Excavation cycle dependent changes of hydraulic properties of granular soil at the tunnel face during slurry shield excavations

摘要

Increased pore pressure heads in front of the tunnel face are frequently observed during excavations with slurry shields. It is expected that the excess pressures are caused by a flow in front of the tunnel face resulting from the interaction between the face supporting fluid (bentonite slurry) and the soil. The flow is assumed to be unsteady from two points of view. First, it is influenced by a time dependent change of hydraulic properties of the soil at the tunnel face. The time dependent change is governed by the interaction with bentonite slurry. Second, the area of the soil on the tunnel face with changed hydraulic properties is disturbed periodically by passing cutting tools which are rotating with the cutting wheel. After each passing of the cutting tool through a particular point at the tunnel face, new local flow is induced. The aim of this paper is to investigate experimentally the transient changes of hydraulic properties of granular soils due to interaction with bentonite slurries. The time dependent change of the soil properties is compared with excavation data of real shield machine describing the time scale of the cutting process. Based on the comparison, the hydraulic properties of soil on the real tunnel face during excavation can be evaluated. Thus, a better assessment of the pore pressure heads in front of the tunnel face can be performed.