A new non linear mechanism able to generate avalanches based on soil   mechanics
  We propose a general mechanism based on soil mechanics concepts, such as dilatancy and friction, to explain the fact that avalanches stop at an angle smaller than they start: the mechanism involved is linked to the fact that the stress field near the free surface of a pile built with inclined strata obeys always the plasticity criteria, even when the slope is smaller than the friction angle. It results from this that the larger the slope angle the smaller the mean stress and the smaller the maximum principal stress. So when the pile rotates to generate the next instability the granular material is submitted to a decrease of the mean stress, resulting in an increase of its yielding angle, which becomes larger than the friction angle. The slope starts then flowing at an angle larger than the friction angle. 
P. Evesque