Influence of time effects on instability of granular materials

Results of previously performed series of triaxial compression tests designed to expose the type of behavior displayed by granular materials have shown that materials that exhibit non-associated flow do not obey the stability postulates by Drucker and by Hill. Triaxial tests on fully and paltry saturated specimens of sand have been performed under drained and undrained conditions to study the regions of stable and unstable behavior. Granular materials may become unstable inside the failure surface if the state of stress is located between the instability line and the failure surface for the material. Presented here are three series of experiments conducted on sands that compress during shear. In one series, experiments are performed to study the effect of strain rate on the location of the instability line. In the second series, the sand is allowed to creep under drained conditions resulting in outwards movement of the yield surface, and the sand consequently becomes stiffer in the region above the stress point. Increasing amounts of creep therefore require increasingly high stress differences to render the sand unstable. In the third series of experiments the saturated sand is allowed to creep under undrained conditions, and this results in increasing pore pressures and possible instability because the stresses may be brought into the region of potential instability. Thus, time effects play important roles in the stability of soils. (C) 1997 Elsevier Science Ltd.