Stick-slip and force chain evolution in a granular bed in response to a grain intruder.

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 concentrate. 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.