Slurry Shield Tunneling: New Methodology for Simplified Prediction of Increased Pore Pressures Resulting from Slurry Infiltration at the Tunnel Face Under Cyclic Excavation Processes

摘要

Within mechanized tunneling, slurry shields are used for excavations in soils with unstable tunnel face due to the possibility to support the tunnel face with pressurized slurry (bentonite suspension). Two key conditions have to be fulfilled to stabilize a tunnel face. These two conditions are sufficient face support pressure in the excavation chamber and the pressure transfer of slurry excess pressure, exceeding the pore pressure, onto the soil skeleton. In practice, the German standard DIN 4126 [Nachweis der Standsicherheit von Schlitzwanden (Stability analysis of diaphragm walls), Deutsche Institut fur Normung, 2013] is usually used to predict this transfer. However, DIN 4126 (2013) cannot explain increased pore water pressures measured in practice close to the slurry supported tunnel face during excavation. The increased pore water pressures reduce the efficiency of slurry face support. These pressures are explained by on-going disturbance of the pressure transfer mechanism by periodic rotating cutting tools. The characteristics of disturbance are designated as excavation scale. Another factor of influence is the timespan during which the pressure transfer mechanism can achieve a significant decrease in its own permeability, and thereby to decrease considerably the flow through the tunnel face. By scale comparison of these two processes, a prognosis about occurrence of increased pore water pressures in saturated sands can be derived. It turns out that the different combinations of penetration rate and revolutions per minute of the cutting wheel, while keeping the advance rate constant, would result in different chance for causing increased pore pressures. Consequently, an excavation strategy for reducing the chance for increased pore pressures is suggested with respect to three reference slurry shields.