The thermo-mechanical (TM) behaviour of the energy pile (EP) group becomes more complicated in the presence of seepage, and the mechanism by which seepage impacts the EP group remains unclear.In the current work, a 2 x 2 scale model test bench of EP group was set up to investigate the TM behaviour of EP group with seepage. The test results indicate that the heat exchange performance of EP group with seepage can be significantly enhanced, but also leads to obvious differences in the temperature distribution of pile and surrounding soil along the seepage direction, and thus causes evident differences in the mechanical properties between the front pile and the back pile in pile group. Compared with the parallel connection form, the thermal performance of EP group with the series connection form is slightly attenuated. However, the mechanical properties of various piles in the EP group differ significantly. Under the action of seepage, the mechanical balance properties of various piles in the forward series form are optimal, followed by the parallel form, and the reverse series form is the least optimal. A 3-D CFD model was established to further obtain the influence of seepage and arrangement forms on EP group. The findings indicate that seepage can not only mitigate thermal interference between distinct piles but also expedite the process of heat transfer from pile-soil to reach a state of stability. Concurrently, the thermal migration effect induced by seepage will be superimposed along the seepage direction, resulting in the elevation of thermal interference of each pile along the seepage direction, and the superposition of thermal migration effect increases with the time. Under the same seepage condition, the cross arrangement can enhance the thermal performance of EP group, optimize the temperature distribution of pile and soil, and thus the imbalance of mechanical properties among pile groups can be reduced. In addition, the concepts of thermal interference coefficient and heat exchange rate per unit soil volume are introduced to facilitate a more precise evaluation of the thermal interference degree of each pile in the pile group and the heat exchange performance under different pile arrangement forms.The standard deviation and mean value in the statistical method are used to evaluate the equilibrium of mechanical properties of pile group, which is more intuitive to compare the differences in mechanical properties of pile groups under different working conditions.

Energy piles, which serve the dual functions of load-bearing and geothermal energy exchange, are often modeled with surrounding soil assumed to be either fully saturated or completely dry in existing design and computational methods. These simplifications neglect soil saturation variability, leading to reduced predictive accuracy of the thermomechanical response of energy piles. This study proposes a novel theoretical framework for predicting the thermo-hydro-mechanical (THM) behavior of energy piles in partially saturated soils. The framework incorporates the effects of temperature and hydraulic conditions on the mechanical properties of partially saturated soils and pile-soil interface. A modified cyclic generalized nonlinear softening model and a cyclic hyperbolic model were developed to describe the interface shear stress-displacement relationship at the pile shaft and base, respectively. Governing equations for the load-settlement behavior of energy piles in partially saturated soils were derived using the load transfer method (LTM) and solved numerically using the matrix displacement method. The proposed approach was validated against experimental data from both field and centrifuge tests, demonstrating strong predictive performance. Specifically, the average relative error (ARE) was less than 15% for saturated soils and below 23% for unsaturated soils when evaporation effects were considered. Finally, parametric analyses were conducted to assess the effects of flow rate, groundwater table position, and softening parameters on the THM behavior of energy piles. This framework can offer a valuable tool for predicting THM behavior of energy piles in partially saturated soils, supporting their broader application as a sustainable foundation solution in geotechnical engineering.

The incorporation of PCMs in energy piles holds significant potential for revolutionising thermal management in construction, making them a crucial component in the development of next-generation systems. The existing literature on PCM-integrated energy piles largely consists of isolated case studies and experimental investigations, often focusing on specific aspects without providing a comprehensive synthesis to guide future research or practical applications. To date, no review has been conducted to consolidate and evaluate the existing knowledge on PCMs in energy piles, making this review the first of its kind in this field. Up until now, this gap in research has limited our understanding of how PCM configurations, thermal properties, and integration methods impact the thermal and mechanical performance of these systems. Through thoroughly analysing the current research landscape, this review discovers key trends, methodologies, and insights. The methodology used here involved a systematic search of the existing SCI/SCIE-indexed literature to ensure a structured review. Based on the SLR findings, it is evident that current research on PCMs in energy piles is focused on improving thermal efficiency, heat transfer, and compressive strength. Furthermore, precise adjustments in melting temperature significantly impact efficiency, with PCM integration boosting thermal energy extraction by up to 70 % in some cases, such as heating cycles, and saving up to 30 % in operational costs. PCMs also reduce soil temperature fluctuations, improving structural integrity through minimising axial load forces. However, challenges remain, including reduced mechanical strength due to voids and weak bonding, high costs, and complexities such as micro-encapsulation. We acknowledge that there are gaps in addressing certain key factors, including thermal diffusivity; volume change during phase transitions; thermal response time; compatibility with construction materials; interaction with soil, creep, and fatigue; material compatibility and durability; and the long-term energy savings associated with PCM-GEP systems.

Composed of a raft and pipe piles with embedded heat exchange devices, a pipe-type energy piled raft foundation can enhance both foundation performance and energy utilization efficiency. An urgently needed thermomechanical analysis method would facilitate the optimization design and the broader adoption of this fundamental form. Therefore, this paper proposes an efficient method for the thermo-mechanical analysis of pipe-type energy piles with a raft in layered transversely isotropic media. The pile-soil and raft-soil interaction equations are derived by coupled finite and boundary element method. A simplified approach is then proposed and applied to tackle the pile-raft-soil coupling interaction. The correctness and efficiency of the method are verified through comparisons with a field test and two finite element numerical cases. Finally, parametric analyses are conducted to investigate the influences of temperature increment, pile thickness, raft thickness, and soil anisotropy on the performance of the pipe-type energy piled raft foundation.

Energy pile is a green, constant-temperature utilization technology with dual functions of heat exchange and load bearing. Improving its heat transfer efficiency has always been one of the main directions of scholars' research. This study discussed the optimization of heat transfer buried pipe parameters, modification of pile materials, and improvement of working fluid performance within the pipes. Additionally, based on the research achievements of the research team in recent years regarding heat transfer enhancement in energy piles, a comprehensive heat transfer enhancement system is summarized, aiming to provide new ideas and methods for the study of heat transfer enhancement in energy piles. The optimization status of different buried pipe types and pipe parameters is also summarized. The heat transfer performance and mechanical properties of different modified concrete materials are studied. A comparison and analysis of the heat transfer performance and flow characteristics of different types of circulating mediums with nanofluids are conducted, providing new approaches to improve the heat transfer performance of circulating mediums. Finally, discussions and prospects were made on the external environmental conditions around the pile, thermal interference phenomena of pile groups, energy storage concrete, the long-term stability of nanofluids, benefit assessment, and ecological evaluation. These efforts aim to promote research on energy piles both domestically and internationally.

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.

Energy pile groups transmit geothermal energy and have attracted widespread attention as one of new building energy-saving technologies. Accurately predicting the time-dependent behaviors of energy pile groups is a challenge, given the complex thermal and mechanical interactions between piles, surrounding soils and the pile cap. This study presents a semi-analytical solution for analyzing energy pile groups within heat exchangers. Utilizing the transformed differential quadrature method, a flexible coefficient matrix for the saturated surrounding soils is acquired, which accounts for both consolidation and heat transfer. The piles are segmented, and the discrete solving equations considering thermal stresses and expansion are formulated. To accurately reflect the interactions among piles-to-piles, piles-to-soils and piles-to-pile cap, the coupled matrix equations are constructed with involving both the displacement coordination and the force equilibrium at the pile-soil interface as well as the pile cap. The validity of the proposed solution is confirmed through comparisons with results from onsite tests and simulations using COMSOL. Pivotal parameters including temperature variations, pile spacing, and the relative stiffness are discussed through examples. Compared with traditional simulation and field test, the proposed solution enables fast and accurate prediction of displacement and load distribution across pile groups, facilitating the safety evaluation of heat exchangers.

The operational performance of energy pile (EP) group with seepage is strongly influenced by seepage parameters. In this paper, a model test system of 2 x 2 EP group with seepage is built to study the influences of seepage water level and seepage velocity on thermo-mechanical behaviour of EP group. Also, a numerical model of EP group considering seepage is developed to obtain the variations of thermo-mechanical behaviour of EP group under different seepage parameters. The findings demonstrate that an augmentation in seepage water level can enhance the heat exchange performance of EP group, but it also exacerbate the imbalance of mechanical properties between piles in the short term, in which the seepage only have a significant effect on the temperature of piles and soil below the seepage water level. Increasing seepage velocity and circulating flow rate can strengthen thermal performance of EP group and improve the equilibrium of pile axial force and displacement between the pile groups, but increasing seepage velocity also increases the imbalance of mechanical properties between the front and back rows of pile group. At the same time, compared to the circulating flow rate, the change in seepage velocity has a dominant impact on the thermo-mechanical characteristics of EP group. Moreover, when the seepage angle is within 0-45 degrees, increasing the seepage angle can effectively improve the heat transfer performance of EP group, and the temperature distribution of pile and soil is obviously different for different seepage angles, in which the mechanical properties of EP group have the best equilibrium when the seepage angle is 30 degrees for current simulation conditions.

This study focuses on predicting the impacts of a heating-cooling cycle on the pullout capacity of energy piles installed through a soft clay layer. Geotechnical centrifuge physical modeling was used to evaluate temperature, pore water pressure, volume change, and undrained shear strength profiles in clay layers surrounding energy piles heated to different maximum temperatures to understand their impacts on the pile pullout capacity. During centrifugation at 50 g, piles were jacked-in at a constant rate of penetration into a kaolinite clay layer consolidated from a slurry in a cylindrical aluminum container, heated to a target temperature after stabilization of installation effects, cooled after completion of thermal consolidation requiring up to 30 hours (1250 days in prototype scale), then pulled out at a constant rate. T-bar penetration tests were performed after the heatingcooling cycle to assess differences in clay undrained shear strength from a baseline test. The pullout capacity of an energy pile heated to 80 degrees C then cooled to ambient temperature was 109 % greater than the capacity in the baseline test at 23 degrees C, representing a substantial improvement. The average undrained shear strength measured with the T-bar at a distance of 3.5 pile diameters from the pile heated to 80 degrees C was 60 % greater than at 23 degrees C but followed the same trend as pile capacity with temperature. An empirical model for the pullout capacity was developed by combining predictions of soil temperature, thermal excess pore water pressure, thermal volumetric strain, and undrained shear strength for different maximum pile temperatures. The empirical model predictions matched well with measured pullout capacities.

This paper presents a study on model tests of single energy piles subjected to cyclic axial loads in sand and the development and validation of a 3D thermo-mechanical finite element model. The model accurately simulated the behavior of the pile-soil interface under cyclic shear loads. A subsequent parametric analysis examined the effects of the number of loading cycles and the loading amplitude on the vertical dynamic response characteristics of energy piles. The results showed that under heating conditions, the maximum variation in compressive thermal stress in the energy pile gradually decreased, with its location shifting upward along the pile shaft. A critical cyclic amplitude ratio was identified: below this threshold, the rate of increase in pile tip resistance continuously increased while the average pile side resistance weakened progressively. The presence of a static load accelerated the weakening of the average pile side resistance to some extent. As the number of loading cycles increased, the settlement rate of the energy pile gradually degraded. The cumulative settlement rate at the pile top increased with the cyclic amplitude ratio, peaking before slightly declining. In comparison, the static load ratio had a relatively minor influence on cumulative settlement.