In this study, a numerical simulation of rock joints and direct shear tests were carried out using PFC3D. The influence of the micro-properties on the shear behavior of the joints was examined, and a new technique for simulating JRC and JCS values of a joint by PFC3D was proposed. Particles were generated and bonded into a 10x2x3.6 cm3 block, and a rough joint with a JRC value of 10–20 was produced by defining the joint-contacts along a predefined joint surface. The bond strength between the particles with joint-contacts was reduced by up to 70 percent in order to simulate the decrease in joint wall strength due to weathering and alteration. The shear stress was loaded by horizontally moving the boundary walls at a constant velocity, while the normal stress was controlled by a numerical servo algorithm. Micro-cracks and contact force distribution, as well as the shear and normal displacements were recorded during the shear test. The shear behavior of the simulated joint at a given stress corresponded well to those observed in the laboratory tests. The cohesion responded more sensitively to the joint roughness and contact bond strength of the joint than the peak friction angle. A three dimensional profile scanning technique was proposed to measure the actual roughness of the simulated joint surfaces. The shear strength estimated from Barton's empirical model was in good agreement with those computed in the PFC, within a 6.7 % error for all cases. In addition, the relation between the JCS and contact bond strength of joint was obtained.
Joint is one of the most important factors in understanding and estimating the mechanical behavior of rock mass. The shear behavior of joint comprises the complicated phenomena, such as normal dilation, asperity failure and a change in contact area due to an undulating surface. This makes constitutive models for the joint behavior involve a large number of assumptions and uncertainties. In terms of the explicit representation of a joint, the particle flow code (PFC) can be attractive among joint simulation tools. PFC is a commercial code developed by ITASCA Consulting Group based on DEM theory. It represents a material as an assembly of rigid spherical particles that move independently of one another, and interact only at the contact points. The calculation scheme used in PFC requires only simple laws and a few parameters to govern interactions at the particle level to represent the behavior of a material including a joint. On the other hand, other tools mainly use constitutive (stress-strain) relations which involve many obscure parameters and assumptions. Therefore, the PFC can simulate the effect of the joint roughness and the asperity failure in a direct and realistic manner. Moreover, an explicit finite difference scheme makes it possible to observe the transmission of forces exerted at the contacts and track the propagation of bond breakage events at each stage.
