<p>Seepage is one of the primary causes of instability in earth-rock dams. As the weak link in seepage control systems, the soil-structure interface exhibits particularly prominent seepage erosion issues, which often directly trigger catastrophic failures. However, research on the characteristics and mechanisms of seepage erosion at this interface remains relatively scarce. In this study, a visual seepage apparatus and nuclear magnetic resonance (NMR) technology were employed to systematically investigate the effects of three critical factors—clay content, compaction degree, and hydraulic gradient duration—on the evolution process, microscopic mechanisms, and critical hydraulic gradient of seepage erosion at the soil-structure interface between an earth-rock dam and a culvert. Results show that the interface seepage erosion process can be divided into three distinct stages: seepage stabilization, seepage transition, and particle erosion. Notably, hydraulic gradient duration significantly alters the phased characteristics of this process. Microscopic analysis indicates that clay content, compaction degree, and hydraulic gradient duration collectively influence the impermeability of the interface by differentially regulating pore-filling effects, initial structural compactness, and pore evolution processes. Analysis of variance (ANOVA) demonstrates that clay content is the most significant factor influencing the critical hydraulic gradient, with an optimal clay content maximizing the interface’s resistance to seepage failure. Predictive models for critical hydraulic gradients of the interface were established based on nonlinear regression analysis, providing valuable reference for quantifying the stability of interfacial seepage. These findings hold significant theoretical value for the impermeability design and safety assessment of earth-rock dams.</p> Graphical Abstract <p></p>