Abstract: During tunnel excavation in phyllite formations, the stress concentration effect in surrounding rock and the mechanical effect of beddings significantly influence the deformation and failure of rock masses, commonly resulting in asymmetric deformation. To investigate the impact of phyllite bedding dip angle on the deformation and failure of surrounding rocks, this study examined the deformation characteristics, stress evolution, and progressive failure mechanisms of cavity-containing phyllite specimens with five bedding dip angles (0°, 30°, 45°, 60°, and 90°) under compression. Experimental results indicate that the failure load of cavity-containing phyllite exhibits a U-shaped trend with increasing bedding dip angle. By adopting the digital image correlation (DIC) technology, the influence of bedding dip angle on the failure mode of cavity-containing specimens was analyzed. At a dip angle of 0° , high strain concentration zones developed at the roof and floor of the cavity, resulting in tensile failure. When the dip angle ranged from 30° to 60°, strain concentration bands propagated along the beddings due to the combined effect of cavity stress concentration and bedding plane mechanics, thereby inducing shear-slip failure. At a dip angle of 90°, strain concentration bands extended vertically, leading to tensile-splitting failure along the beddings. The mesoscopic failure mechanisms of cavity-containing phyllite specimens were studied using the combined finite-discrete element method (FDEM) incorporating the cohesive zone model (CZM). Based on the experimental results, the feasibility and applicability of this modeling method for simulating the mechanical behavior of cavity-containing layered rock specimens were validated. Numerical simulation analyses of biaxial loading tests reveal that confining pressure not only enhances the bearing capacity of the specimens and reduces the anisotropy ratio, but also induces transitions in the failure modes of specimens with varying dip angles.

Key words: phyllite, bedding angle, digital image correlation, failure mechanism, finite-discrete element method