GRAIN MOTION IN A 2-D GRANULAR MATERIAL UNDER SLOW CYCLIC DEFORMATION

It is demonstrated experimentally that 2-D embankments of granular mat erial, which are slightly confined and submitted to quasi-static cycli c bulldozing exhibit spontaneous grain convection and grain diffusion: on an average, the grains dive down along the bulldozer wall, rise up at 45 degrees from the vertical after having reached the bottom and e nd their loop by moving parallely to the lid towards the bulldozer. Gr ain motion exhibits randomness, so that grain mixing is achieved after a few cycles. Grain rotation occurs during convection; rotations are distributed inhomogeneously too. Large spatial fluctuations and interm ittency are observed during both diffusion and rotations. Mixing proce ss is briefly studied; it exhibits complexity. Changes of boundary con ditions, of sample size and shapes and effect of grain elasticity are investigated briefly. We interpret these phenomena within the scheme o f classical soil mechanics, using both a finite element technique and the yield design theory with simple boundary conditions. Specially ded icated experiments demonstrate, however, that use of such simple flat boundary conditions are able to explain neither the exact deformation process nor the real back and forth motions of grains. An analogy with convection pattern in horizontally vibrated samples is made.