Pressure infiltration of sandy foam during EPB shield tunnelling in saturated sand

Experiments on infiltration of pressurised sandy foam into saturated sand have been carried out in a laboratory setup that provides a comparable hydraulic gradient as in real tunnels. The sandy foam used was comparable to the excavated soil (foam-water-sand mixture) that can be expected in the mixing chamber of an EPB (Earth Pressure Balance) shield. It appears that a higher FER_m (effective foam expansion ratio) is more effective to form an impermeable layer in the sand. Furthermore, the permeability of sand for water flow through the foam decreases with the FIR (foam injection ratio). In practice, a FIR of 35 - 40 is recommended for a sandy ground and the situation with high sand fraction of the sandy foam should be avoided in the field because there will be no impermeable or low permeable layer formed at the tunnel face.