Thermal Response of an Energy Pile Subjected to Long-Term Dynamic Operation of a Building Ground Source Heat Pump System

The thermal response of energy piles induced by long-term dynamic building thermal loads may differ from short-term steady-state predictions. This study presents a full-scale field test of an energy pile subjected to the operation of a building ground source heat pump system. Spanning a nearly complete annual cycle of 312 days, the evolution of the pile’s heat transfer performance, thermomechanical behavior, and soil temperature field under dynamic and steady thermal loads was compared. Results indicate that long-term dynamic thermal loads facilitated radial heat diffusion in the soil, mitigating near-pile heat accumulation. Consequently, the pile temperature changes during dynamic thermal load tests decreased compared to short-term steady-state conditions. Both pile and soil temperature changes exhibited hysteresis loops with respect to cumulative heat transfer quantity, confirming the thermal inertia of the pile–soil system. Notably, driven by thermal lag and radial heat diffusion, the mobilized side resistance experienced relaxation, increasing the degree of freedom (DOF) in the upper and middle pile sections by 4.5%–23.5%. This resistance relaxation led to a reduced maximum thermal stress and a 133% increase in the pile head thermal displacement rate relative to steady-state conditions. These findings provide new insights into evaluating the long-term thermomechanical behavior of energy pile foundations.