Anisotropic behaviour of bulk solids and its effect on silo design

The anisotropic material behaviour is a characteristic property of bulk solids. Prior research work proved that the measured unconfined yield strength of cohesive bulk solids is lower when the direction of the collapsing stress and the consolidation stress does not coincide. A new powder tester has been developed to measure both the unconfined yield strength in the uniaxial direction, which provides consolidation and collapse stresses in the same direction, as well as the anisotropic unconfined yield strength, which means consolidation and collapse stresses are in orthogonal directions. The friction between the walls and the bulk solid is minimised by the use of rubber membranes which deform uniformly with the bulk sample. The uniaxial and the anisotropic unconfined yield strengths have been measured for a limestone powder at different consolidation stresses. Experimental results as well as their application on the design for funnel flow silos are discussed. To avoid piping two design criteria exist for the design of a funnel flow silo: Jenike assumes a passive state of stress to prevail close to the silo outlet (Jenike, 1961 [1]); Johanson assumes an active state of stress (Johanson, 1969 [2]). While the approach of Jenike leads to good results, the estimation of the critical outlet diameter in a silo with an active state of stress is not satisfying. The pipe dimensions for an active state of stress were measured using a silo centrifuge. The application of the design criteria gives reasonable and safe design value only if the anisotropic behaviour of the bulk solid is considered which was measured by the new powder tester.