Full-scale in situ experimental study on the bearing capacity of energy piles under varying temperature and multiple mechanical load levels

Although the thermomechanical behaviors of energy piles have been investigated through a limited number of full-scale tests with constant loads, the effect of multiple load levels on the bearing capacity of energy piles has not been fully implemented into these in situ tests in the past. We report six full-scale in situ tests on bored energy piles under heating or cooling conditions and multiple mechanical load levels in Shanghai, China. Each pile was tested with 24 h of mechanical loading, 48 h of heating or cooling, and 10 h of thermomechanical loading. We monitored the inlet and outlet water temperature, pile and soil temperature, pore water pressure in the soil, axial force of the pile, and pile head displacement, and analyzed the thermomechanical responses of the energy pile and the surrounding soil, as well as the bearing capacity of the energy pile. Our experimental results show that the mechanical responses induced by the temperature change in the pile were affected by mechanical load level. Larger mechanical load reduced the influence of temperature on the settlement of the pile top and on the variation of the axial force in the pile, and at the same time weakened the negative skin friction effect caused by heating and enhanced the positive skin friction effect caused by cooling. Furthermore, pore water pressure caused by changes in soil temperature decreased slightly, and excess pore water pressure increased with increasing mechanical load level and dissipated rapidly in low-permeability coefficient soil. Finally, we comprehensively discussed the vertical bearing capacity of the energy pile based on the thermomechanical responses of the pile and the surrounding soil.