Cohesion degradation and friction mobilization in brittle failure of rocks

The progressive process of brittle failure was studied using the results of laboratory damage-controlled tests on the samples of Lac du Bonnet granite and Indiana limestone under uniaxial and triaxial loading conditions. A similar trend of decreasing crack damage stress, peak stress and cohesion and increasing friction angle with increasing damage was observed for both rocks which confirmed the previous findings from the results of uniaxial tests. While the absolute strength of Lac du Bonnet granite is much higher than Indiana limestone, the normalized crack damage stress curves for both rocks were similar. It was shown that cohesion degradation is mainly a function of accumulated damage and is not sensitive to confining stress. Friction mobilization, on the other hand, dependents on both the damage level and confining stress and decreases with increasing confinement. A nonlinear Cohesion Weakening Friction Strengthening (CWFS) model was proposed to capture the process of brittle failure at the laboratory scale. The results of the model were in reasonable agreement with experimental stress-strain curves at different levels of confining stress. A simplified version of the CWFS model was proposed for in situ applications and implemented in the finite difference code, FLAC3D for numerical modeling of four underground excavations at the Underground Research Laboratory in Canada. In all cases, the CWFS model closely captured the observed zone of brittle failure around the excavation. The proposed in situ CWFS model eliminated the problematic behavior of the current linear CWFS models by using identical plastic strain thresholds for residual cohesion and residual friction angle. It was shown that the proposed CWFS model offers sufficient versatility to be applied to a wide range of geomaterials with strain softening and strain hardening behavior. Empirical guidelines were proposed for estimating the parameters of the proposed in situ CWFS model for good quality rock masses.