Permeability characteristics of filter cake and pressure transfer on face during slurry shield tunnelling

The slurry pressure can be transferred to the soil skeleton of the tunnel face through the filter cake and becomes the effective stress, which therefore is important for the tunnel face during slurry shield tunnelling. From the laboratory infiltration tests, three key factors affecting the formation of filter cake are discussed: the hydraulic gradient on the tunnel face, the sand content of slurry and the soil particle size. A lower hydraulic gradient leads to longer time of filter cake formation. A filter cake is more easily formed for the fine sand than the medium and coarse sand, but the filter cake is easily damaged. Although a filter cake is harder to be formed for the medium and coarse sand, the slurry-infiltrated zone is thicker and thus is more significant for the stability of the tunnel face. The permeability coefficient of the filter cake is more than 100 times lower than that of the slurry-infiltrated soil. The permeability coefficient of the filter cake is less than 10^-8 m/s, while that of the slurry-infiltrated soil is between 10^-7~10^-5 m/s. A lower permeability coefficient of the filter cake or slurry-infiltrated zone leads to a smaller distribution zone and a higher decrease rate of the excess pore pressure. When the slurry is mixed with sand, the slurry infiltration rate is higher than that of the clean slurry. The permeability coefficient of the slurry-infiltrated zone increases with the increasing slurry density, and the decrease rate of the excess pore pressure decreases with the increasing slurry density.