Experimental and Theoretical Study of Shear Instability of Rock Joints in the Direct Shear Test

The stability of a rock joint was often dominated by one or more major asperities along the slip surface with a large bearing capacity to resist instability. In this study, direct shear tests were performed on artificial split rock joints. The shear failure mechanism of major asperities was revealed and a physical model was developed to describe the shear instability of the rock joint, by coupling a strain-softening constitutive model (based on the Weibull distribution) with a friction model (based on the piecewise function). The effects of constant normal load (CNL) and asperities on cutting-tooth behavior and shear instability characteristics of a rock joint (e.g., sudden jump values Delta u(1) of shear displacement, elastic energy release Delta U-1, and shear instability proneness K-2) were studied. The results showed that the shear instability of the rock joint was mainly affected by the CNL and major asperities. The physical model fits well with the shear stress-shear displacement curves of the rock joint. The necessary condition of shear instability (which is K-2 <= 1) and predicted sudden jump values Delta u(1) of shear displacement were derived by using cusp catastrophe theory. The cutting-tooth effect of major asperity becomes more obvious with the increase of CNL. Here, Delta u(1), Delta U-1, and K-2 all decrease with the increasing CNL. The proposed physical model and the shear instability analysis may improve the understanding of the unstable shear failure behavior of rock joint both in the laboratory and in nature. (c) 2021 American Society of Civil Engineers.