Multidirectional Flow Apparatus for Assessing Soil Internal Erosion Susceptibility

Suffusion is a selective erosion of fine particles within the matrix of coarse soil particles under the effect of seepage flow. Suffusion can induce important modifications in the hydraulic and mechanical characteristics of the soil. At the scale of an earth structure, the flow direction can vary, and the soils that compose the structure and its foundations can be heterogeneous. Thus, to ensure the safety assessment of hydraulic earth structures, the suffusion tests need to match with the reality of the body of dikes or dams with a possible horizontal or vertical flow and heterogeneous soils. In this paper, a new multidirectional flow apparatus was developed for characterizing soil susceptibility facing suffusion process and to study the effect of flow direction. The apparatus allows the specimen tests under a vertical or a horizontal flow, and it permits selection of the layer in which the flow is injected. Two cohesionless soils that slightly differ from each other in their gradations were used to prepare eight specimens by compaction per layer. Suffusion tests were performed under multistage hydraulic gradient, and the suffusion susceptibility was evaluated by the energy-based method. In the case of specimens composed of one soil, whatever the flow direction compared to the interface between soil layers, tested soils appear slightly more erodible under vertical downward flow. These results highlight the combination effect of the hydraulic loading and the gravity on the detached particles. In the case of specimens constituted by a layer of each tested soil, the results show that under a perpendicular flow to the interface between soil layers, the suffusion susceptibility is mainly influenced by the more resistant soil. But conversely, in the case of a parallel flow to the interface between both soils, the suffusion susceptibility is mostly influenced by the less resistant soil.