Shear velocity-based uncertainty quantification for rock joint shear strength

The shear strength of rock joints is an important property required in order to analyze the stability of rock slopes and tunnels. However, estimation of the shear strength of rock joints for in situ conditions is a complex task due to various influencing factors present in the field. Among these factors, the shear velocity or the shear displacement rate along the rock joints are important parameters which are relatively less studied since their effect is considered to be of second order compared to other factors. However, some recent studies in the literature suggest that shear velocity has a significant influence on the shear strength of rock joints, and hence the shear strength of joints estimated at low shear velocities in laboratories cannot be used under in situ conditions where the possibility of higher shear velocities exist due to the presence of different factors, such as blasting, excavation, and thermal and seismic loads. In this paper, we have addressed these issues in three steps. In the first step, an experimental study on jointed rock specimens is presented to investigate the influence of the displacement rate on the shear strength of rock joints. In the second step, a probabilistic method is developed based on the experimental results and the compiled data from the literature to estimate the in situ shear strength of joints under higher displacement rate conditions, i.e., blasting, excavation, and seismic loads from laboratory-estimated shear strength at the International Society for Rock Mechanics suggested low displacement rates. In the third step, a case study of a Himalayan rock tunnel was used to demonstrate the described approach. It was observed that the shear strength of discontinuities reduced with ncreasing shear velocity and that the rate dependency was higher for low-density rocks and under high confining stress. Further, a considerable effect was observed on the probability of failure of the rock tunnel when the effect of shear velocity was considered in the stability analysis.