Analysis of Soil and Structural Thermal Response of a Field Scale Geothermal Energy Pile-Paper

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This study examined the thermal and thermo-mechanical response of a field-scale geothermal energy pile at Monash University under continuous heating, continuous cooling, and alternating daily cooling and heating. Each operating mode was assessed over 552 hours. Researchers monitored pile strain and temperature, inlet and outlet water temperatures, and soil temperatures at distances of 0.5 m and 2 m from the pile.<br /><br />Thermal effects on the surrounding ground were strongest near the pile and diminished with distance. At 2 m, temperature changes were small, suggesting limited potential for thermal interaction between piles at typical or greater spacing. Alternating operation produced cyclic changes in pile temperature, strain, and stress. In all modes, the initial thermal response was greatest because of the strong temperature difference between the pile and surrounding ground, then decreased as conditions approached steady state.<br /><br />Greater changes in pile temperature produced greater thermal strains and stresses, with a broadly linear relationship. The highest reported thermal stress was about 4 MPa. The rate of stress change with temperature change was 175 kPa/°C at the most constrained measurement depth.<br /><br />After testing, thermal strains recovered to within 20 microstrain of their undisturbed values. The results therefore indicate that, for the tested soil profile and operating conditions, continuous and alternating thermal loading did not materially affect the pile’s elastic response or cause substantial permanent thermal deformation.

Keywords

geothermal energy pile

thermal response

thermo-mechanical behavior

continuous heating

continuous cooling

alternating cooling and heating

pile strain

thermal stress

soil temperature

elastic response