Differences between reverse and normal shear in failure characteristics of layered rocks

Abstract

The study of the bedding plane effect has important implications for stability analysis of deep rock masses. However, the differences in shear mechanical behavior between natural layered rocks under reverse and normal dip conditions are still not well understood. For this , a full-angle shear test of shale with 0°≤ψ≤180° (ψ is the bedding plane inclination angle, defined as the angle of clockwise rotation from the shear surface to the laminar surface) was carried out. The shear mechanical properties and differences in failure modes of shale under different bedding plane inclination angles were extensively analyzed. Additionally, the analysis results were supplemented and verified with discrete element simulations. The results are as follows. Firstly, the minimum shear strength is achieved when shearing parallel to the bedding plane. The strength reaches a maximum at ψ=30° and local peaks at 90° and 135°. The shear strength is relatively higher when shearing in the reverse direction. For ψ>30°, the shear strength generally decreases with ψ. Secondly, according to the differences in the shear mechanical behavior under various ψ, the layered rocks are divided into three groups: bedding tension and matrix shear group (ψ=15°-60°), matrix shear group (ψ=75°-120°), matrix and bedding shear group (135°-180°). Thirdly, In the pre-peak stage, stress drop phenomenon only exists in the matrix shear group. In the post-peak stage, stress drops in a “step-like” manner for bedding tension and matrix shear group. Fourthly, tension and shear failures coexist, with shear failure being predominant. Lastly, the number of shear cracks of layer is dominant when shearing parallel to the bedding plane. The number of shear cracks in the matrix is the highest at 90°. At ψ= 30°, the maximum number of tensile cracks is observed in the bedding plane, followed by shear cracks in the matrix. The shear cracks are mainly observed in the bedding and matrix at ψ= 150°. The study reveals the anisotropic characteristics and differences in reverse and normal dip shear of layered rocks. The results provide a scientific basis for improving anisotropic mechanical models and analyzing disaster mechanisms and surrounding rock stability.

Keywords

• layered rock
• anisotropy
• mechanical character
• failure mode
• discrete element