Abstract: A model of joints with five different roughnesses has been built successfully by using the particle flow code(PFC) in two-dimensions; and a series of numerical simulation direct shear tests which under 5 different constant loads have been done. We can observe the morphology damage and crack evolution mechanism of five different roughnesses joints under different constant loads from the viewpoint of mesomechanics. At the same time, the influence of JRC value and particle friction coefficient of joint on the joint shear strength is analyzed. And a joint plane shear strength parameters and relations with JRC values are back analyzed. Detailed results are as follows: normal constant load larger is, the greater the peak shear stress of joints is, the smaller the dilatancy phenomenon is, the greater the range of joints morphology injury is. With the development of shear, the surface contact area of the upper and lower joints reduced; microcracks produced along the joint surface. As the shear displacement continues to increase, the number of microcracks increase significantly, and not limited to the vicinity of the joint plane; but to enter inside the model. The peak shear stress of joints increases with the increase of joint roughness (JRC value) and the particle frictional coefficient of the joints. Joint shear strength parameters Cj and ?j increase with the increase of the JRC value. The results can provide reference and basis for the indoor tests and engineering applications.

Key words: roughness, direct shear, cracks, mesomechanics, shear strength