Model test and numerical analysis for the face failure mechanism of large cross-section tunnels under different ground conditions

Face stability protection is a critical issue during large cross-section tunneling. This study investigates the evolution mechanism of tunnel face instability with different ground conditions based on model tests and numerical simulations. Eight groups of model tests were conducted to study the failure behavior of the tunnel face and to characterize the ground displacement and stress distributions. Three failure modes including front extrusion, forward caving and backward caving were defined based on the model test results. Finite element limit analyses were performed using the strength reduction method to simulate the failure behavior. The simulation results were compared with those of the model tests, validating the effectiveness the numerical techniques used in the study. Parametric studies were conducted to investigate the effects of ground strength and unit weight, and tunnel unsupported length, height and burial depth on the tunnel face stability. The simulation results showed that the effect of the gravity load on face stability weakens with the increase in the burial depth due to the arching effects as observed in model tests. Since the ground strength, unsupported length and tunnel height significantly influenced the face stability, the bench method with different unsupported length was compared with the full-face method. The effects of upper bench length and height on the face stability under different ground conditions were investigated. This study explored the face failure mechanism of large cross-section tunnels excavated under different ground conditions. The results can provide a reliable reference to evaluate face stability and safety control during tunnel excavation.