Changes in shaft resistance and pore water pressures during heating of an energy foundation

Abstract

This study focuses on the evolution of shaft resistance during operation of a geothermal energy foundation installed in a saturated glacial till layer. Energy foundations are a sustainable alternative to traditional space heating and cooling approaches for buildings. Despite efficient operational performance, there are still valid concerns regarding the effects of heating on the structural performance of foundations. To investigate the effect of heating at the soil-pile interface, four drilled shafts are utilized as energy foundations on the Urbana-Champaign campus of the University of Illinois and instrumented. Although the energy foundations are not yet operational, a theoretical investigation is possible to understand the effects of heating on the evolution of thermally induced pore water pressures and the shaft resistance of an energy foundation. A thermo-poroelastic numerical model is validated against an analytical solution, then is used to analyze the thermo-mechanical response of the soil-structure system under different conditions. The results indicate that the evolution of pore water pressure is affected by the rate of heating and the hydraulic conductivity of the surrounding soil, as expected. Higher pore water pressures are generated in the case of low hydraulic conductivity and higher rates of heating. Prior to the dissipation of excess pore pressures, the changes in shaft resistance are variable and influenced by the thermally-induced deformation of the foundation and the surrounding soil.