Bedding Effect of Carbonaceous Slate Failure Behavior at Direct Shear Tests

Abstract In this study, the carbonaceous slate of Muzhailing tunnel was selected to explore the deformation and failure mechanism of tunnel layered slate under direct shear conditions. Five sets of direct shear tests were carried out on slate with different bedding dip angles (β: 0°, 30°, 45°, 60°, and 90°). The strength parameters, failure mode, fracture topography, dissipated energy, and acoustic emission (AE) characteristics were analyzed in detail to study the bedding effect of slate failure. The results show that the cohesion and internal friction angle of slate increases and decreases linearly with β from 0° to 90°, respectively, and the shear strength of bedding plane is lower than that of matrix. Additionally, the relative roughness (δ) of gray-scale image was defined, and the linear relationship between δ and root-mean-square height (Sq) was established to improve the efficiency of evolution of fracture topography. Furthermore, the shear energy consumption per unit area (us) was calculated, less sensitive to the variation of β. Moreover, a larger AE count and energy release rate were observed when the shear stress decreased, and a relative quiet period was observed before the peak loading. Additionally, both the count and energy release rate linearly decreased with the increase in β, and the proportion of tensile microcracks for vertical bedding is slightly higher than that for horizontal bedding. The main frequency of vertical bedding slate is smaller than that of horizontal bedding, i.e., the size of microcracks of slate with β of 90° is relatively larger. Generally, the frequency bandwidth of slate became narrower when β was varied from 30° to 60°, and the main frequency increased, indicating that the degree of shear failure of matrix is weakened. In contrast, the degree of cracking along the bedding plane increased.