Investigation on tunnel face failure under varying seepage conditions in saturated sand using coupled CFD-DEM and analytical modelling

Abstract

Tunnels are increasingly being constructed in saturated sandy ground with high hydraulic head and permeability. However, limited attention has been given to face stability under various seepage conditions, which is critical for the safety of shield tunneling. To address this gap, a computational fluid dynamics-discrete element method (CFD-DEM) model was developed to simulate face failure under tunnel face seepage (TFS). The validated numerical model unveils that the specific discharge decreases exponentially with the increase of distance away from the tunnel face, but the increase of maximum specific discharge at tunnel face q_face,max can expand this influence range from 0.5D to 2D. Due to the seepage force directed toward the tunnel face, tunnel face unloading induces a loosening zone with a shape of wedge and inverted frustum shape, where the inclination angle decreases as q_face,max increases. Meanwhile, although the pressure afforded by the tunnel face can be alleviated by the soil arching effect, the TFS can weaken the soil arching effect and raise the limit support pressure (LSP). Based on the numerical results, the distribution of seepage force was derived and introduced into an equilibrium analysis. Thereby, an innovative analytical model for the estimation of LSP under TFS was proposed and verified. Additionally, the parametric study on the analytical model indicates that the pore pressure on the tunnel face center is inversely proportional to q_face,max, with the decreasing ratio negatively correlated to the void ratio e and the spherical seepage reduction factor η₀. Moreover, the LSP is positively correlated with q_face,max, e, and C, and negatively correlated with η₀ and internal friction angle.