Experimental study on the diffusion behavior and parameter sensitivity of expansive polymer grouting under seepage erosion conditions

Aging urban underground pipeline systems increasingly suffer from leakage-induced subsurface cavities, posing significant threats to surface infrastructure stability. Expansive polymer grouting has been widely applied as an efficient trenchless remediation method. However, the dynamic diffusion behavior of polymer grout under sustained seepage conditions and its performance at different seepage stages remain insufficiently understood. In this study, a laboratory-scale seepage–grouting coupled system was established to investigate polymer diffusion characteristics under varying hydraulic heads, seepage stages, particle gradations, and grouting durations. The evolution of diffusion distance was quantified, and a multi-factor regression model was developed using response surface methodology (RSM). Results show that under constant-flow injection, polymer diffusion exhibits pronounced stage-dependent behavior, including rapid expansion, vertical development, lateral spreading, and stabilization, with strong nonlinear characteristics. The seepage stage significantly influences the cavity development state and diffusion morphology. Late seepage conditions are associated with a higher degree of cavity interconnection and enhanced lateral spreading, resulting in greater final diffusion distance. Increasing hydraulic head intensifies hydraulic scour and downstream migration, occasionally inducing abrupt diffusion increments. Fine-grained gradation improves diffusion uniformity and coverage. Grouting duration is identified as the dominant controlling factor, substantially enhancing resistance to hydraulic disturbance and increasing filling integrity. The regression model demonstrates high predictive capability (R² = 0.976), with parameter sensitivity ranked as: grouting time > seepage stage > hydraulic head > particle gradation.