In-situ tests and thermo-mechanical bearing characteristics of friction geothermal energy piles

Geothermal energy pile is the new pile technology that combines the ground source heat pump technology and building pile foundation. Due to the pile as the heat transfer carrier of ground source heat pumps, the bearing characteristics of energy piles are different from those of the conventional engineering piles because of thermo-mechanical coupling. In-situ thermo-mechanical tests on drilled friction geothermal energy piles of a Kunshan project are performed. The temperature, stress distribution of pile shaft and displacement of pile tip are investigated under multi-level loadings and heat exchange cases. The distribution curves of temperature-induced loading, axial force and friction resistance of pile shaft are obtained, and the thermo-mechanical bearing properties and load transfer characteristics of drilled friction energy piles are analyzed. The results indicate that the additional temperature loading is induced in energy pile shaft by temperature change. The characteristics of axial force and the temperature-induced loading of energy piles are affected by pile loading and constraint of pile toe. The bearing properties of the energy piles is not the same with that of the engineering piles by loading only. Under heating condition, the negative friction resistance of pile shaft is produced at the upper and middle parts of pile, but the effect of negative friction resistance decreases by the increasing loading. While under cooling condition, the negative friction resistance of pile shaft is produced near the pile toes. So the load transfer characteristics of energy piles are changed by the thermo-mechanical coupling effect. Under the design working loading, the head of energy piles is uplift under heating case and dropped under cooling case. The displacement of energy pile head at heating process is almost restored after temperature is recovered, but the temperature-induced settlement of energy piles at cooling process remains after cooled. So the settlement characteristics of energy piles are changed by the thermo- mechanical coupling effect.