Mechanical interpretation of dry granular masses impacting on rigid obstacles

The evaluation of impact forces exerted by flowing granular masses on rigid obstacles is of fundamental importance for the assessment of the associated risk and for the design of protection measures. Empirical formulae are available in the literature estimating the maximum impact force; most of them are based on over-simplifying hypotheses about the behaviour of the granular material. For practical applications, formulations based on either hydrodynamic or elastic body models are often employed. These formulations require the use of empirical correcting factors. In this paper, the same DEM method is used to investigate the relationship between the evolution with time of the impact force and the micromechanics of the granular mass. In fact, considering the dynamic nature of impacts, the impulse value is fundamental for the dynamic response of the barrier, and the mere information about the maximum impact force is not sufficient to design protection works, or assess the vulnerability of structures. Information about contact forces and particle velocities will be discussed and critically compared with macroscopic results. In order to progressively introduce the complexity of the impact phenomenon, four geometrical and mechanical conditions are considered: (a) vertical front, confined flow, bonded material; (b) vertical front, confined flow, purely frictional material; (c) vertical front, free surface flow, purely frictional material; (d) inclined front, free surface flow, purely frictional material.