Numerical simulation of pile geothermal heat exchanger with spiral tube considering its thermo-mechanical behavior

Pile geothermal heat exchanger (PGHE), which utilizes the building foundation piles as part of the geothermal heat exchangers (GHEs) for a ground-coupled heat pump (GCHP) system, has been attracting the interests of researchers and engineers. However, the continuous heat rejection/extraction of the PGHE to/from the piles will cause significant temperature variations (up to 25?) of piles and the surrounding soil, which can influence the mechanical behavior of the pile foundation severely. A modified direct shear apparatus has been developed to investigate the interface behavior between soil and pile. Then, based on the experiment results, the thermo-mechanical behavior of PGHE with spiral coils was investigated by a 3-D simulation model. The thermal loads induce additional compressive stress when the temperature rise, and the local compressive stress can reach to 9.35MPa near the heat exchanger pipe. Additionally, heat extraction led to a decrease of friction angle and normal contact pressure at the interface between soil and pile, and as a consequence, the shear force decreases with the temperature drop. Compared with no thermal disturbance, the ultimate friction resistance of pile is weakened by 15.37%