Friction force between wall and granular matter for a silo with unsaturated stress

For a silo with unfixed wall, the pressure on its bottom will increase linearly with the height of granular material filled in it. Because no Janssen stress saturation occurs, its mechanic states differ from that of usual silo. By vertically pulling and pushing the wall of the silo, we measured the maximum static friction force of the wall, as functions of the load applied on the silo top and the mass filled in the silo. It is observed that the force increases with the mass, linearly for the pulling case, while exponentially for the pushing case, of which value for the latter may be one order of magnitude larger than that for the former. The force also increases with the load, linearly for both cases. The measurements can be well fitted with the theoretical results of continuous mechanics, assuming that ratio between horizontal and vertical stresses keeps the same everywhere inside the silo. The results support that the assumption made by Janssen for usual silo also suitable for the silo which has no stress saturation.