Loading rate-dependent frictional resistance of a sawcut fracture in granite for prediction of thermoshearing

Abstract

The coupled TM behavior of fractured rock masses should be evaluated for the long-term safety of the deep geological repository. Thermally induced fracture shear slip, i.e., fracture thermoshearing, might cause increase in fracture permeability. In this study, the thermally induced shear slip of a sawcut granite fracture was investigated in laboratory. Multistage biaxial shear test, consisting of mechanical shear test and thermoshearing test, was conducted on the true triaxial apparatus, and acoustic emission (AE) signals were recorded throughout the experiment. Effect of loading rate on frictional resistance of the fracture was investigated by conducting mechanical shear tests, and we found that friction coefficient of the fracture decreases in proportion to the increase in the logarithm of loading rate. Under mechanical loading rates of 1 10-3 MPa/s and 5 10-3 MPa/s, the failure envelopes with friction coefficients of 0.78 and 0.74 were obtained, respectively. Thermoshearing test was conducted via heating in the direction of the constant minimum principal stress while expansion was restricted in the other direction of the maximum principal stress. A maximum loading rate caused by thermal stress was observed at around 0.9 10-3 MPa/s. Shear slip of the fracture occurred during the heating, and this can be well predicted by the Mohr-Coulomb failure envelope, which was obtained from the mechanical shear test at a similar loading rate.