Soil microstructural changes induced by suffusion: x-ray computed tomography characterization

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.