Bedding Dip Angle Effect on Asymmetric Deformation and Failure of Tunnels in Soft-Hard Interbedded Rock: An FDEM Study

Deep-buried tunnels excavated in soft-hard interbedded rock masses under high in-situ stresses frequently suffer from severe asymmetric squeezing. To elucidate the multi-scale controlling mechanism of the bedding dip angle (β), a finite–discrete element method (FDEM) model was developed on the Abaqus platform using the Muzhailing Tunnel as an engineering prototype. The model couples the Mohr–Coulomb criterion for rock matrix plasticization with zero-thickness cohesive elements for interface debonding and shear slip. Following specimen-scale validation against multi-orientation uniaxial compression tests (average peak strength error of 4.67%) and a qualitative consistency check against field failure patterns, the model was employed to investigate the evolution of surrounding-rock deformation, interface damage, and matrix plastic zones across β=0–90∘. Results demonstrate that maximum radial convergence responds non-monotonically to the bedding dip, peaking at β=45∘ (74.47 mm), which is approximately 52% larger than the minimum at β=60∘ (48.97 mm), defining a prominent deformation amplification window within 40–50∘. An asymmetry index (AI) quantifies the pronounced bias induced by inclined strata, reaching a maximum of 0.153 at 75∘. Dominant deformation zones migrate systematically along the perimeter governed by the geometric tangency between bedding planes and the excavation profile; uniquely, intense interlayer shear slip at 30∘ shifts the peak convergence anomaly to the right sidewall. Crucially, macroscopic convergence, interface damage, and matrix plasticization evolve asynchronously, where modest radial convergence may mask substantial subsurface interlayer slip. The asymmetric instability is fundamentally a progressive, cross-scale process driven by the synergy between discontinuous interface slip and continuous matrix shear yielding, providing theoretical guidelines for non-symmetric support design in deep stratified ground.