Sheared force-networks: anisotropies, yielding and geometry
  A scenario for yielding of granular matter is presented by considering the ensemble of force networks for a given contact network and applied shear stress $\tau$. As $\tau$ is increased, the probability distribution of contact forces becomes highly anisotropic, the difference between average contact forces along minor and major axis grows, and the allowed networks span a shrinking subspace of all force-networks. Eventually, contacts start to break, and at the yielding shear stress, the packing becomes effectively isostatic. The size of the allowed subspace exhibits simple scaling properties, which lead to a prediction of the yield stress for packings of arbitrary contact number. 
Jacco H. Snoeijer, Wouter G. Ellenbroek, Thijs. J.H. Vlugt, Martin van   Hecke