Depending on the stress state, a bulk solid, regarded as the elastic-plastic material, can exhibit workhardening, worksoftening or perfectly plastic properties. During continuing motion, the solid flow can be assumed as incompressible, and the stress field satisfies a condition of critical state which can be identified with the Coulomb yield condition. The model of Coulomb material is introduced; such material satisfies the Coulomb yield condition and the flow law associated with this condition by the plastic potential rule. In general, the Coulomb material is stronger than the considered bulk solid in the critical state. The kinematic approach and the familiar upper bound theorem are applied in order to determine lower bounds on pressure exerted by the flowing material on bin walls and to determine flowability criterion. In the latter section, the flow induced by a horizontally moving rough plate is analyzed and two flow patterns are considered.

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