A unified thermo–hydro–mechanical load-transfer framework for energy piles: Quantifying interfacial softening

Abstract

Energy piles, which serve concurrently as structural foundations and ground source heat exchangers, exhibit complex, coupled thermo-hydro-mechanical (THM) load-transfer responses that are often poorly predicted by conventional models. Current methodologies predominantly simplify the interaction, focusing primarily on temperature-induced pile expansion while overlooking crucial changes in the surrounding soil properties and interface behaviour. This paper presents a novel, unified load-transfer approach designed to accurately capture the nonlinear, multi-factor performance of energy piles embedded in multi-layered soils. The model's uniqueness lies in the simultaneous incorporation of advanced constitutive relationships that account for the temperature dependence of key geotechnical parameters, including thermal expansion/shrinkage of pile materials, radial thermal stress, total stress, particle contact area ratio, pore-water pressure, internal friction angle, effective cohesion, overconsolidation ratio, and suction stress. This framework explicitly integrates the effects of thermal softening of the soil skeleton and the generation of thermally induced excess pore-water pressure. The complex non-linear equilibrium is solved using an iterative Neutral Plane (NP) procedure to precisely determine the distribution of axial forces and skin friction. The predictive capability of the model is rigorously validated against three distinct full-scale field tests across diverse soil types: sandy silts, granular soils, and high-plasticity clays. Results show that the proposed method achieves high accuracy, with an average relative error ranging from 3% to 8.2% across all validation cases. Crucially, the analysis demonstrates that thermal effects significantly decrease or increase interface resistance depending on site characteristics, an observation that cannot be replicated when only pile expansion is considered. This work provides a robust, physics-based predictive tool essential for mitigating design risks associated with THM coupling, advancing the safe and efficient integration of geothermal energy systems into foundational engineering practice.