Green's function probe of a static granular piling

We present an experiment which aims to investigate the mechanical propertie s of a static granular assembly. The piling is a horizontal 3D granular lay er confined in a hox. We apply a localized extra force at the surface and t he spatial distribution of stresses at the bottom is obtained (the mechanic al Green's function). For different types of granular media, we observe a l inear pressure response whose profile shows one peak centered at the vertic al of the point of application. The peak's width increases linearly with in creasing depth. This Green's function seems to he in at least qualitative a greement with predictions of elastic theory.