UNDRAINED INSTABILITY OF VERY LOOSE HOSTUN SAND IN TRIAXIAL COMPRESSION AND EXTENSION .1. EXPERIMENTAL-OBSERVATIONS

The undrained behavior of very loose Hostun RF sand in triaxial compre ssion and extension tests is described. The samples are consolidated i sotropically or anisotropically along constant effective stress ratio paths. Very loose sand exhibits partial liquefaction, deviator peak st ress at relatively low to very low axial strain, gradual post-peak str ess reduction to a small residual value at large strains, sharp loss o f effective mean pressure due to generation of large pore pressure and overall volume reduction. The instability line of Lade is examined in the case of extension tests and extended for anisotropic samples. It is shown that monotonic and anisotropic consolidation strongly influen ces the instability concept. A higher positive anisotropic consolidati on level produces a greater slope of the instability line in compressi on and a reverse trend can occur in extension. Complex stress history can develop a fossilized instability line depending on the amount of a xial strain attained. Effective strain ratio increment at peak has an asymptotic stabilization effect. (C) 1997 by John Wiley & Sons, Ltd.