Gas-induced fracturing in liquid-saturated clay-rich materials presents challenges in understanding and predicting fracture behaviour, due to the complex mechanical and transport properties of clays and the compressibility of gas. This paper introduces a novel experimental device for visualising fluid-driven cracks in clays. The device allows for the induction and observation of two-dimensional cracks in clay-rich, low-permeability materials through the injection of gas or water. The experimental setup comprises precision instrumentation for measuring compression forces, displacement, and fluid pressure, along with high-resolution imaging capabilities. Preliminary tests with Helium gas injection into Boom clay samples demonstrate the device's ability to track fracture evolution. This innovative experimental tool offers insights into the mechanisms governing fluid-driven fractures in clay-rich materials and provides a means to validate numerical models.

The effect of the load level on long-term thermally induced pile displacements and the impact of cyclic thermal loads on the bearing capacity of energy piles are investigated via a full-scale in situ test in Delft, The Netherlands. The pile was loaded to a specific target of 0, 30, 40, or 60% of its calculated ultimate bearing capacity. At the end of each loading step, up to ten cooling–natural heating cycles were applied. The pile behavior during monotonic cooling and cyclic cooling–natural heating in terms of the displacement along the pile is reported, with a focus on permanent displacements. During monotonic (pile/ground) cooling, a settlement of the pile head and an uplift of the pile segment near the pile tip were observed in all four tests. In addition, under higher mechanical load, the pile head displacement was larger while the uplift was lower due to the imposed mechanical load. During cyclic thermal load, under zero mechanical load, pile head displacement was fully reversible while permanent uplift of the lowest pile segment was observed and attributed mainly to the permanent dragdown of the surrounding soil. Under moderate mechanical loads (30 and 40%), thermal cycles induced an irreversible pile head settlement, which stabilized with an increasing number of cycles. In addition, a permanent pile settlement along the pile was observed at the end of these tests. Under high mechanical load (60%), the irreversible settlement along the pile continued to increase with only a slight reduction in rate, being higher compared to moderate mechanical loads. In this test, a normalized pile head settlement of 0.124% was observed after ten thermal cycles. The permanent settlement of the pile under thermo-mechanical loads was mainly attributed to the contraction of sand beneath the pile tip and thermal creep at the soil–structure interface. The pile bearing capacity was observed to increase after thermo-mechanical tests, mainly due to the residual/plastic pile head displacement, which in turn densified sand leading to an increase in tip resistance.

This paper presents quantitative data from a field test on a new type of energy pile, called a displacement cast in situ energy pile. The test pile was installed in a multilayered soft soils and subjected to a continuous cooling for 3 months, with no mechanical load. Afterwards, the pile was loaded to a specific target of 20 or 60 % of its calculated ultimate bearing capacity and then subjected to up to five thermal cycles. Under zero mechanical load, the results revealed that the compressive/tensile stresses coexist along the pile. Under low mechanical load (20 %), thermal cycles induced irreversible residual contractive strains and stresses as well as a limited pile head settlement. Under high mechanical load (60 %) and extreme operating conditions, i.e., negative temperatures which could have indicated a frozen interface, further irreversible settlements observed at the end of this test. Mechanical pile tests however indicated no impact of stress history (including the freezing test) on the shaft resistance and the overall pile-bearing capacity.

Evaluating the temporal stability and risk of unsaturated soil slopes during rainfall is essential for early warning and emergency response to landslides. However, limited research has been conducted on the transition timing of sliding mechanisms, instability/failure time and the integration of sliding consequences into quantitative risk assessment. In order to extend the research in this field, the Random Finite Element Method (RFEM) is used in this paper to investigate the influence of spatial variability of hydraulic properties (related to the fundamental parameter porosity) on the temporal stability and risk of soil slopes subject to rainfall. The findings indicate that the advancing speed of the wetting front is more rapidly in zones with low porosity than that in zones with high porosity. As rainfall progresses, the sliding mechanism of the slope shifts from deep sliding to shallow sliding. The homogeneous case tends to underestimate the rise in groundwater levels, leading to an overestimation of slope stability. Hydraulic boundary conditions significantly affect slope stability, making it crucial to consider horizontal (or near the toe of the slope) drainage conditions in practical applications. Additionally, the time of instability/failure predicted in the homogeneous case may be delayed compared to the actual conditions. Both probability of instability/failure and risk increase with continued rainfall. Compared to scenarios where the spatial variability of internal friction angle is not considered, the probability of instability/failure and risk will be higher when the spatial variability of internal friction angle is additionally considered. Risk-based assessment can define the risk levels, reflecting the severity of sliding consequences. Furthermore, the Malin slope failure record from the Chibo region of India is used to validate the effectiveness of the proposed approach. The probabilities of slope failure align well with actual observations, and the risk-based assessment provides additional information into the Malin landslide. This paper proposes a general model for studying the performance of heterogeneous slopes subject to rainfall, providing valuable guidance for landslide early warning systems and the scope and timing of emergency measures taken.

The Cutter Soil Mix (CSM) energy wall represents a novel type of energy geostructure, merging the environmental and cost benefits of the CSM technique with ground source heat pump technology. This innovation necessitated comprehensive research to understand the thermal, physical and mechanical characteristics of the material, and optimise energy performance of the CSM energy wall. Laboratory tests and finite element numerical modelling, replicating a full-scale CSM test site in Amstelveen (the Netherlands), were employed to evaluate energy performance under varying demand scenarios: heating only, and combined heating and cooling. Key aspects examined include the influence of horizontal connection pipes on energy extraction and injection, and the impact of U-loop configuration on heat exchange rates over short and long terms. Findings indicate that energy demand significantly affects system performance, with combined heating and cooling demands enhancing long-term heat exchange rates compared to heating-only scenarios. The study demonstrates that non-insulated connection pipes increase overall heat exchange rates, especially under heating-only conditions, while reducing the number of U-loops decreases thermal interaction and enhances energy extraction rates per activated area.

Three-dimensional and spatial variability effects on slope failure processes are investigated for an idealised slope stability problem with the random material point method (RMPM). A 45 degree slope is brought to failure by either its own weight or by a combination of its own weight and an additional surface load applied at the crest. The ultimate failure load and potential failure processes are studied for various (heterogeneous) material strength profiles. In 3D, failures tend to spread sideways and backwards. For the slope geometry considered, the resistance to initial and secondary failures in 3D simulations tends to be higher than in 2D simulations, probably due to the additional resistance from the ends of the failure surfaces. The failure behaviour changes when a depth trend in the material strength is introduced. A depth trend in the material strength triggers a flow-like failure process, instead of distinct (approximately) circular failure surfaces which are encountered in a material without a depth trend. The flow-like behaviour causes an expansion in the failure zone in all directions while avoiding (where possible) local strong zones.

This study presents a thermo-hydro-mechanical framework to model hydrothermal systems within a simplified faulted synthetic reservoir, replicating current production scenarios in The Netherlands and Germany. The reservoir, composed of porous and permeable sandstone, and the confining layer, made of porous but less permeable shale, undergoes a process where cold water is injected and hot water is extracted. A fault, situated 750 meters from the injection well, is investigated to examine the conditions when fault slip could occur. Various fault and formation stiffnesses are modeled to assess their impact on fault stability. Our analysis reveals that stress changes induced by hydrothermal operations can lead to fault reactivation, with the stiffness contrast between the reservoir and confining layers playing a significant role in when and where fault reactivation can occur. Stiffer confining layers lead to reactivation occurring more closely associated with the passage of the cooling front. In contrast, a stiffer reservoir results in greater and more gradual stress changes, making reactivation more closely related to the total volume of cooled rock.

Subsurface schematisations are a crucial geotechnical problem which generally consists of filling substantial gaps in subsurface information from the limited site investigation data available and relying heavily on the engineer’s experience and occasionally geostatistical tools. To address this, schemaGAN, a conditional Generative Adversarial Network (GAN) to generate geotechnical subsurface schematisations from site investigation data is introduced. This novel method can learn complex underlying rules that govern the subsurface geometries and anisotropy from a big database of training cross-sections, and can produce subsurface schematisations from Cone Penetration Tests (CPT) in an insignificant timeframe. To test and demonstrate the performance of schemaGAN, a database of 24,000 synthetic geotechnical cross-sections with their corresponding CPT data was created, including spatial variability and gradually spatially varying layers. After training, the effectiveness of schemaGAN was compared against several interpolation methods, and it is seen that schemaGAN outperforms all other methods, with results characterised by clear layer boundaries and an accurate representation of anisotropy within the layers. SchemaGAN’s superior performance was confirmed through a blind survey, and in two real case studies in the Netherlands, where the model demonstrates better predictive accuracy for known CPT data.

Energy Quay Walls (EQWs) are innovative energy geostructures which exchange thermal energy with both soil and open water while providing a structural function. A full-scale EQW with thermally activated sheet piles was tested, measuring 8.4 m in length along a 1.75m deep canal, with the sheet piles embedded 13m into the underlying soil. Two different length heat exchangers were used: shallow (3 m length) loops primarily extracting thermal energy from the open water, and deep (15 m length) loops extracting energy also from the soil. The shallow loops demonstrated a high heat extraction rate per activated surface area (∼200 W/m2 at 8 °C water temperature, compared with ∼60 W/m2 for the deep loops), with their performance closely linked to the open water temperature. The shallow loops did not require time to restore surrounding temperatures, indicating stable long-term performance, yet can extract the least energy at the coldest time periods. In contrast, the deeper loops exhibit greater stability across varying open water temperatures and achieve the highest total energy extraction per quay wall length (∼900 W/m at 8 °C water temperature, compared with ∼600 W/m for the shallow loops). Realistic operation of the deep loops lowered the soil temperature by ∼2 °C.

Research into the impact of innovative sustainable energy experiments and demonstrations is crucial to diversifying, scaling up, and accelerating the sustainable energy transition. Although there is vast research into sustainable energy experiments and demonstrations, research literature offers a fragmented collection of findings. A coherent overview of themes and insights regarding the transformative impact of innovative sustainable energy experiments and demonstrations on sustainable energy systems from the past, present, and near future is lacking and necessary to increase experiments and demonstrations' impact on the sustainable energy transition. The research in this study fills this knowledge gap by providing such an overview and yields novel insights into the organized function and impact of experiments and demonstrations. It spans a broad spectrum of sustainable energy technologies, the empirical domains where these are invented, developed and applied, and the stakeholders involved. The overview is the outcome of a Delphi study in which the insights of 47 international scientific research experts in sustainable energy experiments and demonstrations are bundled and explained. This study presents a thematic overview of the significant insights regarding past and current sustainable energy experiments and demonstrations and outlines a research agenda for the future. Policymakers, practitioners, and scientists can leverage this to inform their sustainable energy policies, business strategies, and research programs.

Energy Quay Walls (EQWs) are innovative energy geostructures with the unique capability to exchange heat with both soil and open water. Although previous laboratory testing demonstrated a promising energy efficiency for this type of system, its novelty necessitated thorough research to advance comprehension of its thermal behaviour and optimise energy efficiency. This paper conducts an in-depth examination of EQWs, employing numerical models validated against real data from a full scale test in Delft, The Netherlands.

Two Finite Element numerical models were developed to (i) reconstruct the undisturbed (i.e. pre-geothermal activation) temperature profile within the soil and (ii) conduct a comprehensive (3D) analysis of heat exchange processes in an EQW application (i.e. during geothermal activation), calibrating relevant parameters with field test data, providing valuable insights into its energy efficiency. Following validation, the geothermal activation model was employed to assess the impact of the flow regime within the heat exchanger pipes and the velocity of the open water on the energy efficiency of the EQW system. Additionally, the contributions of soil, water, and air to the energy gain are investigated. The results indicate that the primary source of energy gain is from open water, and the dominance of this contribution is further increased by the presence of turbulent flow within the heat exchanger pipes. However, the soil can play a key role in short term energy delivery. Furthermore, this study emphasises the importance of the open water movement, revealing a 48% reduction in energy extraction for fully stationary water scenarios.

Cold water injection into geothermal reservoirs is a common, sometimes necessary, technique for multiple reasons including the replenishment and stimulation of the reservoirs, and the disposal of waste water. The injection of cold water results in a thermo-hydro-mechanical (THM) impulse, which can cause near-wellbore cracking. A method is presented to simulate coupled thermo-hydro-mechanical processes, including the re-activation of existing fractures and fracturing of the rock matrix. The model is based on the finite element method, and utilises a newly developed cohesive interface element to represent discontinuities. The interface element belongs to the family of zero-thickness elements and is triple-noded. It is developed to allow the simulation of longitudinal and transversal fluid/heat flow. The cubic law is used to simulate the fracture transmissivity as a function of its aperture, while a elasto-damage law is used to characterise the mechanical response of the discontinuity. The method is successfully verified against analytical solutions for hydraulic fracturing (KGD model) and for the thermo-hydraulic response of a single fracture (Lauwerier’s problem). As numerical oscillations are observed due to the high Péclet number, an artificial diffusion is added to stabilise the numerical solution with sufficient accuracy. Qualitative validation is achieved against experimental data of cold water injection in granite samples. Fracture branching is observed in the case with large cooling shock, while a single fracture is induced in the case with smaller cooling shock, as was observed in the experiment. The validation demonstrates the capability of the proposed model to simulate fracturing processes under THM couplings.

Underground Thermal Energy Storage (UTES) technologies are essential for advancing low-carbon heating and cooling systems, particularly in urban areas where space constraints and retrofitting challenges pose significant barriers. In this study the performance of a system of novel coaxial diagonal borehole heat exchangers (BHE) is analyzed during September–December 2024.

The Home Smart Energy (HSE) system, implemented in Medemblik, Netherlands, features a nine-borehole diagonal array arranged in a circular configuration. The boreholes are drilled at a 60° or 45° angle to depths of up to 40 meters, operating in a closed-loop coaxial setup. A brine mixture of water, operates with a flow rate of 3100 l/h, and 14% glycol lowers the freezing point below 0°C, allowing the system to supply higher capacities. The heat pump extracts the heat from the BHE’s, supported by solar thermal collectors to charge the BHE’s in summer, ensuring efficient year-round heating. An extensive monitoring framework, including Distributed Temperature Sensing (DTS), provides detailed insights into system performance during operation.

The HSE system demonstrated consistent performance under varying configurations and conditions. With all nine boreholes active, the system achieved a seasonal Coefficient of Performance (COP) ranging from 3.8 to 5.2, with daily energy outputs averaging 125 to 220 kWh/day. During December 2024, tests were conducted using three boreholes in different configurations at a reduced flow rate of 2800 l/h. These tests showed that borehole arrangement moderately influenced system performance, with the adjacent configuration achieving slightly higher energy outputs and COP, compared to the dispersed configuration.

The system also demonstrated significant energy cost savings of €954 during November and December 2024, attributed to a reduction in gas consumption by over 700 m³ compared to the previous year. These findings confirm that diagonal shallow co-axial borehole arrays are a scalable and sustainable UTES solution, offering substantial energy savings and CO₂ reductions in dense urban settings.

Energy quay walls (EQWs) are an innovative type of energy geostructure (EG), capable of exchanging thermal energy with both soil and open water. In this work, a validated 3D finite element numerical model is employed to conduct a parametric analysis aimed at identifying the most important design- and site-dependent parameters for optimising EQW energy performance. The Taguchi Experimental Design statistical method is employed to explore the parameter space for two types of heat exchanger loops used in EQW installations: loops incorporated into the structural elements and add-on panels. The most influential design parameter on the energy performance is shown to be the number of U-loops, which can significantly improve the energy yield (up to ∼50%). The effects of reduced inlet temperature (up to ∼35%), enlarged pipe cross-sectional area (up to ∼26%) and increased heat exchanger fluid velocity (up to ∼20%) are also significant for the EQW thermal performance. Among site-specific factors, the presence of a deep water body (up to ∼100%) with high temperature (up to ∼62%) is confirmed essential for achieving high energetic performance, while a high open water flow velocity (up to ∼32%) and elevated soil thermal conductivity (up to ∼23%) are influential in the short-term thermal output.

At present, over half of all primary energy used in Europe is used for heating and cooling. Therefore, decarbonizing the heating supply is essential to achieve climate targets. Underground thermal energy storage is a key enabling technology for the energy transition to buffer the large seasonal mismatch between thermal energy demand and sustainable thermal energy production capabilities. In Delft, a High-Temperature Aquifer Thermal Energy Storage (HT-ATES) system will be installed at the campus of Delft University of Technology (TU Delft). It will be integrated in the wider heating system on and around the TU Delft campus, which itself is undergoing a transformation to optimally supply sustainable thermal energy. The district heating network will be extended and utilize the thermal energy from a geothermal doublet producing heat at around 75-80°C with a flow rate of ~350m3/hr. Excess energy produced by the geothermal well in summer will be stored in the HT-ATES system, and will be utilised when demand exceeds production throughout the winter. The HT-ATES system will comprise of 7 wells (3 hot wells of 80°C and 4 warm wells of 50°C) to a depth of approximately 200m, with storage in an unconsolidated sedimentary aquifer between 160-200m depth. It is designed so that the instantaneous excess power from the geothermal project can be stored and demand from the district heating network be extracted from the system.

The HT-ATES system at TU Delft is partially funded by local stakeholders and the European commission within the PUSH-IT project and has two primary goals: (i) to reduce carbon emissions on TU Delft campus , and (ii) to create a unique demonstration, education and research infrastructure. The complexity of a HT-ATES requires innovative solutions during the entire system life cycle. The scientific programme that is initially planned within the project is therefore focusing on various research fields and includes:

- Characterisation of the subsurface formations including mechanical, hydraulic, thermal, and chemical properties.
- Evaluation and monitoring of the biological conditions and microbial diversity, and potential impact on water quality.
- Innovations in drilling and completion, monitoring and performance.
- Quantification of the system performance and system impact during multiple storage cycles and the full lifecycle of the HT-ATES. This will include extensively monitoring temperature distribution and water quality in the subsurface to characterise behaviour and improve models.
- Demonstrate and develop the implementation of HT-ATES in an urban setting, including control of the system in the built-environment and transforming the conventional heat network to a future-proof heat network.
- To allow access to other universities or institutions with active programmes in the field of Geothermal Science and Engineering to jointly carry out research and perform experiments.
-Societal engagement and legal evaluation for improving the just energy transition.

A geothermal doublet has been installed in a sedimentary reservoir for direct-use heating on the TU Delft campus, targeted to supply around 25 MW of thermal energy at peak conditions. This contribution presents the implementation and initial data collection from the doublet, including an initial evaluation of the logging and coring campaign. Nearly half of Netherlands natural gas consumption is allocated to heating, and the on-campus CO2 emissions from heating exceed 50%. The doublet has been designed with two primary aims of research and commercial heat supply, with the wells being completed in December 2023. The project will be operated by a commercial entity, and built into a larger thermal energy system including a high temperature underground storage system, with the first energy production planned in 2025. The research questions relate to field-scale geothermal operations, e.g. how reliable is the long-term energy production?, how do materials perform in the long-term? and how can geothermal projects be best monitored? The research programme involves the installation of a wide range of instruments alongside an extensive logging and coring program and monitoring network. The doublet has been cored, with substantial continuous samples from the heterogenous reservoir, alongside a large suite of open hole well logs in the reservoir and through casing logs in overlying geological units. A fiber-optic cable will monitor distributed pressure throughout the producer reservoir section, at approximately 2300m depth, which will be installed during commissioning. A local seismic monitoring network has been installed in the surrounding area with the aim of monitoring very low-magnitude natural or induced seismicity. The project is a key national research infrastructure and is being incorporated into the European EPOS (European Plate Observing System, https://www.epos-eu.org/), such that accessibility and data availability will be as wide as possible. All observations will be included in a digital-twin framework that will allow to make better decisions in future geothermal projects.

Numerous full-scale in situ tests have been conducted to assess the effect of thermal cycles on the pile response. However, those studies investigated the response of only precast and cast in-situ energy piles, with limited focus on the impact of the applied mechanical load on the pile response. This study presents the results of a field test conducted on a new type of energy pile, i.e. a displacement cast in-situ energy pile in multilayered soft soils, subjected to different fixed mechanical loads while undergoing simultaneous thermal cycles. Four tests were carried out, each corresponding to various axial loads ranging from 0 % to 60 % of the pile's estimated bearing capacity. After applying the axial load on the pile head (0 %, 30 %, 40 %, or 60 % of the bearing capacity), the pile was subjected to up to ten thermal cycles. The highest magnitudes of thermal axial strains were observed near the pile top due to the lowest restraint provided by the made ground layer in all tests. Under zero (0 %) mechanical load, the thermal axial strains near the pile head were elastic and recoverable, while residual strain was observed near the toe. Under reasonable working mechanical loads (30 %, 40 %, or 60 %) residual strains were observed near both the pile head and the toe, with higher residual strains observed under higher mechanical loads. The results indicate that the cyclic thermal loadings could induce an increase in the compressive stress in the energy pile, attributed to the drag-down effects of the surrounding soil. The compressive stress induced by drag-down effects counteracts thermally induced tensile stress and thus leads to an insignificant effect on the energy pile during cooling. A limited impact of the shaft capacity was observed and was mainly attributed to the drag-down of the surrounding soil and thermal creep along the pile-soil interface.

This paper investigates the implementation of a nonlocal regularisation of the material point method to mitigate mesh-dependency issues for the simulation of large deformation problems in brittle soils. The adopted constitutive description corresponds to a simple elastoplastic model with nonlinear strain softening. A number of benchmark simulations, assuming static and dynamic conditions, were performed to show the importance of regularisation, as well as to assess the performance and robustness of the implemented nonlocal approach. The relevance of addressing stress oscillation issues, due to material points crossing element boundaries, is also demonstrated. The obtained results provide relevant insights into brittle materials undergoing large deformations within the MPM framework.

Ground source heat pumps (GSHP) coupled to shallow borehole heat exchangers (BHE) represent a low emission technology to provide space heating and cooling. However, ongoing long-term heating or cooling of the ground caused by unbalanced loads leads to a performance decline and in the worst case to a system shutdown. Enhanced regeneration can increase the system efficiency, reduce the necessary borehole length or compensate unbalanced loads. In this study, a literature review about the regeneration of shallow BHE fields to counteract ground thermal imbalance is conducted to give an overview about the state-of-the-art and identify research gaps. The most common heat sources for artificial regeneration in heating-dominated applications are space cooling and solar thermal flat-plate collectors, while the most common heat sinks in cooling-dominated applications are space heating and cooling towers. In heating-dominated applications, mostly single buildings are studied. There is a lack of studies on district heating and cooling applications, which are especially needed as the benefit of regeneration increases with system size. There is also a lack of long-term, large system size experimental work to validate theoretical studies.

Heating and Cooling constitute a major part of society's final energy use and a significant contributor to greenhouse gas emissions. The world society ought to mitigate climate change through decarbonisation, which must include the transition to low-temperature, sustainable and renewable heating and cooling technologies. Shallow Geothermal Energy is one of the most energy efficient and least greenhouse gas emitting available alternatives to provide space heating and cooling. The decarbonisation of the heating and cooling sector may have to comprise both individual systems and shared electrified heating and cooling systems from renewable sources of energy, where economies of scale and synergies between different types of consumers can be exploited. To this end, the focus of this paper is on the integration of shallow geothermal energy technologies into district heating and cooling systems. A key contribution of this work is the illustration of a number of practical case studies, highlighting the potential of existing shallow geothermal systems for DHC networks, which, as front runners in adopting such technologies, serve as paradigms for future development. Follows a discussion providing an outlook over the next 25 years. All in all, the future of utilizing shallow geothermal energy for district heating and cooling seems to be promising to play a pivotal role in sustainable urban development and decarbonizing the heating and cooling sector.