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The mechanical response of granular beds under applied stresses is often characterized by repeated cycles of stick-slip. Using the discrete element method, we examine stick-slip from a concentrated force loading--imposed by a single grain that is drawn through a densely packed, periodic granular bed via a stiff virtual spring. Force chains continually form and collapse ahead of the intruder grain. A comprehensive characterization of the birth-death evolution of these load-bearing structures, along with their surrounding contact cycles, reveals a well-defined shear zone of around eight particle diameters from the intruder, encapsulating: (i) long force chains that form buttresses with the fixed bottom wall for support, (ii) a region where the collapse of the most stable, persistent three-cycles preferentially occur to the point where they are essentially depleted by the end of the first cycle of stick-slip, and (iii) an inner core where force chain buckling events concéntrate. Dilatancy is greatest in this inner core, and in the region next to the free surface. During slip, secondary force chains briefly form behind the intruder: these transient force chains, most of which comprise only 3 particles, form in the direction that is roughly perpendicular to the intruder motion. [ABSTRACT FROM AUTHOR]

The concept of "random loose packing" (RLP) has evolved through extensive study of loose packings of spheres, which has resulted in an accepted definition as the loosest packing that can be obtained by pouring grains. We extend this consideration to packings of nonspherical grains (ellipsoids) formed by slow settling in a viscous liquid, and perform a detailed analysis of the structural properties of the resulting packings. We find that as in the case of spheres the loosest ellipsoid packings are generated for grains with high interparticle friction. However, unlike spheres, these packings cannot be considered random as they have a significant degree of orientational ordering that increases with the grain's aspect ratio. This demonstrates that applying sedimentation or pouring techniques that have become part of the commonly held definition of RLP, will not generate random packings of ellipsoids. The consequences for the accepted definition of RLP and its applicability to nonspherical grains is discussed. [ABSTRACT FROM AUTHOR]