Laboratory study on the thermo-mechanical behaviour of a phase change concrete energy pile in summer mode

Abstract

Phase change concrete energy pile (PCCEP) is a kind of underground energy structure with economy and efficiency. A set of model experimental system of PCCEP was built in the laboratory to assess the effects of phase change process, inlet water temperatures, intermittent modes, mechanical loads and thermal cycles on its thermo- mechanical behaviour in summer mode. The results indicate that compared with traditional energy pile, the PCCEP can improve heat transfer performance, reduce the thermal deformation, thermal stress and pile tip soil pressure. The total heat exchange amount are found to be 2900.5 and 3162.7 kJ for traditional energy pile and PCCEP, respectively, and the corresponding pile top displacement, peak stress and pile tip soil pressure are respectively 0.12 mm, -196 kPa, 9.2 kPa and 0.109 mm, -179 kPa, 8.4 kPa. Increasing inlet temperature can enhance thermal performance of PCCEP, but it also leads to the increase of pile top displacement, pile thermal stress and pile tip soil pressure. For different intermittent operation modes, the longer the operation time, the greater the total heat storage capacity, but the lower the hourly average heat storage capacity, and the greater the pile top residual displacement, pile thermal stress and pile tip soil pressure. The total heat exchange amount are found to be 3688, 4090, 4527 and 4925 kJ for intermittent modes of 8 h:16 h, 10 h:14 h, 12 h:12 h, 14 h:10 h, respectively, and the corresponding pile top residual displacement, pile tip soil pressure and peak pile stress are respectively 0.015 mm, 0 kPa and -157 kPa, 0.021 mm, 1 kPa and -179 kPa, 0.029 mm, 1.7 kPa and -199 kPa, 0.045 mm, 2.3 kPa and -207 kPa. The settlement accumulation can be incurred when the PCCEP subjected to pile top mechanical load, and the larger the pile top mechanical load, the more obvious the settlement accumulation, and the greater the pile stress and pile tip soil pressure.Thermal cycles will result in the increase of temperature rise degree of pile and soil, and cause the accumulation of pile temperature rise, pile stress and pile top displacement.