- E Aharonov: Lamont-Doherty Earth-Observatory, Columbia University, Route 9W, Palisades, New York 10964, USA.
We numerically model two-dimensional systems of granular aggregates confined between two rough walls and demonstrate that at a critical grain volume fraction nu(c) an abrupt rigidity transition occurs. The transition has first-order characteristics, although the elastic constants undergo a second-order transition. Densely packed grains, with a volume fraction nu>nu(c), display an elastic-plastic rheology. Loose packings, with nu