Abstract: 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.

Key words: slurry-foam-conditioned sand, bentonite slurry injection ratio, pressurized vane shear test, shear strength characteristics, strength weakening mechanism