Support pressure transfer at a slurry supported tunnel face due to time dependent decrease of soil permeability

Within mechanized tunnelling, the excavation chamber of a slurry pressure balanced (SPB) shield is filled with the bentonite suspension as support medium. A compressed air cushion is applied to generate a hydrostatic fluid pressure, which corresponds to the support pressure required for the stability of the tunnel face due to earth pressure and groundwater pressure. In reference to the support pressure, the bentonite suspension penetrates into the grain structure and transfers the support pressure onto the soil.In the case of bentonite suspensions, the hydrostatic support pressure is transferred to the grain skeleton time-dependent if a zone is formed at the surface or to a certain penetration depth in the near-surface area of the soil whose permeability is smaller than that of the soil itself. In this zone, the pressure difference between the support pressure and the pressure of the soil to be stabilised is converted into an effective stress acting on the grain structure. The formation of a zone of low water permeability in the soil depends on the penetration behaviour of the support medium.This paper presents the experimental results of advanced column tests, where the decrease of the soil permeability due to the penetration of bentonite suspension is measured over time for various combinations of soil and bentonite suspensions. Boundary conditions of the soil are described as particle size distribution, bulk density and initial permeability according to Darcy's law, bentonite suspension is rheologically characterized in terms of yield point and viscosity. In order to examine the impact of the permeability evolutions in front of the slurry shield TBM, numerical investigation is carried out. The finite element numerical model is calibrated and validated in accordance with physical and rheological observations along with laboratory measurements from the advanced columns test under controlled hydro-mechanical boundary conditions. After verification of the numerical 2D model, it is extended to a full scaled mechanized tunnelling 3D problem that to analyse the hydraulic flows and effective stress variations in the soil in front of the TBM and their influence on the face pressure transfer process from the TBM to the soil skeleton.