A giant overshoot effect in the Janssen granular column

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We present new experimental results on the mechanical behavior of static granular assemblies confined in a vertical column. Our measurements confirm, for the first time, the universal Janssen's scaling for the stress saturation curve. We show consequently, in the context of isotropic elasticity, a relation between the Poisson ratio and granular packing fraction. Moreover, using a systematic study of the overshoot effect created by a top mass equal to the saturation mass, we show behaviors reproduced qualitatively by isotropic elastic materials but in the case of a granular assembly of a spectacular amplitude. These experimental results are strong tests for any theory of granular matter.