Analytical Model for Tunnel Face Stability in Composite Strata Considering the Arching Effect

During tunnel construction, the excavation face frequently encounters heterogenous weathered composite strata that considerably reduce tunnel face stability. To assess the stability of the tunnel face under such conditions, a 3D analysis model is established using the limit analysis approach. The model comprehensively assesses the failure of the entire tunnel face, incorporating the Hoek-Brown criterion to account for the nonlinear failure characteristics of rock. In addition, improvements are made to the shape of the sliding surface in the cover layer to consider the influence of the arching effect. The proposed model is validated by comparisons with existing theoretical methods and numerical simulation results. Further parameter analysis is conducted to investigate the influence of stratum parameters, stratum thickness and tunnel depth. The results indicate that tunnel face instability is more likely to occur when the rock mass quality of the higher layer is greater than that of the bottom layer. The rock mass in the bottom layer has the most remarkable impact on the tunnel face stability compared with the cover layer and higher layer. The stability of the tunnel face initially decreases and then increases with the tunnel depth due to the rock-arching effect. These research results can provide useful references for tunnel face support design in composite strata.