Thermally induced slip of a single sawcut granite fracture under biaxial loading

Abstract

Long-term safety of a deep geological repository for high-level radioactive waste disposal in fractured crystalline rock should be evaluated. The potential for fracture slip due to coupled thermal–mechanical loading—and consequently for an increase in permeability in the surrounding rock formation—raised concerns. We investigated the slip of a single sawcut granite fracture inclined 42° to the horizontal under biaxial mechanical (M) loading and thermo-mechanical (TM) loading in the laboratory. The M loading test was conducted in four stages, and at each stage, the minimum principal stress was kept constant while the maximum principal stress was increased. Shear displacement of the fracture was calculated based on the linear variable differential transformer measurements and elastic deformation of rock matrix was excluded. Eight acoustic emission (AE) sensors were installed in the loading plates. When fracture slip occurs, shear stress drops, and the rate of shear displacement accelerates. Both the peak shear displacement rate and the peak AE event rate appear near the onset of fracture slip. Our laboratory results on the onset of fracture slip can be evaluated by the linear Mohr–Coulomb (M–C) failure criterion. For the TM loading test, the fracture was loaded to a stress state corresponding to a Mohr circle that lies below the M–C failure envelope. Next, heating was applied to the four surfaces of the specimen via the loading plates. During the heating, the minimum principal stress was kept constant while the expansion in the direction of the maximum principal stress was restricted. As a result, the maximum principal stress increased due to the thermally induced stress, and slip of the fracture finally occurred when the Mohr circle had reached the linear M–C failure envelope.