Title : Behavior of rock joints under cyclic loads
Abstract:
The presence of the discontinuity in the form of joints is one of the most significant factors causing instability in the rock mass. On the other hand, dynamic loads, including earthquake and blasting induce cyclic shear loads along the joints in rock masses; therefore, failure of rock mass exacerbates along the joints due to changing shear resistance. Hence, it is important to evaluate the shear behavior of rock joint under cyclic condition. In the present study, synthetic rock joints with regular and irregular asperity were prepared by plaster of Paris. Regular joints were simulated by keeping regular asperity angles of 150 -150 and 300-300, and irregular rock joint which is closer to natural joints were replicated by keeping the asperity angles of 150-300. The sample size and amplitude of roughness were kept the same for the both regular and irregular joints respectively. Shear test was conducted on these joints using a large-scale direct shear testing machine by keeping the frequency and amplitude of shear load constant. This experiment examined with different normal stress values. Shear strength and its envelope of irregular joint were found to be higher than regular joint at different cycles of shearing. The shear strength envelope of irregular joint (150-300) was very close to that of regular joint (300-300) at low normal stress (slope condition), and large difference was observed at higher normal stress (underground structure condition). With the increase of the number of shear cycles, the shear strength decreased for all the asperity angles but the rate of reduction was more in case of high asperity angles. In addition, results indicated that peak shear stress increases with increasing normal stiffness at the first cycle, but the influence of normal stiffness decreases with an increase in the number of shear cycles. The mechanism of shearing for regular and irregular joints was different under cyclic condition at low normal stress. The shear strength and degradation of asperities on regular joints between loading and unloading were the same, whereas for irregular joints, they were different. Joint degradation was predominant on the slope of asperity with higher angles on irregular joints until two angles of asperities became equal under shear cycles, and then they started behaving as regular joints for subsequent cycles. This process of shearing was not observed at high normal stress.
