Face stability analysis of shallow shield tunnels in dry sandy ground using the discrete element method

Controlling the face stability of shallow shield tunnels is difficult due to the inadequate understanding of face failure mechanism. The failure mechanism and the limit support pressure of a tunnel face in dry sandy ground were investigated by using discrete element method (DEM), which has particular advantages for revealing mechanical properties of granular materials. The contact parameters of the dry sand particles were obtained by calibrating the results of laboratory direct shear tests. A series of three-dimensional DEM models for different ratios of the cover depth to the diameter of the tunnel (C/D = 0.5, 1, and 2; i.e., relative depth) were then built to simulated the process of tunnel face failure. The limit support pressure, failure zone and soil arching were discussed and compared with other methods. The results of DEM simulations show that the process of tunnel face failure can be divided into two stages. With the increase of the horizontal displacement of the tunnel face, the support pressure decreases to the limit support pressure and then increases to the residual support pressure. The limit support pressure increases with the rise of relative depth and then tends to be constant. In the process of tunnel face failure, the failure zone is gradually enlarged in size and expands to the ground surface. The numerical results also demonstrate that soil arching occurs in the upper part of the failure zone and the soil becomes loosened in the failure zone. Consequently, the comprehensive analysis of tunnel face failure may help to guarantee safe construction during tunneling.