Inherent anisotropic stiffness of weathered geomaterial and its influence on ground deformations around deep excavations

Abstract

The shear modulus G0 at very small strain (0.001% or less) is an important parameter for predicting ground movements of many geotechnical structures. Recent advances in laboratory testing enable the measurement of shear moduli in different planes of a soil specimen for the evaluation of stiffness anisotropy. Most studies of stiffness anisotropy have been conducted on sedimentary soils and clean sands, and the anisotropic stiffness of weathered material has not yet been fully investigated. In this study, the degree of inherent stiffness anisotropy of completely decomposed tuff (CDT) was evaluated through multidirectional shear wave velocity measurements using bender elements. Tests were performed on both natural (undisturbed) Mazier and block samples and the results were compared. CDT clearly exhibits inherent stiffness anisotropy, with a stiffness ratio between the shear modulus in the horizontal and vertical planes (Ghh/Ghv) ranging from 1.26 to 1.36. The stiffness parameters derived from the laboratory tests were utilized in numerical analysis of the influence of the inherent stiffness anisotropy on ground deformations around a hypothetical but typical multipropped deep excavation. For the given soil models and parameters used, the maximum computed wall deflection and ground settlement due to the pumping of groundwater prior to any excavation were 8% and 19% greater, respectively, than those of an isotropic analysis. The maximum wall deflection and ground settlement because of the combined effects of the pumping and recharging of groundwater inside the site and the subsequent multistage excavations were 15% and 10%, respectively, less in the anisotropic analysis.Key words: inherent anisotropic stiffness, shear modulus, excavation, ground movement, volcanic soil, weathered material.