Although phenomenological constitutive models have the ability to exhibit most salient macroscopic features of granular materials, they generally do not provide any convincing interpretation of them: their basic origin remains hidden. It is now well established that the micro-structure of granular materials plays a significant role in their overall constitutive behavior. In the past few years, a great deal of theoretical and experimental research has been devoted to this domain, giving rise to efficient micro-mechanical approaches. First, this paper reviews the micro-directional model, which is a micro-mechanically based constitutive relation. Then an extension is proposed to describe the possible collapse of force chains. This micro-structural adjunction is shown to be sufficient to simulate work-hardening and softening mechanisms. A granular assembly, containing a multitude of frictional contacts whose orientation is generally anisotropically distributed, exhibits various other typical features such as the nonassociated character of plastic strains. The micro-structural origin of this feature is investigated, and further conclusions related to the existence of a regular or a singular flow rule are drawn.