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This work focuses on the development of a three-dimensional shield tunneling model coupling the finite-difference method (FDM) with the discrete-element method (DEM) and evaluates its tunneling performance in boulders. The tunneling process of cutterhead rotation, cutting the formation, soil entering the chamber, and discharging of the screw conveyor is simulated to explain the characteristics of particle movement in front of the tunnel face. The range of the soil failure zone under different shield discharge ratios is determined by the correlation between the discharge ratio and the tunnel surface stability. The evolution process of the soil arching effect in the boulders along with shield tunneling is investigated to understand the mechanisms of the delay settlement in the boulders. Subsequently, the shield discharge ratio is optimized by investigating its influence on the surface deformation. Finally, the average stress distribution on the shield cutterhead in the tunneling process is analyzed to provide the scheme of the cutterhead structure and arrangement. This work can be referred for evaluating the shield tunneling performance and optimizing the excavation design scheme of shield machines during tunneling construction under similar geological conditions. The coupled finite-difference method (FDM)–discrete-element method (DEM) calculation method proposed in this study was successfully applied for boring tunnel equipment performance analysis in Beijing Metro Line 16. This shield tunnel was excavated in the strata with boulders rich in large size and high strength, and the previous tunneling experience shows that the shield is prone to encountering accidents such as tool damage and cutterhead jamming by large boulders when tunneling in such strata. Based on the present parametric analysis and calculation results, the suitable engineering suggestions have been proposed to enhance the shield equipment specifically, such as strengthening the central and peripheral cutters and the wear resistance of the cutterhead sides. In addition, the cutterhead opening and band-type screw conveyor were also adjusted to allow large size stones to pass through the cutterhead and discharge from the soil chamber as much as possible instead of being broken by the cutters. The adoption of these techniques has made it possible for the earth pressure balance shield (EPBS) machine to continuously excavate 1,300 m in boulders without cutter exchanges. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]