Thermo-mechanical behavior of an energy retaining pile for geothermal extraction

A field test was conducted to assess the thermo-mechanical behavior of a full-scale energy retaining pile (ERP) adjacent to a utility tunnel during geothermal extraction. Numerical models were also established and calibrated using measured data. The extracted thermal power, temperature, thermally induced strain and stress, and bending moment of the ERP were analyzed. Additionally, a comparative analysis was conducted using validated numerical models to assess the impact of the air temperature (T_air) inside the adjacent utility tunnel on the thermo-mechanical behavior of the ERP. The findings highlight that the extraction thermal power of the tested ERP was 57 W/m, with the short-term geothermal extraction operation yielding even higher values of 200–250 W/m. The operation of the adjacent high-temperature utility tunnel can lead to an average increase of approximately 15 % in the extracted thermal power of the ERP. Additionally, during the geothermal extraction, regardless of the value of T_air, the ERP primarily reflected in changes to the axial thermo-mechanical behavior. However, the higher-temperature utility tunnel can result in a notable bending moment of the ERP prior to the geothermal extraction operation. Thus, considering the influence of T_air on the thermo-mechanical behavior of the ERP becomes crucial during the preliminary design phase.