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....[ Read more ]

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<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 (G_hh/G_hv) 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 fiom 0.001% to 1%. The effect of stress path rotation on stiffness is significant for strains less than 1%.

The deformations of the excavation in CDT are small. The database for deformations of excavations in decomposed materials in Hong Kong showed that the wall and ground deformations are generally small. The effect of modelling shear stiffness anisotropy of soil is more significant on ground surface settlement than on lateral wall deflection. The computed maximum ground surface settlement is larger and the ground surface settlement trough is narrower when the stiffness anisotropy of CDT is considered. The computed strains are localised at the back of the wall near the final level of excavation. Numerical experiments showed that the modulus ratios E_h/E_v and G_vh/E_v play an important role in predicting deformations induced by excavations.