Abstract: A rational support force is a crucial factor ensuring the stability of the shield tunnel excavation face. However, limited research has been conducted on the support force concerning the rectangular shield tunnel excavation face. To clarify the failure forms of tunnel excavation face of rectangular shield tunnels and determined an accurate design method for limit support forces, the soil failure forms on the tunnel face of rectangular shield tunnels is modified based on the Mohr-Coulomb yield criterion by optimizing the traditional three-dimensional wedge model. An active and passive soil failure calculation model tailored for rectangular shield tunnels is established and corresponding formulas for calculating the limit support force are derived. Additionally, finite element analysis software Plaxis3D is employed to simulate the active and passive limit failure states of shield tunnel excavation face, thus to validate the proposed soil failure mode and the variation law of active and passive limit support forces. The results demonstrate a strong agreement between the proposed soil failure mode and the numerical simulation results. The findings reveal the influence of soil cohesion, internal friction angle, burial depth ratio, and groundwater level height on the limit support force. The passive limit support force is positively correlated with the above-mentioned parameters, while the active limit support force decreases with increasing cohesion and internal friction angle.

Key words: rectangular shield tunnel; excavation face stability, active failure, passive failure, limit support force