Thermo-mechanical behavior of cast-in-place energy piles

An energy pile induces heat exchange with the ground formation by circulating heat carrier fluid through a heat exchange pipe, which is encased in pile foundation. During heat exchange, temperature variation in energy pile generates thermally-induced stress due to the different thermo-mechanical behavior between the pile and surrounding ground, and the restriction of pile deformation. A series of full-scale field tests was performed to identify the thermo-mechanical behavior of a cast-in-place energy pile equipped with 5-pair-parallel U-type heat exchange pipe. During the field investigation, each cooling and heating test lasted for 30 days, including a 15-day operating period and 15-day resting period, and the thermal stress generated in the energy pile was monitored. The maximum thermal stress was evaluated to be 2.6 MPa in the cooling test, which is about 10% of the design compressive strength of concrete. In addition, a finite element (FE) numerical model was developed to simulate the thermo-mechanical behavior of the energy pile. In the numerical analysis, relevant boundary conditions and interface model were determined by comparing with the field measurement. Finally, a parametric study was performed to estimate the thermal stress and deformation of a cast-in-place energy pile for various ground conditions.