Effect of load frequency and amplitude of displacement on soft synthetic rock joints under cyclic shear loads

If dynamic loads propagate in the rock mass, they can generate vibrations or cyclic movements along the joints with different frequency and displacement. Hence, evaluation of rock joints behaviour under different cyclic movements significantly contributes to increasing the safety and stability of rock mass. In this study a series of cyclic shear tests was conducted on soft synthetic rock joints (with asperity angle of 30 degrees- 30 degrees) by shear apparatus to investigate the effects of shear load frequency and amplitude of displacement on the joint. Varying load frequencies (0.01, 0.05 and 0.1 Hz,) and displacement amplitudes (+/- 4, +/- 6, and +/- 8 mm) for the three different normal stresses (0.1, 0.5 and 1 MPa) were applied. The results indicated that the first peak shear stress on the joint was significantly affected by the frequency of cyclic shear load and the subsequent peaks of shear stress were largely influenced by the amplitude of displacement. Reduction of post peak shear stress was significant for the first cycle as the load frequency increased. For low normal stress, shear load frequency of 0.05 Hz and one-third of asperity base length as displacement were the certain values that shear mechanism changed on slopes of the joint asperities. Shear mechanism also demonstrated that asperity diminishing did not depend on load direction at high frequency. Whereas, at low frequency of loads, it depended on the direction of starting cyclic load. Comparison of shear strength envelopes indicated an opposite behaviour of the joint between load frequencies and displacement amplitudes.