Permeability of filter cake and its influence on face stability of slurry shield-driven tunnels

The hydraulic conductivity of filter cake is important for the stability of tunnel face when the slurry shield is excavated. The behaviors of slurry penetration and filter cake formation are investigated based on the infiltration column tests. The change of hydraulic conductivity of filter cake with time is obtained from tests. The permeable filter cake is formed on the tunnel face during advancing because the filter cake is destroyed by cutter and re-built. The scarcely-permeable filter cake is formed during assembling of linings because the cutter wheel stops rotating. A two-dimensional numerical model considering the permeable filter cake is established for calculating the pore pressure in the filter cake and stratum. The numerical results show the maximum pore pressure when the slurry shield is advancing. The seepage forces on the filter cake and failure zone are computed according to the pore pressure. The distributive ratios of the seepage force in the filter cake to the seepage force in the stratum are obtained under different slurry pressures. The efficiency of stabilizing the tunnel face deteriorates when the slurry pressure increases. For a higher slurry pressure, more pore pressure drops beyond the failure zone. A new calculation approach for the limit slurry pressure is proposed based on the limit equilibrium method. The results show that the limit slurry pressure is overestimated by the existing methods.