Effect of joint type on the shear behavior of synthetic rock

The shear behavior of the discontinuities of rock is important because it is closely related to the stability of a rock mass. The scientific challenge lies in the understanding of how different types of joint are related to the failure criterion. In the current study, direct shear tests are used to investigate the shear behavior of continuous planar joints, stepped joints, and discontinuous open joints. The joints were cast in a synthetic rock made of plaster, sand, and water and tested under normal stresses that ranged from 50 kPa to 3.5 MPa. The shear behavior of both the continuous and discontinuous joints has been found to be dependent on the normal stress. At normal stresses above the magnitude of the tensile strength, continuous and discontinuous joints displayed either strain weakening or brittle behavior. Results with the combination of all joint types indicated that the shear strength of the different types of joint increases sharply at low normal stress, and then approaches a lower bound residual strength envelope at high normal stress. At normal stresses of less than the tensile strength (1.84 MPa), the strength is dominated by cohesion, while at normal stresses greater than the tensile strength, friction appears to dominate the shear strength. For open joints, the shear stiffness is independent of the normal stress. For closed joints, the shear stiffness will increase as the normal stress increases, particularly evident below a normal stress of 1 MPa. Increasing the normal stress reduces the brittleness index of rock samples from 1 to 0. A primary reason for this non-unique failure envelope was the large dilation that occurred at high normal stresses. This dilation was attributed to grain crushing, and the roughness resulting from this crushing and gouge formation as shearing occurred.