Abstract: When tunnels pass through fault fracture zones, the surrounding rock is prone to instability. To address this issue, the study proposes an instability analysis model for open-type TBM tunnels that considers the excavation-disturbance effect based on catastrophe theory. First, a simplified mechanical model is established, and the rock-mass constitutive behavior is described using a Weibull distribution function. A fitting formula for TBM excavation-disturbance parameters is developed using tunneling parameters, and the constitutive model is modified to incorporate groundwater-induced weakening. Second, by integrating the simplified mechanical model with the modified constitutive model, a cusp-catastrophe-based analysis model for surrounding-rock instability in TBM tunnels crossing fault fracture zones is constructed, and quantitative evaluation parameters for the instability criterion are derived. Finally, the reliability and accuracy of the instability criterion are verified through a TBM tunnel case study. The results indicate that the shear resistance of the surrounding rock predominantly governs its stability, while excavation disturbance and groundwater also exert notable influences. As the disturbance parameter and groundwater pressure increase, the risk of surrounding-rock instability rises. Grouting reinforcement effectively improves surrounding-rock stability by increasing the shear modulus 1.2－1.8 times the prereinforcement value. The critical disturbance parameter increases from 0.06 to 0.38－0.95, and the critical water pressure increases by 21.9%－101.2%. Field application of chemical grouting shows that the grout effectively fills rock-mass fractures, substantially enhances shear resistance, and improves the stability of the surrounding rock in fault fracture zones.

Key words: instability criterion, catastrophe theory, tunnel surrounding rocks, disturbance parameters, open-type TBM, excavation-disturbance effect