Effect of bentonite slurry on the function of foam for changing the permeability characteristics of sand under high hydraulic gradients

During earth pressure balance (EPB) shield tunnelling in sandy ground, not only foam but also other conditioning agents need to be injected reduce the permeability of muck and avoid water spewing out screw conveyor. Permeability tests were carried study characteristics conditioned sand under high hydraulic gradients. A low bentonite slurry injection ratio (BIR) enhanced workability foam-conditioned sand. As gradient increased, initial coefficient stable period became shorter or disappeared. The BIR had a more significant effect on than (FIR), this gradually weakened as increased. foam–bentonite slurry-conditioned decreased by approximately an order magnitude compared with that With addition slurry, suitable can accept higher content (w) lower FIR, resulting ranges w FIR are flexible. Finally, mechanism stabilizing action was discussed considering interaction between bubbles fine particles.