Shear Characteristics and Failure Mode of Hard Brittle Marl with Parallel Discontinuous Structural Plane

Engineering practice shows that the failure of rock bridges between structural planes which makes the discontinuous structural plane evolve into a continuous sliding fracture plane leads the instability of many slopes. In order to further understand the shear failure mechanism of rock mass with discontinuous structural plane, the direct shear tests under different normal pressures were carried out on the hard brittle limestone samples with different structural plane dip angles. The test results show that the fracture surface of rock bridge is mainly tensile failure, while both sides of the fracture surface are mainly shear failure. Both two flanks and the middle fracture surface show that both ends of the fracture surface are mainly shear failure, while the middle of the fracture surface is mainly tensile failure. When σ is large and α is small, the rock is mainly shear failure. When σ is large and α is large, or σ is small and α is small, the rock is mainly tension-shear mixed failure. When σ is small and α is large, the rock is mainly tensile failure. The failure mode of the sample may vary with the change of the structural plane inclination and normal stress. But overall speaking, it can be summarized as five typical failure modes that are STS–STS–STS, STS–T–STS, S–STS–S, S–S–S and T–T–T. The peak shear stress of the sample increases with the increase in normal stress, and decreases first and then increases with the increase in the dip angle of the structural plane. When α = 0°, the peak shear stress is the largest; when α = 45°, the peak shear stress is the smallest. When α is small, the shear strength of the sample is mainly affected by the inclination angle θ of the rock bridge. When α is large, the shear strength of the sample is mainly affected by the length L of the rock bridge.