Effect of conditioning schemes on mechanical properties of EPB shield soil

Abstract

Shearing conditioned sands under different pressure conditions requires understanding their mechanical properties with various conditioning schemes to ensure feasible conditioning during earth pressure balance (EPB) shield tunnelling in sandy strata. This paper investigates the effect of bentonite slurry and foam, conditioned with sodium carboxymethyl cellulose (Na-CMC), on the workability of sand with varying water content, to obtain preliminary conditioning schemes. The conditioned sands with suitable workability are tested using a pressurized vane shear apparatus to obtain their mechanical properties, including compressibility, resilience, and shear strength before and after resiling. Results indicate that Na-CMC can reduce filtration loss of bentonite slurry and foam expansion ratio (FER) of foam, whereas enhance the stability of them. The foam injection ratio (FIR) of foam-conditioned sands and the maximum slurry addition of slurry-foam-conditioned sands decrease linearly with increasing the initial water content of sand, provided the conditioned sands have suitable workability. Increasing the FIR can remarkably increase the void ratio of conditioned sands, which maintains high compressibility and resilience, and low shear strength before and after resiling. The addition of bentonite improves the workability of sand. The conditioning schemes are feasible when the total stress is lower than a specific critical value. The critical total stress can be increased by increasing FIR and reducing the water content of sand or slurry-conditioned sands. This paper provides critical guidance for the design of conditioning schemes.