Fabric transitions and stress-path effects in gap-graded soils: Interplay between gap ratio, fines content, and fine particle activity

Mechanical behavior of gap-graded soils is influenced by the evolving role of fine particles within the load-bearing fabric, which undergoes significant reorganization during loading. This study investigates fabric evolution and fine-particle activity in gap-graded assemblies by varying the gap ratio (GR = 2, 5, and 7), fines content (FC = 5–50%), and loading path. Three-dimensional Discrete Element Method (DEM) simulations of drained triaxial compression and extension are performed on dense bimodal mixtures representing underfilled, transitional, and overfilled fabric states. Beyond conventional fabric descriptors, the mechanical redundancy index ( I r ) is introduced to quantify the redundancy of contact-network constraints relative to particle degrees of freedom, thereby directly assessing the mechanical stability of fine particles within the load-bearing skeleton. Together with the stress-reduction factor ( α ), particle connectivity, and contact-fabric anisotropy ( a cn ), these metrics comprehensively characterize the evolving micromechanical state of fines. The results show that stress-path sensitivity is most pronounced within the transitional fabric regime, particularly for GR = 5 and 7, corresponding to FC ≈ 20–30% and ≈ 30–35%, respectively. Within this range, compression is associated with increased fine-particle participation and contact stability, whereas extension is associated with contact loss, reduced mechanical redundancy, and higher fabric sensitivity. Stress-path effects diminish in underfilled and overfilled states. Furthermore, the transition of fines from inactive to load-bearing is gradual rather than binary and governed by the combined effects of gap ratio, fines content, and loading path. These findings provide a micromechanical basis for understanding conditions preceding suffusion initiation in internally unstable soils.