Stress propagation in granular media: Breaking of any constitutive state equation relating local stresses together by a change of boundary conditions

The stress response of a granular assembly subject to different changes of boundary conditions is studied experimentally in order to define the stress propagation characteristics. These results demonstrate that no simple and single relationship between local stresses exists, which would be imposed by the local structure of the granular assembly only in general. On the contrary, it is demonstrated that these results are controlled by the boundary conditions themselves and that a tiny change of them may lead to strong variations in the incrememental stress relation; furthermore, in other cases, these changes may generate large variations of the stress field and can allow to understand partly the fluctuations already observed in these media. PACS: 46.10+z ; 46.30.-i ; 81.05.Rm