Ultimate bearing capacity of energy piles in dry and saturated sand

The influence of thermal loads on the ultimate bearing capacity of energy piles is examined. Five laboratory model tests were carried out to investigate piles equipped with U-shaped and W-shaped heat exchangers in dry and saturated sand. The pile load-displacement relationships were investigated for one, three, and five heating-cooling cycles and under three different pile temperatures. The results show that the ultimate bearing capacity, in dry sand at high soil relative density, increased as pile temperature increased. After one heating-cooling cycle, the ultimate bearing capacity reduced slightly. Compared with dry sand, the thermo-mechanical response in saturated sand was less obvious and the reduction of pile ultimate capacity after one heating-cooling cycle was smaller. A reduction in the ultimate bearing capacity of 13.4% was observed after three heating-cooling cycles in dry sand, while a reduction in ultimate bearing capacity of 9.2% was observed after five heating-cooling cycles in saturated sand. The more noticeable reduction of ultimate bearing capacity in dry sand was related to the larger temperature variation which would induce more degradation at the pile-soil interface. In addition, the pore water viscosity in saturated sand may contribute to less degradation at pile-soil interface during heating and cooling.