Three-dimensional static and dynamic behavior of kaolin clay with controlled microfabric using combined axial-torsional testing

A combined axial-torsional testing system was developed to investigate the effect of principal stress rotation and microfabric on the three-dimensional mechanical behavior of Kaolin clay. Uniform and reproducible specimens having a shape of a hollow cylinder were obtained using a slurry consolidation technique. Desirable microfabric (flocculated or dispersed) was obtained by adding appropriate dispersant into the clay slurry during the specimen preparation stage. Precise stress-paths corresponding to a fixed rotation of the major principal stress axis were achieved by using Proportional-Integral-Derivative feedback control technique. The Kaolin clay specimens were tested under a variety of stress paths associated with a constant principal stress rotation angle (β) under undrained conditions. Typical test results for both types of microfabric, such as effective friction angle, undrained shear strength, stress-strain relationship, and pore pressure evolution are presented as a function of β-value. New approaches for data analysis and visualization are also reported for providing a convenient way of incorporating the effect of principal stress rotation. Two established constitutive models for clay soils (Modified Cam-Clay and Lade's three-invariant approach) were evaluated for the present experimental data. Substantial discrepancies were observed between the experimental data and predictions. A new 3-D model that includes a loading-history-dependent plastic potential was proposed with improved predictions. The evolution of shear band was vividly visualized and recorded during the loading process in the tests using a digital image analysis technique. The theory of strain localization was reviewed and applied to the Mohr-Coulomb model and the model developed in this study. Significant disagreement was observed between the experimental observation and the theoretical predictions related to the initiation of strain localization and the inclination of developed shear bands. Characteristic behavior of Kaolin clay under dynamic loading, such as threshold behavior and stiffness degradation, was determined. The relationships of applied shear stress and excess pore pressure with residual strain were obtained under dynamic loading and have implications to cohesive soil behavior under earthquake type loading. This research is likely to have a significant impact on the understanding of the three-dimensional constitutive behavior of cohesive soil from both experimental and modeling points of view.