This paper investigates the progressive shear behavior and asperity degradation of rock joints in the direct shear test using PFC2D. PFC is a discrete element code in which the intact material is represented by assemblies of circular particles bonded together. Traditionally, joints have been modeled in PFC by removing bond between particles or the smooth joint model. However, these approaches are not able to reproduce the shear behavior of rock joints in the direct shear test. A new developed method called "shear box genesis" is used in this study to study the shear behavior of rock joints. Three saw-tooth triangular joint profiles with the base angles of 15°, 25° and 35° are generated and the mechanism of asperity degradation of these profiles under different normal stresses are studied. Three mechanisms of sliding, asperity wearing and shearing are observed in the numerical models and it observed that as the asperity base angle increases, the transition from sliding to shearing mechanism takes place at a lower normal stress. A good agreement is found between the peak shear strength of numerical models and Patton and Ladanyi and Archambault models. In order to study the process of asperity degradation during the shear test on a real rock joint profile, direct shear test on standard profile of JRC18–20 is carried out. The peak shear strength of numerical models under different normal stresses are compared against the Barton model and a good agreement is found.
The stability of near-surface rock structures such as rock slopes and crown pillars in blocky condition is strongly dependent on the shear behavior and mobilization of the friction of joints (Swan and Zongqi, 1985). Understanding the shear mechanism of rock joints in a given situation is not easy to ascertain and depends on several parameters such as joint surface roughness, the properties of intact material, loading conditions, weathering of the joint surface and infilling materials. A number of studies have been carried out to experimentally and analytically investigate the effect of joint roughness on the shear behavior of rock joints. Patton (Patton, 1966), Ladanyi and Archambault (Ladanyi and Archambault, 1980) and Barton (Barton and Choubey, 1977) models are the most widely used models for estimation of the shear strength of rock joints. However, a robust method to consider the effect of joint roughness on the shear behavior of rock joints is still not available. Quantitative description of the joint roughness and unknown mechanism of asperity degradation during the shearing process are the main challenges in understanding the shear behavior of rock joints.
