Stability of slurry supported tunnel face considering the transient support mechanism during excavation in non-cohesive soil

Slurry shields are known to be a reliable excavation method in non-cohesive soils under groundwater level. In such conditions, they can actively support the tunnel face while minimising of the support pressure fluctuations. Two fundamental conditions must be fulfilled to stabilize a tunnel face: A sufficient face support pressure in the excavation chamber and an efficient pressure transfer of slurry excess pressure onto the soil skeleton. At the time of introduction of the slurry shield, the theories to describe the pressure transfer were transferred from diaphragm wall technology, in which the bentonite slurry supports the open trench. The objectives of this thesis are to understand the consequences of simultaneous slurry penetration and tool excavation process at the tunnel face. A further aim is to characterize the pressure transfer and resulting tunnel face support efficiency for various combinations of slurry penetration and excavation scales. Two hypotheses about the pressure transfer during slurry shield excavation resulting from the interaction between cutting tools and pressure transfer mechanism are formulated. Case A and Case B of the interaction at the tunnel face are introduced based on local comparison between slurry penetration and tool cutting depth. The Case A stands for higher cutting depth than slurry penetration depth, while Case B represents shallower cutting depth than slurry penetration depth. It is concluded that the pressure transfer efficiency in Case A is significantly reduced due to increased pore water pressures outside of slurry penetrated zone during excavation. Based on the obtained results, it is recommended to conduct the excavation with a type of interaction according to Case B.