Experimental characterization of particle migration regimes in unsaturated gap-graded soils: density-dependent patterns under rainfall infiltration

This study examined rainfall-induced shallow internal erosion mechanisms in gap-graded soils, characterizing subsurface erosion patterns under controlled rainfall in-tensities (60, 90, 120 mm/h). Combining theoretical and experimental analyses of particle stress distributions and force chain evolution, we employed 1D soil column tests on specimens at densities of 1.7, 1.8, and 1.9 g/cm 2 . Particle migration dynamics across depths were quantified using drag force models and pore structure analysis. Results iden-tified soil density as the critical control: Medium-coarse particles (2–0.075 mm) migrated significantly in low- (1.7 g/cm 2 ) and medium-density (1.8 g/cm 2 ) soils across all rainfall intensities, forming distinctive “m” or “n” shaped mass fraction curves. High density (1.9 g/cm 2 ) suppressed mobility, shifting curves to linear/gamma distributions. Fine particles (< 0.075 mm) enriched selectively only in medium-density specimens; enrichment was absent under high density (enhanced seepage resistance) or low density (preferential loss). Hydraulic gradient variation and intermittent pore blockage were dominant controlling factors. Soil densification governed coarse particle transport thresholds by altering force chain networks, while fine particle dynamics depended on pore structure metastability. These mechanisms informed a four-pattern classification system ( “m”, “n”, linear, “Γ” ) correlated with rainfall-density parameters, providing a theoretical framework for as-sessing internal erosion risk in unsaturated gap-graded soils and designing mitigation strategies.