DEM and theoretical analyses of the face stability of shallow shield cross-river tunnels in silty fine sand

A discrete element numerical simulation is used to evaluate the face stability of a shallow shield cross-river tunnel through silty fine sand. The micro mechanism of silty fine sand is first studied with direct shear tests and the micro parameters are calibrated. A shallow shield cross-river tunnel model is then conducted considering the water pressure and buoyancy force. Failure mechanism of the soil in front of the tunnel face is analyzed by measuring the characteristics of shear strain, sliding fraction, volumetric strain, particle displacement, and the cohesive failure zone. The results show that a wedge model in front of the tunnel face is first formed and as the displacement of the piston movement increases, the area of the failure zone in front of the tunnel face is enlarged. Base on the numerical results, a new analytical model for shallow shield cross-river tunnels in silty fine sand is proposed. The theoretical solution of limit support pressure of the proposed model is deduced. The normalized support pressures calculated by the present model are in good agreement with other existing well-established methods that the proposed model can be availably applied to the shallow and deep cross-river tunnels.