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.

Energy piles are often operated in the form of pile group, and their thermo-mechanical (TM) behaviors are strongly affected by the operational strategies. A model experimental bench of energy pile group with the layout of 3 x 3 was established to investigate the TM behaviors of energy pile group under different start -stop time ratios and pipe connection forms. The test results show that under the current test conditions, raising the startstop time ratio leads to the increase of heat storage and extraction amount by the pile group, but the temperature of soil around the pile is not well recovered, which in turn results in the rise of deformation degree of pile. For the connection forms of pile buried pipe, the heat storage amount under three kinds of series connection forms is less than that under the conventional parallel connection form. However, the mechanical properties of pile group under the series connection forms are better than those of parallel connection form, which helps to alleviate the pile deformation degree. A 4 x 4 group pile model was developed to further find the effects of two operation strategies, namely, opening partial piles at different locations in winter mode and non -uniform intensity operation of piles in the inner and outer zones, on the TM characteristics of energy pile group. The results showed that opening partial piles increased the axial force of pile in the running piles relative to opening all the piles. Meanwhile, opening eight side piles extracted more heat per month during the heat extraction period relative to opening four internal piles and four corner piles. The four kinds of internal and external zoned non -uniform strength operation modes have different energy storage and thermodynamic properties. The most suitable mode should be selected considering the actual situation.

Energy piles, as innovative energy underground structure, serve the dual purpose of shallow extracting geothermal energy while bearing the upper building load. There are few studies on the thermomechanical properties of energy piles under combined horizontal and vertical loads. The temperature change of pile body under combined horizontal and vertical loads will result in variations in pile bending moment, horizontal and vertical displacement, etc. This paper investigated the deformation characteristics of energy piles under combined vertical and horizontal loads through model tests with 10 heating-cooling cycles applied to the piles. The results showed that the heating-cooling cycles under combined load led to further increase in the pile bending moment, particularly affecting the middle of the pile, with the maximum increase in pile bending moment reaching 117%. Additionally, the heating-cooling cycles caused cumulative displacement at the top of the pile. The vertical displacement of the test pile increased by 0.201 mm, and the increase in horizontal displacement due to the thermal cycles reached 1.46% D (D is the diameter of the pile). Simultaneously, the heating -cooling cycles induced a forward tilt of the pile, with the tilt angle reached 1.88x10(-3) rad after 10 heating-cooling cycles and gradually increasing with the number of thermal cycles. Moreover, the soil pressure in front of the pile decreased during heating, while increased during cooling.

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.