Effect of water pressure on permeability of foam-conditioned sands for EPB shield tunnelling

Water spewing is a potential risk when Earth Pressure Balance (EPB) shields pass through water-rich sandy strata, and may even cause ground instability. A low permeability of excavated sands via active conditioning is required to avoid water spewing. This study investigated the effect of water pressure on the permeability of foam-conditioned sands using laboratory permeability tests. The water pressure, for the first time, is decoupled with the hydraulic gradient, owing to a newly developed permeameter with the controllable downstream hydraulic pressure in the laboratory. The results show that the permeability is significantly affected by the water pressure, and the effect is also predominantly dependent upon the Foam Injection Ratio (FIR). The initial permeability coefficient increases with an increasing water pressure, while the initial stable period duration decreases. The water-plugging structure formed by foam bubbles and sand particles is prone to be damaged under high water pressure. The underlying mechanism of water pressure in modifying the permeability of conditioned sands is also examined from a particle-scale perspective.