Instability and liquefaction of granular materials

Stability postulates by Drucker and by Hill are applicable to solid metal. When applied to granular material exhibiting nonassociated flow, they imply that the material may be unstable when exposed to certain stress paths inside the failure surface. Series of triaxial tests on fully saturated and partly saturated specimens have been performed under drained and undrained conditions to study the regions of stable and unstable behavior. The stability postulates by Drucker and by Hill do not capture the conditions for stable and unstable behavior of granular materials. The experimentally determined conditions for which granular materials are stable, conditionally unstable, and unconditionally unstable are discussed.