Résumé
We used a Contact Dynamics algorithm to investigate some of the statistical properties of the contact network in quasistatically driven assemblies of rigid particles containing from 500 to 4000 particles in two dimensions. We found that the contact network at every stage of deformation is composed of two complementary subnetworks: a "strong" percolating subnetwork of the contacts carrying a force larger than the mean force (containing about 40% of contacts), and a "weak" subnetwork of the contacts carrying a force lower than the mean force. In the strong subnetwork, all contacts are nonsliding and the probability distribution of forces is well fitted by an exponential decay. Almost the whole dissipation takes place inside the weak subnetwork, where the number of forces decays as a power law. The weak subnetwork contributes only to the mean pressure (about 25%), whereas the strong subnetwork takes over the whole deviatoric load. The strong subnetwork bears the primary anisotropy induced by shear with the same orientation as the stress tensor, but it gives rise to a secondary anisotropy inside the weak subnetwork with an orientation orthogonal to that of the stress tensor.