Novel findings on the dilatancy and non-coaxiality of sand under generalised loading

The dilatancy and non-coaxiality of sand are investigated under generalised loading including rotation of the stress principal axes (PA), in drained and undrained conditions. Weak non-coaxiality is exhibited under radial loading being dependent on the stress direction relative to the deposition direction, as well as on state, and gradually diminishing beyond peak failure. On the contrary, non-coaxiality is stronger and persists in the post-peak regime when sand is subjected to rotation of the stress PA. In conditions involving undrained rotation of the stress PA at constant total stress principal values (PV), distinct non-coaxiality patterns and elastic–plastic coupling are observed under increasing stress ratio, while a steady state is ultimately attained. This study shows that sand exhibits strong non-coaxiality and contracts immediately upon initiating the rotation of the stress PA at constant effective stress PV very close to critical state (CS), while it was previously dilating on the failure surface in a coaxial deformation mode, under radial loading; the phenomenon becomes increasingly intense as CS is approached. The degree of non-coaxiality and associated contractancy becomes higher when the previous shearing becomes more persistent, in terms of shear strain accumulation, while the influence of pre-shearing is stronger than that of the degree of stress rotation and the level of η = q/p′, p′, e, and b = (σ′2 − σ′3)/(σ′1 − σ′3), but diminishes gradually during stress rotation.