Numerical study of energy piles in unsaturated soils

Available literature on energy piles typically assumes surrounding soil to be either completely dry or fully saturated. However, in realistic scenarios, the surrounding soil is often unsaturated. The actual saturation condition of the soil surrounding an energy pile depends on the hydraulic properties of the surrounding soil and the location of the groundwater table relative to the pile tip. It is crucial to account for the real soil saturation conditions while analyzing energy piles to accurately estimate the thermal performance of energy piles and the hydro-thermal (HT) behavior of the surrounding soil, which is essential for a full thermo-hydro-mechanical (THM) modeling of energy pile systems. To this end, this study presents a fully coupled HT formulation for unsaturated soils, along with a thermal formulation for completely dry and fully saturated soils. These formulations are numerically implemented within a finite element method (FEM)-based framework and employed to simulate a three-dimensional field-scale energy pile to examine the effects of saturation conditions and hydraulic properties of the surrounding soil on the thermal performance of energy piles. Furthermore, the study investigates the hydraulic behavior of the surrounding unsaturated soils and highlights its implication on the thermal response of unsaturated soils surrounding energy piles.