Numerical analysis of the thermo-mechanical behavior of energy piles

A finite element model is developed to investigate the thermomechanical behavior of the energy pile in detail. In the model, soil is regarded as a kind of thermo-elastic-perfectly plastic material, and the interaction between the pile and soil is modeled by contact elements. In order to save the computing time, the U-tubes in the energy pile are simulated by line elements, which are proved to be suitable for calculating the temperature of the pile. To deal with the thermomechanical multi-field problem, the sequential coupling method is utilized. The simulation results show that the thermally induced stress and deformation in the pile can be significantly influenced by the properties of the soil, the applied mechanical load and the restraint condition at the pile head. Long-time simulations of the energy pile with cyclic heat injections and extractions indicate that the thermal cycles would induce an unrecoverable settlement of the pile, and the ultimate bearing capacity of the energy pile may need to be redefined in view of its longtime performance.