Evaluation of the Active Limit Support Pressure for Shield Tunnel Face in Clay–Sand Interface Mixed Ground

Although many calculation methods for tunnel face support pressure have been proposed, they are difficult to use in practical applications. In tunneling, the tunnel support pressure is currently being determined by empirical methods. In this study, five special geological sections were selected based on the shield tunnel project between Wangfujing and Qianmen Stations of Beijing Metro Line 8 to evaluate the different support pressure ratios λ by using the particle flow code (PFC). Both the stress distribution in the ground and the settlement under different support states were analyzed. A simplified limit-equilibrium method was developed in combination with the limit state in the PFC model. The present method was compared with the classical limit-equilibrium calculation methods to predict the active limit support pressure in the tunnel face. The values of the minimum limit support pressure ratio λmin of the tunnel face were suggested. The results showed that such ratio should be set to 0.6–0.7, which implies that the change in the support pressure cannot be less than 0.6 times of the true strata stress before excavation. The present calculation method of the limit support pressure is simple and effective and can be used in the design and construction of shield tunnel engineering.