Cross-anisotropic deformation characteristics of natural sedimentary clays

The cross-anisotropic elasticity parameters were experimentally determined for six sedimentary clays of different ages and origins at both natural and reconstituted states by using high-precision triaxial apparatus. All the parameters and their dependency on the effective stress are satisfactorily described by a simple, conventional power-law model. The patterns of the inherent anisotropy exhibited close agreement between the natural and reconstituted states, suggesting that the development of natural microstructures due to ageing and diagenesis does not play a primary role in determining the anisotropy patterns in clays. Despite different degrees of stiffness anisotropy observed in the six clays, relatively well-defined correlations encompassing all the tested clays were found between certain stiffness parameters. While fully characterising the drained elasticity moduli is still a challenging laboratory task, these correlations will help estimate some of them based on more readily measurable undrained moduli. All the exponents in the stiffness model equations, representing the dependency of the moduli on effective stress states, were generally smaller for older natural clays, whereas they were broadly constant for reconstituted samples, indicating a cementation-like effect in older, aged clays. Poisson ratios, $ν′hh$⁠, $ν′hv$ and $ν′vh$⁠, were generally very small, with secant $ν′vh$ values becoming gradually larger at larger strains.