Shear behaviour of regular and irregular rock joints under cyclic conditions

Abstract

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 surface and correct evaluation of shear strength and deformations of irregular joints are very important for 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 correctly evaluate the shear behaviour of regular and irregular rock joint under dynamic condition. In the present study synthetic rock joints are proposed with plaster of Paris and regular joints are simulated by keeping regular asperity with asperity angles 15⁰-15⁰- and 30⁰-30⁰, irregular rock joints are proposed by keeping the asperity angles 15⁰-30⁰ and 15⁰-45⁰. The sample size and amplitude of roughness is kept same for both regular and irregular joint and it is 298*298*125 mm and 5mm respectively. Shear test have been performed on these joints on a large scale direct shear testing machine by keeping frequency and amplitude of shear load under cyclic condition as a constant and varying the normal stress. Shear strength of rock joints increases with increase in asperity angle and normal load during the first cycle of shearing. With increasing number of shear cycle, shear strength reduces for all the asperity angles but rate of reduction is more in case of high asperity angle. The test results also indicate that there is no effect of shearing cycle on shear strength of 15⁰-15⁰ rock joints on low normal stress and it is almost constant from first to last cycle of shearing. Mechanism of the shearing for regular and irregular joints is different under cyclic condition at low normal stress. Shearing and degradation of joint asperities on regular joint between loading and unloading are same, but for irregular joints are different at low normal stresses. Shear strength and joint degradation is more on the slope of asperity with higher angle on irregular joint, till two angles of asperities become equal during cycle shearing and it starts behaving like regular joints.