Thermomechanical Soil–Structure Interaction in Single Energy Piles Exhibiting Reversible Interface Behavior

Analytical solutions for displacement, strain, and stress in a single energy pile provide rational, mechanics-based qualitative and quantitative understanding of thermomechanical load transfer mechanism in energy piles. To this end, thermomechanically induced axial displacement, strain, and stress evolutions are derived for different tip restraints and for several different loading scenarios including net heating, net cooling, and compressive and tensile mechanical loads. In addition, the analytical solutions are successfully validated against centrifuge test results. The analytical solution for the location of a thermal null point indicates that it depends on the geometry and stiffness of the pile, stiffness of the soil, and amount of head and tip restraints. In addition, the thermal null point moves upward into the top half of the pile with increase in the ratio between the amounts of head and tip restraints. It is also found that formation of a tension zone in energy piles is likely for several different load scenarios. At a constant mechanical load, the length of a tension zone depends on the geometry of the pile, ratio of the soil and pile stiffness, and the magnitude of thermal load. An increased magnitude of a net cooling combined with a constant compressive axial force increases the length of a tension zone and magnitude of tensile stress in the pile. On the contrary, the length of a tension zone and magnitude of tensile stress decrease with the increase in magnitude of heating at a constant tensile axial force. In summary, the analytical solutions and their features presented herein provide a fundamental, rational, mechanics-based framework for advancing the understanding of a load transfer mechanism and soil structure interaction in energy piles, thus contributing directly toward their safer design and wider use, and increased sustainability of civil engineering infrastructure.