  We study experimentally the distribution of local stresses in a granular
material confined inside a vertical cylinder. We use an image correlation
technique to measure the displacement field of the container induced by the
forces exerted by the grains on the inner wall. We describe an optimization
procedure based on the linear theory of elastic shells to deduce the
distribution of these forces from the measured displacement field. They
correspond to the stress field of the granular material close to the
container's inner wall. We first confirm the validity of Janssen's description
for various experiments, including the influence of the bead diameter and the
effect of an additional mass on top of the granular column. We then apply this
method to determine the stress field during the gravity-driven discharge of a
silo through an aperture.
