An anisotropic-nonlinear-damage constitutive model and crack evolution mechanism for layered rock tunnels

As underground engineering extends to greater depth and more complex strata, tunnels increasingly pass through layered rock mass, and the instability of surrounding rock induced by crack propagation after tunnel excavation has become more prominent. Therefore, an anisotropic-nonlinear-damage constitutive model (ANDC) was established. Before yielding, the model treats layered rock mass as a transversely isotropic elastic material, uses a nonlinear strength criterion to describe rock mass strength, and introduces separate damage variables for the rock matrix and the bedding plane to characterize the deformation response and strength degradation of the rock matrix and the bedding plane, respectively. On this basis, the model was secondarily developed through the C++ programming language and embedded into finite difference software, thus establishing a numerical method for mechanical response analysis during tunnel construction in layered rock mass. Validation cases show that the proposed model can effectively capture the deformation and failure characteristics of tunnels in layered rock mass and has good potential for engineering application. The study further revealed the mechanisms of crack evolution and instability in layered rock mass tunnels: (i) Under different bedding plane inclinations, the overall crack evolution of surrounding rock follows a progressive process of local shear crack initiation-tensile crack initiation-circumferential propagation-radial extension-continuous crack development-tensile-shear composite instability. (ii) Bedding plane inclination significantly controls the crack initiation location, crack propagation path, and final failure mode of the tunnel; (iii) Shear crack evolution is jointly affected by unloading of the shallow surrounding rock near the tunnel boundary and by the transfer of the peak deviatoric stress; (iv) Tensile action mainly occurs near the tunnel boundary, and the evolution path of tensile crack is controlled by the migration of the peak tensile stress.