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 (epsilon < 0.001%); (ii) stress-strain behaviour and anisotropy at small strains (0.001%< epsilon < 0.1%); and (iii) shear modulus degradation with strain (0.001% < epsilon < 1%). The performance of a deep excavation in CDT was back-analysed. The effects of stiffness anisotropy on deformations associated with the excavation were studied through numerical modelling. The effects of cross anisotropic stiffness parameters on deformations were further investigated by conducting a numerical parametric study.; CDT possesses stiffness anisotropy at very small strains. The degrees of inherent anisotropy (Ghh/Ghv) of the block and Mazier specimens of CDT were found to be 1.48 and 1.36 respectively. The higher shear modulus in the horizontal plane is attributed to the horizontal layering structure of CDT resulted from geologic processes such as metamorphism. Stiffness anisotropy was shown to be path dependent for strains ranging from 0.001% to 0.1% as revealed from different cross coupling behaviour along different stress paths. The shear modulus of CDT drops significantly as deviatoric strain increases from 0.001% to 1%. The effect of stress path rotation on stiffness is significant for strains less than 1%. (Abstract shortened by UMI.)...