Estimation on bearing characteristics of a large-diameter and extra-long energy pile under various actions of mechanical and thermal loads

Energy piles, combining structural support with ground heat exchange, are increasingly used for sustainable building energy solutions. However, the thermo-mechanical behaviour of large-diameter, extra-long energy piles under real operational conditions remains inadequately understood. This study investigates the bearing characteristics of a 1.5 m diameter, 58.3 m long cast-in-place concrete energy pile installed beneath a liquefied natural gas tank, subjected to staged mechanical and thermal loading. A full-scale in situ experiment was conducted, simulating realistic load conditions, including static mechanical loading, combined thermal–mechanical loading during heating and cooling cycles, and cyclic loading effects. The results indicate that the pile exhibited high heat exchange efficiency, with rates of 115.6 and 130.3 W/m during heating and cooling, respectively. Axial expansion due to thermal loads led to the formation of a neutral plane at approximately −39 m depth, redistributing shaft resistances along the pile. Under heating, negative shaft resistance developed in the upper (−1.5 to −14 m) and lower (−39 to −51 m) parts, while positive resistance was concentrated in the mid-depth. Upon reloading, the negative resistance diminished, demonstrating mechanical dominance in pile behaviour. The pile head settlement reached −11.6 mm under maximum static loading (24 000 kN), with an additional −7.3 mm settlement observed during combined heating–loading. These findings confirm the feasibility of large-diameter energy piles for deep foundations, highlighting their stable thermo-mechanical response and informing future design optimisation for cyclic thermal and mechanical actions.