This paper investigates the shear behavior of rock joints in a direct shear test using PFC2D. PFC2D is a distinct element software in which the rock is represented by a dense packing of non-uniform sized circular particles bonded at their contacts and damage process under loading is represented by breakage of the bonds. Two approaches have been used in PFC for generation of joints, namely: bond removal method and the smooth joint model. In bond removal method, joints are generated by removing bonds between particles within a specified distance on either side of the joint track. It is found that this approach is unable to reproduce the sliding behavior of joints due to micro-scale roughness of joint surface and reduction of friction coefficient of unbonded particles or increase of unbonded band width cannot improve this deficiency. In the smooth joint model, particles can overlap and pass through each other rather than move around one another. This method is investigated by undertaking direct shear test on planar joints. It is found that at shear displacements more than the minimum particle size, particle interlocking occurs, due to the inability of this approach to recognize the upper and lower blocks particles. To overcome this problem, a new shear box genesis method is proposed. In order to study the ability of the suggested method in reproducing the shear behavior of rock joints, direct shear tests on three joint profiles of saw-tooth triangular joint with the base angle of 25°, standard JRC 12-14 and 18-20 are carried out. Results of numerical models are compared against the Patton, Ladanyi and Archambault and Barton and Choubey models and good agreement are found.
The main characteristic of a rock mass that distinguishes it from other materials is the presence of discontinuities.