Calculation of Limit Support Pressure for EPB Shield Tunnel Face in Water-Rich Sand

With the rapid development of the tunnels constructed under the rivers and seas, the research on face stability of shield tunnel in water-rich sand has important theoretical value and engineering application significance. In addition to the loads exerted by overlaying strata, the tunnels constructed in water-rich strata are usually subjected to high hydrostatic pressure or seepage forces, which are apt to cause the ground collapse of the shield tunnel face. The distribution of hydraulic head field around the tunnel face is critical to assess the impacts of the seepage forces on the tunnel face stability. This paper investigates the axisymmetric problem of the face stability of the shield tunnel under a seepage condition within the framework of limit equilibrium analysis. First, numerical simulations are carried out in this paper to analyze the distribution rules of total hydraulic head and pore water pressure near the tunnel face of the shield tunnel under the condition of stable seepage with different cover depths. Then, based on the distribution rules of total hydraulic head, new formulas for predicting the total hydraulic head along the horizontal and vertical directions are proposed and compared with the numerical simulations in this paper and existing approximate analytical solutions. Second, the classical axisymmetric limit equilibrium model is revised by incorporating the new approximate analytical solutions of hydraulic head field to determine the failure modes and the limit support pressures with a numerical optimization procedure. Lastly, the comparisons of the results obtained from the theoretical analysis model in this paper and the existing approaches are conducted, which shows that the failure mechanism proposed in this paper could provide relatively satisfactory results for the limit support pressures applied to the tunnel face.