Shear strength of bubble-slurry-conditioned sand under total vertical pressure based on void characteristics

The shear strength of chamber soil in earth pressure balance (EPB) shield tunneling is crucial as it impacts tool wear and tunneling efficiency. In water-rich, coarse-grained soil layers, EPB shield tunneling often uses a combination of slurry and foam to reduce soil permeability. A novel, environmentally friendly bubble-slurry has been demonstrated to enhance fluidity and significantly lower the permeability of water-rich coarse-grained soils. This study investigates the shear strength of bubble-slurry-conditioned soil through undrained shear tests using a self-made pressurized vane shear apparatus. Tests were conducted on sands conditioned with bubble-slurry under pressures ranging from 0 to 300 kPa. The results indicate that the strain softening in the bubble-slurry-conditioned sand decreases with increasing pressure. Pressure shifts the void ratio of the conditioned sand from above the maximum void ratio emax of unconditioned sand to below emax. The shear strength of the conditioned sand transitions from being governed by the cohesion of bubble-slurry to being governed by internal friction of the conditioned sand. Moreover, the pressure enhances the shear stress required for bubble splitting and increases resistance to bubble migration within sand pores. This contributes to the observed increase in the shear strength of bubble-slurry-conditioned sand.