  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.
