Infiltration of pressurised foam into saturated sand has been investigated in a laboratory setup that provides a comparable hydraulic gradient as in real tunnels. It appears that the permeability of sand for foam decreases as FER of the foam increases until a limit FER (approximately 15 for the foaming agent and sand used in this experiment). For higher FERs the permeability remains more or less constant. It was found that a ‘dry’ foam (FER = 20) is not essential for formation of a low permeable layer in the sand, even a ‘wet’ foam (FER = 5) can form a low permeable layer. When the discharge is less than that corresponding to the drilling velocity of a TBM, there will be no foam spurt and consequently no zone with low permeability. Without a low permeability plastering at the tunnel face the pressure applied through the foam will be less effective to stabilize the face.