International audienceWe use DEM simulations on a simple 2D model of a granular material to test for the applicability of elastoplasticity (yield criterion, flow rule) to the response to stress increments of arbitrary directions. We apply stress probles in a three-dimensional stress space to various intermediate states (investigation points) along the biaxial compression path, and pay special attention to the influence of the magnitude of the increments The elastic part of the material response is systematically identified by building the elastic stiffness matrix of well-equilibrated configurations. The influences of the contact stiffness level and of the dominant strain mechanism, contact deformation (I) or network rearrangement (II), are considered. Stress increments sharing the same principal directions as the stress state in the ubvestigation point comply with a standard (single-mechanism) elastoplastic model with a Mohr-Coulomb criterion and well-defined flow rules and plastic moduli. Stress increments with principal axis rotation entail a response which is satifactorily modeled by superimposing 3 plastic mechanisms, 2 of them symmetrically corresponding to shear stresses of both signs. The full dependence of strain increments on sterss increments is thus parametrized with three flow rules, two of which are essentially symmetric

A granular soil REV located on the upstream side of the erosion pipe front is modelled numerically, at the grain scale, by coupling the Discrete Element Method (DEM) with the Lattice Boltzmann Method (LBM) for the representation of the solid and fluid phases, respectively. The implementation of DEM follows a standard molecular dynamics approach and the interactions between grains are regulated by unilateral contacts and breakable bonds. A synopsis of the LBM scheme is provided, with focus on the implementation of non-slip conditions for moving boundaries and use of the Multiple Relaxation Time approach for improved numerical stability. The coupling scheme is described along with the criteria for setting the numerical parameters of the two methods. After a "dry" preparation procedure, the numerical REV is tested under fully saturated conditions and increasing pressure difference. Backward erosion is observed and a micromechanical inspection of the granular phase suggests that arching through force chains and the breakage of tensile bonds are prominent resistance and degradation mechanisms, respectively.

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In recent years a discussion could be followed where the pros and cons of the applicability of the Cosserat continuum model to granular materials were debated [Bardet, J.P., Vardoulakis, I., 2001. The asymmetry of stress in granular media. Int. J. Solids Struct. 38, 353-367;Kruyt, N.P., 2003. Static and kinematics of discrete Cosserat-type granular materials.