Plastic strain-induced small strain stiffness anisotropy. A multi-scale experimental evidence

The paper summarises the main results of a research into the very small strain shear stiffness of a clayey soil, as measured in vertical and horizontal directions, Ghv and Ghh respectively, by means of T-shape horizontal bender elements fitted in a stress-controlled triaxial system. The testing programme, carried out on a reconstituted clay, was designed to investigate the evolution of the anisotropy ratio Ghh/Ghv along isotropic (η = 0) and anisotropic (η  0) virgin radial paths. The results show that the small strain stiffness anisotropy smoothly adapts itself to the imposed strain history. The experiments also show that the complete modification of the directional elastic properties of the soil requires the virgin radial compression path to be extended along a new direction up a stress level at least four times larger than that experienced previously. Finally, the variations in the clay elastic anisotropy have been related to the changes in clay fabric as investigated by means of scanning electron microscopy (SEM). In this context, the variation of the fabric orientation has been quantified through a specific digital image processing.