Non-uniform thermal strains and stresses in energy piles

This paper presents the non-uniform thermally induced strains and stresses that develop in the cross-sections of energy piles when subjected to operational energy demands. The thermal operation of energy piles depends on the thermal demands of the supported building. These thermal demands vary seasonally, daily and even hourly, which generates non-uniform temperature changes over the pile's cross-section throughout the lifetime of the system. Therefore, non-uniform thermal strains and stresses develop over the cross-section of energy piles. Numerical analyses performed in this study indicate that the maximum temperature changes and the thermally induced strains and stresses occur near the location of the ground loops. Further, contradicting the current understanding that heating the piles creates only thermal compressive stresses, while cooling them creates only thermal tensile stresses; tensile and compressive thermal stresses were found to coexist during heating and cooling over the same pile cross-section due to the non-uniform temperature changes. Furthermore, the magnitude of the tensile stresses that develop due to non-uniform temperature changes is crucial when the reinforcing cage is cut short of the pile toe. Finally, ignoring the non-uniform temperature changes while processing the temperatures and strains measured in full-scale tests will result in errors in approximated thermal stresses.