Abstract: Currently, it is challenging to accurately evaluate tunnel face stability during shield tunneling, and the limit equilibrium methodbased models cannot accurately describe the failure mechanism of tunnel face instability. Therefore, an improved threedimensional model for shallowburied tunnels is proposed. The results of the improved model are compared with the existing theoretical models, physical tests, and numerical simulations. The errors are less than 10%, validating the rationality of the proposed model. Furthermore, the influence of soil parameters, tunnel diameter, and burial depth on the critical support pressure and failure mechanism characteristics derived from the improved model is investigated. The results reveal the following: (1) The internal friction angle and cohesion of the soil significantly impact the critical support pressure. (2) The critical support pressure remains constant as the burial depth and diameter of the tunnel increase to a certain value, due to the soil arch effect. (3) The internal friction angle of the soil has a greater influence on the characteristics of the failure mechanism than cohesion and burial depth.

Key words: shield tunnel, tunnel face stability, critical support pressure, improved failure model, limit equilibrium method