New findings on the evolution of the instability surface of loose sand

The conditions that trigger the unrestrained flow deformation of loose anisotropic sand are investigated. An instability surface (IS) is defined in the deviatoric plane. It comprises the transient-peak states at which flow instability is triggered when isotropically consolidated sand is subjected to monotonic undrained loading at various fixed directions of principal stress, alpha, under constant mean total stress, p, and fixed stress parameter, b = (sigma 2 '-sigma 3 '/(sigma 1 '-sigma 3 ') = 0.5. Generalised undrained loading including rotation of the sigma 1-axis is also imposed on anisotropically consolidated sand. The mobilisation of the instability stress ratio, sin phi(ip) = (sigma 1 '-sigma 3 ')/(sigma 1 '+sigma 3 '), that corresponds to stress direction alpha via the IS locus, generally, triggers flow under loading with both fixed and rotating sigma 1 '-axis. Novel results are also presented: loose sand is subjected to undrained principal stress rotation at constant deviatoric stress, yet the previously established IS is crossed stably and flow is triggered after stress rotation is imposed on the failure surface, while a non-flow diffuse instability is triggered on the failure surface under increasing stresses and decreasing stress ratio. The experimental results indicate that the triggering of flow instability depends on the stress-strain history as well as on the incremental stress direction. It is also shown that both diffuse and localised instabilities occur preferably at stress states corresponding to unfavourable deformation kinematics.