Investigation on thermo-mechanical behavior of reinforced concrete energy pile with large cross-section in saturated sandy soil by model experiments

Abstract

Energy piles are a new type of heat exchange systems with buried pipes in a pile foundation, which optimize a ground source heat pump system for the utilization of shallow geothermal energy. In this study, based on the principle of similarity, the thermo-mechanical behavior of the model energy pile with a large cross-section in saturated sandy soil was experimentally evaluated. The pre-cast model concrete pile with a diameter of 0.2 m and length of 1.5 m was buried in saturated sand in a steel box with dimensions of 2.5 m × 2.5 m × 2.0 m (length × width × height). The pile was heated using water in the polyethylene (PE) pipe, which was connected to a water cycle temperature controller. At a constant inlet water temperature of 55 °C, three thermal cycles were carried out with the same heating and cooling periods and different water flow rates. The temperature distributions in the pile and soil, in addition to the pore pressure, soil pressure, and displacement of the pile, were monitored to clarify the thermo-mechanical behavior of the pile and soil. The heat transfer efficiency was analyzed based on the temperature difference and water flow rates. The measured strain at different locations in the pile under cyclic thermal loading revealed that the uneven strain that developed in a pile body should be considered for its long-term application. Furthermore, focus should be directed toward the long-term unrecoverable displacement of the energy pile due to the thermal plastic strain and thermal consolidation of the soil.