Impact of a damage-based cohesion degradation–friction angle reinforcement model on tunnel surrounding rock stability

This study addresses the stability of surrounding rock in tunnels constructed through carbonaceous phyllite. A novel damage constitutive model, termed the DP-CWFS model, is proposed by integrating the Drucker–Prager strength criterion with the cohesion weakening–friction strengthening (CWFS) model. The model is implemented in the FLAC3D numerical simulation software via a dynamically linked library (.dll) compiled using Visual Studio 2015. The constitutive model is validated against laboratory triaxial compression test results. The findings demonstrate that the proposed DP-CWFS model effectively captures the elastic behavior, post-yield strengthening, strain-softening characteristics, brittle failure behavior, and residual strength of carbonaceous phyllite. Based on this model, numerical simulations were conducted to analyze stress redistribution, plastic zone evolution, and deformation patterns following excavation of the Yaowangmiao Tunnel in Shaanxi Province, China. The simulation results reveal that when excavation-induced damage is considered, the stress release in surrounding rock is significantly intensified, the plastic zone expands markedly, and the predicted deformations are in good agreement with field monitoring data. The case study confirms that the DP-CWFS model can reliably describe the mechanical evolution of soft rock tunnel surrounding rock under complex geological conditions, offering theoretical and technical support for tunnel stability assessment and support design optimization.