Range of support pressures for slurry shield and analysis of its influence factors

The earth pressure balanced shield supplies a passive support for the tunnel face. However, the slurry shield offers an active support for the tunnel face with slurry (liquid medium). The key work for slurry support is to choose a suitable slurry and a support pressure to form a slurry cake and maintain its integrity. Based on the theory of soil fracturing and the wedge-prism model, the upper and lower bounds of slurry support pressures are supplied. Slurry support pressures are analyzed with consideration of hydrostatic pressures, soil properties, tunnel diameters and earth covers. It is suggested that the lower bound of the support pressure is mainly affected by hydrostatic pressure and soil friction, especially by the hydrostatic pressure. The slurry support pressure can be usually set as 20 kPa more than the hydrostatic pressure; the upper bound of support pressure is soil fracturing (seepage failure) pressure, mainly affected by hydrostatic pressure and earth cover. The soil fracturing (seepage failure) pressure can be increased by increasing the thickness of earth cover. And then the ranges of slurry support pressure can be also enlarged. However, the ranges of slurry support pressure can only be moved but not enlarged by increasing the hydrostatic pressure. The range length of slurry support pressure is also affected by the soil friction rather than other factors mentioned above. Considering the setting of slurry pressure in shield cutter replacement condition, the ratio of earth cover to tunnel diameter should be larger than 0.8~1.0