Abstract: Energy pile is a new type of pile foundation technology that bears the load of the upper building and transmits the shallow geothermal energy. Heat transfer efficiency and thermal induced stress are two key problems must be solved before energy pile can be widely applied. Present studies mainly focus on the thermo-mechanical coupling characteristics of single pile, while the researches on the thermo-mechanical characteristics of pile-raft foundation are relatively limited. Based on the pile-raft foundation, field tests on heat transfer efficiency and thermo-mechanical characteristics under different velocities were carried out. The temperature and stress changes of both energy pile and raft under cooling condition were measured and analysed. The influence caused by different velocities was also discussed. The results show that when the energy pile is running at the velocities of 0.8, 0.4, 0.2 m/s, the corresponding heat transfer efficiency are around 75.0, 82.5, 63.8 W/m. Under the influence of raft restraint, the maximum constrained tensile stress of pile is about 38.9 kPa, which is generated in the top of the pile; and the maximum constrained tensile stress of raft is about 47.2 kPa, which is in the bottom side of the raft. As the velocity of flow decreases, the temperature of pile rises slightly is increased and the value of tensile stress decreases. The influence of short-term cooling condition on the temperature of the raft is negligible.

Key words: pile-raft foundation, energy pile, test velocity, heat transfer efficiency, thermal-mechanical coupling, field test