Comparison of slip behaviour in critically stressed saw-cut granite fractures under mechanical and thermal loading

Deep geological repositories (DGRs) undergo successive mechanical loading, heating, and natural cooling stages during their service life, during which shear slip may occur along fractures and critically-stressed faults. However, the slip behaviour and stability of fractured rock can vary substantially among these stages. This study investigated the slip stability of a saw-cut granite fracture during mechanical shear and thermoshearing induced by heating or natural cooling using a polyaxial testing system equipped with acoustic emission (AE) monitoring. Bi-axial loading experiments were conducted on a cuboid granite specimen with a side length of 100 mm containing a saw-cut fracture, under minimum principal stresses of 1, 2, and 3 MPa. Distinct slip behaviours were observed across the three stages. During mechanical loading, the fracture exhibited either stable sliding at a velocity of approximately 0.1 μm/s when the normal stress was below 4.0 MPa or stick-slip sliding with peak slip velocities of 26–363 μm/s when the normal stress exceeded this threshold. During heating, the fracture underwent stable sliding at a very low slip velocity of 0.014 μm/s. In contrast, natural cooling induced abrupt slip, with peak slip velocities of 297–381 μm/s. AE event locations indicated that fracture failure was spatially heterogeneous, suggesting that slip may have occurred locally rather than uniformly along the entire fracture surface. A thermomechanical model was further developed to resolve the evolution of temperature and thermal stress fields, revealing pronounced thermal stress heterogeneity during heating. The distinct slip behaviour during the heating may arise from the combined effects of heterogeneous thermal stress, extremely low preparatory slip velocity, and temperature-dependent evolution of friction. These findings highlight the strong dependence of thermoshearing behaviour on the thermal loading path and the coupled evolution of thermal stress, deformation and friction.