Evolution of Shaft and Tip Resistance in Energy Piles throughout a Full Heating-Cooling Cycle

Recently derived analytical solutions for soil structure interaction in single energy piles are implemented and extended to investigate load transfer mechanism of energy piles subjected to combined thermo-mechanical loads. To this end, a semi-floating energy pile was subjected to a complete heating-cooling cycle combined with mechanical compressive load, in presence of a head restraint. It was found that the pile shaft and tip are engaged in the transfer of thermo-mechanical load to the soil during heating and a small portion of cooling phase. Nevertheless, once the compressive force at the tip induced by mechanical load is balanced by tensile force at the tip induced by cooling, no further transfer of load at the pile tip is possible with increased cooling. At this stage a semi-floating pile undergoes the transition of the load transfer mechanism from the semi-floating to the fully floating mode whereby the entire thermo-mechanical load is transferred through the pile shaft. Furthermore, the thermo-mechanical load is accommodated by generation of compressive axial stress in the pile during heating. This stress can be significantly larger than the mechanically induced stress. On the contrary during cooling the load is accommodated by generation of tensile stress that can develop within the large portion of the pile.