Face Stability Analysis of Shield Tunnel in Sandy Ground Using 3D DEM

Based on the model test carried out by Chambon and Corte, the three-dimensional discrete element method (3D DEM) was used to study the face stability of shallow shield tunnels in sand, and the face failure mechanism was investigated from microscopic perspectives. A three-dimensional flexible stress boundary was implemented in the numerical model, and the support provided by air or fluid in the chamber for a tunnel face was simplified as specified normal pressure acting on face particles. Pressure was decreased gradually to 0 kPa, and ground deformation was closely recorded. Thus, the limit support pressure could be determined naturally. The tunnel excavation process was incorporated by deleting the particles that flowed into the tunnel, and its effect on tunnel stability was considered. Results show that when C (tunnel buried depth)/D (tunnel diameter) ≤ 1.0, the limit support pressure first increases with buried depth and then tends to be constant. The ratio of the limit support pressure to the initial support pressure decreases with buried depth. The support pressure at which ground settlement accelerates abruptly is smaller than the limit support pressure. The failure zone directly propagates up to the ground surface. In engineering practice, attention should be paid to the ground surface settlement and limit support pressure to keep the tunnel face safe. When C/D ≥ 2.0, a stable soil arch exists above the tunnel crown and extends upwards to approximately 0.7D-1.3D and 0.9D-2.3D.