The anisotropic small strain stiffness of completely decomposed tuff and its effects on deformations associated with excavations

The small strain stiffness anisotropy of saprolitic soils is less investigated as compared with other soils. Stiffness anisotropy at small strains is considered as an important soil characteristic to the prediction of ground movements. Compared with other geotechnical structures such as tunnels and shallow foundations, the understanding of the effects of stiffness anisotropy on the ground movements associated with excavations is not well studied and deserves more attention. The objectives of this study are to investigate the stiffness anisotropy of a natural completely decomposed tuff (CDT) at small strains and to understand the effects of small strain stiffness anisotropy on ground deformations induced by deep excavations. A new bender element probe was developed to determine the velocities of shear waves with different polarization. Three series of triaxial tests with multidirectional shear wave velocity measurements and local strain measurements were conducted to study: (i) shear stiffness anisotropy at very small strains (ε<0.001%); (ii) stress-strain behaviour and anisotropy at small strains (0.001%<ε<0.1%); and (iii) shear modulus degradation with strain (0.001%<ε