Bentonite slurry modification through enzyme-induced carbonate precipitation for the filter cake formation in high-permeability sand during slurry shield tunneling

This study proposed a new bentonite slurry modification method through enzyme-induced carbonate precipitation (EICP) for the filter cake formation in high-permeability sand during slurry shield tunneling. The properties of pure bentonite slurry, bentonite-CMC slurry and EICP-modified slurries were measured. The micro-evolution process of EICP-modified slurries was evaluated through glass slide experiments, and slurry infiltration tests were conducted to explore the effects of slurry properties on filter cake formation. The results show that CaCO₃ precipitations generated during the EICP reaction densely coat and bond bentonite particle, promoting dispersed particles to aggregate into large bentonite particle-CaCO₃ clusters. With the reaction progresses, these clusters further interconnect to form a network structure in slurry. Compared with pure bentonite slurry and bentonite-CMC slurry, EICP-modified slurries exhibit significantly higher density, particle size and viscosity. Meanwhile, the 24h bleeding rates of all EICP-modified slurries are below 5%. In infiltrate tests, the pure bentonite slurry and bentonite-CMC slurry fail to form a filter cake, whereas the EICP-modified slurries rapidly form a low-permeability filter cake. Increasing CaCl₂-CO(NH₂)₂ concentration further accelerate filter cake formation and reduce internal filter cake thickness. The EICP-modified slurry with a CaCl₂-CO(NH₂)₂ concentration greater than 1mol/L is recommended for slurry shield tunneling in high-permeability strata.