Calculation Model for Loosening Earth Pressure of a Shield Tunnel Based on the Influence of Steady Seepage

Water seepage occurs occasionally in the undersea shield tunnels or high groundwater level shield tunnels after long-term use. The influence of the seepage field on the tunnel top load cannot be ignored under any circumstanc. Seepage effects are rarely considered in the existing theory of loosening earth pressure. Therefore, this study proposes an analytical model to evaluate the loosening earth pressure, considering the seepage effects. The shield tunnel is in a steady seepage state; therefore, the pore pressure and seepage force around the tunnel are determined based on the conformal mapping of complex variable methods. According to the effects of the principal stress axes rotation, the lateral pressure coefficient of calculation model is obtained. Based on Terzaghi's assumption, an analytical solution of effective vertical stress is deduced using the limit equilibrium method. Seepage and principal stress axes rotation are analyzed to obtain the analytical solution. The analytical model is verified via a parametric analysis and using a FLAC numerical model. The result show that when the relative permeability between the soil and lining is high, the effective stress acting on the tunnel is high; permeability coefficient variations have negligible influence on the effective loosening earth pressure when it goes beyond a certain value. The effective loosening earth pressure decreases with the increasing tunnel seepage rate; a higher surface water head leads to lower effective loosening earth pressure at the top of the tunnel. The effect of arching will be weakened with increase in the head height inside the segment; further, larger displacement of soil mass strengthens the arching effect. The theoretical results are consistent with the numerical results. © 2019, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.