Tunnel face instability in composite strata during shield tunneling: a case study

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Accurately predicting tunnel face stability during shield tunneling in composite strata remains a significant challenge due to the pronounced heterogeneity and contrasting mechanical behaviors of stratified geological formations. To address this issue, this study establishes a refined coupled FDM–DEM numerical model of a tunnel boring machine (TBM), capable of realistically capturing the excavation-induced disturbance process. A parametric study was conducted to evaluate the effects of the composite ratio and the chamber fill ratio on tunnel face behavior, including failure zone development, TBM thrust, and torque evolution. The results indicate that tunnel face instability evolves through a typical three-stage process: initial disturbance, failure development, and failure propagation with arch reformation. The extent of failure is positively correlated with the composite ratio and negatively correlated with the chamber fill ratio. A higher composite ratio amplifies disturbance effects due to increased exposure to weaker strata, while greater chamber fill enhances face support and suppresses failure propagation. These findings enhance the understanding of tunnel face failure mechanisms in composite strata and support the rational design of shield tunneling parameters under complex geological conditions.