Novel findings on the dilatancy and non-coaxiality of sand under generalised loading
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RESEARCH PAPER
Novel findings on the dilatancy and non-coaxiality of sand under generalised loading P. K. Triantafyllos1
•
V. N. Georgiannou1 • Y. F. Dafalias2,3 • I.-O. Georgopoulos2
Received: 22 February 2020 / Accepted: 15 October 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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 g = q/p0 , p0 , e, and b = (r0 2 - r0 3)/(r0 1 - r0 3), but diminishes gradually during stress rotation. Keywords Anisotropy Dilatancy Instability Non-coaxiality Principal stress rotation Stress–strain history effects
1 Introduction Shear deformation of granular materials is associated with movement of grains along directions diverging from that of the applied shear stress and this fabric-related behaviour is the origin of dilatancy [34, 78, 105], defined as the volumetric and shear incremental plastic strains coupling. The internal constraints associated with the shape and packing of particles as well as with the orientational distribution of
& P. K. Triantafyllos [email protected] 1
Department of Geotechnical Engineering, National Technical University of Athens, University Campus Zografou, Heroon Polytechniou 9, 15773 Zografou, Greece
2
Department of Mechanics, National Technical University of Athens, University Campus Zografou, Heroon Polytechniou 9, 15773 Zografou, Greece
3
Department of Civil and Environmental Engineering, University of California, Davis, CA 95616, USA
contact normal vectors and non-spherical particles, commonly called fabric, constitute one of the origins of noncoaxiality, the latter defined as the non-coincidence of the principal dir
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