Pressures from flowing granular solids in silos

Abstract

Large displacement flows in granular solids occur whenever silos are discharged. Measurements of pressures during flow, combined with visual observations of flow patterns and control tests on the ensiled solids, have revealed several phenomena contributing to pressure variations during flow. Not only are the pressure variations time dependent within a single silo, but significant systematic differences are found between one silo and another, even when the two are superficially identical and contain similar ensiled materials. One serious outcome is that silo pressures are quite unsymmetrical even in symmetrical silos, and this is a most dangerous phenomenon for the safety of the silo structure. The loss of symmetry can largely be traced to inhomogeneity and anisotropy in the ensiled material, developed by the initial packing during filling. Thus, unless the mechanism of packing is understood, the mechanics of these solids cannot be modelled. The sensitivity of granular materials to stress history and the role of geometrically imperfect boundaries present other complications in interpreting observations and understanding the mechanics of silo pressures and flow regimes. No current theory of silo loads covers these phenomena, which themselves are only illustrative examples of current shortcomings. More comprehensive constitutive models are needed for application to silos and large-scale granular solids flows.