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. ... Read more

The mechanical behaviour of soils under non-isothermal conditions is a crucial factor in the analysis and design of energy geostructures. To investigate this behaviour, thermal direct shear tests were conducted in this study within the practical operational temperature range (8-45°C) of energy geostructures. Normal stress was applied in the range of 100 and 400 kPa. Within the scope of the study, continuous heating and cooling was adopted as thermal loading protocol and therefore, the effect of creep was not examined. Furthermore, the impact of soil type on the temperature-dependent shear strength parameters of soils was investigated. Results show that hardening behaviour was observed for normally consolidated (NC) clays while there was no change in the behaviour of sand. There was an increase in apparent cohesion for both heating and cooling episodes. Angle of friction remained unchanged for sand during tests while in NC clays lower friction angle is observed at higher temperatures. Obtained experimental results are reported and used to discuss the practical implication of the dependency of soil strength parameters on temperature in design and performance of energy geostructures. ... Read more

The thermal volumetric behaviour of soils plays a critical role in designing energy geostructures, withstanding temperature fluctuations. This study examined, for the first time, the thermal deformation of the partially saturated kaolin clay (matric suction of 0–300 kPa) within the operating temperature range of energy geostructures (8 °C to 45 °C), subjected to varying most recent stress histories (normal stress of 0–400 kPa). The thermal cycle has been applied to samples with identical hydro-mechanical stress histories initiated by heating or cooling from room temperature. Scanning electron microscopy (SEM) tests have been performed to investigate the impact of temperature on soil microstructure as a potential determining mechanism of thermal deformation. The volumetric deformation associated with thermal cycles is analysed, considering the concurrent role of the overconsolidation ratio and the most recent stress. Secondary thermal consolidation (i.e., particle rearrangement) mainly depends on the most recent stress history, occurring only in samples heated beyond the yield limit, with cooling preventing secondary thermal consolidation. A clear relationship between thermal volume change and matric suction was observed, with higher matric suction resulting in less pronounced particle rearrangement. Further SEM analysis revealed that heating beyond the yield limit alters the soil microstructure permanently, with cooling showing no impact. ... Read more

In unsaturated soil mechanics, the soil–water retention curve (SWRC) continues to play an important role, since it provides the necessary links between the properties and behaviour of unsaturated soils with a variety of engineering challenges. The temperature has been identified as the main factor influencing SWRC as compared to a variety of other parameters. The goal of this research is to describe theoretical and experimental aspects of the temperature effect on unsaturated soil water retention phenomena. Theoretically, a brief review of the constitutive laws governing the thermal-hydro-mechanical (THM) behaviour of unsaturated soils is presented, along with links between variations in suction with water content, temperature, and void ratio. It also provides a broad framework that would to be well adapted to describing many specific circumstances. Through a closed-form predictive relationship that is developed in this framework, the effect of temperature is examined. By using this relationship, the soil–water retention curve at arbitrary temperature could be determined from one at a reference temperature, therefore significantly decreasing the number of tests necessary to describe the thermo-hydro-mechanical behaviour of a soil. Besides, the SWRC of kaolinite clay was also measured at three different temperatures in an experimental program. The test findings reveal that when the temperature rises, the SWRC decreases significantly. The experimental results were then integrated with sixteen other available data sets covering a wide range of soil types, densities, and suction to create a complete verification program for analytical models. The proposed model has a good performance and reliability in forecasting the fluctuation of non-isothermal SWRC than any existing model, according to statistical assessment results. The analytical model can be used to examine the thermo-hydro-mechanical characteristics of unsaturated soils in numerical simulations. ... Read more

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]. ... Read more

Energy geostructures have been identified as a cost-effective mitigating strategy for the adverse impact of climate change. Operation of energy geostructures results in temperature fluctuation and subsequent water migration, particularly at the soil–structure interface, determining the shear response of soil and soil–structure interface. This state-of-the-art paper brings together experimental data from direct shear tests carried out on the soil–structure interface from several laboratory investigations, presenting a comprehensive review to gain a thorough understanding of the interface response in different thermo-hydro-mechanical states, which is critical in the analysis and design of energy geostructures. First, the evolution of shear strength parameters, i.e., adhesion and friction angle, with matric suction and temperature, are investigated. Then, a more detailed analysis of the impact of matric suction and temperature on the shear strength of the soil–structure interface is provided. Furthermore, a comprehensive discussion is provided in this section on the role of the most recent stress history in determining the non-isothermal shear strength of an interface. Data on the effect of matric suction and temperature on shear parameters of the corresponding fundamental soil is reviewed as a reference to the interface behaviour throughout the study, revealing potential underlying mechanisms. In general, a higher matric suction results in higher shear strength of the interface, whereas non-isothermal variations in adhesion and friction angle may lead to a higher or lower shear strength of a saturated interface. ... Read more

The analysis and design of energy geostructures are mainly characterised by the mechanical behaviour of the soil–structure interface in non-isothermal conditions. In this study, direct shear tests are conducted to investigate the shear behaviour of soil and soil–structure interface in the practical temperature range of energy geostructures (i.e., 8–45 °C). The interface in this study is formed of kaolin in contact with concrete specimens with different roughness. Tests are performed on normally consolidated and overconsolidated interfaces following the unloading/reloading paths to better understand the impact of thermal strain on the interface behaviour. The volumetric response of the interface is observed to be highly influenced by the thermal strains experienced during heating/cooling. The soil stress level and the most recent soil stress history are identified as the primary determinants of thermally induced changes in interface shear strength. For normally consolidated interfaces, the temperature increase led to higher adhesion and slightly lower friction angle, whereas higher adhesion and identical friction angles were found for tests conducted on cooled specimens. Temperature does not seem to affect the shear strength of overconsolidated interfaces. Finally, a conceptual understanding of the temperature effect on interface shear behaviour is provided by analysing data from the literature. ... Read more

The multi-physical phenomena, particularly water content and temperature variations, governing the behaviour of soils should be considered in the design and analysis of the energy geostructures. Soil temperature and water content variations impose a significant risk on the stability and serviceability of existing and future geostructures. Although potential failure modes, impacts at a system scale, and the response of saturated soils to thermal loads are previously discussed, interpretation of the thermo-hydro-mechanical behaviour of partially saturated soils in the context of energy geostructures is not thoroughly investigated. In this regard, this paper brings together the experimental data from several laboratory investigations to attain a comprehensive understanding of the partially saturated fine-grained soils response under thermo-hydro-mechanical loading, which plays a vital role in the analysis of the soil behaviour and energy geostructures in contact with them. In this paper, the effect of thermal loading in different matric suctions and hydraulic loading at different temperatures on soil preconsolidation stress, water content variation, thermal and hydraulic conductivities, and compression indexes are studied. Furthermore, soil thermal deformation is studied in detail for different overconsolidation ratios and matric suctions. ... Read more

In unsaturated soil mechanics, the soil-water retention curve (SWRC) continues to play an important role, since it provides the necessary links between the properties and behaviour of unsaturated soils with a variety of engineering challenges. Temperature is acknowledged as one of the most influencial variables on the change of SWRC when compared to several other parameters. The goal of this research is to describe theoretical and experimental elements of the temperature effect on unsaturated soil water retention phenomena. Surface tension, contact angle, void ratio, particle size, and water density are five temperature-dependent factors that were accounted for in a closed-form model of non-isothermal SWRC. The SWRC of kaolinite clay was also measured at three different temperatures in an experimental program. The test findings reveal that when the temperature rises, the SWRC decreases significantly. The experimental results were then integrated with sixteen other available data sets covering a wide range of soil types, densities, and suction to create a complete verification program for analytical models. The proposed model has a good performance and reliability in forecasting the fluctuation of non-isothermal SWRC than any existing model, according to statistical assessment results. The analytical model can be used to examine the thermo-hydro-mechanical characteristics of unsaturated soils in numerical simulations. ... Read more

A new framework is developed to extend an existing small-strain shear modulus (Gmax) model to determine Gmax of unsaturated silty soils along different paths of the soil water retention curve (SWRC) including the scanning loops. The suitability of the proposed framework is validated against experimental results of a series of bender-element tests performed in this study and data reported in literature. Measured values of Gmax showed a slight hysteresis in the Gmax measurements along the scanning curves of the SWRC, with lower values along the wetting scanning paths. However, results indicated that the value of Gmax was recovered once the main drying path was reached. Results of this study also indicated that Gmax behavior of the silty specimen along the scanning curves was stress dependent. The model was observed to follow the experimental data along different paths of the SWRC including scanning curves. ... Read more

Hydro-mechanical behavior of unsaturated collapsible soils can be drastically affected by saline infiltrations. Saline infiltrations into the underlain soil layers of the urban transportation and lifelines due to deicing or existence of traffic pollutants has been a common issue of concern for years in many countries. Additionally, many reports have been published so far, indicating the construction of roads and railways on unsaturated collapsible soils. In this research, hydro-mechanical characteristics of an unsaturated collapsible loessial soil diluted with saturated solutions of three different salts that are frequently involved in transportation infrastructure (namely NaCl, CaCl2 and KCl) are investigated by conducting scanning electron microscopy (SEM), filter paper, uniaxial compression and oedometer tests. Obtained test results have been analyzed by simultaneous consideration of the matric and osmotic suctions in the soil texture and the applied stresses. Test results revealed that the soil texture and hydro-mechanical responses of the soil are considerably affected by the salt type, saline degree of saturation and applied loading paths. Results also imply that there is a critical saline degree of saturation (corresponded to each loading path and each mixing salt type), at which the magnitude and modality of the osmotic and matric suctions within the soil fabric are changed. In addition to experimental studies, empirical constitutive models are presented in this paper to predict the changes in strength, stiffness and yield stress of the tested collapsible soil subjected to different road salts. Comparisons show very good agreement between the laboratory test results and the model predictions. The achievements presented in this research can be directly implemented for better prediction of the behavior of unsaturated collapsible soils subjected to salinity. In addition, the result can be implemented in a suitable stabilization plan when the soil and salt involved in transportation infrastructures. ... Read more