Group Performance of Energy Piles under Cyclic and Variable Thermal Loading

Abstract

Energy piles, a new type of heat exchanger that serves dual purposes, have gained increasing attention due to the growing energy demand and corresponding carbon emissions. Depending on intended operational requirements or by accident, energy pile groups may be subjected to cyclic and variable thermal loadings. This study presents the results from a series of full-scale field tests of a 2×2 energy pile foundation. The energy pile group was operated partially or fully during the tests to investigate the potential effects of cyclic nonuniform thermal loadings. The results indicated that the cyclic thermal loadings could induce an increase in compressive stress of piles at the end of the experiments. Further comparative analysis showed that the residual compressive stress was attributed mainly to the drag-down effects of the surrounding soil. The compressive stress induced by drag-down effects will overlap with the thermally induced stress, and thus lead to a more significant effect on the energy piles. If the drag-down effects are presented, the stress change may exceed the theoretical upper bound of thermal stress and may be underestimated. In addition, cyclic nonsymmetrical thermal loadings could induce accumulation of differential settlement and group tilt in the energy pile group. It was shown that the group tilt caused by cyclic and variable thermal loadings can be acceptable for general engineering structures. The group tilt will not accumulate further when subjected to symmetrical thermal loadings. A reasonable arrangement of operational piles still can be used to avoid unwanted group tilt generation.