Performance of a Prestressed Concrete Pipe Energy Pile during Heating and Cooling

Energy pile, piled foundation incorporating pipe loops for ground source heat pump systems, has been implemented in more and more countries. However, the mechanical performance of energy pile has not been comprehensively studied. To improve the understanding on the performance of energy pile, a field study on energy pile is carried out in this study. A prestressed concrete pipe pile installed with water pipe loops inside was tested as an energy pile in the field. The performance of energy pile during heating and cooling was investigated by using vibrating-wire stress gauges and temperature sensors. The field tests reveal that the temperature influence distance of the energy pile on surrounding soil is approximately 0.5 m from the center of the pile after heating for 60 h. The axial force of energy pile increases/decreases with the increase/decrease of temperature at different rates at different depths. The neutral plane shifts from above 7.7 to below 7.7-m depth because of heating. The magnitudes of shaft resistance increase with temperature increase and decrease with temperature decrease.