Abstract: During the construction of large-section rectangular pipe jacking in highly permeable silt and silty-sand strata, the Earth Pressure Balance (EPB) mode is prone to spoil blow-out, which severely constrains construction safety and efficiency. Based on the Jianning West Road River-Crossing Channel Project in Nanjing, this paper systematically investigates soil conditioning in highly permeable silt and silty-sand strata. Through laboratory experiments, the influence of three conditioning agents—foam, bentonite slurry, and carboxymethyl cellulose sodium (CMC) solution—on soil slump and permeability coefficient under different injection ratios was quantitatively analyzed. The micro-mechanisms of soil improvement were further elucidated through scanning electron microscopy (SEM). Results indicate limitations of individual conditioning materials: foam enhances workability but lacks impermeability, while bentonite and CMC significantly reduce the permeability coefficient yet deteriorate or eliminate soil flowability, thus failing to meet construction control requirements for slump (10-20 cm) and permeability coefficient (≤1×10⁻⁵ cm/s) simultaneously. Further research reveals that composite conditioning with bentonite and CMC achieves a permeability coefficient as low as 5.61×10⁻⁶ cm/s at an injection ratio of 15%, while maintaining slump within the construction requirements, demonstrating excellent synergistic improvement effects. The study demonstrates that for the construction of large-section rectangular pipe jacking in highly permeable silt and silty-sand strata, composite soil conditioning utilizing foam in conjunction with bentonite or CMC is required to synergistically improve soil flowability and impermeability.