A characteristic feature of granular materials is the existence of arches. These force chains alter the isotropy of a granular packing and can stop the flow of grains during the discharge of a silo. Therefore, the knowledge of the arches stability is crucial for the applications involving granular materials.
Breaking arches is not an easy task : increasing the vertical load, that leads to their formation, usually makes them more stable. Therefore, different kind of stresses must be applied. This can be achieved by means of an external vibration.
In the experiment presented in the paper  recently published in Physical Review Letters, we have investigated the shape of the arches that block the exit orifice of a two dimensional silo. We have submitted the silo to an external vibration to establish the relationship between the features of the arches and their resistance against vibrations. The main result is that the presence of a geometrical defect in the arch is the most important issue regarding its endurance. These defects are quantitatively defined in terms of contact angles. We also provide a model that helps to understand the reason for the weakening.
 C.Lozano, G.Lumay, I.Zuriguel, R.C.Hidalgo, A.Garcimartin, Phys. Rev. Lett. 109, 068001 (2012) – PDF