Pressure infiltration characteristics of foam for EPB shield tunnelling

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.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.P