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This thesis examines selected issues related to the performance of earthfill embankments constructed on soft clay foundations. The primary objectives of the thesis are: to extend an existing elastic-viscoplastic (EVP) constitutive model to describe the influence of micro-structure and strength anisotropy on the engineering response of soft clay, to investigate the impact of clay structure on the performance of a full-scale test embankment on soft clay, and to evaluate the significance of three-dimensional effects on the behaviour of three test embankments constructed on soft clay foundations. Firstly, in this thesis, generalized EVP theory is used to evaluate the viscous response of 19 clays reported in the literature. It is shown that the viscous response of clay, including rate-dependent and time-dependent behaviour in different types of experiments, can be quantitively characterized using a unique set of viscous parameters. A practical methodology to determine the EVP constitutive parameters is provided. Next, an existing EVP constitutive model is extended to account for the influence of micro-structure and anisotropy on the engineering response of rate-sensitive natural clay. Microstructure and the process of destructuration are mathematically simulated using a state-dependent fluidity parameter. The EVP model also incorporates a structure tensor that can be used to describe strength anisotropy of natural clay. The extended structured and anisotropic models are shown to the responses of undisturbed structured clays, such as Saint-Jean-Vianney clay, Gloucester clay, and St. Vallier clay. Lastly, four case studies are used to investigate the impact of microstructure and destructuration on the performance of embankments founded on soft clay and the effects of 3-dimensional geometry on test embankment behaviour. The Gloucester test embankment is studied using the structured EVP model. This case is used to examine the impact of destructuration on strength gain in the Gloucester foundation during staged construction. In addition, three embankment cases in Vernon British Columbia, St. Alban Quebec, and Malaysia are studied using 3-dimensional finite element analysis to examine the impact of 3-dimensional geometry on the performance of test embankments. Keywords. elastic-viscoplastic, viscosity, rate-sensitivity, natural clay, microstructure, anisotropy, case study, three-dimension.
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