Effects of residual foam and defoamer on flocculation efficiency during in situ muck recycling for EPBS tunneling

In situ recycling technology provides an effective approach for treating earth pressure balance shield (EPBS) muck with high water content and residual foaming agents. Within this process, slurry flocculation serves as a critical dewatering stage, in which defoamers are essential to control foam and maintain processing efficiency. Therefore, this study examines the interactions between foam, defoamer, and flocculant in a scaled flocculation tank. The materials included a novel hydroxy silicone oil-glycerol-polypropylene ether (H-G) defoamer and anionic polyacrylamide (APAM) as the flocculant. The influences of residual surfactant concentration, defoamer dosage, and flocculant dosage on flocculation kinetics, muck recovery rate, and supernatant quality (turbidity and total suspended solids) were systematically evaluated. Additionally, advanced characterization of the flocculated supernatant was performed through integrated particle size distribution analysis and optical microscopy, which revealed the impact of residual foam and surfactants on flocculation. Results indicate that the foam-formed network structure in the flocculation tank hinders floc sedimentation, thereby reducing the muck recovery rate and increasing turbidity and total suspended solids (TSS) in the discharged supernatant. More critically, even after foam collapse, residual surfactants adsorb onto soil particles via hydrophilic groups, increasing inter-particle repulsion and thus inhibiting aggregation. This dual physical-and-chemical mechanism is the primary cause of the observed low muck recovery rate and persistently high turbidity/TSS in the supernatant. This work provides novel insights into the mechanistic role of residual foam in flocculation, offering critical guidance for optimizing the in situ recycling of EPBS muck.