Effects of Cyclic Temperature Variations on Thermal Response of an Energy Pile under a Residential Building

Abstract

The effects of daily cyclic temperature variations on the thermal response of an energy pile built under a six-level residential building are examined. The axial and radial thermal strains along the length of the pile followed stable, linear reversible paths during daily active heating and cooling cycles corresponding to a pile temperature range of 10°C to 23°C (ΔT of −8°C to 5°C) around a baseline temperature of 18°C. The stable responses of the thermal strains indicate that plastic deformations did not occur in the pile during the daily cyclic temperature changes coupled with the mechanical load in the pile corresponding to 52% of its estimated ultimate capacity. A complex distribution of axial thermal stresses with depth was observed in the pile, with higher stress magnitudes near the pile ends particularly at the end of cooling because of larger temperature changes in the cooling cycle. The magnitudes of radial thermal stresses were considerably smaller than the axial thermal stresses along the length of the pile and are not anticipated to play a significant role in the development of thermomechanical loads in the pile. The temperatures over the cross section of the pile were uniformly distributed at the end of cooling and heating at all depths, while the axial thermal stresses had a nonuniform distribution but with magnitudes less than the calculated ultimate capacity of the pile.