Modelling the infiltration behaviour of foam into saturated sand considering capillary resistance for EPB shield tunnelling

With reference to earth pressure balance (EPB) shield tunnelling, the pressure infiltration of foam into saturated sand is investigated through model study. The model accounts for the companion liquid flow during the foam spurt that was experimentally measured in a previous paper. A micro-stability model is established to predict the maximum foam penetration depth that is based on the minimum pressure difference over an individual foam bubble through the pore throats. From tests on three different sands, the micro-stability model compared well with two sands but underestimated the maximum penetration depth for the third. This is attributed to a case where many bubbles are small enough to flow through the larger pore throats unhindered. Further results from numerical simulation are in accordance with the measured discharge behaviour during the foam spurt. The general agreement suggests that the model could explain the foam infiltration behaviour and can be used to describe the foam spurt during foam infiltration that can be expected in EPB shield tunnelling.