Pressure infiltration characteristics of foam for EPB shield tunnelling in saturated sand – part 1: ‘clean’ foam

Abstract

Infiltration of pressurised foam into saturated sand was investigated in a laboratory set-up that provided an hydraulic gradient comparable to real tunnels. Four different stages were identified in the infiltration: ‘foam spurt’ (the foam infiltration into the pores of the sand); water flow with reduced permeability; water flow with increased permeability; and a residual stage. It appears that the infiltration depth of foam into sand decreases with increasing foam expansion ratio (FER) of the foam 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 also found that a ‘dry’ foam (FER = 20) is not essential for formation of a low-permeability layer at the sand surface; even a ‘wet’ foam (FER = 5) can form a low-permeability layer. Furthermore, a foam-infiltrated zone is essential to achieve a low-permeability layer. In these tests, the flow velocities were lower than the normal excavation velocity of a tunnel-boring machine. Thus there was no foam-infiltrated zone and consequently no zone with low permeability during drilling. By comparing the pore pressures measured using pore pressure transducers with and without hydrophilic filter papers, the capillary pressure in the foam-infiltrated sand was found.