Study on the thermo-mechanical coupling behavior of phase change backfill materials and its influence on the borehole and surrounding ground

To address concerns regarding structural stability in engineering applications of phase change backfill materials (PCBM), the thermo-mechanical influencing mechanism of PCBM on boreholes and the surrounding ground throughout phase-change cycles was investigated. PCBMs with an admixture of capric acid-lauric acid/expanded graphite (CA-LA/EG) and an admixture of CA-LA were prepared and tested, and multiple intermittent cycles were simulated. The results showed that the temperature gap between the borehole and surrounding ground caused a non-axisymmetric thermal stress distribution, resulting in tensile stress in the surrounding ground at shallower depths. It was found that the phase change process influenced stresses through two approaches: reducing support of PCBM to the borehole, and narrowing the thermal gap across the U-loop, borehole and surrounding ground. In comparison to concrete backfilling, the CA-LA/EG improved the heat transfer rate of U-loop by 6.42 % at most, reduced the maximal tensile stress in radial and tangential directions in surrounding ground by 27.66 % and 25.42 % respectively, amplified the stress difference in the circumference of borehole wall by four times. Under the assumed conditions, no mechanical failure would occur according to the failure criteria. The findings provide fundamental insights for stability analysis and wellbore design in preventing potential failure.