Numerical Analysis of the Long-Term Performance of Energy Piles in Sand

Abstract

Energy piles are piles equipped with heat exchange pipes through which a heat-carrying fluid circulates and exchanges heat with the ground. This technology couples the structural role of classical pile foundations with the energy supply of heat exchangers. During heating and cooling processes, the temperature of the energy pile and the ground will change seasonally. Due to the thermal displacement incompatibility between the pile and the soil, the load transfer mechanism of energy piles is different to that of conventional piles which are only subjected to mechanical loadings. In order to improve the understanding of the long-term performance of energy piles in sands, a series of coupled thermal-stress finite element analyses were carried out. In the analyses, the bounding surface plasticity model was used to describe the nonlinear behavior of sands under monotonic and cyclic loadings. The thermally induced displacement and axial force in the pile, the thermally induced change in the soil stress, and the ultimate pile resistance after thermal cycles were discussed. The numerical results indicated that the soils around the energy pile were subjected to cyclic mechanical loadings caused by repeated temperature variations. The accumulation of plastic strains resulted in a significant increase in the pile head settlement for the free head pile and a significant decrease in the pile head reaction force for the restrained head pile. During the reloading stage, the thermally induced decrease in the shaft resistance was compensated by the soil dilatancy, the ultimate pile resistance after thermal cycles did not change remarkably.