Effects of silt clay content on rheology and filter cake characteristics of water-bentonite slurry in slurry shield tunneling

Water-bentonite slurry is widely used in slurry shield tunneling due to its ability to stabilize the tunnel face and transport excavated soils. As the shield advances, natural soil is eaten by the cutting teeth and inevitably mixes with fresh water-bentonite slurry, among which silt clay-sized soil particles are difficult to remove, impeding reuse of the slurry and posing challenges for tunneling safety and efficiency. Therefore, it is crucial to evaluate the performance and adaptability of slurry containing silt clay soil. In this study, non-swelling silt clay (kaolin) was added to water-bentonite slurries of varying bentonite concentrations, creating water-bentonite-kaolin slurries with densities ranging from 1019 to 1200 kg/m³. The rheological properties of these slurries were evaluated using a concentric cylinder rheometer, while the hydraulic characteristics of the resulting filter cakes were examined via modified fluid loss tests. The results showed that due to the addition of kaolin, higher-density slurries displayed increased rheological parameters. Slurries with higher bentonite concentrations showed pronounced non-linear rheological profiles, attributed to bentonite's strong thixotropic properties. Both higher bentonite concentrations and slurry densities produced thicker filter cakes with extremely low hydraulic conductivities (approximately 10⁻⁹ m/s). Pure bentonite slurries formed filter cakes with the highest void ratios and greater water retention, whereas denser slurries exhibited reduced pore space. Furthermore, pressure gradients under laminar flow conditions in a specific pipe were quantified, and a reference chart was developed to preliminarily determine the slurry's capacity to suspend soil particles. The implications for slurry shield tunneling practices were discussed.