Experimental Study on Filter Cake Formation in Slurry Shield Tunnelling in Cobble-gravel Stratum Using Slurries with Various Properties

Given the problem of filter cake formation of the slurry shield that passes through a highly permeable cobble-gravel stratum in the Tuan-Jiu Project Phase II under the South-to-North Water Diversion Project in Beijing,the slurries with different densities and different sand quantities are prepared to conduct the filter cake formation test,and the filter cake formation effect is analyzed from aspects such as slurry seepage discharge and permeability coefficient of filter cake. As the results show, in a highly permeable stratum with coefficient of permeability of 10-1 cm/s magnitude, the bentonite-clay mixed slurry with density <1.25 g/cm3 is difficult for filter cake formation, but the slurry with density of 1.3 g/ cm3 can form a slurry skin + permeation zone type filter cake. If the bentonite-clay mixed slurry with density of 1.15 g/cm3 is given a proper amount of sand, it will form a slurry skin + permeation zone type filter cake, and the seepage discharge and filter cake formation time are noticeably decreased as compared with the slurry without sand and with the same density. As more sand is added, the seepage discharge of slurry decreases slightly, and the void ratio and permeability coefficient of the formed filter cake show also the trend of decreasing.When the sand content is 5%, the permeability coefficient of the formed filter cake reaches the 10-8 cm/s magnitude.