Three-Dimensional Mechanical Behavior of Kaolin Clay with Controlled Microfabric Using True Triaxial Testing

Clays in their natural state are mostly anisotropic because of their modes of deposition. Many boundary value problems in geotechnical engineering involve soil elements that are subjected to stress paths with varying relative magnitudes and orientation of principal stresses. In clay, these soil elements may also have different orientations of its microfabric. It is a well-known fact that the variation in relative magnitudes and orientation of principal stresses and the microfabric of clay have significant effect on the stress-strain, pore pressure and shear strength behavior of clay. In order to study this behavior of clay from a normally consolidated to heavily over-consolidated state, a series of strain controlled true triaxial undrained tests with constant intermediate principal stress ratio (b-value) are performed on cubical specimens of Kaolin clay with controlled microfabric. The true triaxial device used in the present research has flexible boundaries and uses three-axis electro-pneumatic ProportionalIntegral- Derivative (PID) based real-time feedback control. A custom developed software that can automatically saturate, consolidate and apply shear stresses along predetermined stress or strain paths is also developed for this study. The cubical specimens are prepared in the laboratory with two distinct microfabric of Kaolin clay; flocculated and dispersed.

A comprehensive analysis of the observed three-dimensional behavior of clay is presented in this dissertation with its emphasis on the following factors; the effect of relative magnitudes and orientation of principal stresses, the influence of consolidation history and its role in the anisotropy of clay, and the effect of change in soil's microfabric on its mechanical response. Strain localization issues are discussed in light of the experimental observations. A method for determining the onset of localization is proposed, which is eventually used in the identification of failure state of stress and strain. The effect of change in the test boundary conditions and specimen shape on the observed mechanical response of clay is evaluated by comparing the results obtained from true triaxial tests using cubical specimens, conventional triaxial tests using cylindrical specimens, and combined axial-torsional tests using hollow cylinder specimens. Two well-recognized constitutive models (modified cam clay model and single hardening model) are evaluated for the present true triaxial test data, and limitations in predicting various aspects of soil behavior within reasonable accuracy are identified. Based on the experimental observations and the past theories on the normalized behavior of clays, a dynamic failure surface as a function of stress state and the pre-consolidation history is proposed. Formulation of a new rate independent elastoplasticity model is presented, which is based on the normalized clay behavior observed in this study. Validation of the proposed model from this study shows significant potential for predicting the three-dimensional mechanical response of clay.