49 / 2025-04-24 12:55:17

Impact of Stress Path on Contribution of Fine Particle to Soil Fabric in Gap Graded Internally Unstable Soils

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

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.