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.

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.

In this paper, numerical simulations of a special energy pile, which constitutes a spiral-injected pipe and one straight discharge pile for Geothermal Heat Pump Systems (SGHEs-P(parallel)), were conducted by Fluent software. The effects of the spiral pitches on the heat transfer rate based on the G-function method and peripheral soil temperature of the pile were investigated under continuous and intermittent operation strategies. The impact of spiral tube sizing on the surface heat transfer coefficients was studied. The results indicated that SGHEs-P may be preferred for office buildings under intermittent operation conditions. For a short period, the temperature profiles and heat transfer efficiency of SGHEs-P were mainly influenced by the fluid type, length of the spiral tube, and spiral pitch. The smaller the spiral pitch, the more uniform the temperature distribution, and the better the heat transfer effect, but the heat transfer per unit depth of pile decreased. The average temperature variation curve of the soil around the energy pile with different spiral pitches was simulated and obtained over time. Meanwhile, the impact of spiral radius, spiral pitch, and spiral tube radius on the convective heat transfer coefficient was also presented. Through data fitting, the formulas for the correction coefficients of spiral radius, spiral pitch, and spiral tube radius on convective heat transfer coefficient were obtained, respectively.

The combination of phase change materials (PCMs) with building materials is a flourishing technology owing to the low-temperature change of the materials during phase change and the potential for enhanced heat storage and release. In this study, a new type of PCM energy pile, in which 20 stainless steel tubes (22 mm in diameter and 1400 mm in length) filled with paraffin were bound to heat exchange tubes, was proposed. An experimental system monitored by a fiber Bragg grating (FBG) to study the thermo-mechanical behavior of energy piles and surrounding soil was established. Both the PCM pile and the ordinary pile, with the same dimensions, were tested under the same experimental conditions for comparison. The results indicate that the temperature sensitivity coefficient calibration results of the FBG differ from the typical values by 8%. The temperature variation is more obvious in the ordinary pile and surrounding soil. The maximum thermal stress of the ordinary energy pile is 0.5 similar to 0.6 times larger than that of the PCM pile under flow rates ranging from 0.05 m(3)/h to 0.25 m(3)/h. The magnitudes of the pore water pressure and soil pressure variations were positively correlated with the flow rates.