Pressure infiltration characteristics of foam for EPB shield tunnelling in saturated sand – part 2: soil–foam mixture

Experiments on infiltration of pressurised soil–foam mixture into saturated sand have been carried out in a laboratory set-up that provides a hydraulic gradient comparable to real tunnels. The soil–foam mixture used was comparable to the excavated soil (soil–foam mixture) that can be expected in the excavation chamber of an earth pressure balance (EPB) shield. It appears that a higher effective foam expansion ratio (FER_m) is more effective to form a low-permeability layer in the sand. Since there will be an interaction between air, water and sand in the soil–foam mixture, the infiltration characteristics of the soil–foam mixture are a little different from that of the foam. It was shown that a larger air fraction at a given solid fraction reduces the foam flow in the sand. The water permeability of foam-infiltrated sand decreases with an increasing air fraction and a decreasing liquid fraction for a given solid fraction, and also decreases with an increasing air fraction and a decreasing solid fraction for a given liquid fraction. The situation with high sand fraction should be avoided in the field because there will be no impermeable or low-permeability layer formed at the tunnel face. Furthermore, the water permeability of foam-infiltrated sand decreases with the foam injection ratio (FIR) and an appropriate FIR is important for EPB shield tunnelling in saturated sand.