**A new non linear mechanism able to generate avalanches
based on soil mechanics**

** **

**Abstract:
**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 principle stress. So when the pile rotates to generate the next
instability the granular material is submitted to a decrease of the mean stress
so that its apparent friction angle j_{peak} increases, becoming larger than the friction
angle j . The slope starts then flowing at an angle
larger than the friction angle.

*poudres & grains* **11** (3), 42-48 ( september 2 000)