Shear behaviour of regular and irregular rock joints under cyclic conditions

Rock masses often have sets of joints or fractures and almost all failures are due to the presence of these discontinuities. Natural joints have irregular surfaces, and the correct evaluation of shear strength and deformation of these irregular joints is very important for the analysis and engineering design of rock structures. These joints are often subjected to dynamic loads because of earthquake and blasting during mining and rock cutting. Hence, it is important to correctly evaluate the shear behaviour of regular and irregular rock joints under dynamic conditions. In the present study, synthetic rock joints are prepared with plaster of Paris and regular joints are replicated by keeping regular asperities with asperity angles 15 degrees-15 degrees and 30 degrees-30 degrees. Irregular rock joints are prepared by keeping the asperity angles 15 degrees-30 degrees and 15 degrees-45 degrees. The sample size and amplitude of roughness are kept constant for both regular and irregular joints, at 298x298x125 mm and 5 mm respectively. Shear tests have been performed on these joints with a large scale direct shear testing machine by keeping the frequency and amplitude of shear loads constant under cyclic load conditions and varying the normal stress. The shear strength of rock joints increases with increase in the asperity angle and normal load during the first cycle of shearing. With the increase in the number of shear cycles, the shear strength reduces for all the asperity angles, but the rate of reduction is greater in the case of high asperity angles. Test results indicate that the shear strength of irregular joints is higher than regular joints at different cycles of shearing at low normal stresses. The mechanism of the shearing for regular and irregular joints is different under the cyclic conditions at low normal stresses. Shearing and degradation of joint asperities on regular joints between loading and unloading are the same, but for irregular joints they are different at low normal stresses. Shear strength and joint degradation are more significant on the slope of asperity with higher angles on the irregular joint, until two angles of asperities become equal during the cycle of shearing and it starts to behave like a regular joint.