On the applicability of cross-anisotropic elasticity to granular materials at very small strains

The paper examines the validity of assuming that granular material behaviour can be considered as cross-anisotropic, linear elastic, within a kinematic ‘kernel’ yield surface that is dragged through stress space with the current effective stress point. Data from more than 300 high-resolution triaxial probing and bender element tests performed on 15 samples are reported in which both small stress cycles and shear wave measurements in both the vertical and horizontal directions were conducted on sand and glass ballotini specimens under various effective stress states. It was found that behaviour is not fully elastic. However, provided the effective stress ratio, R, was kept below around 2·2, most features of the very small-strain behaviour could be described in terms of strongly anisotropic compliance terms that varied with void ratio and current effective stress state, apparently independently of the specimen's stress history. The anisotropy depended on the effective stress states. Specimens loaded to R values exceeding 2·2 that experienced shear-induced dilation followed different patterns of behaviour. The concept of a kinematic closed elastic yield surface, or ‘kernel’, proved to be an appropriate way of describing the pronounced effects of recent stress history on the apparent limits to the linear range observed in a wide range of stress probing tests.