A Non-Isothermal Viscoplastic Framework for Assessing Shaft Resistance in Energy Geostructures

This study presents a coupled thermo-hydro-mechanical (THM) numerical investigation of shaft resistance degradation in geothermal piles embedded in low-permeability clays. A thermo-viscoplastic constitutive model is developed to simulate the pile-soil interface behavior under non-isothermal conditions. The model captures the evolution of excess porewater pressure, effective stress reduction, and their combined impact on shaft bearing capacity during thermal activation. Parametric simulations are carried out for a range of fluid temperatures (21-50 • C) and soil permeabilities (10-6-10-14 m 2), representative of in-situ conditions. Results show that undrained heating leads to substantial excess porewater pressure buildup, especially at low permeability and high temperature, resulting in significant reductions in zero-thickness interface element shear strength. A shaft resistance reduction function is proposed to quantify this phenomenon. The numerical predictions are benchmarked against full-scale energy-pile measurements, showing close agreement in temperatures, strains, uplift, and stress. The findings underline the critical role of thermal pressurization in energy pile design and provide an enhanced modeling strategy for performance evaluation in fine-grained soils.