Experimental investigation of the evolution of elastic stiffness anisotropy in a clayey soil

The paper presents the main results of a research study into the very small strain shear stiffness of a clayey soil, as measured in vertical and horizontal directions, G(hv) and G(hh), respectively, by means of T-shaped horizontal bender elements fitted in a stress-controlled triaxial system. The testing programme was designed to investigate the evolution of the anisotropy ratio G(hh)/G(hv) along isotropic (eta = 0) and anisotropic (eta not equal 0) virgin radial paths. In particular, a reconstituted clay has been chosen as tested material in order to unequivocally relate the observed evolution of its stiffness anisotropy to the specific stress-strain histories imposed in laboratory. The results show that the small strain stiffness anisotropy smoothly adapts itself to the imposed strain history, achieving different constant values ranging from 1 to 1.2 for eta ranging from 0 to 0.8. The experiments 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. In this context, the variation of the fabric orientation has been quantified through a specific digital image processing.