Investigation of potential dragdown/uplift effects on energy piles

This study focuses on the interpretation of axial strains in a pair of full-scale energy piles beneath an 8-story building measured over the course of five years of geothermal heat pump operation. Although the cyclic temperature changes imposed upon the energy piles are consistent during each of the years of operation, the axial strains at different depths appear to show diverging trends. Evaluation of the profiles of thermal axial strain under different instances of extreme heating and cooling in each year of operation indicates that predominantly contractile strains are being superimposed atop the thermo-elastic expansion and contraction of the piles, especially near the toe of the piles. An evaluation of the trends in mobilized coefficient of thermal expansion during different heating and cooling cycles indicates that the superimposed contractile strains on the pile are not affecting the thermo-elastic expansion and contraction of the energy piles. Accordingly, the superimposed contractile strains were determined to be due to the effects of dragdown or uplift of the surrounding soil on the piles. The observed dragdown or uplift may be caused by thermal effects on the subsurface surrounding the piles or long-term mechanical compression of the subsurface under the applied building load, and deserve further study using more advanced analyses.