Microstructure of shear bands and its relation to the mechanisms of dilatancy and failure of dense granular soils

Microstructural changes taking place in two shear bands of Toyoura and Ticino sands were examined by means of an X-ray method and optical measurements using a microscope and thin sections. The following must be taken into account in order to produce a realistic microstructural deformation model for granular soils: (1) shear band boundaries are not straight, but are gently curved with different inclination angles to the major principal stress from section to section; (2) extremely large voids are produced in shear bands, and the resulting local void ratio can be larger than the maximum void ratio determined by standard methods; (3) the particle orientation changes sharply at shear band boundaries, so that a high gradient of particle rotation can be developed within a relatively narrow zone during the shear banding process; (4) particle rotation, on average, takes place in parallel with the corresponding macroscopic rotation in the continuum sense; (5) the thickness of shear bands is about 7 to 8 times the mean particle size. In addition to these observations, photoelastic pictures taken from a biaxial test on a two-dimensional assembly of oval rods were analysed and used in proposing a microstructural deformation mode. In the model, the main microstructural change during the strain hardening process is the setting up of columns extending parallel to the major principal stress direction. The columns start buckling at the peak stress, and the buckling columns tend to concentrate in shear bands during the strain softening process, which causes not only the growth of the extremely large voids but also the particle rotation in shear bands. On the basis of this model, the following two points are emphasized: (1) rotation stiffness at contacts must be one of the important components controlling the strength of granular soils, and (2) the presence of rotational restraint at contacts must be taken into account in the formulation of static equilibrium conditions in a shear band and in the interpretation of residual strength of granular soils.