Experimental study of foam infiltration from foam-sand mixture into saturated sand for EPB shield tunnelling: A mechanism study

This paper investigates the foam infiltration behavior from foam-sand mixture, resulting from foam conditioning of excavated sand inside of the excavation chamber of an Earth Pressure Balance (EPB) shield, into saturated sand at the tunnel face. The laboratory investigation was conducted using one dimensional element tests that simulate foam infiltration from the mixture into the saturated sand in front of the tunnel face. Pore pressures at different locations are measured near the sand surface, representing the tunnel face, while the sand column could provide a comparable hydraulic gradient to field conditions. Influences from foam injection ratio (FIR), remaining water content and sand type are examined. In the case of fine sand, it is found that there is little foam infiltration from the foam-sand mixture into the sand surface. Sedimentation of the foam-sand mixture on top of the saturated sand column is the dominating process that results in a dense mixture containing less liquid content with a smaller permeability. While for coarse-grained sand, foam infiltration will result in a foam infiltration zone with a lower permeability than the sedimented foam-sand mixture. Plastering effect at the tunnel face will increase with an increase in FIR and a decrease in remaining water content. Further analysis indicates that the sand matrix plays an important role in determining the permeability of a foam-sand mixture, as a higher density of sand results in a more tortuous seepage path for water. Mechanisms are discussed and implications for field applications are described.