Permeation behavior and filter cake formation of graded sand modified bentonite slurries in highly permeable strata

In highly permeable strata, the rapid formation of a dense filter cake is critical to the safety of slurry-shield tunnelling beneath rivers and seas. However, conventional bentonite slurries often struggle to develop an effective cake under these conditions. To investigate the permeation behavior of bentonite slurries modified with graded sand in highly permeable strata, based on the Nanjing Heyan Road river-crossing tunnel project, a field-reflective column infiltration experiment was established to systematically examine the permeation-cake-formation behavior of five graded sand gradations and three dosages (5-15 g/L) in 2-5 mm formations. Results indicate two end states: permeation zone, or a coexisting filter cake with a permeation zone. The cake consists of a graded sand skeleton interwoven with bentonite fines, with a small fraction of sand entering pore throats to produce localized internal clogging. Surface retention shows a pronounced negative relationship with permeation flux and exhibits saturation; in the 4-5 mm formation, the maximum surface retention at 15 g L⁻¹ is comparable to that at 10 g/L (0.55 kg/m²), both corresponding to the lowest flux. Filter cake formation is jointly governed by gradation-pore-throat matching and dosage, and low dosages struggle to establish a continuous filter cake. For formations with a permeability coefficient below 5.29 × 10⁻² m /s, and with particle-size matching between the sand and the formation's pore diameter, forming a dense filter cake requires a minimum retention of 0.51 kg/m ² on the excavation surface graded sand surface. Based on these results, an empirical criterion centered on particle-size matching and surface retention, together with mix-design recommendations, provides a verifiable design basis for face-pressure stabilization and slurry selection in river-crossing shield tunneling.