**1. Introduction**

Internal erosion is one of the main causes of instability of earthen hydraulic structures (dike, levee, dam, etc.). The disorders observed on recent structures underline the need for a better understanding and quantification of the phenomena that govern internal erosion. The entrainment and transport of grains by the internal flows affect the granulometric distribution and modify the porosity, as well as the mechanical and hydraulic characteristics.

Dike failures are much more numerous than those of dams due, on the one hand, to the variability of hydraulic solicitations and, on the other hand, to the length and heterogeneity and sometimes the age of dykes and levees, which make monitoring and maintenance difficult.

Numerous dam failures have occurred worldwide, some of them reported by Foster et al. [1]. The main cause of these failures has been identified as being linked to a channeling phenomenon that occurred in the foundation soil or in the dam structure. To ensure the viability of hydraulic infrastructures, it is essential to take into account the vulnerability of the soil to infiltration [2, 3]. Normally, in the case of unconsolidated soils, which are generally made up of slightly cohesive sand particles, we find that water flow velocity plays a very important role in the erosion phenomenon that can occur. Interpreting and understanding the underlying mechanisms and quantifying the effects of relevant variables on this erosion phenomenon is of great practical importance. Soil erosion due to liquid flow can be modeled using a variety of approaches. These include continuum-based models and discrete models, which use certain parameters that are calibrated using laboratory tests or field observations to predict when internal soil erosion begins to occur and the expected erosion rate. Several models for predicting soil erosion rates at the solid/fluid interface have been developed in the literature [3, 4]. One of the most important tests used to predict erosion is the tube erosion test (HET). A model for interpreting the HET with a constant pressure drop has been developed by Bonelli and Brivois [4]. This model provides a characteristic erosion time that depends on the initial hydraulic gradient and the soil erosion rate.

One of the aims of this work is to describe the turbulent two-phase fluid flow that causes erosion inside the porous soil sample, taking into account the influence of the variable clay concentration in the flow fluid. A computational fluid dynamics (CFD) approach will be used to study the shear stress developing at the water/soil interface, which represents the main mechanical action at the origin of surface erosion.
