Microstructural changes during shear deformation in two natural sands and one artificial assembly of oval rods are investigated with special interest in the micromechanisms leading to the generation of shear bands. Anisotropy is gradually apparent, in a strain--hardening process, as load carrying columns develop and extend parallel to the major principal stress direction. Buckling of these columns, which starts near the peak stress, causes strain softening accompanied by the generation of shear bands. As a result, large voids are produced in shear bands between the buckling columns, and the resulting local void ratio can be larger than the maximum void ratio. Particle orientation changes sharply at shear--band boundaries, so that a high gradient of particle rotation can be developed within relatively narrow shear zones during the shear-banding process. Based on these findings, a microdeformation model is proposed, which emphasizes that the development of induced anisotropy in the strain--hardening process is a necessary condition for the generation of shear bands in granular soils.
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