Abstract: Energy piles are an innovative building energy-saving technology that serves the dual purposes of structural load-bearing and geothermal energy exploitation. The impact of different operational modes and long-term service on the heat transfer performance of the pile and the thermal response of the surrounding soil cannot be overlooked. However, there is a relative scarcity of systematic research on the evolution patterns of thermal responses between piles and soil under long-term service across various operational modes. Addressing this gap, model tests of energy piles in saturated soft soil conditions were carried out under different operational modes. A hydrothermal numerical model for energy piles was developed and validated against experimental findings. The study primarily focused on examining the effects of long-term service and atmospheric temperature on the thermal response of energy piles and the extent of thermal influence on the soil under different operational modes. The results reveal that in short-cycle tests, a longer thermal recovery period corresponds to a higher heat transfer power upon completion of the same thermal cycle. The pile temperature demonstrates a pattern where the central section heats up sharply, while the ends experience a milder temperature rise. Atmospheric temperature exerts a notable influence on the temperature of the upper soil layer, and the thermal influence range of the pile foundation progressively expands in a spindle-like manner. During long-cycle simulations, the proportional disparity in heat exchange powers among various operational modes progressively widens over time, peaking at up to 60%. The maximum temperature differential within the pile foundation consistently hovers around 3 ℃, with relatively gentle fluctuations in the surrounding soil temperature. In the initial phase of the cycle, the thermal influence radius increases linearly with depth until it reaches a certain value and then stabilizes; in subsequent phases, it continues to expand linearly with depth. A longer thermal recovery period within the cycle correlates with a relatively smaller thermal influence radius. Nevertheless, over extended operational periods, the discrepancies in thermal influence radii among different modes gradually diminish.

Key words: energy pile, model test, operation mode, long-term thermal responses, thermal impact range of pile foundation