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

Abstract

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

Research and Experimental Application of New Slurry Proportioning for Slag Improvement of EPB Shield Crossing Sand and Gravel Layer

“…At present, the improvement additives mainly include foam agents, dispersant, clay minerals and flocculant. The clay minerals are predominantly bentonite and clay [26,27], which are mainly used to 2 of 20 supplement the fine particles in order to reduce the internal friction angle of the soil mass in the pressure chamber [28]. Foam agent is composed of special foam agent and compressed air, which can be used to improve the fluidity [29,30] and impermeability [31,32] of the excavated soil mass.…”