Green's function probe of a static granular piling

We present an experiment which aims to investigate the mechanical properties of a static granular assembly. The piling is a horizontal 3D granular layer confined in a hox. We apply a localized extra force at the surface and the spatial distribution of stresses at the bottom is obtained (the mechanical Green's function). For different types of granular media, we observe a linear pressure response whose profile shows one peak centered at the vertical of the point of application. The peak's width increases linearly with increasing depth. This Green's function seems to he in at least qualitative agreement with predictions of elastic theory.