During the processing, transport or storage of granular materials it is important to be able to predict not only the bulk behaviour but also the likely damage sustained by the granules. In order to shed some light on this problem, this paper describes a mechanistic model which gives an insight into the way the granules respond when bulk granular materials are deformed. It extends the ideas of critical state soil mechanics by incorporating both local deformation at the granule contacts and rearrangement of the granules to account jointly for any bulk deformation. As illustrations, drained and undrained triaxial tests are simulated with axial symmetry and qualitative agreement with published experimental results is obtained. Some fully triaxial drained tests are also simulated in which a shear band is given the opportunity to appear. This allows the model to predict a failure surface similar to the Mohr-Coulomb criterion whilst using a yield surface with a circular cross-section on the plane of constant pressure. A maximum principle is provided for this model which can be used to obtain approximate solutions to boundary value problems.