Long-term thermomechanical performance of energy piles in sand: A semi-analytical solution using interface constitutive model

Energy piles combine foundation support and heat exchange functions, offering significant potential for geothermal applications, but they face challenges from cyclic thermal loading that affects their structural integrity. This paper presents a theoretical method for examining the long-term thermomechanical behavior of energy piles in sand. This method is based on two load-transfer models: a cyclic interface constitutive model that captures pile side–soil interaction and a cyclic hyperbolic model modified by Masing's criterion to represent pile tip–soil interaction. These models incorporate various phenomena at the pile–soil interface, including hysteresis, hardening, softening, degradation, and particle breakage, all of which arise from repeated shearing induced by thermal cycling. The proposed method has been rigorously validated, and comprehensive parametric studies have been conducted. The results demonstrate high consistency among predicted, measured, and simulated values, validating the accuracy and reliability of the current method for analyzing the thermomechanical behavior of energy piles. Pile head displacement exhibits a linear correlation with temperature increment during pure thermal loading. Thermomechanical loading increases the ultimate bearing capacity of energy piles under heating but decreases it under cooling, with the latter effect being more pronounced due to the reduced contribution of the pile-end resistance. Thermal cycling increases pile head settlement and axial force, while reducing upper shaft resistance and increasing the lower shaft resistance. Additionally, superstructure loads, cyclic variations in ground temperature, and their coupled effects have a significant impact on the long-term behavior of energy piles.