Abstract: For the tunnel excavated in layered rock mass, the failure mechanism of surrounding rocks is different from that of isotropic surrounding rocks. For unsupported tunnel, the failure mode can be summarized into mixed failure, V-shaped notch breakout failure and spalling of bedding. The failure modes are controlled by multiple factors such as the physico-mechanical properties of the rock mass, in situ stress, and the shape of the tunnel. In this study, the combined finite-discrete element method (FDEM) is used to understand the failure mechanism of horizontally layered rock mass, including the influences of strength parameters (such as cohesion, internal friction coefficient and tensile strength), deformation parameters (such as Young’s modulus), in situ stress and tunnel span. The study results show that the mixed failure is the basic failure mode of the layered rock mass, and its failure mechanism is the conjugate shear fractures F3 generated by the horizontally concentrated stress continuously propagate into the surrounding rocks near the centerline of tunnel; at the same time, shear-slip fractures F1 parallel to the bedding plane are generated, resulting in slab-like rock fragments. The tensile fractures F2 perpendicular to the bedding plane are generated due to the contact squeezing effect between the left and right slab-like rock fragments and thus flip to the tunnel. With the increase of the rock mass strength, the increase of the lateral pressure coefficient or the decrease of the tunnel span, the fractures F1 disappear, and the fractures F2 are oblique to the bedding plane, resulting in V-shaped notch breakout. When the rock mass strength or the lateral pressure coefficient is further increased, the fractures F3 are blocked at the junction of the bedding planes, resulting in spalling of bedding.

Key words: horizontally layered rock mass, buckling failure, finite-discrete element method (FDEM) numerical simulation, failure mode of surrounding rocks, circular tunnel