Mechanical behavior and constitutive modelling of natural structured soils

The engineering properties of natural soils are sensitive to their soil structure. Understanding and accounting for soil structure are important in the design and analysis of geotechnical structures. In this thesis, both laboratory tests and numerical analyses were conducted to conceptualize and quantify soil structure and its effect on deformation mechanisms.; Laboratory tests were performed to investigate the effect of soil structure on stiffness degradation, anisotropic properties and time dependency. Deformations over a wide range of strains (10{dollar}sp{lcub}-4{rcub}{dollar}% to 10%) were measured by a pulse transmission technique, a torsional shear device, and a triaxial device. Flocculated and dispersed kaolinite, undisturbed Pisa soils, and San Francisco Bay Mud were tested.; The stress-strain behavior at small strains was greatly affected by soil structure. Soils with the same origin had similar degradation characteristic even if the plasticity varied considerably. As deformation continued towards the failure state, the soil structure broke down, and the shear resistance was more controlled by the frictional forces at particle contacts.; The initial anisotropy, measured by pulse transmission tests, affected the deformation in the pre-yield region. In the post-yield region, the stress-induced anisotropy resulted in differences in deformation between vertically and horizontally cut specimens.; The observed time-dependent deformations were interpreted by viscous creep acting at small strains and dislocational creep at large strains. The effects of creep and stress path on drained triaxial creep deformation demonstrated that the coupled analysis of porewater migration and soil deformation was necessary to predict time-dependent deformation.; Two models were developed to simulate the observed soil behavior. For an anisotropic non-linear model, non-linearity was modelled by {dollar}G/Gsb{lcub}max{rcub} = 1/(1 + agammasp b),{dollar} and anisotropy was expressed by {dollar}A = Gsb{lcub}hh{rcub}/Gsb{lcub}vh{rcub}.{dollar} An elasto-viscoplastic model was based on the modified Cam-clay model including rate dependent isotropic preconsolidation pressure. Various types of stress-strain-time behavior were simulated adequately by these models for a variety of soils. Finite element analysis of loading on a rigid footing showed that non-linearity, anisotropy, stress path and creep affected the stress distribution and deformation under the footing in both drained and undrained analyses.