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 (FERm) 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.