Thermomechanical response characteristics of full-scale prestressed high-strength concrete energy pile under the flexible constraint of cushion layer

Energy piles serve a dual function of harnessing geothermal energy and supporting structural loads. Existing studies of energy piles mainly focus on free pile heads or rigid pile-raft connections, with limited attention to flexible cushion connections. The study analyzed the heat exchange performance and structural response of a prestressed high-strength concrete (PHC) energy pile whose pile head was flexibly constrained by a cushion layer in an existing building under continuous heating mode through field tests and numerical investigations. In addition, numerical simulations were conducted to analyze cooling conditions, and the effect of soil thermal expansion coefficient and cushion layer strength on the thermomechanical characteristics of the energy pile was considered. The results showed that the PHC energy pile experienced significant additional compressive or tensile stress after heating or cooling, resulting in overall compression or tension of the pile and a downward shift of the neutral point. The pile-soil axial stress ratio increased with the rise in pile temperature and decreased with the reduction in pile temperature. The constraint level at the pile head under the connection form of the cushion layer was between the free pile head and the pile-raft constraint. The variation in the soil thermal expansion coefficient did not cause a significant change in the constraint of the pile. Increasing the strength of the cushion layer to a certain extent increased the constraint stress of the pile, but the effect gradually weakened along the pile depth.