Measurements and Numerical Simulations of Inherent Stiffness Anisotropy in Soft Taipei Clay

Soils are generally known not to be isotropic materials. The inherent anisotropy of clays results from the deposition process, which tends to induce a horizontal bedding plane in the soil layer. In this paper, the anisotropy of clays was studied to obtain more accurate analytical results for geotechnical problems, especially for deep excavations in soft clay with nearby structures. A series of K0-consolidated undrained triaxial compression (CK0UC) tests thus was conducted on tube samples of natural Taipei silty clay with multidirectional bender elements. A newly designed triaxial testing system equipped with a high-precision servo motor and local strain sensors was developed. A new soil suction-control system was also developed to perform soil saturation for triaxial tests. The suction-control system was used to reduce the change in the void ratio during saturation in the triaxial tests. Soil samples were retrieved from a site near a well-documented excavation case in Taipei. The anisotropy ratios for both the shear modulus and the undrained Young's modulus were obtained by performing small-strain triaxial tests. The test results indicated that the anisotropy ratios of the shear moduli at the end of reconsolidation ranged from 1.15 to 1.44. Soils with higher overconsolidation ratio values had higher anisotropy ratios. Numerical simulations for an excavation case history were performed using a developed small-strain soil model that incorporates the anisotropy in the soil stiffness. The analysis results showed that differences between the anisotropic and isotropic models for the wall displacements, ground-surface settlements, and lateral soil movement behind the wall ranged from 10 to 43% at specified depths.