Microscopic origins of quasi-static deformation in dense granular assemblies

We present numerical simulations of biaxial tests on dense packings of nearly rigid grains. Homogeneous samples are prepared under hydrostatic pressure, and then submitted to a stepwise increasing deviator stress. The use of both static and dynamical methods allows us to identify two different regimes according to whether macroscopic strains result from the mere deformations of the contacts themselves, or are essentially due to rearrangements of the packing. Some ensuing differences in terms of mechanical behaviour in the macroscopic limit of large systems are discussed.