The face stability of slurry-shield-driven tunnels

During the excavation of a tunnel through soft water-bearing ground, a temporary support is often required to maintain the stability of the working face. In a slurry shield, this support is provided by a pressurized mixture of bentonite and water. Slurry-shield tunnelling has been applied successfully worldwide in recent years. Under extremely unfavorable geological conditions, however, face instabilities may occur. This paper aims at a better understanding of the mechanics of face failure when using a bentonite slurry support. The complex interrelations between the various parameters (shear strength and ground permeability, suspension parameters, slurry pressure, geometric data of the tunnel, safety factor) are studied. Attention is paid to the time-dependent effects associated with the gradual infiltration of slurry into the ground ahead of the tunnel. Related topics, such as the stand-up time, soil properties and the effect of advance rate, are discussed quantitatively.