Laboratory-scale pull-out tests on a geothermal energy pile in dry sand subjected to heating cycles

Ground source heat pumps coupled with energy piles operate intermittently, subjecting the piles to temperature cycles throughout their lifetime. The research presented in this paper focuses on studying the thermomechanical behavior of energy piles subjected to heating cycles. Laboratory model tests were performed at the soil-structure interaction (SSI) facility at Lehigh University. A fully instrumented model energy pile, embedded in dry sand, was subjected to different number of heating cycles followed by axial pull-out loading. Baseline (room temperature), five heating cycles (5HC), and 100 heating cycles (100HC) tests are reported in this paper. The soil was instrumented with temperature sensors and pressure sensors, while the pile was instrumented with temperature sensors, strain gauges, and pressure sensors. The test results showed that the peak pull-out loads for the baseline, 5HC, and 100HC were 2794 N, 3633 N (30% higher than baseline), and 3559 N (27% higher than baseline), respectively. It was also found that subjecting the pile to large number of daily heating cycles induced small degradation in the load transfer or the peak pull-out load in dry sand.