Experimental insights into frictional resistance and slip pattern of granite fractures and implications for thermoshearing prediction

Rock fractures or faults could be reactivated by the thermal stress generated during the decay process of the high-level radioactive waste in deep geological repositories (DGRs). Understanding thermoshearing behavior and its influencing factors are important for the long-term performance assessment of DGRs. We designed multistage mechanical (M) shear tests and thermomechanical (TM) shear tests on three 100 mm-cubic granite specimens, each containing a single inclined sawcut fracture with distinct microroughness of 8–15 μm. M test results have shown that the static friction coefficient of the granite fracture decreases in proportion to the increase in the logarithm of the loading rate within the range of 1–15 kPa/s. For the given heating and boundary conditions, thermal loading rate, i.e., thermal stress increment with heating time, is measured to be around 1 kPa/s in the fractured granite. Thermoshearing can be well predicted by the linear Mohr–Coulomb failure envelope deduced from M shear tests employing a loading rate that is comparable with the thermal loading rate. The granite fractures exhibited two distinct slip patterns during the mechanical shearing, i.e., stick-slip observed in the smooth fracture and stable sliding in the relatively rough surface. In contrast, the mechanical loading rate (1–15 kPa/s) investigated in this study appears to not influence the slip pattern. Unlike those in M shear tests, thermoshearing in both smooth and relatively rough fractures show stable sliding with a very slow peak velocity of around 0.002 μm/s.