This paper employs the Particle Flow Code in 2 dimensions (PFC2D) to simulate the shear behaviors of rock joints under direct shear tests. A series of rock joints and regular joints with various saw-tooth angles are investigated. The shear resistance - shear displacement relationship, shear strength parameters, crack propagation, and failure modes are observed and investigated. Based on the numerical simulation results and application of the regular joint models, the simulation results compared well with the shear strength models of Patton  and Zhang  for higher and lower normal stresses but only compared well with the model of Zhang  for critical normal stresses. For observation of crack propagation, the initial micro-cracks appeared near the saw-tooth interfaces at 60% ~ 70% of the peak stress state before the peak. Soon, micro-cracks began to propagate into a macro-crack until the stress state reached the peak. The simulated failure modes are compare well with Zhang . This paper further established the chart of the distribution of failure modes based on the saw-tooth angle and the normal stress conditions
Rocks usually accompany inherent discontinuities, including joints, cleavage, and foliation, which were occasioned by tectonic stresses and pressure release. For engineering activities of tunnel excavation and slope stability, a rock joint is one of key components that could lead to tunnel collapse or slope failures during construction. Several factors could influence the mechanical behaviors of a rock joint, i.e., roughness, persistence, wall strength, aperture, and spacing, of which roughness is one of the most important factors for rock sliding.
The natural rock joint geometry never follows a specific regular pattern. Hence, most of the shear strength criteria of a rock joint have been developed by empirical analysis [1-2]. Several scholars simplified the rock joint geometry as a saw-tooth shape and established analytical solutions by observing the results of experimental tests [3-4]. Although shear strength criteria can be developed by experimental tests, the sliding failure process is still difficult to observe. Numerical simulation is one of approaches that can help to understand the failure mechanism, and the use of the discrete element method (DEM) is a magnificent demonstration of this capability [5-13]. To understand the relationship between the joint failure process and its shear strength, a fundamental investigation is required (a simplified joint geometry (saw-tooth), a basic sliding test (direct shear test, DST), and a 2 dimensional numerical analysis were considered). Hence, this paper used the Particle Flow Code in 2 dimensions (PFC2D, a DEM program) to simulate the mechanical behaviors of saw-tooth rock joints under the direct shear test. The shear resistance - shear displacement relation, the shear strength parameters, the crack propagation, and the failure modes will be discussed in this paper.