Abstract: The mechanical behaviour of granular materials depends on their grading. Crushing of particles under compression or shear modifies the grain size distribution, with a tendency for the percentage of fine material to increase.  It follows that the frictional properties of the material and the critical states are modified as a consequence of the changes in grain size distribution and the available range of packing densities.  The seminar will illustrate some problems connected with the selection of appropriate descriptors of particle shape (such as e.g., roundness, angularity, and roughness) and the results of an extended experimental investigation of the evolution of the grading of an artificial granular material, consisting of crushed expanded clay pellets under different loading conditions. The changes of grading of the material after isotropic, one-dimensional and constant mean effective stress triaxial compression are described using a single parameter based on the ratio of the areas under the current and an ultimate cumulative particle size distribution, which are both assumed to be consistent with self-similar grading with varying fractal dimension.  Relative breakage is related to the total work input for unit of volume.