This study investigates internal erosion in gap-graded gravelly soils, focusing on the critical fine content transition zone (30–35%). By integrating µ-CT imaging, geometric criteria analysis, and a novel predictive model based on the graded erosion principle, we demonstrate that fine particle content governs the transition between suffusion and piping erosion modes. Experimental results reveal that exceeding the 30–35% threshold shifts the soil fabric from a stable coarse-grained skeleton to an “over-filled” structure, where coarse particles float within a fine-dominated matrix, drastically increasing piping susceptibility. A multi-criteria assessment framework validates the limitations of traditional geometric criteria (e.g., Cu, Kenney & Lau) for transitional soils, while the grading entropy criterion offers enhanced robustness. The proposed graded erosion model successfully predicts particle size distribution evolution toward stable Fuller limits and porosity changes by incorporating particle-size-sensitive erosion rates with physical thresholds. This integrated methodology advances the understanding and prediction of internal erosion in gap-graded soils, supporting the design of resilient geotechnical structures and improving infrastructure risk assessment.
Zhu et al. (Sun,) studied this question.