Strain Distributions in Full-Scale Energy Foundations (DFI Young Professor Paper Competition 2012)

Abstract This paper focuses on the thermo-mechanical response of two full-scale energy foundations installed at the new Denver Housing Authority Senior Living Facility in Denver, Colorado. The energy foundations were formed by attaching heat exchanger tubes to the inside of the reinforcement cages of drilled shafts. The heat exchange tubes were connected to a ground-source heat pump system, which circulates a methanol-water mixture through the tubing to absorb or shed heat into the foundation and surrounding soil or rock. Instrumentation was incorporated to assess the heat exchange response of the foundations as well as the thermo-mechanical strains during construction and typical building operation. The temperature changes within the foundations are stable during heating and cooling operations and the corresponding thermal axial strains are within acceptable limits. The thermal axial strain profiles measured for both foundations follow trends expected for end-bearing boundary conditions with greater strains near the top of the foundations (upward expansion). The mobilized thermal expansion coefficients inferred from the instrumentation confirm that side shear stresses provide resistance to thermally induced movements. The results from this study indicate that energy foundations can be implemented in new buildings to gain improved heat exchange capabilities without major impacts on the foundation performance, for little added installation cost.