Experimental study on the mechanisms of bentonite slurry penetration in front of a slurry TBM

Abstract

Abstract Penetration of bentonite slurry reduces the effective support pressure at the tunnel face and thus the face stability during slurry shield excavation in saturated sand. The reduction of effective support pressure depends on the penetration distance of the bentonite slurry but also the hydraulic characteristics of the slurry-penetrated soil. The penetration of bentonite slurry, pressurized against saturated sand, therefore, was investigated in a laboratory set-up to improve the understanding of the mechanisms of two stages of the penetration: mud spurt and filter cake formation. The permeability of sand for bentonite slurry (kb), the permeability of filter cake for water (kc) and the Peclet number (Pe) at initiation of filter cake formation were calculated from the experimental results. It appeared that the values of kb and kc depend on the bentonite concentration of the slurry. Adding bentonite to the slurry decreases both kb and kc. Due to the thixotropic nature of the bentonite, the permeabilities estimated from the viscosity of fluid are approximately a factor 2 to 3 higher than the measured ones. The estimated Pe is a bit higher than the expected value. It was also found that filter cake is only formed on the surface of the sand in front of the shield tunnel, thus is very vulnerable to damage. The excess pore water pressure caused by slurry shield tunneling will only be negligible due to filter cake formation during standstill.