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Stability of the tunnel face is a key design aspect for slurry shield tunnelling. Two requirements have to be fulfilled during excavation in order to achieve a stable tunnel face. The first requirement is a sufficient slurry pressure in the excavation chamber of the shield to counterbalance the acting pore water pressure and the earth pressure. Moreover, the fraction of the slurry pressure in the excavation chamber, which exceeds the pore pressure in the ground, has to be applied as effective stress onto the soil skeleton. This transfer of effective stress can be formulated as the second requirement. The pressure transfer theory currently being employed in the practice expects either a filter cake formation on the tunnel face or a penetration zone through which the pressure transfer is assured. However, this practical assessment approach was developed originally for diaphragm wall technology and therefore the time interval of the excavation process is neglected. Hereby, it is assumed, that the transfer mechanism on the tunnel face is built-up immediately and steadily. This assumption is not valid for the tunnel face of the slurry shield since the transfer mechanism is periodically destroyed by rotating cutting tools. In this paper, the time-related conditions on the slurry supported tunnel face are experimentally simulated. It is focused on the cases with deep slurry penetration. First, existing theories dealing with the pressure transfer are reviewed. Then, a new experimental devices and approaches are introduced. The investigations show that a relation exists between the efficient support pressure transfer during excavation and the pore pressure distribution in the slurry penetrated zone at static conditions. |
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