Discrete-based computational geomechanics: towards describing realistic saturation conditions or grain shapes

The settlement of geotechnical structures, e.g., earthfill dams, is one possible case of an engineering process where the mechanics is dictated by the micro-scale kinematics of a granular material, here, soil. While the Discrete Element Method (DEM) appears as an appropriate numerical method to numerically simulate those situations, it still requires appropriate implementations that should account for all pertinent micro-scale material features, aiming for quantitative numerical results.

As a first example, a possible presence in the inter-granular pore space of immiscible fluids, e.g., air and water, would induce capillary effects which favor mechanical strength. Aiming for an unified constitutive description of soils whatever their saturation, analytical stress homogenization formulas are first derived in order to isolate the various contributions to the material internal forces in a partially water-saturated granular material. At variance with the classical Bishop’s effective stress proposal, the equations demonstrate a fully tensorial nature (with a possible deviatoric component) of the stress contribution from the fluids mixture. Using a DEM implementation of a simple configuration of spherical particles with a small liquid content forming distinct capillary bridges, this deviatoric nature is emphasized in a number of configurations. The DEM simulations also enable one to investigate the constitutive nature of the stress contribution from the interparticle contact forces. The latter is actually shown to unify failure description between dry and partially saturated conditions, and to bear a limited stress-strain effective nature.

As a second example, particle shape is also a key feature of the soil that rules how particles reorganize, e.g., rotate, in the material bulk. Both a convex polyedra and a more general Level Set approaches are discussed for a proper DEM description of the latter. It is first illustrated, using polyedra, how a realistic shape description is a physically-based alternative to enriched contact model formulations. Then, the precision, computational costs, and versatility of the Level Set approach, as implemented into the YADE code, are discussed on a number of shapes of increasing complexity, from spheres to superquadrics and rock aggregates.