Field study on heat transfer performance and thermo-mechanical properties of pre-bored PHC energy pile

This paper introduces a type of pre-bored PHC energy pile which can increase the survival rate of heat exchange pipes as well as improve the heat exchange performance of energy pile through the unique installation method. A group of field tests were conducted to study the heat transfer performance and thermo-mechanical properties of pre-bored PHC energy pile, and the distributed fiber optic sensors (DFOS) were equipped in test pile to measure the variation of temperature and strain of pile shaft in the test process. The field test results showed that the installation method of pre-bored PHC pile largely increased the survival rate of heat exchange pipes; the average heat transfer efficiency of pre-bored PHC energy pile was 81.3 W/m under summer conditions, and 65.8 W/m under winter conditions. The maximum thermally induced stress in pile shaft was about 2 MPa under summer conditions, while the maximum thermally induced stress in pile shaft was - 1.6 MPa under winter conditions. The influence of tensile stress in pile shaft under winter conditions on the structural safety of pre-bored PHC energy pile should be specially considered as the tensile strength of PHC pile shaft was much smaller than its compressive strength.