In-situ tests on structural responses of energy piles during heat exchanging process

Little information is available regarding the impacts of heating and cooling processes on the geotechnical performance of piled foundations incorporating pipe loops for ground-source heat-pump systems (so-called energy piles). A pile-loading test that couples thermal loading cycles with a constant external mechanical load is undertaken to investigate the behavior of an energy pile installed in Xinyang, Henan, China. The pile-loading test is carried out over a period of about four weeks, during which the thermal and mechanical loads are jointly applied in order to simulate the working conditions of energy piles. Using the vibrating-wire extensometers, the temperature and strain profiles of the test pile are monitored. Meanwhile, the load and movement at the pile head, the ambient air temperature and the inlet/outlet temperatures of circulating fluid in the pipes embedded in the pile are also recorded using the conventional instrumentation methods. The additional thermal stresses mobilized in the pile shaft are calculated based on the measurements, and the structural responses of an energy pile can be understood according to the simplified mechanism. The additional thermal stresses (tensile or compressive) superposed to the mechanical stresses mobilized in the pile during the heating and cooling processes are also subjected to the restraint conditions at the ends of a pile. The additional thermal stresses can possibly exceed the limit design stress values specified by a design code, which needs to be carefully considered in the structural design of an energy pile.