Internal erosion plays a major role in destabilizing hydraulic structures such as foundations, dams, and levees. Over the last century, nearly 50% of dam failures have been attributed to internal erosion mechanisms like concentrated leak erosion, backward erosion, contact erosion, and suffusion. Among all modes of internal erosion, this study focuses on suffusion, which involves the detachment and movement of fine particles through the soil matrix due to seepage forces. Soils prone to suffusion, known as internally unstable, experience detachment, transport, and migration of fine particles when geometrical, stress states and history, and hydraulic conditions are met. The internal stability of soil against internal erosion is closely related to the role its fine particles play in soil’s stress matrix or fabric. In internally unstable soils (suffusive), fine particles loosely occupy voids between coarse particles, leading to vulnerability under impact of seepage forces. Conversely, when fine particles are integrated into the soil skeleton and contribute to force chains, both coarse and fine particles help maintain internal stability. In metastable structures, fine particles provide lateral support within the soil skeleton, further enhancing the soil's resistance to internal erosion. Fine particles can play different roles—inactive, semi-active, or active—within the soil stress matrix. These roles are influenced by various controlling parameters, such as stress states, hydraulic conditions, and soil fabric. The contribution of fine particles to the stress matrix can be assessed through fabric indicators, which reflect changes in particle arrangement, interparticle contacts, and force transmission within the soil. Recent studies on suffusion have highlighted the impact of stress conditions, particularly in earthen structures like embankment dams, where varying stress and hydraulic conditions lead to anisotropic stress states. The maximum principal effective stress forms angles between 0 and 90 degrees relative to flow direction, potentially altering the soil's pore structure and fabric. These changes, caused by erosion, affect the soil's mechanical properties by modifying interparticle contact networks. In this research, three-dimensional DEM simulations were conducted to examine the effects of isotropic compression, drained triaxial compression, and extension stress paths on fine particle erodibility and their role in stress transfer mechanisms. This was achieved by adjusting the gap ratio (GR), defined as the ratio of the smallest coarse particle to the largest fine particle, from 2 to 7, and varying the fine content between 15% and 50% to cover stable, unstable, and transitional soil structures. The simulations involved cubical assemblies of gap-graded soils composed of spherical particles. Gravitational effects were excluded to reduce segregation, inhomogeneity, and anisotropy in particle packing. The analysis of variation in micro-mechanical parameters such as the stress-reduction factor ( ), particle connectivity status including the proportion of rattlers and chained particles, contact force transmission including the strong contact network, along with the evolution of contact fabric anisotropy (ø ), was used to illustrate the evolving role of fines in the soil stress matrix under different stress paths. The results indicate that changes are more pronounced in the transitional state of soil fabric, which consists of semi-active fine particles, compared to those that are either inactive or fully active. Consequently, the effect of stress paths on the susceptibility of fine particles is greater for semi active particles. Additionally, an increase in the gap ratio reduces contact fabric anisotropy, as coarse particles dominate the contact network, leading to a more isotropic fabric during shearing. Furthermore DEM results indicate that triaxial extension weakens the role of contact fabric anisotropy in transitional soils, making the fabric less directional and more prone to instability compared to the triaxial compression state.

Internal Erosion; Gap-Graded Cohesionless Soil; Discrete Element Modelling; Stress Path; Fine Particle Contribution.