Stability of granular materials in postpeak softening regime

Triaxial compression tests on sand have been performed to study stability and plastic yielding in the postpeak softening regime. Softening occurs in dense sand sheared at low confining pressure. The experiments show that the stability postulates by Drucker and by Hill are not applicable to sand in the softening regime. Plastic yielding, as well as elastic behavior, can occur along the same stress paths, and plastic strains may be obtained along stress paths with increasing, constant, and decreasing stress differences. Granular materials, in which softening occurs, can go through peak failure more than once. The behavior is entirely controlled by the type of volume change exhibited by the granular material and by the drainage conditions imposed on the material. A yield surface of the conventional type cannot be employed in the softening regime. The yield surface is more likely to take the form of a barrier or fence.