Pull-Out Response of a Laboratory-Scale Energy Pile Subjected to Cooling Cycles

During winter, energy piles are subjected to cooling cycles due to the cyclic operation of the ground source heat pump (GSHP). The cyclic operation of the heat pump causes the energy pile to contract and expand axially and radially, affecting the soil–pile interaction. The effects of cooling cycles on the thermomechanical behavior of energy piles have not been fully investigated. A fully instrumented model concrete energy pile was installed in dry sand and subjected to cooling cycles. Cooling was achieved by circulating a heat exchange fluid with a target inlet temperature of 20°C lower than the initial temperature of the soil ($\mathrm{\Delta }T=-20°\mathrm{C}$). Following the application of thermal loading, pull-out axial load tests were performed to evaluate the effects of cooling cycles on the load–displacement response of the energy pile. Baseline (room temperature), 5-cooling-cycle (5CC), and 100-cooling-cycle (100CC) tests were performed and summarized in this paper. During the thermal and mechanical load applications, temperature in the soil and the pile, axial strains, radial strains, and radial pressures at the soil–pile interface were measured. For the $1g$ scaled energy pile and test conditions (pile in dry sand subjected to cooling cycles with $\mathrm{\Delta }T=-20°\mathrm{C}$), the peak pull-out loads decreased by 32% and 38% for the 5CC and 100CC tests, respectively.