  The ability of grains to rotate can play a crucial role on the collective
behavior of granular media. It has been observed in computer simulations that
imposing a torque at the contacts modifies the force chains, making support
chains less important. In this work we investigate the effect of a gradual
hindering of the grains rotations on the so-called critical state of soil
mechanics. The critical state is an asymptotic state independent of the initial
solid fraction where deformations occur at a constant shear strength and
compactness. We quantify the difficulty to rotate by a friction coefficient at
the level of particles, acting like a threshold. We explore the effect of this
particle-level friction coefficient on the critical state by means of molecular
dynamics simulations of a simple shear test on a poly-disperse sphere packing.
We found that the larger the difficulty to rotate, the larger the final shear
strength of the sample. Other micro-mechanical variables, like the structural
anisotropy and the distribution of forces, are also influenced by the
threshold. These results reveal the key role of rotations on the critical
behavior of soils and suggest the inclusion of rotational variables into their
constitutive equations.
