Three-dimensional mechanical behavior of kaolin clay with controlled microfabric using true triaxial testing.

摘要

Clays in their natural state are mostly anisotropic because of their modes of deposition. The mechanical behavior of clay is largely influenced by the stress history, stress paths (relative magnitudes and orientation of principal stresses), and the microfabric of clay. To study the mechanical behavior of normally consolidated to heavily overconsolidated clays, 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 has flexible boundaries and uses three-axis electro-pneumatic Proportional-Integral-Derivative (PID) based real-time feedback control. Software that can automatically saturate, consolidate and apply shear stresses along predetermined stress or strain paths is developed for this study. The cubical specimens are prepared in the laboratory with two distinct microfabric of Kaolin clay; flocculated and dispersed.; The three-dimensional behavior of clay is discussed with emphasis on the following factors; the effect of relative magnitudes and orientation of principal stresses, the influence of consolidation history, and the effect of change in soil's microfabric on its mechanical response. Strain localization issues and their significance in defining the failure conditions are discussed. 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 test data, and limitations in predicting various aspects of soil behavior are identified. Formulation of a new rate-independent elasto-plasticity 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.