Pore-Scale Flow Measurements at the Interface between a Sandy Layer and a Model Porous Medium: Application to Statistical Modeling of Contact Erosion

Contact erosion is potentially initiated at the interface between two soil layers by a groundwater flow within the coarser material. Once eroded by the flow, particles from the finer soil are transported through the pores of the coarser layer. Fluvial dikes are often exposed to this phenomenon. Small-scale experiments combining refractive index matching medium, planar laser-induced fluorescence, and particle image velocimetry were carried out to measure the flow characteristics in the vicinity of an interface between a model granular medium and a fine graded sandy layer. Longitudinal velocities and shear-stress distributions were obtained in Darcy flow conditions. They revealed a long tail toward large values, which reflects the spatial variability of the constrictions in the pores network. Taking into account these distributions can improve the modeling of contact erosion by going beyond the simple use of mean quantities, like Darcy velocity, as is usually proposed in the literature. This is done by successively considering the variability of the porous flow and also that of the critical shear stress. As a main consequence, the resulting global erosion rate is now nonzero for any value of the mean shear stress and there is no longer a stress threshold. Finally, the effect of paving at the sand surface can also be added to the statistical model and makes it possible to account very satisfactorily for previous contact erosion tests at the sample scale.