Some properties of a continuum model for yield and flow of granular materials are presented in which a physical quantity analogous to grain rotation is incorporated by using a reduced Cosserat model. The intrinsic spin is introduced as a new primitive kinematic quantity and satisfies the equations of rotational motion. The stress tensor, in general, is not symmetric and, in addition to the Coulomb&mdashlMohr yield condition, a rotational yield condition governs the anti-symmetric part of the stress tensor.

The objective is to model the essential mechanical properties of rate independent and frictional granular materials based on a hypoplastic continuum approach, assuming isothermal and homogeneous deformations. The evolution equation of the stress tensor is described using a tensor-valued function which is non-linear and homogeneous of first order in the rate of deformation. By including a dilatancy parameter and a pressure dependent relative density the transverse strain sensitivity and the influence of the mean pressure and current density on the incremental stiffness can be reproduced for an initially loose and dense granular material with a single set of constitutive constants.