The shear modulus at very small strains (0.001% or less), Go, is a useful parameter for characterizing the non-linear stress-strain behaviour of soil and for predicting ground deformations of soil structures subjected to static and dynamic loadings. Anisotropic shear moduli at very small strains can be determined by measuring multidirectional shear wave velocities using bender elements in the laboratory. Up to now, relatively little attention has been paid to the small strain behaviour of weathered geomaterials, especially at unsaturated states. In this study, four series of tests were carried out on intact and recompacted triaxial specimens to study the inherent and stress-induced anisotropic shear moduli of a completely decomposed tuff (CDT) at both saturated and unsaturated states. State-dependent semi-empirical equations are proposed to consider the effects of net mean stress, matric suction and void ratio on the anisotropic shear moduli of the unsaturated soil.
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