Micro-scale investigation of the role of fabric on internal stability of gap-graded soils

Internal stability of gap-graded soils susceptible to suffusion is governed by soil fabric, which evolves through particle rearrangement. Using three-dimensional Discrete Element Method (DEM) simulations, this study examines the impact of fine content (FC), gap ratio (GR), and relative density ( D R ) on the role of fine particles in fabric evolution and their potential for erodibility. Stress in fine particles is evaluated using particle- and contact-based stress partitioning, together with fabric indicators such as the stress reduction factor (α). Particle connectivity is assessed through fine coordination ( Z avg fine ), mechanical coordination ( Z m ), and the mechanical coordination number of fine particles ( Z m fine ). To advance beyond conventional binary rattler methods, a novel fabric index, I rattler , is introduced as a continuous, coordination-based measure of fine particle inactivity, enabling more accurate detection of suffusion onset and fabric transitions. Contact force transmission across different contact types is also analyzed, with emphasis on the strong contact network and its variation with FC and GR, highlighting their influence on packing stability with respect to D R . The DEM framework is validated against previously published experimental and numerical studies on gap-graded soils. Results demonstrate that for GR = 4–7 with FC between 20–35 %, soils occupy a transitional zone where densification promotes a shift from F–C to F–F contact dominance, thereby stabilizing the fine matrix. In contrast, at GR = 20, fines remain largely inactive until FC ≈ 35 %, with stability attained abruptly at higher FC, marking a geometric upper bound for fine contribution under extreme particle size disparity.