Experimental study of pore water pressure development in soil when foam infiltrates into saturated sand

Foam is often used as an additive during tunneling in soft ground conditions in an Earth Pressure Balance (EPB) shield. Field measurements show that excess pore pressure builds up in front of the tunnel face during drilling and dissipates during standstill. This excess pore pressure will have consequences over the tunnel face stability. In this study, the pressure transfer mechanism of a slurry driven shield is reviewed as a starting point for the mechanisms to be expected during drilling with an EPB shield. Foam infiltration tests were conducted to study the pore water pressure development during foam infiltration into saturated sand. The main focus is to investigate the change of pore water pressure during foam infiltration that can be expected during drilling of a tunnel. The results indicate that a pressure drop can be realized through the foam infiltrated area, thus the supporting pressure can be applied on the soils in front of a tunnel face. Further pressure development reveals that this foam infiltrated area functions similarly as an internal cake in a slurry shield. Mechanisms of the pressure change are discussed.