling potential, bentonite clay is preferred in high level waste (HLW) repositories. The long-term heat released by nuclear waste causes changes in the structure of the buffer material bentonite clay. Hence, it is critical to determine and improve the short and long term engineering parameters of bentonite clay under high temperature. Various fiber materials are used to improve the mechanical properties of the clayey soils [1,2]. In this study, it was investigated short-term and long-term compressibility behavior of bentonite with glass fiber additive under high temperature.

The bentonite used in the study was activated Ca-bentonite. Bentonite was in powder form and all of its particles were smaller than 75 µm. The liquid and plastic limit values of the Ca-bentonite is 270% and 63%, respectively. The specific gravity is 2.60 and the natural water content is 7%. Glass fiber is a silica-based material obtained by melting glass, passing it through thin wires and cutting it. It has high tensile strength and modulus of elasticity and is resistant to heat. The glass fibers used in this study were3 mm long, 13 microns in diameter, and have a specific gravity of 2.60. To examine the compressibility behavior, one-dimensional consolidation tests were conducted on the samples according to ASTM D2435 (2020). Glass fiber was added corresponding to 1% by dry weight of bentonite. The liquid limit value of the bentonite-glass fiber mixture was found to be 265%. Bentonite and bentonite-glass fiber mixtures were prepared at a water content of 1.5 times their liquid limit values. Tap water was used in the tests. For a uniform moisture distribution, the samples were kept for 24 h in a closed container. All samples were placed in a cell made of plexiglass with a height of 3 cm on a rigid steel ring with a diameter of 7 cm and a height of 1.9 cm and kept under seating pressure of 12.25 kPa for 14 days. At the end of 14 days, the sample in the steel ring was placed in the oedometer cell. The oedometer cell modified for high temperature. The heat coil, thermostat probe and thermocouple placed inside the oedometer cell in order to maintain constant 80 °C temperature inside the cell. The temperature was increased by 5 °C per hour to prevent excess pore water formation. For long-term behavior, after the samples were subjected to seating pressure for 14 days, samples were cured for 6 months in thermal water pools at 80 °C. Samples were kept in a clamped mold inside the thermal pool to avoid any volume change.The tests were performed under three different thermal conditions; under room temperature, high temperature (80 °C) and curing in a heat pool at constant temperature of 80 °C for 6 months.

The stress-strain graphs of the samples are shown in Figure 1. Since the e-logp graphs of high plasticity clays are non-linear, different compression index and swell index values of each loading level occur. Compression and swelling moduli values obtained from the stress-strain plot were used to explain the compressibility behavior. Based on the initial conditions, all samples are fully saturated. Initial dry densities of all samples varied between 2.64-2.94 kN/m3. Initial and final void ratio values for additive-free bentonite samples were found to be 7.68 to 2.00 for the room temperature sample, 8.02 to 1.15 for the short term 80 °C sample, and 8.49 to 1.68 for the long term 80 °C sample. In bentonite-glass fiber mixtures, 8.78 and 2.83 in the room temperature sample, 8.01 and 2.40 in the short-term 80 °C sample, 7.33 and 1.68 in the long-term 80 °C sample.The compression deformation moduli were determined when the applied stress is increased from 24.5 to 784.5 kPa and the swelling deformation moduli were determined when the stress was decreased from 784.5 to 49 kPa. Compression and swelling moduli values for each experiment are presented in Table-1. In the presence of 80 °C, the compressibility of additive-free bentonite increased while swelling (rebound) decreased. This result is in agreement with the literature [3]. When the long-term and short-term behavior of bentonite were compared, an improvement was observed in both compressibility and rebound behavior. Decrease in the rebound behavior of bentonite under high temperature can be explained by the decrease in water retention capacity and weakening of the bonds of water molecules with clay minerals. In the presence of 1% glass fiber, it was observed that the plastic deformation increased in different thermal conditions. In the presence of 1% glass fiber, it was observed that the plastic deformation increased in three different thermal conditions. Since glass fiber occupies less space than bentonite particles in terms of volume, it increased the compressibility. However, it was seen an improvement in total rebound behavior by reducing the permanent deformation of bentonite.

Minerals Dissolution Effect on the Mechanical Properties of Synthetic Carbonate Rocks

Katia Galindo — 2023-10-04

Injection of CO₂ and water in saline aquifers or oil reservoirs causes changes of pressure, saturation and concentrations that affect the state of stress and promote chemical reactions in the host rock, resulting in porosity and permeability variations. It is therefore a coupled hydro-mechanical and chemical (HMC) problem. Numerical simulation of multiphase and multicomponent flow of CO₂, oil and water with mechanical coupling allows realistic modeling of the reservoir and cap rocks. Carbonate reservoirs are geological formations composed mainly of minerals such as calcite and dolomite which can dissolve or precipitate in the medium when injecting a fluid of chemical composition and temperature different from those of the fluids initially contained in the rock. Water-weakening due to matrix acidification of carbonates is a well-known phenomenon that can be modeled by including mineral concentrations as state variables in the stress-strain behavior of the material.

The objective of this work is to characterize synthetic carbonate rocks through microtomography and petrography techniques, focusing on a comparative analysis before and after load application and degradation with a reactive fluid [1]. The synthetic rocks were subjected to physical characterization (mineralogy, computed tomography and porosity) and mechanical characterization (uniaxial compressive strength and Brazilian tests) before and after the dissolution process. The petrographic analysis verified an increase in both intergranular and intragranular porosities after dissolution. The microtomography analysis quantified the maximum increase in porosity, from 11.8% to 41.3% in the two-dimensional analysis and 31.6% to 52% in the three-dimensional analysis of the porous structures. Furthermore, the pores were quantified according to their area, and data was obtained on the orientation of the pores, providing insight into the preferred paths of fluid flow. It was also observed that the microtomography technique was an effective tool for characterizing fractures in the samples before and after dissolution [1].

Dissolution tests were also performed in a modified oedometer cell adapted to measure horizontal stress. The dissolution phase was conducted using water and an acid solution to evaluate the influence of the pH on the mechanical behaviour of the samples. When the sample in the oedometric cell is exposed to an acid solution under constant vertical load of 400kPa, vertical displacement takes place (volume decrease of the sample) and horizontal stress increases (Figure 1). The synthetic rock used in this experiment is mainly composed by calcite, with small additions of calcium hydroxide, and the reactive fluid is water acidified with acetic acid (with 10% concentration). This material is manufactured in laboratory in order to have greater control of its constituents and reproducibility of experimental results. During the acidification phase of the experiment, the sample was subjected to an acid flow with a pressure differential of 12kPa for 7h. The behavior in Figure 1 was also observed by [2] and [3]. The model for degradation of carbonatic soft rocks proposed by [2] was implemented in a finite element code capable of performing coupled THMC analyzes in porous media [4]. This model is based on the Critical State Theory with the introduction of a bonding variable that controls the size of the yield surface by modifying the tensile strength and the pre-consolidation stress of the material. In Figure 1 it is also possible to see that the HMC coupled simulation of the oedometric test was able to reproduce the experimental results in terms of volumetric strain and horizontal stress. In Figure 2 tomographic images of the synthetic carbonate rock before and after exposure to the acid solution in an oedometric cell under constant vertical load are presented. Such analyses are crucial for the injection of reactive fluids, such as CO₂, at high depths. Dissolution of minerals will affect not only the porosity and permeability of the reservoir rock but also, possibly, the stress state in the vicinity of the injection well.

Simulation of complex triaxial tests with HySand, a new multisurface constitutive model in the hyperplastic framework

Luc Simonin — 2023-10-04

Offshore wind has a key role to play in the energy transition. The majority of offshore wind turbines (OWTs) are bottom-founded with monopiles [1]. Monopiles account for up to 35% of the installation cost of the OWT [2, 3]. Optimising the design of monopiles can thus lead to significant savings and increased competitiveness of OWTs in the energy market.

The PISA project [4] has led to the optimisation of the design under monotonic lateral loading of monopiles founded in sands, clays, and layered profiles [5 ,6, 7]. Because of this optimisation, along with the increasing size of OWTs, and the deeper waters where they are installed, design under cyclic loading is becoming crucial. There are, however, significant shortcomings in current design methods to predict the effects of cyclic loading on a monopile: accumulated rotation, changes in stiffness, energy dissipation and strength.

The PICASO project, a joint research project between the University of Oxford and Ørsted, seeks to develop a new design method for monopiles under cyclic loading in sands and clays. Similarly to PISA [8,9], this design method will use the finite element method to simulate the behaviour of monopiles. For PICASO, such simulation requires constitutive models able to capture the behaviour of soil under cyclic loading. However, available advanced constitutive models for sand face severe limitations [10, 11] and are not satisfactory for the simulation of cyclic loading.

HySand [12, 13], a family of models developed in the hyperplastic framework, fills this gap. HySand_base is a 14 parameter multisurface plasticity model with non-linear elasticity, shear plasticity and hardening, and two plastic volumetric mechanisms resulting in non-associative plasticity: one dilation mechanism which value depends on the evolving density and anisotropy of the sample, and one density dependent consolidation mechanism.

This document presents comparisons between the database on Karlsruhe fine sand by Wichtmann [14] and results of simulations with HySand_base. The focus will be on complex tests that other models fail to simulate adequately, such as undrained strain-controlled cyclic tests. Data and simulation with HySand_base of such test on medium dense sand are presented in Figure 1.

HySand_base performs well across densities and types of triaxial tests. This gives confidence in its existing three-dimensional implementation in finite element codes, and demonstrates its value to future design processes.

Evidence of gas formation and venting in organic soils: experimental evidence and modelling approach

Inge de Wolf — 2023-10-04

Peatlands have been recognised to provide a natural carbon sink thanks to waterlogged conditions, which keep summertime temperatures relatively low, increase their water holding capacity, decrease the organic soil decomposition rate by creating anoxic conditions and eventually keeping high water table. However, unfavourable environmental conditions due to increasing temperatures and more frequent droughts will reduce water retention of peats and the summertime insulation, in turn increasing their temperature sensitivity and their decomposition rate [1]. As a result, peatlands may start inverting their positive cycle and emitting greenhouse gases, including CO₂ and CH₄ [2], which suggests better investigating how increasing climate stresses will affect the efficiency of peats in the greenhouse gases cycle and CO₂ sequestration.

Some evidence of gas production from increasing decomposition rate in the Netherlands is coming from continuous pore pressure measurements in saturated layers below the water table, which are monitored to assess the safety of the water defence and the transportation infrastructures. Increasing water pressure in closed piezometers compared to vented ones seem to suggest that gas is produced and capped in the ground, until the breakthrough pressure is reached and the gas vents from cracks opened in the soil matrix. Besides the environmental issues, increasing gas production from decomposition is becoming of concern for the stability of embankments made of organic soils, where the effective stress may be lowered to such an extent to endanger their stability. As a matter of fact, in the last ten years, gas overpressure has been claimed to be a triggering or a contributing factor in few small failures experienced by regional dykes in the Netherlands. In spite of the evidence [e.g. 3] and the risk increasing with heat waves and drought events, the role of gas on the coupled hydromechanical response of organic soils has been seldom investigated nor properly understood yet.

In the section of Geoengineering at TU Delft, a research effort has been undertaken in the last years to investigate in depth the role of gas formation and venting on the coupled hydro-mechanical response of organic layers in the subsoil of water defence embankments. Preliminary laboratory tests performed on peats to fill this gap showed the role of increasing gas content on their compressibility and on the mobilised shear strength at given strains [4, 5]. The volumetric response of peats including gas was tentatively interpreted with a simple non-linear elastic model, which proved able to model the experimental results [6].

A similar model was used to numerically investigate the relevance of gas production and venting on the response of a regional dyke in the Netherlands, where gas bubbles from venting were observed after excavating - unloading - the toe of the dyke during a stress test. Fully coupled three-phases hydromechanical numerical analyses were performed with CODE_Bright [7] to include gas overpressure. A gas content of 6% in volume was artificially generated in the peat layer, capped by a clay layer on top, and let reaching an equilibrium distribution, which depends on the stress-strain response of the different layers and their volumetric compressibility (Figure 1(a)). Gas venting is triggered by simulating excavation at the toe of the dyke, which allows gas escaping after the capping clay is removed.

The variation in the operative stress, on which stiffness and strength are assumed to depend [6], is shown in Figure 1(b) over gas generation and venting. In spite of the small amount of gas generated, the predicted overpressure is enough to bring the operative stress to zero in the upper meter of soil at the toe of the embankment due to the light weight of peat and cover soil, which temporarily reduces the factor of safety of the water defence against global stability. As soon as the gas overpressure is released, the operative stress increases above the effective stress which would characterise saturated conditions, bringing the system back to safer conditions.

These preliminary analyses are supporting an undergoing experimental and numerical thorough effort to better quantify the dynamics of gas generation and venting in organic soils to reduce the hazard associated with increasing climatic stresses.

Experimental study on drilling of soft rocks

Shwetabh Yadav — 2023-10-04

Drilling is commonly occurring material deformation process in vast range of industries including manufacturing, mining, petroleum, infrastructure, biomedical etc. Due to its three-dimentional nature and multi-axial deformation of material, drilling is a very complex process involving severe plastic deformation of material, high strain and strain rates, volume change, tempretarure gradient etc. In addition to this, drilling in rocks becomes further more complicated due to additional complexity added by brittle fracture. Therefore, it becomes important to have a complete understanding of the process and the effect of drilling paramerets on the material response in order to make it more energy efficient and improve the productivity in industrial applications.

An attempt to understand the drilling in metals has been made by using a simplistic approach of decoupling the process into simpler process where, the penetration of the drill into the material and the rotatory motion has been correlated to indentation and cutting, respectively [1, 3, 8]. Similar framework has been adopted for rocks by many researchers where drill bits consist of many polycrystalline diamond compact cutters mounted on a tingesten carbide matrix [2, 5, 7]. In order to understand the interaction of these bits with rock, simplified models have been proposed for the cutters mounted on these bits [6]. This research work intends to understand the drilling mechanism experimentally by comparing drilling response (thrust force and torque) with the findings of intentation and cutting studies. Also, the force response (thrust and torque) were compared with the results obtained from the model.

Drilling experiments were performed on samples prepared using gypsum, which is an ideal model material for soft rocks [4, 9]. Sample porosity was controlled by controlling the water content during casting of the samples (46 mm x 46 mm x 20 mm). Twist drills with helix angle of 30⁰ and 60⁰ were used. The diameter of the drill (6 mm) was kept significantly smaller (~ 8 times) than the width of the sample. The tip angle was kept constant (120⁰) for both the drills. The penetration rate was varied from 1 – 100 mm/min at constant rotation speed of 100 rpm. The penetration rate in combination with the rotation speed can be seen as depth of cut per revolution of the drill. The thrust force and torque were normalized with respect to the drill radius to compare the signatures with indentation [10, 11] and cutting [12] experiment results, respectively.

On comparing normalized thrust and torque signatures with indentation and cutting results, a remarkable similarity was observed. Figure 1 and 2 show normalized thrust and normalized torque, respectively, as a function of depth of cut per revolution (penetration rate/ rotation speed). Similar to indentation results, Fig.1a shows that the thrust force has a strong dependence on material porosity but remained independent of drill geometry (Fig. 1b). This indicated that the drill geometry (which is responsible for cutting the material) does not influence the indentation part (vertical penetration). In contrast to this, the normalized torque showed dependence on both, drill geometry and porosity (Fig. 2) which was evident from studies performed gypsum cutting [12]. Also, the torque signatures were remarkably similar to cutting forces and cyclic peaks were observed which indicated cyclic removal of the chips during cutting. The drilling results were compared with the theoretical model as well [6]. However, results corresponding to the 30⁰ helix angle drill did not align with the model predintions. Suitable modifications in the model was suggested. Based on some preliminary experiments performed by imaging the drilling process, it was believed that there exists an optimum helix angle for which the model presdintions hold good.

This study shows experimentally that drilling, which is a complex 3 dimentional process, can be studied by decoupling it into simpler 2 dimentional processes: cutting and indentation. The similarity between the experimental findings of cutting, indentation and drilling suggest that the findings of cutting and indentation studies can be clubbed together and can be used to understand drilling better.

Thermo-hydro-mechanical behaviour of a deep plastic clay formation

Núria Sau — 2023-10-04

Several applications which are framed in the energy geotechnics field, particularly the design of energy geo-structures, energy geo-storage, enhanced geothermal energy systems and radioactive waste disposal, require a thorough thermo-hydro-mechanical (THM) characterisation of their host soil/rock formation. The present study comes up from the need to investigate the THM behaviour of deep Ypresian clays (Ycs, 300 to 400 m below ground) since they are one of Belgium’s potential host rock formations for deep geological disposal of heat-emitting and long-lived radioactive waste.

The impact of temperature on the hydro-mechanical response of deep clays has been extensively tackled for the last four decades. Nevertheless, to the authors’ knowledge, the study of the evolution of radial stresses during drained thermal paths under oedometer conditions has never been addressed. Such information would allow for drawing a better-defined stress state while accurately measuring volume changes. To this aim, the present study used a newly designed and fully instrumented temperature-controlled oedometer cell [3] combined with well-planned test protocols. The local instrumentation consisted of a pore pressure transducer embedded in the soil 3 mm from the bottom boundary, thermocouples at various positions and radial displacement transducers with a localised thin-wall system to estimate radial stress.

Two well-preserved Ycs core samples retrieved at different depths at Kallo (Belgium) and belonging to distinct lithological units were tested (see Table 1). Both slightly overconsolidated samples (Yield Stress Ratios of Ycs between 1.2 and 1.8, [1, 2, 3]) presented high initial matric suctions induced on deep water-undrained sampling. Consequently, the first stage of the well-planned test protocol consisted of loading at constant water content to bring the samples to the large in situ stresses and diminish the induced matric suction. The remaining matric suction was then reduced by soaking with synthetic water equivalent to in situ. Afterwards, drained loading and unloading paths were followed to attain different vertical effective stresses at varying overconsolidation ratios (OCRs) before performing the main research goal of drained thermal paths. The heating-cooling (H-C) cycles consisted of slowly stepwise increase/decrease in temperature (steps of 10ºC) with intermediate stabilising periods of around 15 hours controlled by the embedded pore pressure transducer.

Figure 1 depicts the vertical deformation in terms of the mean effective stress (p’) beyond in situ stress. Changes in radial stresses ruled the variations in p’ during heating-cooling cycles since the vertical effective stress was kept constant during non-isothermal paths. At a slightly overconsolidated (OC) state (Core-48), the drained ‘1^st H-C’ cycle showed a quasi-reversible response in volume changes and radial stresses. At OC state (OCR=2) on Core-103 (‘2^nd H-C’), the response was reversible. However, the induced volume change in normally consolidated (NC) states was not reversible with a contractive response dependent on the stress level. Therefore, the nature of the thermal-induced volume changes aligned with the behaviour observed on other plastic clays. Nevertheless, the stress dependence of the contractive response at NC state differed from the observations made by [4] on NC Boom Clay -the other poorly indurated clay considered as potential host rock in Belgium. As observed in ‘2^nd H-C’ (NC conditions) on Core-48, a reduction of the radial effective stress was only recorded at temperatures higher than 70°C. In contrast, the radial effective stress systematically increased on heating at values < 70°C, tending the sample to lower deviatoric stress. This phenomenon might be

associated with some change in the soil structure on heating that tends to a more isotropic stress state despite the oedometer condition. A clear transition between the elastic and elastoplastic domains was detected during the subsequent loading after each H-C cycle at NC states.

Finally, the study included a thermo-mechanical elastoplastic interpretation: thermal expansion coefficients dependent on temperature and mean effective stress in the elastic domain and thermal softening function on post-yield drained heating.

Thermo-mechanical behaviour of microbial induced carbonate precipita-tion (MICP) sand for geothermal pavements

Xiaoying Gu — 2023-10-04

Introduction

Ground source heat pump (GSHP) or shallow geothermal energy systems are gaining attention for helping combat global warming and the negative effects of urbanisation caused by human activities. In recent decades, energy geo-structure techniques have developed by using subsurface infrastructures to exchange heat with the ground. These techniques can provide space heating and cooling while still preserving the primary structural function. As a result, they have become a valuable part of geothermal energy systems. Researchers have developed an understanding of incorporating ground heat exchangers into foundation piles, retaining walls, and tunnel linings with modest additional cost [1-4]. Pavements are also structures in contact with the ground that have the potential to be used as energy geo-structures (i.e., geothermal pavements), yet, their use has not been extensively studied [5-8].

Soil thermal conductivity is an important factor influencing the efficiency of geothermal pavements since heat transfer in soils occurs primarily by conduction [9]. Microbially induced calcium carbonate (CaCO₃) precipitation (MICP) is an innovative technique for strengthening sandy soils by coating and binding soil grains with calcium carbonate crystals. The distribution and arrangement of the CaCO₃ within the pore spaces play a crucial role in determining the resulting strength of the treated sand. In addition, these crystals can act as thermal bridges to enhance the soil's thermal conductivity [10]. Combining geothermal pavements with MICP sand is still nascent, and the limited number of studies that exist mainly focus on the associated thermal property changes [11]. However, since the principal function of pavements is transmitting (dynamic) loads to the subbase and the underlying soil, the thermal conductivity of the MICP-treated pavement may vary as a result of the applied mechanical loads (e.g., due to the partial or total loss of thermal bridges and/or particle rearrangement). This research thus investigates the changes in the thermal conductivity of MICP-treated sands as they are subjected to quasi-static triaxial compression. The experimental results collected can deepen our understanding of the thermo-mechanical behaviour of MICP-treated sands and provide practical insights for using MICP to reinforce the subbase or underlying soil of geothermal pavements.

Methodology

This research performed a series of quasi-static triaxial tests on MICP-treated Houston sand, fine-grained, high-purity silica sand. Sporosarcina pasteurii (strain designation DSM 33) was used for the MICP treatment of the soil specimen. To study the effect of CaCO₃ content on the thermo-mechanical performance of MICP-treated sand, three cementation solution treatment cycles were applied, yielding theoretical CaCO₃ contents of 0.6%, 1.6% and 2.7% by weight, respectively. Details on the MICP treatment of the samples can be found in [12]. Samples for triaxial testing were treated in cylindrical tubes of 50mm inner diameter and 100mm height. To investigate the influence of the CaCO₃ content on the soil thermal conductivity, MICP-treated specimens were air-dried prior to triaxial testing Triaxial tests were subsequently conducted in dry conditions to isolate the effect of the MICP and avoid the influence of water content on soil thermal conductivity (λ). Furthermore, a new miniaturised transient sensor was embedded in the triaxial samples to monitor the λ changes during the sharing phase [12].

Results

An example of the evolution of the deviatoric stress with axial strain under 50kPa confining stress (σ₃) in the triaxial cell is shown in Figure 1a. Compared to the untreated sand, MICP-treated samples lead to higher peak strength and stiffness. Importantly, λ changes during triaxial testing for different CaCO₃ contents are summarised in Figure 1b. Results indicate that the increase in CaCO₃ content can significantly improve λ, and that λ rapidly decreases post-peak strength due to dilation and CaCO₃ bond breakage. Once the samples reach its ultimate state, λ remains unchanged.

Gas breaktrough behavior of the Spanish reference bentonite

Guillermo García-Herrera — 2023-10-03

The Spanish concept for the geological disposal of radioactive waste implies the use of pre-compacted bentonite, as a main component of the engineering barrier system. The reference bentonite, called FEBEX bentonite, is composed mainly of montmorillonite, has a maximum grain size of 5 mm and a hygroscopic water content of ~14.5%. The aim of this work was to determine the gas breakthrough (BT) pressure on saturated samples of this buffer material.

Samples (20 mm length, 38/50 mm diameter) were uniaxially compacted at 1.5 and 1.6 g·cm^-3 dry densities (DD), directly in the test cell. Their water content were from hygroscopic to around 26% (full saturation at 1.6 DD), most of them on the wet side of optimum. Compaction pressures ranged between 7 and 1.4 MPa, depending on target DD, water content and granulometry of the bentonite. To check the effect of the as-compacted macrostructure, we tested three grain size distributions (full <5 mm, 1.18<x<2.0 mm, and 2.83<x<4.7 mm) with similar water content (hygroscopic); and the full grain size distribution with different water contents (around 18, 22 and 26%). Mercury intrusion porosimetry (MIP) on small compacted samples (for different DD and granulometric fractions) showed that the main differences between them were macrostructural, with minor effect on mesostructure.

A series of long-term gas injection tests were carried out to determine, by constant pressure steps, the BT pressure. Some improvements were introduced in the experimental gas breaktrough (BT) setup and the testing protocol described in [1], including different configurations of gas up and downstream volumes (~150, 75 and 50 cm³), where actual pressure (inlet or outlet) is measured (Fig. 1a). A measurement of gas permeability (on the as-compacted sample) and two injection phases (on the fully saturated sample) configure the BT protocol. Each injection phase consists of a water permeability measurement and a long-term gas injection at increasing/decreasing pressures. The gas injection is intended to elicit at least two BT events (Fig 1b). The cells kept the samples in isochoric conditions during the test: after the saturation phase, the swelling pressure is expected to prevent preferential gas flows between the sample and the cell body. Once the gas injection starts no more water was supplied.

Gas permeability was measured on as-compacted samples with the high-pressure steady-state setup described in [2, 3]. The measurements of gas outflow and other variables, including RH/Temperature, permit to calculate the gas permeability and to estimate the sample drying induced by gas flow during the test. In these samples, there was a linear relationship of gas flow with the difference of square pressures for all dry densities tested. From these measurements, intrinsic permeability ranged from 10^-14 m² to 10^-17 m² (for 1.5 DD and 1.6 DD).

The water permeability measurement before the first gas injection phase allows to obtain a base-line value; the one after gas injection allows to determine if the sealing capacity was compromised by the previous BT events. Average values were in the range (0.7 – 2.0)·10^-18 m².

After [1], the air entry pressure for the FEBEX compacted bentonite is around 25 MPa (for DD 1.6). BT pressures determined in this work were much lower than this value and close to the upper range of the expected swelling pressure, that controls the gas entry and the breakthrough events. Local variations in the swelling pressure due to several factors (material heterogeneity, change in suction, temperature, stress) can direct the high-pressure gas through preferential local paths.

The BT events on pressure-time curves showed a consistent and systematic repetition of BT values (Fig 1b); they had different shapes depending on whether the injection pressure was greater than, close to or just less than the actual BT pressure; and changes in the slope of the curves could indicate different types of flow and the underlying physical concept (from microfracturing, in the case of the instantaneous episodes, to microscopic pathway dilation for the gradual ones).

BT pressures increased with increasing dry density and water content (initial saturation degree) at compaction (considered as initial microstructural state); and the geometry of the sample (decreasing L/D ratio). The BT behavior is considered as a characteristic of a given liquid-solid system (sample): related to the initial state after compaction and the preservation of their structure during the test, at a specified temperature. Changes affecting the structure will affect the BT pressure.

Gas migration in saturated FEBEX bentonite at DD<1.6 was interpreted to occur by the formation and propagation of dilatant pathways.

Numerical gas flow simulation performed to analyze advective gas flow in a compacted clay material

Babak S. Noghretab — 2023-10-02

Burial of nuclear waste in a clay formation can result in the generation of significant amount of hydrogen and other gases due to corrosion of metallic materials under anoxic conditions, radioactive decay of waste, and radiolysis of water. If the rate of gas production exceeds the rate of gas diffusion within the clay porewater, formation and accumulations of a discrete gas phase will occur until the gas pressure becomes large enough to surpass the surrounding material gas-entry pressure; at that critical point occurrence of dilating and advective gas flow is expected [1]. Precompacted bentonite is suggested as a sealing material for the isolation of boreholes, disposal galleries, and deposition holes in a deep geological disposal facility for radioactive waste. Both formation of new porosity and the spread of dilatant pathways have been related to the advective flow of gas in bentonite [2]. Understanding of the processes and mechanisms involved is a key aspect when evaluating the impact of gas flow on repository design and construction of any future facility. A series of gas injection tests on compacted bentonite were carried out at the British Geological Survey within DECOVALEX-2023 (D-23) Task B: MAGIC and LASGIT. The project focuses on the development of new numerical techniques for the quantitative prediction of gas flow in repository systems.

DECOVALEX-2023 Task B started with Modelling Advection of Gas In Clays (MAGIC). The planned approach of the task includes different stages, with an initial conceptual model development phase. In this work, the D-23 MAGIC has been studied based on the same methodology as previously applied in the work for DECOVALEX-2019 Task A - stage 1A (ENGINEER) using the same material properties and modelling strategy [3, 4]. Based on the actual dimensions of the sample, the 3D finite element model includes the geometry of the sample, the injection rod, the shape of the injection tip, the back pressure and the two filters associated to the location of the sensors. Hexahedral elements have been generated utilizing the CODE_BRIGHT software [5], and the random heterogeneity have been defined considering 3 different materials (M1-M2-M3). Layer-by-layer random permeability distribution was assumed (see Figure 1) applying 2/3–1/6–1/6 weighting for intrinsic permeabilities (k_i) equal to 1×10^-21, 1×10^-20 and 1×10^-19 m², for materials M1, M2 and M3, respectively, as well as incorporating different embedded fracture parameters.

LASGIT (LArge Scale Gas Injection Tests modelling) is a full-scale demonstration experiment operated by SKB at the Äspö Hard Rock Laboratory Hard Rock Laboratory at a depth of 420 m [6]. The 3D finite element model geometry is generated according to the real dimension of the test. Tests measurements included pressure and rate of gas inflow, gas outflow volume and pore pressure observed at various points of the sample. The gas test 1 of the LASGIT experiment is modeled in this study using a multiphase flow in porous media approach with the aim of facilitating the creation of favorable gas migration routes assuming permeability heterogeneity (Figure 1(b)), the same strategy as the MAGIC random permeability distribution outlined above. In this case, a coupled hydro-gas 3D FEM numerical model has been developed by the UPC/Andra research team to simulate the gas flow tests using the computer software CODE_BRIGHT, including initial permeability heterogeneity throughout the model. The numerical formulation also includes embedded fractures. The proposed model has been able to reproduce satisfactorily the observed behavior of the test measurements.

Discrete particle methods for granular bentonite material simulation

Joel Torres-Serra — 2023-10-02

Bentonite clays are strategic materials to ensure the sealing capacity of engineered barrier systems (EBS) in deep geological repositories of high-level radioactive waste. Formerly installed in the form of compacted blocks, in recent years, there has been growing interest in EBS design using granular bentonite materials (GBM) due to improved construction processes [1]. GBM range from the powdered and granulated forms, both natural and obtained from crushed blocks, to compressed bentonite pellets, and the mixtures thereof. The effect of thermo-hydro-chemo-mechanical (THCM) conditions on the evolving gap-filling performance following installation has been investigated in full-scale experiments [2] and in laboratory-scale tests [3], with particular focus on bentonite pellet-powder mixtures [4, 5, 6].

Numerical modelling of coupled THCM expansive soil behaviour is widespread using continuum methods, namely the finite element method [7, 8], applicable up to full-scale systems even though at the expense of resolution. At the laboratory scale, discrete particle-based methods accounting for the granular nature of bentonite materials in the early stages of hydration provide insight into pellet shape and local effects on the mechanical behaviour of dry pellet-powder systems as well as hydro-mechanical effects on the swelling capacity of GBM systems.

The compressibility of dry pellet packings and pellet-powder mixture samples is modelled by the discrete element method (DEM). This method has been applied to study the swelling behaviour of pellets represented by spherical particles [9]. The DEM is a powerful tool for reproducing complex granular shapes by non-spherical particle surfaces [10] and also with clumps of spheres (Figure 1(a)), allowing to define mixed material and mechanical contact properties within the pellets. The deformation and fissuring of pellets under oedometer compression is tracked using a cohesive particle model [11], including damage for interparticle contacts in tension, which captures deformation associated phenomena such as surface spalling and pellet reorganisation occurring at stress levels below the diametral fracture. In addition, bentonite powder is represented by loose packings of upscaled spheres matching

the experimental porosity of the fillings, interacting by a classical linear spring-dashpot contact model resulting in the reduced overall compressibility observed in pellet-powder mixtures [5].

Flow through GBM systems is modelled by the new discrete particle-based XMm method [12]. In the XMm, the material is discretised into a set of bentonite units (BU) or particles acting as representative elementary volumes of a homogeneous continuum inside which the mass balance equations are solved at the megastructural (X), macrostructural (M), and microstructural (m) pore levels (Figure 1(b)). Moreover, the mechanical compatibility of the system is obtained from the coupling between strain increments and centroid displacements in each BU, where the contact stress and overlap between neighbouring particles are updated similarly to the DEM approach. The hydro-mechanical problem is posed as a set of ordinary differential equations (ODE) to be solved for each BU, and thus the efficiency of the XMm is a compromise between the typically reduced computational cost of ODE solvers and the number of BU considered. The XMm has been used to simulate the hydration of a powder sample with low heterogeneity [4], successfully capturing the evolution of the displacement field upon saturation in isochoric conditions. This method is applied to reproduce the confined hydration of a heterogeneous pellet-powder system [6]. Assuming 2D axisymmetry, the XMm allows defining different initial material properties in each BU introducing the heterogeneity of the sample. The evolving swelling pressure is reproduced as well as the hydration velocities at the different structural levels, showing the decreasing heterogeneity of the mixture.

Studying water infiltration on bentonite/sand blocks with MRI and X-Ray μCT techniques

Pablo Eizaguirre — 2023-10-02

Radioactive waste management agencies all around the world are focusing their efforts to provide a reliable technical solution to the Deep Geological Disposal (DGD) concept, in order to store high level (HLW) and intermediate level long lived (IL-LLW) radioactive waste. The protection and isolation of radionucleides is guaranteed by a set of barriers. Bentonite/sand blocks -in proportion of 40/60 on dry mass of the mixture- constitute the main core of the sealing barrier in the French concept run by the French National Radioactive Waste Management Agency (Andra). Its high swelling potential (>2MPa) during hydration, its low hydraulic permeability (<10^-11m/s) and its gas entry pressure (<2MPa) make it ideal to seal the DGD galleries excavated inside the host rock used as geological barrier. Bentonite/sand blocks are placed at unsaturated state during construction. Afterwards, water migration –in both liquid and vapour states- from the saturated host rock to the blocks will induce the humidification of the blocks until hydraulic equilibrium is reached, hypothetically reaching the saturation state. As this natural process will take hundreds of years, a multitude of experimental devices have been reported at laboratory [3, 5] and real scale [4] to understand the evolution of water transfer in bentonite mixtures and support the predictions of numerical models [1]. In laboratory experiments, samples made of a compacted bentonite mixture are usually placed inside a rigid cell connected to a water source. The infiltration of water is not measured directly but deduced by relative humidity gradients measured along the specimen height.

Numerical models and experimental results have not yet reached accurate predictions of how long it takes for bentonite mixtures to hydrate until complete saturation or how the unsaturated hydraulic permeability evolves. In this context, there is a motivation to study this phenomena by advanced techniques able to capture, image and quantify water transfer through porous media. Thus, an experimental campaign based on Magnetic Resonance Imaging (MRI) and 3D X-Ray micro computed tomography (µCT) is proposed to analyze water infiltration in bentonite/sand blocks. MRI is extensively used in health sciences to create images where the contrast is given by the water’s hydrogen proton content in the different tissues of the body. For geological media, where water content is much smaller than in biological tissues and relaxation times of the porewater are shorter -from 0.1 to 2 ms for compacted bentonite/sand [2]- the experimental requirements are different and less documented. On the other hand, µCT scanning provides high spatial resolution images based on the contrast between the X-Ray absorption properties of the constitutive phases observed. Its temporal variations, in a bentonite/sand mixture, might be the result of concurrent phenomena, namely, porosity and saturation changes together with heterogenous motions induced by hydromechanical couplings -the swelling of bentonite solids when they absorb water-.

On going MRI and µCT experimental campaigns on a PEEK cell aim at proving the suitability of these techniques to track the evolution of water infiltration in compacted bentonite/sand mixtures, characterizing the water front and the water content along the specimen in function of time. At this stage, preliminary MRI tests have shown the potential of Single Point Imaging (SPI) sequence to quantify, with one-dimensional images, the water content along the bentonite/sand specimen based on a linear correlation with its signal intensity. Preliminary X-Ray radiography tests (Figure 1) run on a cylindric cell of PMMA filled with small blocks of 12 mm in diameter and 10 mm high, prove that X-Ray radiography image analysis gives access to observable and quantifiable increase of absorption associated to the measured decrease in grey level at different transversal sections of the sample. This evolution is linked to the increase of apparent density of the solids -that indeed are absorbing the water-, as described by the Beer-Lambert equation. Even though apparent density increase is goverened by the augmentation of water content, changes in porosity due to the hydromechanical coupling along the sample cannot be neglected. Thus, observing the liquid phase evolution (hydraulics) by MRI, as well as the evolution of the solid/liquid phase (hydromechanics) by 3D-µCT imaging, will potentially provide complementary insight into the mechanisms governing water infiltration in bentonite/sand mixtures.

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Nandini Adla — 2023-10-03

Compacted bentonites are often considered as the buffer material in constructing Deep Geological Repositories (DGR). Bentonite buffer exhibits volume expansion when it comes in contact with water. During saturation, the bentonite swells and generates swell pressure. In a DGR, the swell pressure of bentonite buffer gets generated when the voids formed due to temperature fluctuations are sealed due to closure of voids [1]. The temperature changes in the DGR alter the important hydraulic characteristics of compacted bentonites (i.e., permeability and water retention). The permeability and water retention parameters are crucial to anticipate the hydration rate of the buffer and resulting changes in the swell pressure of the buffer [2]. So, the mechanical response of the buffer (i.e., swell pressure), which is crucial for the performance assessment of the buffer, needs to be investigated by considering temperature-induced changes on the water retention behavior. So far, many numerical models have been developed to capture the swell response of bentonite [1,3]. But very few studies considered the changes in hydraulic parameters of soil subjected to different temperatures and, thereby, their effect on the mechanical behavior of the buffer [2]. Therefore, in the current work, a hydro-mechanical model was developed to assess the swell pressure of the buffer by incorporating the temperature influence on the water retention behavior of bentonite. The simulations were performed for three different constant temperature cases of 20^oC, 40^oC and 60^oC.

The governing differential equations of the Hydro-Mechanical (HM) model were acquired from the past study to investigate the influence of temperature on bentonite buffer’s swell pressure [1]. Richard's fluid flow equation was utilized for capturing the system of two-phase fluid flow (i.e., water and air) in the soil pores. The primary variable in Richard's equation is fluid pressure, and the secondary variables are the degree of saturation and permeability of fluid. The negative pressure head from the in Richard's equation indicates the suction and positive pressures head implies pore pressure. The van Genuchten relative permeability model was adopted to relate the suction and degree of saturation. The Extended Barcelona Basic Model (BBMx) elasto-plastic mechanical constitutive model was coupled with the changes in the suction with the fluid flow [3]. In this formulation, the hydraulic and mechanical processes are coupled by updating the suction values in the mechanical constitutive model at each time step.

The present work considers a 2D axisymmetric geometry with dimensions of 0.05 m (Diameter) and 0.012 m (Height) [2]. Febex bentonite compacted at dry density of 1.65 Mg/m³ was considered for the simulations under constant volume conditions (i.e. confined in all the directions with roller boundaries). The fluid flow in the sample occurs due to atmospheric pressure condition at the bottom boundary, whereas the other boundaries were under no flux condition. The mechanical parameters of the BBMx constitutive model were calibrated from the experimental study of Lloret et al. [4]. However, the temperature influenced hydraulic parameters were evaluated from the experimental investigations of Villar & Lloret [2]. The initial suction of the samples subjected to constant temperatures of 20, 40 and 60^oC are 170, 160 and 130 MPa respectively [2]. From the experimental observations, the soil-water retention curve parameters of the compacted bentonite samples at all constant temperatures were determined. Thereby, the model explicitly incorporated the effect of temperature on the swell behavior of bentonite from the altered water retention parameters. The simulations of all three test cases were performed for the time duration of 30 days. The swell pressures were reported as the surface average values over the sample geometry. The current model did not consider changes in the viscosity and density of the water with temperature and the temperature induced strains.

The calibrated van Genuchten fitting parameters (α – fitting parameter and n – pore size distribution parameter) at different temperatures are detailed in Table. 1. From the study, it was observed that, at high temperatures, the bentonite's water retention parameters were altered (α and n). The van Genuchten fitting parameter (α) was higher for the sample subjected to 60^oC, indicating a lower air entry pressure compared to the sample subjected to 20^oC. Also, a higher value of n for 20^oC sample represents a narrow pore size distribution. Based on the pore size distribution parameter (n), the sample subjected to 60^oC allows a higher water volume into its pores compared to the sample exposed to 20^oC.

After performing the HM simulations with the temperature dependent water retention parameters, it was noted that the variation in temperatures affected the maximum swell pressures of bentonite. The capillary pressure decreased with increased buffer saturation for all temperatures. However, a lower value of suction was observed for higher temperature induced sample (i.e., 60^oC) at a particular saturation compared to other samples. The lower capillary pressure at higher temperatures could be substantiated from Tang and Cui’s past study [5], where interfacial tension decreases as a result of increase in temperature. Because of the lower suction at higher temperatures, the maximum swell pressure was observed to be higher for the sample exposed to 60^oC (Fig.1). In addition, as the sample subjected to higher temperatures has lower value of n, it suggests that the soil has a broad range of pore sizes. The presence of broad range of pore sizes allows additional embedding of water molecules into the interlayers of bentonite. Therefore, the samples subjected to high temperatures adsorbs more water into the interlayers and there by repulsion of diffuse double layer resulting in high swell pressures (Fig.1).

Overall, the simulations suggest that for possible confined conditions inside a DGR, the effect of temperature on the hydro-mechanical response of the buffer depends on water retention behavior of buffer. Therefore, further modelling studies are needed on the water retention behavior of buffer at high temperatures (i.e., 80 ^oC to 200 ^oC) that are prevalent in DGR. These modelling studies will help in understanding the long-term performance of the buffer inside the DGR.

Diffusion of dissolved gases: from lab scale to in situ scale

Elk Jacops — 2023-10-02

Over the last decade, SCK CEN obtained a large set of diffusion coefficients for different gases in different Members of the Boom Clay. These diffusion experiments were performed in the lab on Boom Clay samples with diameter 80 mm and length 30 mm [1, 2]. When performing safety calculations, diffusion of gas is assumed to take place within an approximate domain over 100’s of meters. This led to the question: is the lab-scale diffusion coefficient also valid on a larger (meter) scale? In the past, a similar question was raised for more classical tracers such as HTO and iodide, and several in situ diffusion experiments lead to the confirmation of lab results. Hence, SCK CEN, ESV Euridice and ONDRAF/NIRAS will perform a new in situ diffusion experiment with dissolved neon in the HADES URL with main objective to confirm/improve the current knowledge on diffusion of dissolved gas at a large scale. To perform an experiment under the most relevant conditions for the Boom Clay, i.e. without being perturbed by the presence of an excavation-damaged zone, the MEGAS setup (drilled in 1992) is re-used. The MEGAS setup consists of four piezometers in a 3D configuration. The most suitable location to inject dissolved gas is filter 17. Monitoring will take place in filters 18, 9 and 22, which allows assessing the anisotropy of diffusion (Figure 1 left).

In a geological disposal facility, hydrogen is the main gas that will be produced. Performing experiments with hydrogen is complicated due to microbial conversion of hydrogen into methane. Therefore, this experiment will make use of neon, the best proxy for hydrogen. The principle of this in situ gas diffusion experiment is similar to the experiments in the lab: gas (neon) is dissolved in water, and the water with dissolved neon is circulated over a filter that is in contact with the clay. After some time, dissolved neon will be present in the water in the monitoring filters. By circulating this water with dissolved neon through a vessel with a non-interfering gas phase (e.g. helium), an equilibrium reaction will occur and neon will be present in the gas phase of the monitoring vessel. The monitoring vessels are connected to a gas analyser that performs on line analysis of the neon concentration. By measuring the concentration increase of neon over time in the monitoring vessels at the different filters, the diffusion parameters can be determined. Note that the initial pressure in each vessel will be equal to the in situ pore water pressure, to minimize disturbing the in situ hydraulic field. A summary of the experiment is available from [3].

Prior to the start of the diffusion experiment with neon, and in order to better understand the performance of the setup and the processes in the host formation during the experiment, a trial in-diffusion experiment with helium was performed in filters 8 and 21. Both filters will not be involved in the neon through-diffusion experiment, but the processes and conditions around these filters during in-diffusion tests are considered relevant for the through-diffusion experiment. Two circuits are filled with 50% synthetic pore water and 50% helium at an initial pressure equal to the in situ pore water pressure at respective filters. Due to in-diffusion of helium in the host rock, the pressure in the circuits decreases over time, which in turn induces water inflow to the circuits and a subsequent increase of circuit pressure. The gas and water phase balance in the circuit is therefore dynamically adjusted by the whole system, until a steady state is reached.

A numerical model was set up in COMSOL to predict pressure variations in both circuits. The diffusion coefficient of helium measured from the lab-scale diffusion test was used. The hydraulic conductivity of the clay comes from in situ permeability tests around these filters. If water flow is not considered in the model, the circuit pressure decrease of filter 21 follows the blue curve as shown in figure 1-right. The red curve gives the circuit pressure decrease considering diffusion of dissolved neon in clay as well as water flow under water pressure gradient. The three-month measurements follow well the red curve, which means that water inflow to the circuit plays a significant role in adjusting the circuit pressure. The good agreement between model results and field measurements confirms our lab scale test results. It also implies the good understanding of the in situ experimental system, which lays a good foundation for the long-term neon through-diffusion experiment afterwards.

Multiphase flow gas transport in a deep geological repository

Alireza Hassanzadegan — 2023-10-03

Gas transport in porous media is of interest in many industrial applications, such as the oil and gas industry, geological storage, and deep geological repositories for radioactive waste. In a deep geological repository, gas will be generated due to the corrosion of metallic components and the degradation of organic materials. This leads to a build-up of gas pressure, which may activate gas transport through the host rock as well as the excavation-damaged zone around backfilled galleries.

In order to understand different transport mechanisms involved, numerical simulations were performed, and the results were compared with laboratory data. In the framework of the EURAD/GAS project, a gas pressure-dependent permeability model was implemented into the finite element code OpenGeoSys-6 (OGS-6) [1]. The permeability alteration in this model is a function of gas pressure. The laboratory experiments showed that the rate of permeability change are different at low and high gas pressures. Therefore, the permeability model employed a threshold pressure ( to categorize this behaviour.

and are empirical parameters. Moreover, two other permeability models were employed to study the hydro-mechanical behaviour of the host rock and permeability changes. In the strain-dependent permeability model, the permeability change was related to the elasto-plastic behaviour of the host rock, and in the embedded fracture model, it was related to the opening and closure of fractures [3] [4]. Thus, volumetric elastic strain and equivalent plastic strain are employed to be the controlling variables.

The initial permeability of the intact rock samples were determined by applying a constant pressure at the upstream and downstream of the samples (i.e. constant pressure gradient). The imperical parameters were determined by matching experimental and numerical results.

Two types of gas injection tests carried out by the Institute for Rock Mechanics (IFG GmbH, See Figure 1) were used to investigate the gas transport through Opalinus Clay and to examine the permeability models [2]. The first experiment demonstrates an advective-diffusiion gas transport through the sample and an elastic deformation. The second experiment highlights the formation of a tensile fracture (plastic deformation and preferred flow path). In both experiments the advective transport is the dominant transport mechanism. The strain-dependent permeability model was successfully applied to reproduce the hydro-mechanical behaviour of the host rock in both elastic deformation test and tensile fracture test (see Figures 2 and 3). The hydro-mechanical response of a saturated single phase flow model was compared with the behaviour of a saturated two-phase flow model. Both single phase and two-phase flow models were able to describe the hydraulic as well as mechanical behaviour of the experiments performed. Therefore, one can conclude that in these experiments the water phase (wet-phase) was immobile.

A gas injection test under triaxial conditions was performed by École Polytechnique Fédérale de Lausanne (EPFL) in saturated Opalinus Clay (Fig. 4). The numerical simulations reproduced the hydro-mechanical behaviour of the sample during the gas injection test. A two-phase flow model was applied to simulate the experiment. The relative permeabilities and capillary pressure functions followed Mualem approach and van Genuchten formulation, respectively. The outflow volume and mechanical response of the sample were measured. The experimental results were in good agreement with the numerical ones. The results of the modelling illustrated the penetration of gas into the sample and hence, the displacement of water (see Figures 5 and 6).

Numerical simulations of thermal fracturing in nuclear waste disposal

Fei Song — 2023-10-02

Regarding high-level nuclear waste (HLW) and spent fuel (SF) disposal, underground geological disposal is likely to be the most feasible and effective solution [1]. The coupled Thermo-Hydro-Mechanical (THM) behaviour of host rocks has been widely studied by laboratory tests, in-situ experiments and numerical analyses. In field experiments performed in underground laboratories, pore pressure generation and damage evolution have been observed. In this context, coupled THM numerical analyses can be an essential tool to better understand the coupled multiphysics behaviour of host rocks, buffer materials and canisters. In addition, large-scale field experiments provide a crucial opportunity to demonstrate understanding and to assess the predictive/modelling capabilities under realistic conditions.

A number of hydromechanical (HM) numerical simulations of underground excavations have been carried out and the mechanisms of pore pressure and damage evolution have been studied [2]. Also, numerical analyses have been used to predict the observations of THM behaviour of in-situ heating tests, and there is some research on the damage evolution due to thermal loading [3], but few studies focus on the analysis of strain localization problems, in part due to specific numerical difficulties. However, thermal fracturing behaviour has indeed been observed in experiments and a better understanding of the phenomenon and of shear band evolution is crucial for a proper assessment of the stability of HLW/SF disposal.

In this study, the coupled THM response of a generic HLW disposal facility under high-temperature has been analysed; in particular, the mechanism of thermal pressurization and the corresponding thermal fracturing behaviour have been examined. For this purpose, an advanced elastoplastic constitutive model with damage has been used to represent the mechanical behaviour of the host rock; it incorporates the following features: (1) a hyperbolic approximation to the Mohr-Coulomb yield surface to represent shear and tensile failure modes, (2) stiffness and strength anisotropy, (3) hardening and softening behaviour, (4) viscous effects and (5) a non-local integral type approach for the plasticity component of the model. The adopted overstress theory and the nonlocal formulation are capable of regularising the spatial distribution of plastic strains, eliminating mesh dependency and improving numerical efficiency. The coupling between accumulated plastic strain and hydraulic permeability has been considered in the analyses. Note that the term damage used in this study, refers to a state of the material, instead of the concept associated with damage mechanics theory; in addition, fracture represents localization phenomena in the damaged area.

Numerical simulations have been performed using the finite element method software CODE_BRIGHT. Two-dimensional generic disposal configurations have been carried out for the two cases of unsupported and supported tunnels. The host rock is assumed to be the Callovo-Oxfordian claystone (COx). Three modelling stages have been considered: the excavation stage, the waiting stage and the heating stage, as shown in Figure 1 (a). In the heating stage, the thermal flow at the borehole wall is set as a constant power (200 W/m) and a no hydraulic flow boundary condition is prescribed. A maximum temperature of around 80 °C has been obtained on the drift wall at heating time t = 10 years.

In addition, Figure 1 (b) shows the contours of plastic shear strains and liquid pressures at heating time t = 10 years. Near-field damage is caused by both the excavation of galleries and the effect of thermal pressurization. In the simulations presented, shear bands develop significantly during the heating stage. By comparing two cases in Figure 1 (b), it can be concluded that the installation of liners can efficiently limit the damage evolution and reduce the likelihood of thermal fracturing, significantly improving the stability of HLW disposal. Note that the scales of the two cases in Figure 1 (b) are different. Figure 1(c) shows that a lower pore pressure close to the gallery wall for the unsupported tunnel due to the higher level of damage in this case that results in a larger permeability of the rock in the damaged area.

Apart from the analysis of the mechanism of THM behaviour in a high-temperature environment, numerical simulations have also been carried out of the large-scale PRACLAY Heater test (performed in the HADEs underground laboratory) and of the CRQ heating experiment (performed in the Meuse-Haute Marne underground laboratory). A good agreement has been observed between numerical predictions and experimental data, in terms of both temperature and pore pressure. These numerical validation exercises are, however, outside the scope of this abstract.

Numerical modelling of a large-diameter sealing structure in a deep radioactive waste repository

Matias Alonso — 2023-10-03

There is a consensus that deep geological disposal is one of the most appropriated solution to store radioactive waste, and that argillaceous rocks have great potential as possible geological host formation. In this context, The French National Radioactive Waste Management Agency (Andra), is leading the design of a deep geological radioactive waste repository to be located in the Callovo-Oxfordian claystone (COx), at about 500 m depth (Cigéo project).

Radioactive waste disposal aims to protect the environment over a period of several thousands of years. For this purpose, these facilities are designed following the multi-barrier concept, in which engineered and natural barriers are combined, each one providing a degree of safety to the overall system [1]. The long-term safety of Cigéo relies on the COx claystone, the natural barrier that plays the main role, and also on the so-called sealing structures. These structures, installed after the operational phase at some key positions in shafts, ramps and horizontal galleries, are intended to limit the flow of water and the migration of radionuclides to the biosphere, ensuring the post-closure safety over the entire storage life.

Excavation of underground drifts generally causes damage to the rock around the openings [2]. Controlling the preferential pathways of groundwater through the fractures generated by excavation is a key objective related to the long-term safety of a radioactive waste repository. The sealing structures should be designed not only to seal the drifts so that they do not become preferential radionuclide migration pathways, but also to reduce the permeability of the excavation-induced damaged zone (EDZ) to a low value, re-creating conditions similar to those of the intact host rock.

Andra's current sealing concept for large diameter galleries (~ 10 m) is based on the installation of a confined expansive bentonite-based sealing core occupying the entire cross-section area (Figure 1(a)). The expansive core will be confined by two concrete plugs placed at both ends and the remaining section of the gallery will be filled with a backfill consisting of disaggregated and recompacted excavated COx. Swelling under confined conditions leads to the development of a swelling pressure that the core exerts on the surrounding materials. In this way, as the sealing core is saturated, the EDZ will be gradually compressed by the increasing swelling pressure, favoring the self-sealing of existing fractures, and leading to a gradual recovery of the rock low permeability [3]. The sealing core will also exert a pressure on the concrete plugs. As a result, the plugs will slide, compressing the backfill and releasing some of the swelling pressure in the core. The performance of this type of solution therefore relies on the development and long-term stability of the core swelling pressure, which is strongly related to the stability of the entire system.

Numerical simulation is a potentially useful tool for a better understanding of the behaviour of these engineered barriers. This work aims to assess numerically the phenomenology underlying the response and performance of large diameter sealing structures under real disposal conditions. The simulations presented address the complexity of the problem by considering large scale 3D geometries including the main components of the sealing structure (Figure 1(b)), realistic and advanced constitutive models, complex hydro-mechanical (HM) coupled formulation, and key geometric details at decimeter scale such as lining and interfaces between the different components.

Simulations were carried out with the finite element code CODE_BRIGHT [4]. Particular features of the models include:

a) advanced constitutive laws to capture the non-linear time-dependent anisotropic response of the COx claystone;
b) the development of the fractured zone around excavations;
c) the multi-scale expansive response of the bentonite-based core material.

Physico-chemical processes between bentonite and the nearby materials that could affect the long-term sealing performance, and gas pressurization due to degradation and corrosion of the support elements are not considered at this stage of the work.

These challenging simulations have provided qualitative and quantitative results on key aspects of the performance and long-term integrity of the current sealing concept in the Cigéo facility. From a practical point of view, the performance was evaluated considering the response associated with the capability of the core to reach full saturation at all points and to develop the target swelling pressure as uniformly as possible, the capacity of the stabilizing components (plugs and backfill) to provide long-term stability, and the capability of the sealing core to recompress the EDZ. The influence of the plug-lining interface shear strength was particularly examined.

Results show that the sealing core reaches a full saturation state around 2000 years after its construction, and that final hydro-mechanical equilibrium with hydrostatic conditions and the supporting elements is reached after an additional 1000 years. During this period, the plugs move around 0.5 m in the longitudinal direction due to the pressure exerted by the expansive core. As a result, the expansive core density and swelling capacity reduce due to deconfinement (Figure 1(c)). Despite this, the central part of the core maintains a swelling pressure within the admissible design range: the value of radial swelling pressure obtained is around 3 MPa (≈ 75% of the target swelling pressure). During the post-closure phase, the EDZ is recompressed by the swelling of the core and part of the initial stresses are recovered.

Experimental investigation on infiltration behavior of sodium ion in compacted bentonite

Guodong Cai — 2023-10-02

Because of its high swelling capacity and low hydraulic conductivity, bentonite has been selected as the candidate buffer material in deep geological disposal project for handling high-level radioactive waste. In Japan, seawater infiltration is an important issue in the evaluation of buffer material, as disposal facilities are under discussion for siting in coastal areas [1]. Sodium ion, which is the main ion in seawater, has profound effects on the properties of bentonite, such as hydraulic conductivity and swelling behaviours etc. [2][3]. These changes may be closely related to the infiltration behavior of sodium ion in bentonite. T In this study, for observing the infiltration behavior of sodium ion in compacted bentonite, infiltration tests were conducted on compacted bentonite specimens with thickness of 2 mm, where the specimens were wetted by NaCl solutions with a concentration range of 0 to 1 mol/L. In the testing program, wetting time, specimen dry densities and initial water contents were varied to investigate their effects on sodium ion infiltration. After the infiltration test, final water contents of specimens were measured and leached cations were obtained using benzyltrimethylammonium chloride (BTM) method [4].

Figure 1 shows the results of leached cations and final water content of specimens with 1.5g/cm³ initial dry density for different wetting time. It is found that under the 0.1mol/L NaCl solution case, there is almost no sodium ion infiltration in compacted bentonite with a dry density of 1.5g/cm3 as shown in Figure 1(a). For the case of 0.5mol/L and 1mol/L NaCl solution, the leached sodium cation from specimens increases with the increase of concentration of supplied NaCl solution under the same wetting time. For other exchangeable cations (i.e., Mg⁺, Ca²⁺ and K⁺), there is no appreciable difference among the varieties of NaCl solutions and wetting time. For the final water content measurement results, it is found from Figure 1(b) that the water content increasing speeds in 0.5mol/L and 1mol/L NaCl solutions are significantly faster than that in 0.1mol/L NaCl solution.

A multi-scale model to study gas transport processes in clay materials

Gilles Corman — 2023-10-03

In the field of radioactive waste confinement, the question of gas transfers in clay formations is a crucial issue [1]. A certain amount of gas, such as Hydrogen may be generated during the exploitation phase in the nearfield by the deterioration of the metal components of the system. Since the host medium is characterised by a very low permeability, the mechanisms of gas transport by advection and diffusion within the groundwater remain insufficient to evacuate the gas generated and a free gas phase is formed. If the gas pressure keeps increasing and reaches the minimum principal stress acting on the rock, micro-fractures, known as preferential gas pathways could develop through the rock mass [2], and affect the clay barrier integrity.

There is a growing body of experimental evidences [3, 4] that separation planes such as bedding planes or pre-existing fractures and heterogeneities in clay-rich materials represent preferred weaknesses for the process of opening discrete gas-filled pathways. Capturing the related transport mechanisms therefore requires to go from macroscopic to microscopic scale. In this context, the adopted numerical approach would ideally take into account the effect of each constituent of the material microstructure on the macroscopic gas flow. Yet, direct modelling of the entire microstructure using small-scale models is usually not possible due to the huge computational costs it would require at the scale of a repository. Conversely, indirect modelling of the behaviour of all the micro-constituents by collective closed-form macroscopic constitutive equations using large-scale models [5] has also limits in terms of assumptions formulation and parameters identification. Hence, the use of a multi-scale approach that models the micro-scale effects explicitely on their specific length scale and couples their homogenized effects to the macro-scale is proposed in the present work. Based on a periodicity assumption of the microstructure, the physical and geometrical properties of the microstructure are embedded on a Representative Element Volume (REV) which contains a detailed model of the microstructure constituents, i.e. the pore network, the bedding planes and the bridging planes, while eluding a complex description of the whole microstructure over the domain. From this local description of the material, the macroscopic response to loading can be derived using numerical homogenisation techniques [6].

The multi-scale model proposed here has been implemented in the finite element code Lagamine, and validated against a well established and documented macro-scale THM coupled model [7]. The size and structure of the REV comes from experimental data acquired from scanning images in clay material, like for instance the Boom Clay formation which is envisaged as a potential host rock for a deep geological disposal in Belgium. Such scans reveal the opening of large aperture with a repeated distance in-between, which make it possible to extrapolate a physical idealisation of the microstructure (Figure 1a), built with one of these fractures corresponding to the bedding planes (red) and the pore network substituted by an assembly of tubes (green). Narrow-aperture fissures, known as the bridging planes (blue) were also identified to contribute to the flow normal to the predominant direction of fissure [4], by connecting the natural discontinuities which were initially closed. These micro-scale constituents are defined in such a way as to satisfy the conditions of pore size distribution, macroporosity and intrinsic permeability. For the mechanical behaviour, a simple linear elastic constitutive law was introduced for the tube network while a non-linear stress-strain relationship was used for the interfaces defining the bedding and bridging planes. Concerning the hydraulic behaviour, gas diffusive fluxes were considered and modelled by a Fick’s law while a channel flow based on the Navier-Stokes equations was used for the multiphase flow in the different constituents.

The model has been subsequently applied to simulate a gas injection test parallel and perpendicular to the bedding of initially saturated samples of Boom Clay [4]. This analysis provides a rather good agreement with the experimental results in terms of injection and outflow pressure response, outflow volume and average axial strain along the sample height. In addition, it allows to simulated the creation of a preferential flow pathway along the sample axis (Figure 1b, top), which serves as basis to numerically reproduce the development of random pathway through the sample in plane strain state (Figure 1b, bottom), and aims to improve the mechanistic understanding of the gas transport processes at play in clayey barriers [8].

Effectiveness of self-sealing after gas transport in Boom Clay

Laura Gonzalez-Blanco — 2023-10-03

The study of gas transport in low permeable materials is becoming a significant focus in energy-related geotechnics, particularly for managing deep geological disposal of long-lived and heat-emitting radioactive waste. Argillaceous rocks, studied to host the disposal, may present induced fractures caused by the excavation activities that increase the permeability to liquid and gas. The generation of gases can also lead to an excessive pressure build-up in these saturated media resulting in the development or reactivation of fractures/fissures creating preferential pathways for the gas flow [1]. Nevertheless, these rocks present the advantage of self-seal (via swelling of clay minerals due to re-saturation, consolidation or creep), reducing fissure permeability and potentially restoring the barrier function [2].

This study focuses on Boom Clay, a Cenozoic clay candidate for hosting the repository in the Belgian programme, aiming at characterising the effectiveness of its self-sealing process after gas injection/dissipation tests due to the swelling of clay minerals during re-saturation. This poorly indurated rock presents sedimentary bedding planes that can act as preferential pathways during the gas invasion. In a previous experimental campaign, samples at two bedding orientations (parallel and orthogonal to the flow) were tested under oedometer conditions [3, 4]. The results indicate that gas transport induced slight expansion of the samples in both orientations, increasing their intrinsic permeability, which suggested the opening of preferential paths. Furthermore, the analyses of the pore network using mercury intrusion porosimetry (MIP) and micro-focus X-ray computed tomography (µ-CT) confirmed the development of gas pathways following the bedding direction or interconnecting bedding planes [4, 5].

To assess the self-sealing capacity, oedometer tests were carried out at two bedding directions using an oedometer cell with lateral stress measurement to comprehensively understand the stress state and ensure that gas flow occurred through the sample rather than between the sample-ring interface. Prior to the gas injection stage, water permeability was measured. Subsequently, a gas injection/dissipation stage was performed at constant vertical stress. Immediately after, the sample was placed in contact with synthetic water to allow re-saturation. Finally, the water permeability was measured again. The self-sealing capacity of the clay was assessed by comparing the water permeability at both stages. If the obtained values are similar, the barrier function has been restored. Some tests also included a second gas injection to evaluate the potential pathway reopening. MIP and µ-CT data allowed for evaluating microstructural changes due to these processes.

The experimental results demonstrate that gas can flow at pressures lower than the minor lateral stress, and the computed effective gas permeability is always higher than the initial intrinsic water permeability, pointing to the opening of preferential pathways (Figure 1a). Water permeability after re-saturation showed values comparable to the initial intrinsic permeability, recovering the hydraulic barrier function thanks to the clay minerals’ swelling (Figure 1a). This indicates the good self-sealing capacity of the Boom Clay. The gas permeability calculated during a second injection/dissipation stage is slightly higher than the initial one, suggesting some memory of the previously opened path (Figure 1a). The self-sealing effect was also observed in the CT images performed under unstressed conditions after water-undrained unloading of the tested samples. The fissures detected after the gas injection (Figure 1b left) are no longer visible after re-saturation within the technique resolution (fissures > 40µm) (Figure 1b middle). However, a small proportion of large and disconnected pores were identified in CT images, likely due to some gas exsolution. Regarding the subsequent gas injection, it again led to the development of fissures following the bedding direction detected with microstructural techniques (Figure 1b right).

Air and hydrogen injection tests on saturated compacted bentonite

Markéta Kučerová — 2023-10-03

The concept of the Czech deep geological repository for radioactive waste, as in most other countries that are planning such projects, currently envisages the use of compacted bentonite for the sealing layer around the waste disposal package emplaced in the disposal borehole. The concept considered involves the use of steel-based waste disposal packages. It is expected that gases (mostly hydrogen) will form as a result of the corrosion of the waste disposal package, via the radiolysis of water or due to the development of microbial activity; therefore, a knowledge of the gas transport mechanisms in bentonite is fundamental in terms of the safety assessment of the deep geological repository. It is known that gas flow in water-saturated bentonite occurs mostly via networks of pressure-induced dilatant pathways, a fact that has been supported by the results of numerous laboratory tests involving indirect observations as well as direct detection methods. The current state of the art with concern to gas transport processes was established via the EURAD project [1].

This article presents the work carried out by the CTU in Prague and ÚJV Řež as part of Task 2 of the GAS Work Package of the EURAD project. Since the Czech deep geological repository concept is considering the use of local bentonites, the research programme is focusing on Czech Ca-Mg bentonite (BCV) and its benchmarking comparison with foreign materials such as MX-80 sodium bentonite. Both the CTU and ÚJV laboratory testing programmes involve the performance of gas breakthrough tests on water-saturated compacted bentonite samples using similar methodologies and equipment. The objective of the research is to compare the gas breakthrough behaviour of Czech bentonite with other materials. Moreover, the key contribution of the research concerns the direct use of hydrogen as the gas injection medium for some of the tests, thus providing unique data for comparison with other gases that are typically used as a surrogate for hydrogen.

The basic component of the CTU test equipment comprises a constant volume steel cell which can be used for the determination of the hydraulic conductivity and swelling pressure of bentonite samples when connected to a water permeameter [2], as well as for gas injection testing purposes [3]. Homogeneous samples of the material are prepared via the direct compaction of powdered bentonite in a cylindrical steel chamber of 30 mm in diameter and 20 mm in height. The testing cell, which has been specially modified for the purposes of this project, consists of two pistons and two total pressure sensors positioned between the sample surfaces and the cell flanges measuring axial pressure hereinafter called as total pressure. Initial testing was performed via the injection of the gas into the centre of the sample using an injection needle with the aim of simulating a gas point source. However, due to gas leakage issues, a standard procedure was adopted for the rest of the tests consisting of injection into the base of the cylindrical sample through sintered steel plates. A total of 5 samples of BCV bentonite with dry densities that varied between 1300 and 1500 kg/m³ were prepared for the purposes of the project. All the samples were firstly connected to the water permeameter to ensure their saturation under a constant water injection pressure. The saturation level is checked by the monitoring of the water flow and the total (swelling) pressure. It usually takes several weeks to months to attain full saturation, which is confirmed by the attainment of stable water flow and total pressure values. The samples are then subjected to air injection tests involving incremental increases in pressure at the sample inlet applying small increments of, in most cases, 0.05 MPa until gas breakthrough (detected by gas outflow and decreasing injection pressure) is detected. The pressure at the outlet of the sample is maintained at the atmospheric level and the outflow is measured using a flow meter.

The ÚJV experimental apparatus is equipped with a similar constant volume cell that contains samples of 30 mm in diameter and 15 mm in height. The apparatus enables the use of both air and hydrogen as the injection media. One of the most important components comprises a GDS ELDPC pump that is connected to the outlet from the sample, which is used for the precise measurement of the volume of the fluid which passes through the sample. The pressure in the outflow pump is maintained at the atmospheric level. The procedure for the gas injection testing of fully water-saturated samples involves incremental pressure increases with increments of 0.2 to 0.3 MPa until gas breakthrough is achieved. The pressure is always increased at the moment at which the flow through the sample has stabilised. The initial injection pressure is, in all cases, set at a lower value than the expected swelling pressure of the sample. Following the completion of the gas injection test, the sample is dismantled and its porosity and water content are determined in order to verify whether the sample has been partially de-saturated by the injection of gas. Three different materials are being subjected to testing. In addition to a series of samples of BCV bentonite, samples of MX-80 bentonite have been tested and it is planned that Kunipia bentonite will also be tested so as to complete the data set. Three samples with a dry density of 1400 kg/m³ and three samples with a dry density of 1600 kg/m³ are prepared for each of the materials. One sample is used for air injection testing, the second for hydrogen injection testing and the third is used as the control sample for the determination of the water content and porosity directly following the saturation process.

The results of the CTU testing programme revealed that the time period necessary for the saturation phase for the BCV samples may be in excess of 5 months before the conditions stabilise. The gas injection test on the 1300 kg/m³ sample lasted 3 months, with gas breakthrough occurring at slightly below the measured total pressure. The gas injection tests on the 1400 kg/m³ and 1450 kg/m³ samples have been underway for several months and, in both cases, the injection pressure level has exceeded the total pressures without the detection of gas breakthrough.

The results of the ÚJV tests on the BCV and MX-80 series of samples demonstrated that the breakthrough events in all cases occurred at pressure levels that corresponded to the theoretical swelling pressure values for both air and hydrogen. The breakthrough events for the 1400 kg/m³ and 1600 kg/m³ samples were observed to be very close, with a maximum difference of 0.2 MPa. With concern to the 1400 kg/m³ MX-80 samples, breakthrough was registered at exactly same pressure level (3.9 MPa) as the theoretical swelling pressure as determined by Karnland et al. [4]. The difference between the breakthrough pressures for the 1600 kg/m³ MX-80 samples was more noticeable; the breakthrough with hydrogen occurred at 8.5 MPa and with air at 7.2 MPa. However, it must be noted that, based on known correlations between dry density and swelling pressure, the theoretical swelling pressure for higher dry densities lies in a broader interval and a large range of variability of material properties exists for different material batches. The results of the determination of the water content for the samples subjected to gas injection testing did not reveal any major deviations from the non-loaded samples and the theoretical full saturation values. This result corresponds to the general observation that gas flow through dilatant pathways does not result in the significant de-saturation of the samples, or the de-saturation is so minor that it cannot be detected by means of standard methods.

Modelling gas fracturing experiments in saturated clay using zero-thickness interface elements

Joaquin Liaudat — 2023-10-02

The injection of gas in a liquid-saturated clay-rich material may lead to the formation of narrow channels or fractures created by the mechanical action of gas pressure. However, the complexity of the mechanical and transport properties of clays, combined with the high compressibility of gases, makes it challenging to understand and address this phenomenon. Gas fracturing can occur in a range of engineered processes, for instance the stimulation of sensitive hydro-carbon reservoirs [1], carbon dioxide injection and storage in subsurface reservoirs [2], pneumatic fracturing for enhanced remediation of contaminated soils [3], or gas transport through natural and engineered clay barriers in geological disposal facilities for radioactive waste [4]. Despite the multiple environmental and economic implications associated with gas fracturing it remains difficult to predict and control due to the lack of fundamental scientific insight.

One of the most challenging aspects of understanding gas-driven cracking in clays is the difficulty in visualising the crack formation in real time. In recent years, a new experimental setup has been proposed by the British Geological Survey, which makes it possible to induce and observe the formation of “two dimensional” cracks in clay-rich low-permeability materials by the injection of gas or water [5,6]. In this setup, a thin layer (~1mm) of clay paste is compressed between a 110 mm diameter sight glass and a steel plate, while it is held laterally in place by a ring filter (Figure 1). Then, gas is injected at the centre of the lower platen at a controlled volumetric rate while the cracks induced are registered by a camera through the sight glass.

In order to aid the interpretation of the results obtained with this new device, the authors have carried out a series of numerical simulations performed with a finite element approach recently published [7], and compared them to experimental data. The model uses a fully coupled Pneumo-Hydro-Mechanical (PHM) formulation in 2D, and is implemented in the code LAGAMINE [8]. Two different types of finite elements are used: continuum and interface elements. Continuum elements are used to represent the mechanical and flow processes in the bulk clay material and interface elements are used to represent cracks. Interface elements are introduced a priori in between the continuum elements in order to provide potential cracking paths. The model parameters are such that, as long as the interface elements remain closed, they do not have any significant effect on the response of the model.

These simulations required two additional numerical developments. First, since the model is 2D (plane strain), the tangential, frictional stresses developed at top (clay-glass interface) and bottom (clay-steel interface) were introduced as a body force, using an elastoplastic formulation in terms of the in-plane displacements and the out-of-plane stress. Second, a mixed boundary condition was implemented to make it possible to impose a given volumetric gas injection rate as in the experiments.

The simulations show that the model proposed can reproduce the experimental results, providing valuable insights into the factors controlling the onset and propagation of the gas fractures (Figure 1). These insights have the potential to enhance the performance and safety of projects involving gas fracturing processes

An experimental study on the measurement of gas diffusivities in un-saturated clay based materials

Aadithya Gowrishankar — 2023-10-02

Gas migration in porous media in the context of nuclear waste disposal is dominated by diffusion. The availability of abundant data on diffusion coefficients of different gases in many clay based materials such as Boom Clay, Opalinus Clay and Callovo-Oxfordian Clays provides a good overview of the rates of gas transport through these different clay rocks [1]. Such formations are being considered as potential host rocks for the disposal of high and intermediate level radioactive wastes across Europe. Diffusion coefficients have been studied with particular interest due to the unavoidable gas generation phase in the nuclear waste repository [2]. However, all the diffusion measurements performed so far have been under fully saturated conditions, with little experimental data on gas diffusivities in partially saturated conditions, which have been hypothesized to exist during the early part of the life cycle of the repository [3]. The objective of the current study is to provide an overview of the effect of desaturation on the rate of diffusive transport of gases in partially saturated clay based materials.

The experimental methodology is based on the double through diffusion setup used in previous studies to measure gas diffusivities in saturated clays [1]. However, the setup is modified so as to keep a fixed level of unsaturation in the clay sample. This is done by employing the vapour equilibrium technique, using oversaturated salt solutions in both upstream and downstream reservoirs to maintain a constant relative humidity in the entire setup [4].

Preliminary pore network modelling has already shown that the gas diffusivity increases with a decrease in water saturation of the clay material. However, the predicted trend also suggests that this increase is only marginal and should not normally exceed an order of magnitude. This is shown in the figure below.

All the diffusion measurements are carried out on synthetic clay samples of dimensions 3.8 cm (diameter) and 2 cm (height). In Table 1, the Sw stands for water saturation. The compositions of the synthetic clay samples (60% clay, 20% sand, 20% silt) have been adjusted so as to most closely mimic the mineralogical compostion of Boom Clay. Synthetic samples are used instead of natural samples like Boom Clay because of their better physical response to drying than Boom Clay.

The selected control variable to lower the saturation of the material is suction, which is imposed by changing the relative humidity of the environment using oversaturated salt solutions [4]. Suction can be kept constant given constant ambient temperature and pressure. Thus, the clay sample is conditioned at a certain suction, which in turn leads to desaturation in the range of 70-100% of total saturation depending on the level of suction. For this study, the saturation range has been fixed in between 70-100% of total saturation. This range has been chosen because of what is expected in a real life nuclear waste repository. The first measured diffusion coefficients in two partially saturated samples are shown in Table 1. The initial measurements of gas diffusion coefficients in unsaturated synthetic clays show that the increase in diffusivity of helium is higher than that of argon. Helium is the second lightest known element and has an extremely small molecular size and hence the impact of desaturation on helium diffusivity could be more pronounced than the same for heavier gases.

Transient temperature and water distributions in compacted MX80 bentonite under high temperature gradients

Yu Lu — 2023-10-02

Numerous element-scale and full-scale tests have been carried out on compacted bentonite to investigate its coupled thermo-hydro-mechanical (THM) behavior when used as a buffer material in high-level nuclear waste geological repositories, such as the Full-scale Engineered Barriers Experiment (FEBEX) and its forerunner laboratory mock-up test [5]. In these past studies the applied temperatures were below 100°C, while new repository conditions being explored in the recently started HotBENT project at the Grimsel Test Site involve waste canister temperatures up to 200°C [2]. Recently, Lu and McCartney [3] conducted a tank scale test where a temperature of 200°C was maintained by a cylindrical heating element having a diameter of 12.5 mm at the center of a compacted MX80 bentonite layer. A notable temperate drop in temperature was observed with distance from the small diameter heating element. A similar phenomenon was also reported by Gens et al. (2020) for heating tests on MX80 pellets during heating to 140°C. Measurements from the FEBEX project indicate a more distributed temperature decay away from the heater [3]. This discrepancy between laboratory and field tests provided the motivation for better understanding the temperature and water distribution near a heater under high temperature gradients to better guide the design of laboratory-scale experiments in the future.

A one-dimensional heating test was performed in this study to investigate the coupled THM response of a MX80 bentonite layer in unconstrained conditions during basal heating. Measurements include the transient temperature and volumetric water content redistribution at different vertical distances from the flat plate heater under high-temperature gradients as well as global volumetric strains. Three dielectric sensors (Decagon 5TM) were placed into a compacted MX80 layer having an initial gravimetric water content of 12.2% and a dry density of 1.15 Mg/m³ within a PVC modified Proctor mold (inner diameter of 152 mm and height of 178 mm), as shown in Figure 1(a). Low-density polyethylene sheeting was stretched across the top and bottom of the soil layers to minimize any global loss of water from the compacted bentonite layer during heating. A temperature-controlled hot plate was used as the basal heat source, and a dial indicator was mounted on the top of the soil layer to measure volume changes. Fiberglass insulation was wrapped around the cell to minimize lateral heat loss from the side boundary. A basal temperature of 128°C was maintained during a heating stage lasting 4000 h to ensure THM equilibrium, after which three short-term (96 h for each) cooling-heating cycles were applied. Results indicate that soil temperatures increase rapidly in the first 20 h of heating then gradually stabilize (Figure 1(b)). The volumetric water content increases at the beginning of heating then slowly decreases (Figure 1(c)). This increasing-decreasing trend is due to the upward movement of a wetting front away from the heater due to coupled heat transfer and water flow. An initial increase in volumetric strain (swelling) was followed by a decrease (contraction) as heating progressed (Figure 1(d)). This is consistent with the observed thermal response of normally consolidated expansive soils (e.g., [4]). A sharp monotonic decrease in volumetric strain occurred during the three short-term cooling-heating cycles. The temperature and water content follow nearly identical trajectories in each cycle. The temperature distribution in the bentonite with distance from the heating element after 4000 h of heating is smoother than the sharp drop-off in temperature from the small-diameter heater observed by Lu and McCartney [3], as shown in (Figure 1(f)). The main implication of the results presented in this study is that the surface area of the heater in contact with the surrounding bentonite plays a major role in the temperature distribution and the associated coupled water flow. This is a critical finding that may help guide the geometric design of small-scale laboratory mock-up experiments used to study THM processes in controlled laboratory conditions.

Multi-scale investigation on gas transport behaviour of compacted granular bentonite under partially saturated states

Hao Zeng — 2023-10-03

Compacted bentonites with high swelling potential and low permeability are under consideration as buffer/backfill materials in deep geological repositories of radioactive nuclear waste [1]. Within the short-term service period, the bentonite undergoes progressive saturation developing the swelling pressure, due to the water received from the host rock. Since the progressive saturation first occurs at the boundaries near the host rock, the bentonite close to the fuel canister could remain partially saturated for a long time. Within the long-term running stage, bentonite will withstand the accumulation and transport of gases such as H₂ (mainly generated by the anaerobic corrosion of the metallic overpack of canisters) [2]. The accumulated volume of gas can produce a gas pressure build-up that could break through the bentonite, damaging its microstructural integrity and isolation capacity for radionuclides and contaminants [3]. This issue may be improved by adopting granular bentonite (GB) with an extended particle size distribution (maximum grain sizes of the order of mm) [4]. The compacted GB has a high initial macroporosity, which might permit gas transport at low pressures with less impact on the microstructure. However, up to now, few investigations have focused on the gas transport behaviour of partially saturated compacted GB within a multi-scale perspective.

To this end, gas injection tests were performed on partially saturated compacted MX-80 type GB at different hydro-mechanical states. Effective gas permeability was measured in samples at the as-compacted state and in samples prepared at the as-compacted state and then partially saturated under isochoric conditions without additional vertical stress apart from the generated due to swelling. Additionally, the tests were also conducted on these two types of samples but subjected to initial vertical stress of 3.8 MPa. X-ray micro-tomography in combination with mercury intrusion porosimetry was employed to observe the microstructural characteristics of samples before and after gas injection. The results highlight that the pore network of compacted GB is constituted of inter-granular, inter-aggregate and intra-granular/aggregate pores. Increasing the as-compacted degree of saturation S_r reduces the proportion of inter-granular pores and the effective gas permeability K_a.eff (Figure 1(a)). For the as-compacted sample, gas transport can induce the size of inter-granular/aggregate pores to extend (Figure 1(b)). When the initial vertical stress is applied on the as-compacted sample, K_a.eff decreases, with the closure of inter-granular/aggregate pores. Although the progressive saturation of the as-compacted samples under constant volume causes the reduction of inter-granular/aggregate pores, they are mainly filled with the low-density bentonite gel and the accumulated gas pressure is able to reopen and connect them to form fissures (Figure 1(b)). Thereby, at a given S_r resulting from this saturation process, K_a.eff is similar to that at the same as-compacted S_r (Figure 1(a)). However, the decrease in K_a.eff is significant, when the progressive saturation is conducted under vertical stress. This is because many inter-granular/aggregate pores collapse during the saturation process, and the vertical stress restrains the formation of fissures driven by gas pressure. On the other hand, gas transport does not affect the distribution of intra-granular/aggregate pores within the studied hydro-mechanical states.

The current outcomes provide multi-scale insights into the gas transport behaviour of partially saturated compacted GB and emphasise its microstructural response to gas transport under different hydro-mechanical states. The values of K_a.eff measured in the tested GB are higher than those obtained in powder bentonites but comparable to those of sand/bentonite mixtures at an equivalent porosity [5, 6]. The reason is the GB material presents the granular-type microstructure, with many interconnected inter-granular/aggregate pores, even at high S_r. Once the progressive saturation increases S_r to a value close to 1, K_a.eff will sharply drop, attributed to the significant microstructural modification [7]. Therefore, the initial microstructure (before gas injection) of compacted GB plays a critical role in the gas transport behaviour. This microstructural effect was also confirmed in previous investigations of other soil types [8]. Furthermore, the present work underlines the microstructure of compacted GB is modified during gas transport, dependent on hydro-mechanical states, which can further affect the gas transport behaviour.

Experimental investigations of groundwater inflow-induced piping phenonomenon in compacted bentonite buffers for HLW repository

Minhyeong Lee — 2023-10-02

This study introduces a series of laboratory-scale experimental tests of piping erosion in buffer in order to investigate the phenomenon of piping erosion around the buffer material when rapid groundwater inflow occurs in a disposal repository. In a cylindrical cell, specimens of compacted bentonite were exposed to continuous water injection under varying conditions of flow rate, chemical composition, and flow direction. The erosional behaviors of bentonite buffer in piping are analyzed, including the formation of piping channels, critical inflow water pressure, and self-healing. In addition, X-ray CT scanning was utilized to analyze the post-erosion characteristics of the bentonite buffer. The outcomes will inform the development of requirements for the reference design and construction of a secure HLW repository.

Modelling in the frame of hydro-mechanical infiltration column experi-ment to study the behaviour of binary mixtures of bentonite MX80

Arisleidy Mesa Alcantara — 2023-10-03

Binary mixtures of high-density MX80 bentonite pellets (80% mass ratio at a dry density of 2.0 Mg/m³) and powder (dry density of 1.1 Mg/m³) at hygroscopic water contents are studied by the French Institute of Radiation Protection and Nuclear Safety (IRSN) as an alternative in engineered barrier systems for the long-term disposal of radioactive wastes. The MX80 bentonite was studied by many authors [1–4] with this aim. These mixtures show a dry density around 1.49 Mg/m³ on pouring into infiltration column experiment tests.

An infiltration column cell (100 mm in diameter and 350 mm high) has been developed at a reduced scale of 1/100 of the VSS (i.e., at 1/10 of the in situ VSEAL test) to reproduce asymmetric hydration using independent top (fast injection) and radial (slow injection) water pressure systems, which will undergo fast hydration from the calcareous Oxfordian formation at the top and a slower one by water reaching radially through the Callovo-Oxfordian formation. It also allows performing gas injections at different locations of the core of the bentonite mixture (top and bottom boundaries) and under various hydraulic states. The infiltration column is composed of four transparent cylinders, made of Perspex to visualise processes occurring at the interface during the hydration and gas injection phases (Figure 1a). This cell has been assembled with three stainless-steel injection rings that ensure slow radial water injection through three independent automatic pressure/volume controllers. These stainless-steel injection rings also hold the different lateral transducers (six total stress cells and three water pressure transducers embedded in the soil and three relative humidity probes installed in small dismountable chambers) (Figure 1a). The assembled cell is inserted into a rigid frame composed of two stainless steel disks on the two sides of the cylinder connected by four metallic rods.

A numerical model has been developed to better understand the coupled hydro-mechanical response of this multi-porosity mixture and particularly of its constitiuents upon hydration.The characterisation of pellet and powder components have been focused on their water retention properties, water permeability, compressibility on loading and on suction changes, as well as on their swelling pressure response and the progressive changes of their pore size distribution on wetting. This information at component scale has allowed the upscaling into a 2D plane-strain numerical model based on the discretisation of pellets and inter-pellet powder using Code_Bright FEM [5]. This plane-strain model captures the pellet shape effect without changing the geometry of the pellets.The geometry used in the simulations corresponds to an infiltration column at constant volume mimicking the actual asymmetric hydraulic conditions of the VSS. Figure 2b presents the geometry of the model used at constant volume based on the discretisation of pellets and powder (252 pellets of 8 mm diameter and surrounded by powder filling the entire inter-pellet volume).The generated numerical mixture has a mass-basis proportion of 77.6% pellets and 22.5% powder, which is close to the mixture proportion. Figure 1b shows the three lateral slow-rate hydration boundaries, as well as the fast-rate hydration boundary at the top. The modelling has focused on analysing the global and particularly the local responses: water mass transfer between powder and pellets, evolution of local degrees of saturation and porosities (inside pellets and powder), and mean stress evolution in the pellet and powder domains. Water retention and permeability properties of the components are dependent on porosity and the degree of saturation.

A non-linear elastic model has been considered for the components, in which volumetric deformations are induced by net mean stress and suction changes. Therefore, the pellets and powder have different hydraulic and mechanical properties as a consequence of the different initial properties, but they share the same constitutive relationship of a single structured material. Figure 1c presents the time evolution of porosities (inside the pellets and powder), showing that the mixture tends towards a more homogeneous distribution of porosity as the pellets expand and compress the highly deformable clay powder. During the transient hydration stage, the expansion of pellets at the top of the column evolves faster due to the proximity of the hydration front. Figure 1d shows the water retention curves of the pellets and powder at the initial state and after hydration as predicted by the model, compared to experimental data [6,7]. The predicted water retention curves of the pellets and powder adequately follow the trend of the data reported by [7] and also seem to reproduce the mixture model data suggesting a homogenisation of the retention properties of the material after hydration.

Multi-disciplinary characterization of shrinkage effects in Opalinus clay

Nima Haghighat — 2023-10-02

Due to its low hydraulic permeability and several beneficial mechanical properties, Opalinus Clay formation (OPA) is considered as a potential host rock for high-level radioactive waste disposal. However, several factors during the excavation and operational stages can endanger the long-term safety of the repositories. One of the major safety concerns in the underground geological repositories includes the formation of shrinkage-induced cracks [1]. Shrinkage-induced cracks, i.e., desiccation cracks, are a result of the underground opening being exposed to lower relative humidity (RH) levels during seasonal changes and subsequent shrinkage deformation. Over the last two decades, OPA has been extensively investigated by several in-situ experiments conducted at the Mont Terri Underground Rock Laboratory (URL) in Switzerland [2]. In particular, the recently established cyclic deformation (CD-A) experiment is aimed at the investigation of the coupled hydro-mechanical processes in the sandy facies of OPA due to the seasonal changes and variations in RH [3]. In this study, to further investigate the mechanical parameters of OPA under various climatic conditions and to capture potential desiccation cracking scenarios, a multi-disciplinary approach, including experimental analysis and numerical simulation, has been adopted. In this regard, the required samples for laboratory tests were taken from drilled cores near the CD-A experiment. Additionally, the observations in CD-A regarding the onset and propagation of desiccation fracturing were used as validation point for verification of the employed numerical method.

In order to characterize the RH-dependent mechanical parameters of the OPA, a customized desiccator cell has been configured and set up at the geomechanics and geotechnics laboratory of CAU. The concept of a humidity-controlled rock laboratory test setup originates from [4]. The experimental setup, as well as its schematic representation, is shown in Figure 1(a). The components of the customized desiccator cell can be divided into four parts, including an isolated epoxy glass cell equipped with RH and temperature sensors, a loading apparatus, a salt bath and a micro-camera tube. The experimental layout contains two distinct stages, namely the equilibrium and loading stages. During the first stage and prior to the onset of mechanical loading, the sample is placed inside the cell, where it is allowed to equilibrate with a certain level of RH. Based on the initial water content of the sample and RH in the cell, the equilibrium stage can last 7-10 days. The RH inside the cell, covering the range between 0.19 and 0.88, was achieved and maintained by employing various saturated salt solutions. In this study, the above-mentioned methodology is employed to investigate the strength characteristics of the OPA under different humidity conditions. For such a purpose, a series of semi-circular three-point bending experiments (SCB) has been conducted. Furthermore, given the relative angle between the loading direction and plane of isotropy, denoted as , three types of samples with were considered to account for the anisotropic effects. The experimental observations in terms of peak loads obtained during the loading stage against the varying RH in the cell are illustrated in Figure 1(b). In addition to the general decreasing trend for material toughness as a result of increasing RH, the remarkable effect of structural anisotropy on material strength can be seen. It should be noted that, the observed deviation from the overall trend for peak load response of certain samples can be attributed to the generation of the micro-defects in samples during the preparation or the drilling processes, which can lead to alterations in the mechanical properties of the samples.

To further investigate the shrinkage effects and mechanisms of desiccation fracturing observed in OPA during experimental and field analyses, we utilized and extended the Finite Discrete Element Method (FDEM). In this regard, a hydro-mechanically coupled framework based on the principles of Richards Mechanics is developed to account for the flow in the variably saturated porous medium. The FDEM was originally developed by Munjiza et al. [5]. By combining the principles of the Discrete Element Method (DEM) and Finite Element (FE) analysis, the FDEM is capable of capturing the transition from a continuous to a discontinuous state through the simulation of fracturing and fragmentation processes [6]. To realize the anisotropic mechanical response in OPA, a transversely isotropic stress-strain constitutive law was implemented. Additionally, a strength criterion for each cohesive element was considered based on its relative angle with the macroscopic bedding direction [7].

In order to consider the variably saturated porous media flow within the framework of FDEM, a vertex-centered finite volume scheme is adopted. Based on this approach, a dual grid of polygonal control volumes is subjected to the conservation of the fluid phase. The adopted mass balance equation of water can be expressed as follows

where and are the saturation and pore water pressure, respectively. is the Biot’s coefficient, is the porosity and is the water bulk modulus. In Eq. 1, denotes the fluid flux vector. The flow rate vector, , is evaluated based on the unsaturated Darcy flow law. The Van Genuchten constitutive law was used for the definition of the hydraulic behavior in an unsaturated porous medium where the permeability tensor and saturation state are functions of pressure head [8]. The hydro-mechanical coupling was realized based on the principle of effective stress and achieved by adopting a staggered coupling scheme. The methodology was verified against the available benchmarks in the literature. To capture the desiccation cracking in field scale, a model consisting of a 2D cross section of the CD-A experiment was also considered. The required material properties for the numerical model were obtained from experimental results. In complete agreement with field observation in the CD-A experiment, the desiccation cracks start to develop around the underground opening at RH between 97~98%.

This paper presented a multi-disciplinary approach for the characterization of the shrinkage effects in OPA. In this regard, a combination of experimental analysis and numerical modelling techniques has been employed. The experimental work provided a better understanding of the anisotropic mechanical properties of OPA under different humidity conditions. The developed coupled formulation in the FDEM framework has proven to be successful in capturing desiccation fracturing mechanisms in the field scale. The presented methodology would facilitate further investigation of scenarios in which desiccation fracturing is involved.

Swelling Behavior of a Clay-Pellets Mixture Intended for the Isolation of Spent Nuclear Fuel

Abdulvahit Sahin — 2023-10-03

Deep geological repository is the most favorable option for the safe disposal of high-level nuclear waste (HLW) and spent nuclear fuel (SNF). The design of such a facility relies on the multi-barrier system, encompassing a cylindrical metallic container (encapsulating the HLW/SNF), an engineered barrier system (EBS, around the metallic container); and the host-rock (or natural barrier). The most preferred buffer material to construct the EBS is bentonite clays. They are being considered as potential buffer materials because of, amongst other reasons, their high swelling capacity provides mechanical stability to the metallic canister containing the HLW/SNF; their good thermal conductivity assists to dissipate the heat released by the HLW/SNF; their low permeability delays the flow of water and gas through the system [1].

Bentonites are highly swelling smectites clays that exhibit significant volume increase when wetted under free-swelling conditions, and develop high swelling pressure upon soaking when the volume change is restricted. It is envisaged that the EBS will be built using blocks of compacted bentonites (Figure 1(a)), or a combination of high-density clay-pellets mixtures and blocks of compacted bentonites (i.e., where the metallic container sits, Figure 1(b)). Clay-pellets are also considered as possible seal materials to fill gaps that will be present in this type of system, e.g., the gap between the EBS and the surrounding host-rock [2]. Therefore, a better understanding of the swelling capacity and behavior of the bentonite in these two forms (i.e., compacted-clay and clay-pellets mixtures) is critical for a proper design and evaluation of the long-term performance of geological repositories for HLW/SNF.

Most of the research conducted in this area has been mainly focused on compacted bentonites looking, e.g., at the influence of several factors on their swelling pressure capacity, namely, type of clay minerals, initial dry density, water content (or degree of liquid saturation), and type of water [3]. More recently, investigations have been conducted to study the behavior of high-density pelletized clay mixtures [4, 5], however, relatively less attention has been placed in this type of material. Clay-pellets presents a number of advantage, e.g., they are very suitable for filling (small) technological voids (Figure 1(a)), there is no need for additional in-situ compaction when they are used as a buffer/backfill material (Figure 1(b)), and it is relatively easy to manufacture them.

The granular clay-pellets samples were manufactured, and the constant volume cells were developed at Texas A&M University to investigate the swelling behavior of expansive clays. It has been shown in figure 2 that both materials (compacted clay and pelletized clay) developed similar maximum values of swelling pressures. However, the patterns associated with the swelling pressures evolutions are different, owing to the different pore structures associated with these two materials, which impact on the kinetic of clay hydration and swelling pressure evolutions. The aim of this research is to gain a better understanding of the swelling pressure behavior of clayed materials intended as potential barriers materials for the safe isolation of the HLW/SNF.

Inspecting the role of vapour loss and other model strategies in the mod-elling of a bentonite thermo-hydraulic cell

Laura Asensio — 2023-10-03

Models of engineered bentonite barriers in radioactive waste repositories need to use conceptual and numerical models that consider the most relevant processes taking place. To study such processes, numerous laboratory experiments have been carried out. In this work, we will model the thermo-hydraulic cell presented by Pintado and Lloret [6], since it allows the study of a simple thermo-hydraulic heating test with limited influence of other phenomena. The aim of the study is to conduct several modelling exercises to inspect in a simple way the relevance of different model aspects in this case, such as dimensions simplification and porosity levels, and processes, such as hydraulic and mechanical coupling, water vaporisation and, especially, vapour loss.

In the modelled test [6], a cylindrical Febex bentonite specimen (diameter 38 mm, height 78 mm) was subject to heating for 7 days. Initially, the sample had a dry density of 1.63 Mg/m³ and a water content of 15.3%. A constant heater power of 2.17 W was applied on one end of the sample. At the other end, the temperature was kept constant at 30 ºC. The sample was surrounded with an insulation cover, and the external temperature was of 25 ºC. Temperature was recorded in points of the specimen located at heights z = 0, 20, 38, 60 and 78 mm (Figure 1).

The first modelling exercise conducted is a 1D study solving the steady-state thermal problem in a finite rod, using an analytical solution [1]. Only the bentonite was modelled, and the insulation is introduced as a lateral loss boundary condition. Then, the only parameters needed are the dimensions of the sample (length, section area A, perimeter), the external temperature, the thermal conductivity of bentonite and thermal conductance of the insulation. The conductivity of bentonite is taken as homogeneous and constant equal to 1.13 W/m/K. This exercise identified the conductance of the insulation as 1.09 W/m²/K, which is of the same order of magnitude as that identified by Pintado et al. [5]. In addition, since part of the generated heat is not transferred to the bentonite sample, an efficiency factor to the heater power has been identified of 0.39 to fit the steady-state temperatures. This is consistent with the estimated 60% power loss pointed out by Pintado and Lloret [6].

Subsequently, the transient 1D thermal problem was modelled. The heat equation was numerically integrated in Matlab. A constant global heat capacity of the sample c_G was computed for the initial conditions as the weighted component average [4], taking into account the solid phase, liquid water and vapour. The specific heat capacity values used are c_S = 1091 J/kg/K, c_L = 4184 J/kg/K and c_V = 1900 J/kg/K, for the solid, liquid and vapour components, respectively. The result is c_G = 1100 J/kg/K. This model does not accurately capture the transient experimental temperature values (Figure 1).

Further complexity was then introduced in the model to discard some effects as responsible for the poor fitting to the transient problem. A 2D axisymmetric finite element model was built, solving the thermo-hydro-mechanical problem, with double porosity retention curve as in Navarro et al. [3]. The hydraulic problem included water vaporisation, and the transport of liquid and vapour. The thermal problem was solved as the balance of enthalpy per unit volume h as a sum of component enthalpies [2], taking into account the solid phase, macrostructural liquid water, microstructural water and vapour. A Neumann boundary condition was used for the heater power with the identified efficiency factor, and the heat loss through the insulation was introduced as a Neumann boundary condition with the identified conductance. The thermal conductivity of bentonite was computed as the weighted average between saturated (1.28 W/m/K) and dry thermal conductivities (0.57 W/m/K) [7]. The obtained temperatures were not very sensitive to these changes, since they are similar to the transient 1D thermal problem (Figure 1). The experimental transient results were still not accurately reproduced. This can lead to think that additional heat losses occur that are not considered, such as vapour leakage. Pintado and Lloret [6] estimated a vapour leakage of 0.1 g/day in the test.

Finally, the vapour mass needed to be lost for the model results to reproduce the transient recorded temperatures was estimated. To this end, the evolution of gained enthalpy H in the bentonite sample was studied in the numerical model and from the temperatures recorded, and both were compared. In the numerical model, H_model was obtained by integrating h in the sample volume for all the solved times. For the experimental results, experimental temperature T_exp profiles were integrated for several times to obtain H_exp as , where T₀ is the initial value of temperature. The difference between H_model and H_exp is the greatest around 2 h. The mass of leaked water vapour m_v that would cause such an enthalpy sink can be computed as (H_model – H_exp)/h_v, where h_v is the specific enthalpy of vapour. For the time 2 h, the obtained m_v is 1 g, which gives a constant leakage rate to that time of 15 g/day. Comparing this value to the 0.1 g/day reference [6], it is higher by 2 orders of magnitude, which makes it unlikely that the cause for the discordance between model and experimental temperature results is vapour leakage alone.

Numerical modeling of fractures interaction for EGS

Heber Fabbri — 2023-10-02

Geothermal energy is a promising source of clean renewable energy that does not depend on weather conditions and has the potential of providing base load as well as peaking electrical supply. During the last twenty years, application of the geothermal energy for electricity generation, heating and cooling have increased noticeably in many countries, including USA, Iceland, New Zealand, Turkey, and Indonesia. The worldwide installed geothermal power capacity is approximately 16 MWe, with a growth rate estimated between 3.4% to 5.4% per year over the period between 2015 to 2060, according to the World Energy Council [1]. Geothermal energy has the potential to provide 3% of electricity and 5% of heating with respect to the global demand by 2050 [2].

Enhanced Geothermal System enables producing geothermal energy from Hot Dry Rock (HDR) reservoirs, which are deficient in water and permeability, the two basic components necessary to exploit their geothermal potential (e.g., [3, 4]). EGS implementation requires the drilling of injection and production wells. Through the first one a cold fluid is injected, whereas hot water and/or steam is recovered from the production well. A key feature of this methodology is the network of fractures that connect these two wells. The discontinuities can be either, (naturally) pre-existing fractures, or (artificially) triggered fractures by the thermal shock induced by the contact between the cool injection fluid and the hot natural rock. Water or liquid nitrogen are contemplated as potential injection fluids. This type of project envisages reservoirs at depth with temperature above 180°C. Several field tests in different countries have been conducted to evaluate the feasibility of EGS (e.g., [3, 4, 5, 6, 7, 8, 9, 10, 11, 12]), including the ongoing Utah-FORGE project in the USA (e.g., [13]).

Numerical modeling of the interaction between hydraulic driven fractures (HF) and natural fractures (NF) is still a challenging problem, since the presence of NFs introduce heterogeneity on the hydromechanical properties of the rock mass and affects the geometry of the HF. Therefore, the development of robust numerical methods able to address this kind of phenomena is crucial for the safe application of stimulation techniques. This work presents a finite element (FE) technique, called mesh fragmentation technique (MFT) [14], capable of tackling this type of problem. The MFT consists in introducing solid high-aspect ratio finite elements in-between the regular (bulk) finite elements to simulate both NFs and the formation and propagation of HFs. This MFT has been successfully used to model drying cracks in soils [15, 16], hydraulic fracturing in rocks [17, 18] and natural fractures deformation due to pressure depletion [19]. The fully coupled hydro-mechanical approach has been implemented in the FE program CODE_BRIGHT. The paper presents the main theoretical components and implementation of the proposed approach. Numerical examples are used to demonstrate the capabilities of the proposed technique. Figure 1 presents the pressure contours related to the modeling of the hydraulic fracturing of two neighbor wells in a naturally fractured rock. A very satisfactory performance of the proposed method is observed in all the analyzed cases.

Data-driven discovery of injection-induced inter-slip creep on rock frac-tures

Zhou Fang — 2023-10-02

Injection-induced inter-slip creep on rock fractures reveals how fracture friction recovers from the last slip event and weakens again before the next slip event. Forecasting the occurrence of slip behaviors, either inter-slip creep or dynamic slip, relies heavily on understanding the evolution of friction controls during the inter-slip creep, particularly those significantly modified from the previous slip events. Here we collected the experimental data from a series of fluid injection experiments and built a dual-stage attention-based recurrent neural network (DA-RNN) model to uncover the contributions of controlling factors to the occurrence of slip behaviors.

We conducted two sets of fluid injection experiments on sawcut fractures in Bukit Timah granite using Material Testing System experimental system with Vindum dual syringe pump [1]. We applied a normal stress of 11 MPa on the fracture and a shear stress equal to 80% of the shear strength to simulate a critically stressed fracture. We injected distilled water into the fracture at a constant pressurization rate of 0.05 MPa/s to induce a fracture slip. We also calculated the fluid pressure gradient over the fracture, which is defined as the difference between the injection and monitoring pressures divided by the fracture length. The fracture in the two sets under the same stress and injection conditions exhibits similar slip behaviors before and after the first dynamic slip event, in terms of a dynamic slip followed by an inter-slip creep (Figures 1a and 1b). However, the inter-slip creep continues during the first set, while multiple dynamic slip events appear in the second set.

To better understand the difference of slip behaviors in the two sets, we used the experimental data to train the DA-RNN model, which involves the attention mechanism to predict the slip behaviors based on the most relevant input parameters [2]. The attention distribution of the DA-RNN model reveals an attention increase to normal stress and an attention decrease to shear stress after the first dynamic slip event (Figures 1c and 1d). The minimal attention to shear stress is comparable to the maximum attention to normal stress, indicating that both the stresses control the subsequent slip behaviors. However, the attention to shear stress in the second set dramatically increases and far exceeds that to normal stress. The shear stress thus becomes the dominant control of slip behaviors, promoting the propagation of rupture front and the occurrence of dynamic slip [3]. The attention increase to normal stress during the subsequent inter-slip creep signifies the recovery of asperity contacts, restrengthening the fracture friction and leading to the multiple dynamic slip events. This study demonstrates the data-driven discovery using the DA-RNN model to better understand the evolution of experimental controls and the prediction of slip behaviors during fluid injection.

Lithium Prospectivity in the Northeast German and Thuringian Ba-sins

Alicia Groeneweg — 2023-10-02

Over the years many boreholes have been drilled into the Northeast German Basin (NEGB) in pursuit of the exploration of hydrocarbons. As well as gaining important information regarding petrophysical and geophysical data, legacy boreholes also provide information regarding total dissolved solids in formation water, which sometimes includes lithium concentrations. With the rapid increase in demand for electric vehicles in combination with EU net-zero targets, lithium is a vital component of the future of the energy industry as it transitions away from fossil fuel dependence.

So, where can we find lithium, how much of it is there, and where does it come from? After extracting lithium content data from formation water trace element data, we can answer two of these questions. The data gathered [1] is from boreholes drilled in the NEGB (in the states of Brandenburg, Mecklenburg-Vorpommern, and Sachsen-Anhalt) and the Thuringian Basin. The Northeast German Basin makes up the eastern part of the North German Basin, which, in turn, is part of the Southern Permian Basin. It is located between the Baltic Shield to the north and the Variscan belt to the south [2]. The basin contains up to 12km of Phanerozoic strata [2], the evolution of which began with Late Carboniferous/Early Permian (Stephanian/Autunian) volcanism [3, 4, 5, 6]. In the NEGB increased concentrations of lithium can be found within the Zechstein Staßfurt Carbonate (Ca2) and the Rotliegend. In Brandenburg the highest values of 230 mg/L can be found in the geothermal well doublet Groß Schönebeck which was drilled into sedimentary Rotliegend and Permo-Carboniferous volcanics. Comparable values (190 mg/L) can be found in formation water samples of the Ca2 in the well Lübben 103. In Mecklenburg-Vorpommern, lithium contents of 600 mg/L were encountered in the Rotliegend of the well Gristow 7, with lithium contents in the Ca2 of the nearby well Gristow 6h2 reaching 300 mg/L. The Reinkenhagen field (which targeted the hydrocarbons of the Ca2) has several wells which exhibit lithium contents in the Ca2 of up to 140 mg/L. In Sachsen-Anhalt elevated lithium values are limited to the Ca2. The Fallstein field, originally drilled for the exploration of hydrocarbons, encountered lithium contents of up to 189 mg/L. Spatial distribution of the lithium data can be seen in Figure 1 along with current geothermal projects. Also displayed are areas of proven and suspected geothermal potential.

The Thuringian Basin is located between the Harz Mountains and the Thuringian Forest Mountains, the formation of which was part of the intramontane sudsidence zone of the Variscan mountains [7]. The trough shaped sedimentation area of the basin was formed in the early Mesozoic [8]. In Thuringia, as in Sachsen-Anhalt, elevated lithium values are limited to the Ca2. The fields of Kirchheilingen, Langensalza, and Mühlhausen all have multiple wells which show lithium contents of up to 200 mg/L.

The source of lithium in formation water is not well understood and therefore cannot be clearly defined. A case study in Thurinigia focusing on Zechstein salts in Gorleben and Morlseben made the assumption that lithium content in the salt brines were leached from phyllosilicate bearing units (T4, A3, Ca3, T3) in the Zechstein [9]. Other sources discussed include relict metamorphic brines and organic compounds. A different study focusing on Rotliegend Ca-Cl brines in the

North German Basin found that brines rich in lithium originated from the leaching of lithium rich mica in the Lower Rotliegend (volcanics) [10]. If lithium accumulation in formation waters is related to Permo/Carboniferous rocks, it is possible that deep faults may have acted as conduits for lithium rich brines to migrate into overlying formations [11]. This may be a plausible theory as this dataset shows that elevated lithium contents in the Ca2 can be found within the carbonate margin facies in areas through which major faults run. This is especially evident on the northern basin margin.

Ultimately the data compiled from both basins will be used to perform a lithium in place analysis.

Comparison of two approaches for modelling fracture opening due to cold water injection in geothermal reservoir

Wen Luo — 2023-10-02

Heat and electricity production from deep hot reservoirs through hydrothermal or petrothermal system requires to inject cold fluid in a naturally or artificially fractured medium. Cold water injection in a hot reservoirs causes thermo-hydro-mechanical (THM) coupled processes that may have several influences on operations and long term production [1]. Cold water has a higher viscosity, which means injection can become difficult due to higher flow impedance, while thermal diffusion in the rock matrix causes shrinkage and thus an increasing fracture aperture over time that can lead to flow channeling and a reduction of operation time. A deep understanding of the contribution of coupled THM processes to injection into fractured media is thus important to predict the long-term performance of a geothermal power plant. In this study, coupled THM processes in a single fracture are numerically investigated with two different approaches to model the discontinuity.

The first numerical approach proposes modelling an implicit fracture in a fracture zone using solid elements (Fig.1 (a)). The impact of the fracture is introduced by means of a stress dependent fracture aperture, which in turn determines the fracture transmissivity. This function allows recovery of the opening, and thus the model replicates opening and closure of the fracture. Then, an equivalent continuum permeability [m²] is obtained as follows:

where [m] is the elastic opening, b_r and b_max [m] are the residual and maximum apertures, respectively, a is the stress dependency coefficient; [Pa] is effective normal stress, and [m]the fracture spacing. The combination of the continuum elements to model an implicit fracture embedded in a fracture zone and the evolving permeability has been used successfully used to model elastic opening in volcanic rock [2] and granodiorite rock [3, 4].

In the second approach, zero-thickness interface elements are used to explicitly model the opening of an existing fracture (Fig.1 (b)), following the work by Liaudat et al. [5]. A constitutive law that is capable of describing the fracture initiation, propagation, closing and opening is implemented in this approach, considering heat transfer in the discontinuity. In this approach, the cubic law is used to estimate the longitudinal transmissivity [m³] as a function of the normal fracture aperture:

where [m] is the normal separation and [m³] is a constant value which makes it possible to assign an initial longitudinal transmissivity even if the fracture is mechanically closed.

In both approaches, the simulation of cold water injection requires to consider the heat and hydraulic flows in the fracture alongside mechanical behaviour, since changes in pore water pressure and temperature influence the fracture aperture, thus modifying the fracture transmissivity.

In order to compare both approaches, a simple synthetic model is studied. The model with its boundary and initial conditions are shown in Fig.2. Water with a temperature of 20 °C and pressure varying from 4 MPa – 10 MPa is injected at the centering point of the fracture. Figure 3 shows the injection pressure profile. Fracture permeability evolution is compared in Fig.4, which shows the two different methods capture different characteristics of the natural fracture behaviour. In the continuum method, the fracture is modelled as a continuum element, thus any pressure or stress change will result in corresponding changes in the fracture permeability, which is reflected in Fig.4 showing that the permeability changes with the injection pressure changing, In contrast, in the interface element, Fig.4 shows only when the injection pressure reaches 10 MPa, the permeability increases sharply from the initial value to the peak value. This is because penalty method is used to allow the small overlapping of the two faces of the discontinuity, thus keeping negligible permeability variation, before it opens. The result in Fig.4 also illustrates the effect of thermal stress on the fracture opening, considering the total vertical stress is 10 MPa while injection pressure is below or equal to 10 MPa.

In conclusion, though the peak permeability predicted from two methods are consistent, both around 1.5 x 10 ^-12 D, the behaviour before fracture opens in these two approaches are quite different. For the interface method, permeability variations keep negligible before fracture opens, while for the continuum method, permeability changes with corresponding pressure or stress changes without dependence on the opening of the fracture.

Thermo-hydro-mechanical modelling of geothermal energy extraction in deep mines in spatially heterogeneous settings

Le Zhang — 2023-10-02

With the increasing demand for mineral and alternative energy resources, as well as the gradual depletion of shallow resources, the exploitation and utilization of mineral resources and geothermal energy in deep strata is an effective way to solve the problem of resource shortage [1]. In recent years, as a new type of resource mining mode, the co-mining of deep mineral and geothermal energy has developed rapidly [2, 3]. This method can make use of the original equipment of the mine for geothermal exploitation. However, the deep co-mining system faces two significant challenges: the first is the significant uncertainty inherent in subsurface properties, while the second is the high levels of geostress and temperature associated with deep mining. These challenges are adding some constraints on the practicality of exploiting such systems and limit the feasibility of deep resource co-mining, so that modelling efforts are needed for actual risk assessment.

Consequently, we developed a Thermo-Hydro-Mechanical (THM) coupling framework for geothermal energy exploitation in deep mines using COMSOL to quantitatively characterize the temperature field of the geothermal system and predict the stress field of the mining system, considering the joint effects of large uncertainties and THM coupling. Through SGeMS, the uncertainty and spatial heterogeneity distribution of porosity are first generated. Then, the uncertainty of the hydraulic parameter [4] (permeability), mechanical parameter [5] (elastic modulus), and thermal parameter [6] (heat capacity and heat conductivity) was derived from the porosity. 500 samples were generated within a given uncertainty range, by means of Monte Carlo simulations. The spatial and temporal distributions of the temperature field of the geothermal system, and the stress field of the mining system were simulated, for each sample with COMSOL. Using the distance-based global sensitivity analysis [6], the most sensitive parameters for deep mining are identified, the heat storage capacity of the system and evolution of the maximum stress ratio are evaluated, including uncertainty.

Using control theory for preventing induced seismicity due to fluid injections in a reservoir

Diego Gutiérrez-Oribio — 2023-10-02

Deep Geothermal Energy, Hydrogen Underground Storage and Carbon Capture Utilization and Storage are promising techniques to satisfy the large-scale needs of the energy sector. However, they all depend on the injection of fluids into the earth’s crust, which, in turn, can cause earthquakes [1, 2]. Earthquakes nucleate when large amounts of elastic energy, stored in the earth's crust, are suddenly released due to abrupt sliding over faults. Fluid injections can create new, or reactivate existing, seismogenic faults and, therefore, cause earthquakes [2, 3].

More recently, new results for controlling the earthquake instability of a single, natural, mature seismic fault were obtained [4, 5, 6, 7, 8]. These works were based on the mathematical control theory to stabilize and control the underlying physical system, which is underactuated, strongly non-linear and uncertain. The above works could inspire new approaches for preventing induced seismicity too [4, 9].

In this work, a 3D diffusion equation is considered to model induced seismicity due to fluid injections in a geological reservoir. A robust tracking control is then designed to force the seismicity rate to follow a desired reference, minimize induced seismicity and assure fluid circulation. The designed regulator ensures the control task despite system uncertainties and external perturbations. Simulations of the process are presented to show the reliability and performance of the control approach.

Hydrothermal karst cavities in a Devonian carbonate reservoir analogue (Rhenish Massif, Germany): Implications for geothermal energy potential

Mathias Mueller — 2023-10-02

Deep geothermal reservoirs can provide renewable energy for electricity and heat generation [1]. In the Rhine-Ruhr area of western Germany, where Europe's largest district heating network is located, up to 1,300 m thick carbonates of Devonian age are available in ≥ 4,000 m depth [2, 3]. Near the surface, these rocks contain karst cavities, which host some of Germany´s longest caves [4, 5]. At reservoir depth, abundant karstification may significantly increase the geothermal reservoir potential [6] (Figure 1).

In the Munich area (southern Germany) deep-seated karstified Upper Jurassic rocks are widely used for the city´s district heating network [7]. Most deep-seated geothermal systems are located at a depth of 1,000 m or more at temperatures over 60 °C [8]. Therefore, it is important to assess deep-seated karstified structures elsewhere to assess their geothermal reservoir potential. In the Rhine-Ruhr area, these could have the potential to be used as an alternative renewable energy resource.

This study aims at the geological characterization of deep-seated (hydrothermal) karst cavities in Steltenberg Quarry (western Germany) where Middle/Upper Devonian carbonates (Massenkalk limestone) are present in the vicinity of two regional fault zones [9, 10]. We applied state of the art petrographical, geochemical, palaeothermometrical methods, and U-Pb dating. Here we present the first U-Pb age data of deep-seated hydrothermal karst precipitates in Germany, which formed at the Permian-Triassic boundary (252.4 ± 8.6 My, Table 1). The U-Pb age data of calcite cement veins (LMC 8), which were cutting the near-surface karst cavities before they got dissolved, points to an Oligocene maximum age (30.0 ± 2.81 My) of the karst cavities.