Axial and Radial Thermal Responses of a Field-Scale Energy Pile under Monotonic and Cyclic Temperature Changes

The axial and radial thermal responses of a field-scale energy pile installed in dense sand and subjected to monotonic and cyclic temperatures are examined. It is found that the axial thermal strains in the pile are more restricted to thermal expansion/contraction compared to radial thermal strains. The radial thermal strains are close to that of a pile expanding/contracting freely, indicating minimal resistance from the surrounding soil in the radial direction. As a result, very low magnitudes of radial thermal stresses developed in the pile compared to axial thermal stresses. The pile-soil radial contact stresses estimated from the cavity expansion analysis are up to 12kPa for a pile temperature change of 22.5 degrees C and are likely to stay low for the range of commonly encountered operating temperatures for cast-in-place concrete energy piles installed in dense sand. During cyclic heating and cooling, unstable changes in axial and radial thermal strains were observed initially during initial cycles, indicating a ratcheting response. The changes in strains became more stable over further cycles without significant changes in side friction or pile-soil contact stresses.