Three-Dimensional Seepage Flow Analysis of Slurry Shield with Permeable Filter Cake

The filter cake on the tunnel face has a significant influence on the project of the slurry-driven shield. During the excavation, the only permeable filter cake is allowed to form because the rotating speed of cutter wheel is fast. Through the filter cake, the water of slurry will infiltrate into the soil. The water enhances the hydraulic head and the risk the failure of tunnel face. In order to determine the extent of hydraulic head, the analysis of seepage flow with the considering of the permeable filter should be conduct. A three-dimensional numerical model with the permeable filter cake was established for the analysis of seepage flow. Based on the numerical results, the hydraulic head field was obtained under different conditions. The hydraulic head field shows that the existence of filter cake seriously affects the hydraulic head field near the tunnel face. The hydraulic head tends to a hydrostatic head, at the points where is 2D in front of tunnel face, horizontal 1.5D to the lining of tunnel or soil surface. Three factors are simultaneously considered for assessing hydraulic head. The first is the excavation time, the second is the excess pressure and the third is the hydraulic conductivity. The excavation time and excess pressure have little influence on the distribution of normalized hydraulic head. When the hydraulic conductivity of soil increases, the hydraulic loss in filter cake increases from 0.16 to 0.5 the difference between slurry and hydrostatic pressure. On the contrary, the hydraulic loss in soil decreases when the hydraulic conductivity of soil increases.