Thermo-mechanical effect on the drained creep behavior of a clay rock

The heat released by high-level nuclear waste can cause a significant increase in temperature within the host rock, thereby affecting the long-term performance of a clay-based nuclear waste repository. This study aims to investigate the effects of stress and temperature on the creep behavior of a representative clay rock through a series of experimental creep tests. The tests were conducted under complex thermo-mechanical conditions using a temperature-controlled triaxial testing system. Drained creep tests were performed under various mechanical conditions, including a heating-cooling cycle (40 °C → 60 °C → 80 °C → 60 °C → 40 °C), with each temperature being maintained for approximately one month. The findings indicate the possible existence of a creep threshold stress even at high temperatures, although the temperature may extend the primary creep stage. Beyond the creep threshold stress, experiments conducted at high temperatures (or low confining pressure) exhibited significantly greater creep deformation and higher creep rates compared to those observed at room temperature (or high confining pressure) during the heating stage. Negligible creep deformation during cooling and the irrecoverable creep deformation resulting from deviatoric stress variation during the creep process demonstrate the path-dependent nature of the creep behavior of clay rock under thermo-mechanical conditions. Finally, the microscopic perspective of the creep mechanism in clay rock under thermo-mechanical effects was discussed, including the rearrangement and combination of clay particles, differential thermoelastic expansion among minerals, and the presence of pore water. •A creep threshold stress may still exist, even though the primary creep stage is prolonged at high temperatures.•The temperature elevation accelerates the creep of clay rock beyond the creep threshold stress.•High deviatoric stress or low effective confining pressure can enhance creep deformation.•The creep behavior is strongly influenced by the thermo-mechanical path.•The creep mechanism was discussed from a microscopic perspective.