Coupled Effects of Bedding Angle and Bedding Structure on the Anisotropic Mechanical and Failure Behaviors of Shales: Numerical Simulations on Digital Rocks

Bedding angle and structural characteristics are fundamental attributes of laminated shales, and their interplay inherently governs anisotropic mechanical behavior. A deep understanding of these governing mechanisms is crucial for advancing geo-engineering evaluations across various fields of Earth and Energy Sciences. However, shale's intrinsic heterogeneities complicate experimental research and hinder the unraveling of the underlying physical mechanisms. We integrated geological data into a discrete element model to construct anisotropic digital shales, enabling a combined analysis of the coupled effects of bedding angle and structures on mechanical responses under compressive loading. Results show that Young's modulus increases with bedding angle, while compressive strength displays a V-shaped trend, and Poisson's ratio shows the opposite pattern. Critical bedding angles for minimum strength and maximum Poisson's ratio vary across bedding structures-45$°$ for finely laminated, 60$°$ for laminated, and 30$°$ for massive shales. This finding may help explain the variability of critical angles observed in previous experiments. Strength and modulus anisotropy also differ among shale types: finely laminated shale has the lowest strength but highest modulus anisotropy, while massive shale exhibits the opposite trend. Micro-damage analysis shows that at small bedding angles, axial deformation dominates, while interlayer slip is limited. Increasing bedding angles induces stress concentration along bedding planes, enhancing shear slip and reducing axial strain, resulting in failure concentrated at bedding interfaces. Across varying bedding structures, bedding-plane shear slip reaches the maximum at its critical angle, promoting intensive crack development along bedding planes. Strength variation reflects the internal stress transmission heterogeneities governed by bedding features.