Effect of gradation characteristics on seepage failure in foam-conditioned gap-graded soils for EPB shield tunneling

Muck spewing at the screw conveyor outlet in water-rich coarse-grained soils poses a significant risk to the efficiency and safety of shield tunneling. This study systematically investigates the influence of gradation characteristics on the seepage failure behavior of foam-conditioned gap-graded soils through vertical upward hydraulic tests. The effects of fines content, gap ratio, and foam injection ratio on failure modes and hydraulic responses are examined. Results show that the seepage failure mode is jointly controlled by the combined effects of particle size distribution, fines content, and foam injection. Changes in the gap ratio can trigger transitions between piping and heave failure, and excessively high foam injection can destabilize soil during heave. The pore structure, regulated by gradation, determines the stability of foam within the soil, with a sufficient proportion of fine particles essential for maintaining internal stability and mitigating seepage erosion. These findings provide insights into the mechanisms by which gradation characteristics influence seepage failure and offer guidance for foam conditioning practices in shield tunneling, supporting the theoretical modeling of critical hydraulic gradients in foam-conditioned soils.