  The shear strength of cohesionless granular materials is generally attributed
to the compactness or anisotropy of their microstructure. An open issue is how
such compact or anisotropic microstructures, and thus the shear strength,
depend on the particle properties. We first recall the role of fabric and force
anisotropies with respect to the critical-state shear stress. Then, a model of
accessible geometrical states in terms of particle connectivity and contact
anisotropy is presented. This model incorporates in a simple way the fact that,
due to steric exclusions, the highest levels of connectivity and anisotropy
cannot be reached simultaneously, a property that affects seriously the shear
strength. We also analyze the force anisotropy in the light of the specific
role of weak forces in sustaining strong force chains and thus the main
mechanism that underlies anisotropic force patterns. Finally, we briefly
discuss the effect of interparticle friction, particle shape, size
polydispersity and adhesion.
