COLLAPSE OF DRY SAND

Loose cohesionless saturated materials have proved responsible for a n umber of serious or catastrophic flow slides. Liquefaction failures wi th no obvious triggering mechanism have also been recorded. This pheno menon of sudden liquefaction without a presence of cyclic shear stress es is often referred to as spontaneous or static liquefaction. Results from previously published studies suggest that liquefaction is trigge red not by the undrained loading and generation of pore pressures but by the collapse of the metastable sand structure, which in turn genera tes the driving pore pressures in a saturated material. Hence, the col lapse is a characteristic response of a material to certain stress sta tes rather than a result of some enforced undrained loading. This theo ry is evaluated on very loose dry Ottawa sand. It is shown that the ve ry loose dry sand when subjected to a constant deviatoric stress path significantly changes its behavior at a certain discreet stress state, increases compressibility, and becomes increasingly unstable. This re sults in collapse - vigorous contraction of the specimen. This structu ral collapse appears to be equivalent to the pore-pressure generation in collapsing, very loose saturated sand.