Axial Load Transfer Analyses of Energy Piles at a Rock Site

An axial load-transfer analysis for energy piles is presented in this study that incorporates empirical models for estimating the side shear resistance and end bearing capacity in rock along with associated normalized stress-displacement curves. The analysis was calibrated using results from field experiments involving monotonic heating of three 15.2 m-long energy piles in sandstone. Analyses of the field experiments indicates that poor cleanout of the excavations led to an end restraint smaller than that expected for a clean excavation in sandstone. Specifically, end bearing parameters representative of cohesionless sand were necessary to match the load-transfer analysis to the field experiment results. Parametric evaluations demonstrate the importance of using appropriate rock- or soil-specific empirical models when estimating the side shear resistance and end bearing capacity of energy piles. Specifically, the end bearing capacity and side shear resistance in rock are greater than in soils, leading to more restraint and greater thermal axial stresses. The stiffer side shear restraint in rock was also found to lead to a less nonlinear distribution in thermal axial stress along the length of the energy pile.