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Dynamic loads, including earthquake, blasting, and vibration loads, induce cyclic shear loads along the joints in rock masses; hence, the risk of failure increases on the joints due to changing shear resistance. On the other hand, joints are under different boundary conditions: constant normal load (CNL) and constant normal stiffness (CNS). Normal stiffness increases on the joints with increasing depth, and it can affect shear resistance. For an accurate assessment of joint shear resistance under varying normal stiffness and number of cycles, advanced laboratory shear apparatus is essential for the shear test. Conventional direct shear apparatuses have limitations such as boundary conditions, working under monotonic (static) shear loads only, or cyclic shear loads with no change of frequency and amplitude of shear loads. Therefore, a new large-scale servo-controlled direct shear testing machine was developed to conduct cyclic shear test (as well as monotonic shear test) under CNL and CNS boundary conditions with varying normal stiffness at different frequencies and amplitudes of shear loads. In the present study, the cyclic shear tests were conducted on nonplanar joints under varying normal stiffness. Moreover, the effects of different frequencies and amplitudes of shear loads were investigated. The test results indicate 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 frequency of shear load influences peak shear stress, i.e., peak shear stress increases with increasing frequency. The number of cycles does not affect peak shear stress on the joints at low shear amplitude, but peak shear stress decreases with higher amplitude.

The evaluation of changes in shear resistance on soft (or weathered) rock joints under cyclic shear loads with constant normal load (CNL) and constant normal stiffness (CNS) significantly contributes to increasing the safety and stability of rock slopes and underground structures. In this study, a series of cyclic shear tests were conducted on simulated soft rock joints with regular (15°-15°, 30°-30°) and irregular (15°-30°) asperities under different normal stiffnesses (kn). The results indicated that the first peak shear stress increases with the increase in kn up to the normal stiffness of the joints (knj). Beyond knj, no significant change was observed in the peak shear stress. The difference in peak shear stress between regular (30°-30°) and irregular joints (15°-30°) increases as kn increases. The minimum difference of peak shear stress between regular and irregular joints was observed (8.2%) under CNL and the maximum difference was found (64.3%) on knj under CNS. The difference in peak shear stress between the first and subsequent cycles significantly increases as both the joint roughness and kn increases. A new shear strength model is developed to predict peak shear stress of the joints for different kn and asperity angles under cyclic shear loads.

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A series of cyclic shear tests was conducted on soft synthetic rock joints with 30°- 30° asperities. Large scale shear apparatus was used in the experiments to investigate effects of shear load frequency and amplitude of displacement on the joint behaviour. Varying load frequencies (0.01, 0.05 and 0.1 Hz,) and displacement amplitudes (±4, ±6, and ±8 mm) for 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 is significantly influenced by the frequency of cyclic shear load and subsequent peaks of shear stress are largely influenced by the amplitude of displacement. Reduction in peak shear stress at higher frequency of load is greater than the reduction at lower frequency of load under cyclic movements. Shear mechanism on the joint asperities at low normal stress demonstrate asperity diminishing does not depend on load direction at high frequency. Whereas, at low frequency of loads, it depends on direction of starting cyclic shear load. Low amplitude of displacement has no effect on asperity deformation under cyclic loads. As the amplitude rises, the diminishing of asperities increases significantly. Shear mechanism and asperity diminishing at low normal stress is similar for the both higher amplitude of displacement and lower frequency of load. Comparison of shear strength envelopes indicated an opposite behaviour of the joint at varying load frequencies and displacement amplitudes.