Effects of pile-soil interface degradation on the thermomechanical performance of energy piles

Energy piles are subjected to repeated shearing cycles at the interface due to the expansion/contraction of the pile during heating/cooling, which may affect the mechanical performance of the interface and their serviceability. Based on the pile-soil interface testing, a load transfer (τ-Δs) relationship that considers the cyclic response and strength degradation of the pile-soil interface is proposed and implemented into a three-dimensional finite element model to simulate the thermally induced stress and displacement of the energy pile under combined mechanical and cyclic thermal loadings in a simplified way that can be included in general purpose simulators. The model was shown to realistically capture the residual stresses and irreversible settlement of energy piles after thermal cycles when validated against field tests, emphasizing the importance of considering the shear characteristics of the pile-soil interface when conducting energy pile analyses. The sensitivity of the pile-soil interface response to thermal cyclic conditions and degradation characteristics was systematically investigated, demonstrating the importance of considering shaft degradation and its impact on the pile displacements, which is relevant in engineering practice.