A damage model for full-stage mechanical behavior of rocks and application to excavation response analysis of deep-buried tunnels

A full-stage damage model for rock is proposed by accounting for the intermediate principal stress, strain-rate effects, and nonlinear deformation characteristics. Three independent characteristic strength surfaces corresponding to the initial, peak, and residual states constrain damage initiation, peak load-bearing capacity, and residual load-bearing capacity, respectively. Stress-state-dependent deformability and damage-induced stiffness degradation are incorporated into a unified stress–strain relation, allowing continuous representation of rock behavior from the undamaged state through strain hardening and strain softening to the residual state. The model is further applied to excavation analysis of a deep-buried circular tunnel, and a finite-difference iterative semi-analytical solution is developed. At each radial node, the initial, peak, and residual characteristic deviatoric stresses, damage variable, and stress state are determined iteratively, maintaining consistency between damage evolution and stress updating and avoiding stress oscillations in the strain-hardening zone caused by explicitly lagged updating. Results show that the extent of the excavation damaged zone alone is insufficient to characterize damage severity in surrounding rock; the internal state composition and transition process must also be considered. Increasing the support pressure ratio reduces tunnel-wall unloading, the extent of the excavation damaged zone, and convergence deformation, while progressively shifting the dominant internal state from the residual and strain-softening states toward the strain-hardening state. Increasing the initial in-situ stress enlarges the excavation damaged zone, increases tunnel-wall displacement, and promotes the development of the strain-softening and residual zones. The results advance understanding of excavation-induced damage evolution in deep-buried tunnels.