Shear behavior of rock joints-microscale characterization

Abstract

Reliable estimation of shear strength of rock mass is important for design of tunnels or underground structures. Especially, the assessment of shear strength of a rock joint is critical because the rock joint is one of the weakest points of rock masses due to its discontinuity. Until now, numerous works have been performed through experimental tests, but it is hard to find a microscale study focusing on asperity geometry or asperity size of a rock joint and its spatial distribution. This thesis centered on the shear behavior of rock joints with respect to micro-scale asperities. The main objectives were to identify the shear mechanism of asperities, to explore the effects of microscale characteristics of asperities on joint shear behavior, and to suggest the theoretical model for prediction of shear behavior of rock joints. First, two failure modes of a rectangular asperity are identified depending on its shape and critical aspect ratio: one mode is a dilative failure with a failure plane of $45-φ_f/2$ and the other is a non-dilative failure with shearing of asperity. The critical aspect ratio, which is used as a failure mode criterion of a rectangular asperity, is a function of peak friction angle, cohesion, and normal stress, and is the most sensitive to peak friction angle. Two shear mechanisms of triangular asperities are reviewed: one is a sliding mode and the other is a shearing mode. The critical inclination angle, which is used as a failure mode criterion of a triangular asperity, is a function of peak friction angle, basic friction angle, cohesion, and normal stress. The shear strength of rectangular and triangular asperities can be determined with peak friction angle, basic friction angle, cohesion, normal stress, and geometric condition of an asperity (e.g., aspect ratio α, inclination angle θ). And also the shear behavior of rectangular and triangular asperities can be predicted with the shear modulus and sliding stiffness of an intact material. ...