  We study experimentally gravity-driven granular discharges of laboratory
scale silos, during the initial instants of the discharge. We investigate
deformable wall silos around their critical collapse height, as well as rigid
wall silos. We propose a criterion to determine a maximum time for the onset of
the collapse and find that the onset of collapse always occurs before the
grains adjacent to the wall are sliding down. We conclude that the evolution of
the static friction toward a state of maximum mobilization plays a crucial role
in the collapse of the silo.
