Influence of infiltration and groundwater flow on tunnel face stability

During the excavation of tunnels in saturated soft soils, a pressurized bentonite slurry is often used to support the tunnel face. The slurry should form a filter cake, which is used to transfer the support pressure onto the soil. This filter cake is constantly removed by the cutter teeth of the TBM during excavation and slurry will infiltrate into the soil while the filter cake rebuilds. This results in excess pore pressures directly in front of the tunnel face. To quantify the influence of these excess pore pressures on the stability of the tunnel face, a simple groundwater-flow model has been incorporated in a wedge stability analysis. For a tunnel in fine sand layers, the pore pressure distribution calculated with this model has been compared with field measurements. It is shown that the minimal support pressure is significantly higher, compared with full-membrane calculations.