Linking nonlocal damage-plastic mechanisms to EDZ formation in large-deformation tunnels within CCM framework

Large deformations of rock masses present substantial challenges in deep tunnel support design. Accurate stability assessments within the convergence-confinement method (CCM) framework rely significantly on the appropriate characterization of rock mechanical behaviour. Mainstream plastic models (e.g., perfect plasticity, brittle-plasticity, and strain softening) fail to represent intrinsic damage processes—namely stiffness degradation and crack propagation. To address these limitations, we introduce a finite-strain gradient-extended damage-plastic model to better capture the complex behavior of rocks under loading. Simulations of the unloading process around circular tunnels address: (1) the effect of damage factors on tunnel response; and (2) the influence of large deformation, bifurcation phenomenon, and mesh dependency on predictive reliability. Results reveal that nonlocal damage-plastic modeling enhances the predictive accuracy of CCM and mitigates mesh dependency. The excavation damaged zones (EDZ) and highly damaged zone (HDZ) are partitioned more rationally based on mechanical mechanisms. The progressive transition from a plastic-dominated zone through damage-plastic-coupled zones, and eventually to the damage-dominated zone, is effectively captured. Traditional plastic models may underestimate tunnel convergence by up to 40% in brittle rocks owing to damage neglection. The study further establishes a novel cross-scale correlation between material-level damage-plastic mechanisms and engineering-scale EDZ formation. These insights facilitate the predictive reliability of deep tunneling designs and offer actionable guidance on optimal support in squeezing ground conditions.