Fatigue damage mechanism of soft-hard interbedded rock joints subjected to pre-peak cyclic shear loading

The cyclic shear fatigue damage of rock joints under frequent micro-earthquakes in the Three Gorges Reservoir area has an key impact on the dynamic stability of slopes. The macro-meso fatigue damage mechanism of soft-hard interbedded rock joints is studied via the laboratory pre-peak cyclic shear tests and PFC^2D meso numerical calculation considering the effects of the first-order asperity angle, moisture content, shear rate, shear amplitude, normal stress and cyclic shear number. The results show that: (1) The curve of shear stress versus shear displacement of rock joints experiences six evolution stages, i.e., initial nonlinear compression-shear deformation, approximate linear elastic compression-shear deformation, cyclic shear fatigue damage deformation, compression-shear deformation with slowly rising stress, compression-shear deformation with sharply rising stress and compression-shear deformation with brittle stress drop. (2) Under the same first-order asperity angle, moisture content, shear rate, shear amplitude or normal stress, the peak (residual) shear strength and cumulative shear (normal) displacement of rock joints decrease and increase with the increase of the cyclic shear number, respectively. Under the same cyclic shear number, they increase and decrease with the increase of the first-order asperity angle or normal stress, respectively. Meanwhile, they decrease and increase with the increase of the moisture content, shear rate or shear amplitude, respectively. (3) The macro-meso results are in good agreement on the whole. The number of meso shear fatigue damage cracks of rock joints exhibits the development characteristics of slight increase-steep increase, slow increase and steep increase-slow increase-steep increase in the early, middle and late stages, respectively, with the change of shear displacement; and it shows the development characteristics of steep increase and slow increase in the early and late stages, respectively, with the change of cyclic shear number. (4) The typical evolution process of macro-meso shear fatigue damage of rock joints can be described as three progressive development stages, i.e., compaction-crack initiation failure, cyclic dislocation-penetration failure and separation-gnawing failure, and the macro-meso shear fatigue damage cracks are densely distributed near the rock joints in an approximate inverted U-shape.