Internal erosion is a complex phenomenon which represents one of the main risks to the safety of earthen hydraulic structures such as embankment dams, dikes or levees. Its occurrence may cause instability and failure of these structures with consequences that can be dramatic. The specific mode of erosion by suffusion is the one characterized by seepage flow induced erosion and the subsequent migration of the finest soil particles through the surrounding soil matrix mostly constituted of large grains. Such a phenomenon can lead to a modification of the initial microstructure and, hence, to a change in the physical, hydraulic and mechanical properties of the soil.A direct comparison of the mechanical behaviour of soil before and after erosion is often used to investigate the impact of internal erosion on soil strength (shear strength at peak and critical state) using triaxial tests.However, the obtained results are somehow contradictory, as for instance in Chang's study [1], where it is concluded that the drained strength of eroded soil decreases compared to non-eroded soil while both Xiao and Shwiyhat [2] and Ke and Takahashi [3] have come to the opposite conclusion. A plausible explanation of these contradictions might be attributed to the rather heterogeneous nature of the suffusion process and to the way the coarse and fine grains are rearranged afterwards leading to a heterogeneous soil structure, a point that, for now, is not taken into account, nor even mentioned, in the existing analyses.In the present study, x-ray computed tomography (x-ray CT) is used to follow the microstructure evolution of a granular soil during a suffusion test, and therefore, to capture the induced micro-structural changes. The images obtained from x-ray CT reveal indeed that fine particles erosion is obviously not homogeneous, highlighting the existence of preferential flow paths that lead to a heterogeneous sample in terms of fine particles, void ratio and inter-granular void ratio distribution.

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Internal erosion is a complex phenomenon which represents one of the main sources of risk to the safety of earth hydraulic structures such as embankment dams, dikes and levees. Its occurrence may cause instability and failure of these structures with consequences that can be dramatic. Erosion by suffusion corresponds to the process of detachment and transport, under the action of hydraulic flow, of the finest soil particles within the porous media formed mainly of large grains. Its occurrence usually causes change of the initial microstructure and hence a change in the physical, hydraulic and mechanical characteristics of the soil. In this study, we present first an experimental characterization of the erosion mechanism during its occurrence within a granular soil. Particular emphasis was put on the role of hydraulic conditions in triggering of fines migration. Thereafter, we present a preliminary microstructural characterization of the erosion process through direct visualization by optical techniques of particles migration using crushed glass samples as model materials.

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Suffusion is one mechanism of internal erosion, which occurs in gap-graded or broadly graded soils when the fine particles are detached and transported by the seepage flow through the void space formed by the granular soil skeleton. Suffusion is therefore a particle scale mechanism. During this microscale, the initial soil fabric may change due to both fines migration and coarse grains rearrangement, leading to an increase/decrease of global/local porosity and hydraulic conductivity, besides of a probable appearance of heterogeneity, which can, in turn, impact the mechanical behaviour of the eroded soil. In the literature, suffusion test results give only a macroscopic point of view and fail to quantify the effect of suffusion at the scale of the soil's induced heterogeneities. In this paper, x-ray tomography is used to get microscopic observations of soil sample microstructure evolution during a suffusion test. The results reveal that suffusion is not a homogeneous process; the removal of fine particles takes place mainly around the soil sample circumference leading to a higher void ratio at the periphery. Besides, the inter-granular void ratio decreases significantly but almost uniformly throughout the sample owing to the progressive collapse and reorganization of the coarse grains induced by the loss in fines.

Suffusion is a particular internal erosion process that can lead to important disorders in water retaining structures such as embankment dams and levees. It causes modifications in the soil micro-structure and may modify the mechanical behaviour of the soil leading to deformations at the macroscopic scale. Therefore, the aim of this study is to investigate the consequences of internal erosion on the mechanical properties of the soil. We present such an investigation through numerical and experimental approaches. For the experimental approach, a newly developed suffusion test apparatus is used while for the numerical approach, a model is established based on the discrete element method (DEM) with a one-way fluid-solid coupling.