Face Stability Analysis of EPB Shield Tunnels in Dry Granular Soils Considering Dynamic Excavation Process

The aim of this paper is to study the face stability of earth pressure balance (EPB) shield tunnels in dry granular soils considering the influence of the dynamic excavation process. The analysis was performed using three-dimensional discrete-element method (DEM). In the DEM model, the EPB shield considering the cutting-tool and opening configuration of panel was established, and the support of soil in the chamber was simplified as normal pressure acting on the excavation face particles. The present work has two advantages: (1) it can consider the soil disturbance induced by the dynamic interaction between the tunnel face and shield cutterhead; and (2) it can consider the support provided by the cutterhead panel. The simulation results show that the face stability in granular soils is overestimated without considering the dynamic excavation process. The face failure mechanism is affected by the buried depth of the tunnel and the friction angle of soil. For shallow tunnels with C/D=0.5 (C = buried depth and D = tunnel diameter), the failure zone resembles a wedge directly propagating to the ground surface; for tunnels with C/D=1.0, the failure zone resembles either a wedge with a chimney on top outcropping the ground surface or a closed bulb, depending on the friction angle of the soil; for deep tunnels with C/D=2.0, the failure zone resembles a bulb regardless of the friction angle of the soil.