Fine particles may migrate in the preexisting pores of an internally unstable soil matrix caused by water flow. This migration changes the fine particle distribution and content at different zones and can affect the mechanical properties of these soils. Due to the different roles that fine particles can play in the force chains of an internally unstable soil, the available geometrical assessment methods do not predict post-erosion behavior of the soil. The fine particles may sit loose in the voids, provide lateral support for the primary soil matrix, or participate directly in stress transfer. This will depend on the fine content, particle size distribution, constriction size, relative density, stress path, and particle shape. However, to evaluate the post-erosion behavior accurately, computational modelling or experimental investigation needs to be conducted. A modified triaxial apparatus connected to a water supply system and collection tank was developed to investigate the post-erosion behavior of an internally unstable cohesionless soil under different loading patterns in undrained conditions. This system allowed all test phases to be completed, including erosion inside the triaxial chamber to remove any possible impact of specimen disturbance. The results suggest that the undrained shear strength of the eroded specimen increased at small vertical strains (0-4 %) under monotonic and cyclic loadings, whereas the initial modulus of elasticity remained unchanged. Also, the eroded specimen showed much higher resistance against cyclic loadings, whereas the non-eroded specimen was liquefied during less than five cycles of loading. This improvement was due to a better interlock between coarse particles due to erosion of fine particles. The hardening strain behavior of the noneroded specimen changed to limited flow deformation due to a decrease in the fine content. The flow deformation of the eroded specimen at medium strain may be due to the local increase in lubrication effect of fine particles in the eroded specimen.

It is believed that the relative density (Dr) can affect the internal stability of the gap-graded soils and hence the erosion of their fine particles (i.e. susceptibility to suffusion). This paper investigates the influence of Dr on the contribution of fine particles on soil fabric. A new procedure is proposed to produce samples with target Dr using the discrete element method (DEM). DEM simulations were carried out using spherical particles. Particulate scale analysis of variation of stress reduction factor (α DEM ), the evolution of contact type and fine particle coordination number (Z fine ) with Dr reveals how Dr affects packing stability. The results show that packings with a fine content of 35% and the gap ratio in a range of 4-7 are in the transitional zone in which they are unstable initially and become internally stable as Dr increases. The behaviour of gap graded soil in the transitional zone is governed by fine-coarse contacts, but fine-fine contacts dominate the behaviour when soil becomes internally stable. Both α DEM and Z fine are reliable parameters in determining internal stability of gap graded soils. Finally, the correlation between the results and macro-scale matrix phase diagrams confirms the validity of micro-scale information to describe the underlying phenomenon.

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Suffusion is defined as the migration of fine particles caused by seepage flow through pre-existing pores of a soil structure made of coarse particles. This particle transportation changes the fine particle content and its distribution, possibly impacting the mechanical behaviour of eroded soil. Although limited research has been conducted on the post-erosion mechanical consequences under monotonic shearing, little attention has been paid to the impact of suffusion on the cyclic resistance and liquefaction potential of internally unstable soils. This paper investigates the cyclic and post-cyclic behaviour of a gap-graded cohesionless soil using combined triaxial-erosion apparatus. An internally unstable soil was chosen for the erosion test and was subjected to different seepage flow velocities and durations followed by cyclic loading and post-cyclic shearing. During cyclic loading, the eroded specimens with different residual fine contents behaved in a similar manner to a soil specimen constructed only of coarse particles. Regardless of the seepage velocity and duration, the erosion of fine particles resulted in significant increase in cyclic resistance. It is understood that eroded specimens with lower intergranular void ratios show higher resistance during cyclic loading, highlighting the importance of the intergranular void ratio in understanding the post-erosion mechanical behaviour of soils.

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