A new triaxial apparatus for high-fluidity shield muck: validation and application

To study the shear behaviour of foam-conditioned muck with high fluidity in an earth pressure balance (EPB) shield, a new triaxial apparatus was developed. The newly developed triaxial apparatus are aimed at addressing the issue of specimen preparation for soils with high fluidity, which is not possible in a conventional triaxial apparatus. This is achieved by equipping the triaxial cell with a detachable mesh-shaped cylindrical mould, through which the initial confining water pressure is able to be exerted directly on the soil specimen so that it is able to remain upright with the prescribed size. Dry sands were used to conduct triaxial tests in a conventional triaxial apparatus and the newly developed triaxial apparatus. The testing results of the stress–strain curves and shear strengths of the specimens measured with the newly developed apparatus agree well with those measured with the traditional triaxial apparatus, verifying the reliability of the new triaxial apparatus. Triaxial tests using the new apparatus were then implemented with foam-conditioned shield muck. The testing results showed that the muck exhibited strain hardening with a water content of 45% and a foam injection ratio (the ratio of foam volume to muck volume) of 40%; the internal friction angle was measured to be approximately 0°, and the cohesion was measured to be 0.55 kPa. Interestingly, a shear contraction phenomenon was observed in the foam-conditioned gravelly clay muck under different confining pressures. The newly developed device provides an effective test vehicle for the study of the shear behaviour of foam-conditioned shield muck.