Experimental Study on Impermeability Characteristics of Filter Cake Formed by Lubricating Slurry for Pipe Jacking in Saline Strata

In saline strata, lubricating slurry for pipe jacking deteriorates due to salt ion intrusion, causing stratum instability and increased frictional resistance, which pose major risks in underground construction in coastal and inland saline areas. This study investigates the evolution of filter cake impermeability under salt intrusion and establishes a three-stage evaluation method covering forward grouting, post-damage recovery, and reverse water sealing. Using a self-designed permeation apparatus, comparative tests were conducted on ordinary bentonite slurry (4% bentonite) and salt-resistant composite slurry (5% attapulgite + 5% bentonite + 0.8% HV-CMC + 0.35% NaOH) in sand layers of 0.25–2.0 mm particle sizes. Results show that the salt-resistant slurry achieves a forward pressure attenuation rate of 97% (formation pressure of 3–7 kPa at 250 kPa grouting pressure), versus only 10% for the ordinary slurry (formation pressure of 45–51 kPa)—a difference of approximately one order of magnitude. After mechanical filter cake damage, the salt-resistant slurry restores impermeability within 15–180 s through re-grouting, while the ordinary slurry cannot recover. The salt-resistant slurry resists approximately 20 kPa of reverse groundwater pressure, whereas the ordinary slurry fails at 10 kPa. Optimal performance occurs in 0.5–1.0 mm medium sand, where the filter cake and seepage zone exhibit the strongest synergy, with post-damage recovery of only 15 s. Solid particles retained in the seepage zone provide a substrate for filter cake reconstruction—the core mechanism for impermeability recovery under repeated damage during pipe jacking advancement. A three-stage evaluation framework integrating pressure attenuation rate, recovery time, and critical breakthrough pressure is proposed to guide slurry selection and grouting parameter optimization in saline strata.