A Temperature-Dependent Model for Ultimate Bearing Capacity of Energy Piles in Unsaturated Fine-Grained Soils

Abstract

This study presents an analytical framework to estimate the change in ultimate bearing capacity of energy piles in unsaturated fine-grained soils under drained mechanical loading conditions after drained heating. The framework was developed by extending conventional methods for the ultimate bearing capacity of piles in unsaturated soils to temperature-dependent conditions, where thermally induced changes in the characteristics of the unsaturated soil and soil–pile interface are considered. Specifically, the thermally induced variations in matric suction and effective saturation profiles with depth were incorporated into calculations of the shaft capacity and the end bearing capacity of piles in unsaturated soils. The proposed ultimate bearing capacity model is validated against experimental data for an energy pile loaded to failure in unsaturated Bonny silt, and a good match between measured and predicted values was obtained. A parametric study was carried out to evaluate the effects of infiltration rate and pile aspect ratio (i.e., pile embedment length/pile diameter) on the ultimate bearing capacity of energy piles in unsaturated clay and silt layers subjected to temperatures ranging from 5°C to 45°C. For both soils, the shaft, end bearing, and ultimate bearing capacities vary with an increase in temperature. At the reference temperature, the shaft, end, and ultimate bearing capacities vary monotonically with pile embedment length, while at elevated temperatures they vary nonmonotonically with pile embedment depth. At a given temperature, the parametric study shows that the bearing capacity of energy piles in clay decreases with increasing downward infiltration of water into the soil profile surrounding the energy pile, while in silt it may decrease or increase depending on pile embedment length. The ultimate bearing capacity increases with a decrease in pile aspect ratio at all temperatures. Estimates of the ultimate bearing capacity of energy piles in unsaturated fine-grained soils from the framework are a critical part of thermomechanical soil–structure interaction analyses needed to design energy piles, so this study contributes toward the widespread application of this emerging technology in practice.