The prediction of the force needed to move blades through a bed of cohesionless granules

Abstract

A previous paper described the measurement of the force required to move a plane vertical blade horizontally close to the surface of a level bed. This paper discusses methods of predicting those blade forces using modified techniques of theoretical soil mechanics. For blades close to the surface of the granular bed the shape of the material profile must be taken into account. It is demonstrated for steel blades in ballotini that force calculations employing a kinematic model of material failure lead to predictions of force above the experimental values. Such models are therefore preferable for design purposes to calculations based on assumed stress distributions which predicted forces less than experimental. It is concluded that an assumption that the granular material fails in a logarithmic spiral surface leads to the prediction of forces close to the experimental. Difficulties arise, however, because solutions are not always unique and because of doubt as to boundary conditions at the blade face.