During an earth pressure balance shield tunneling in water-rich coarse-grained stratum

muck spewing or discharge stagnation often occurs. Only foam conditioning cannot improve the fluidity and anti-permeability of poorly graded muck due to foam dissipation and loss. Therefore

bentonite slurry must be added. Understanding the large shear deformation of slurry-foam-conditioned poorly graded sand under chamber pressure can optimize the conditioning scheme to ensure safe tunneling. In this study

a series of pressurized vane shear tests on slurry-foam-conditioned poorly graded sand was conducted. The results show that increasing the bentonite injection ratio (BIR) raises the initial void ratio and saturation of conditioned sand under pressure

resulting in a decrease in effective stress

peak strength

and residual strength. However

the shear-rate dependence of shear strength and effective internal friction angle are found to be insensitive to BIR. Moreover

increasing BIR reduces the yield stress of the conditioned sand after large deformation. A comparison of the large shear deformation under pressure of three types of conditioned soils with similar workability at atmospheric pressure indicates that increasing the slurry or foam injection ratio can enlarge the void ratio and reduce the effective stress under pressure. Notably

slurry-foam-conditioned sand exhibits lower effective stress and shear strength than foam-conditioned sand. The suitable conditioning parameters for testing poorly graded sand are FIR=20% and BIR=10%–15%. Microscopic images reveal the dual weakening effect of bentonite on the shear strength

including the bentonite adsorption on the surface of sand particles to reduce inter-particle friction and the deceleration of foam degradation

leading to a uniform distribution of small-size bubbles in pores and effectively weakening the soil skeleton strength.