Three-dimensional experiments and modeling of anisotropic clay

Graduation date: 2011This dissertation presents the results of a research effort conducted to better understand the stress-strain, volume change, shear band formation, and strength characteristics of normally consolidated anisotropic clay under fully three-dimensional stress states. A series of consolidated drained true triaxial tests with a constant mean effective stress and constant Lode angles during shear are performed on cross-anisotropic kaolin clay. All tests are performed in a fully automated true triaxial testing apparatus. The relative magnitude of the intermediate principal stress, expressed in terms of the b-value, and initial cross-anisotropy show significant influence on the stress-strain, volume change, strength, and shear band formation characteristics of the clay. Shear bands form and appear to cause failure in all true triaxial tests performed, except in triaxial compression. The initiation and development of shear bands is observed to take place when the clay undergoes volumetric contraction. The lower strength exhibited in the shear bands is caused by alignment of the clay particles. This shear band mechanism is different from that observed in granular materials in which the lower shear strength is reached due to dilation in the shear bands.\ud \ud This dissertation also presents a study on the simulative capabilities of anisotropic and isotropic elasto-plastic bounding surface models under fully three-dimensional stresses. The models assume the associated plastic flow rule and are based on bounding surface elastoplasticity and the critical state theory. The predictions of the models are compared to the stress-strain, volume change, and strength characteristics observed in drained true triaxial tests with a constant effective mean stress and constant Lode angles during shear on normally consolidated anisotropic kaolin clay. The bounding surface models can predict the influence of intermediate principal stress on the behavior of the clay but with magnitudes considerably different from those observed in the experimental results. The effects of initial cross-anisotropy on the behavior of the clay, however, cannot be fully predicted by the anisotropic bounding surface model. These incapabilities are largely due to the use of associated plastic flow rule. More realistic predictions can also be obtained by incorporating a model feature that allows bifurcation from the uniform stress-strain conditions into a localized failure mode or shear bands. \ud \ud This dissertation also includes the details and results of a laboratory study on the influence of non-plastic silt content on the stress-strain, volume change, and strength characteristics of transition silt-clay soils. Series of one-dimensional consolidation tests, isotropic consolidation tests, and triaxial compression tests on three soils of similar base clay but different non-plastic silt contents are performed on cross-anisotropic specimens. The specimen preparation method, testing procedure, and testing conditions of each testing type are controlled to be consistent. Both vertical and horizontal specimens have been tested. It is observed that the tested silt-clay soils are less compressible with increasing silt content during one-dimensional and isotropic consolidation tests. During drained and undrained triaxial compression tests, the normally consolidated soils of the same consolidation stresses show larger values of stiffness, drained and undrained shear strengths, and slightly stronger volumetric contractive tendencies with increasing silt content. The silt-clay soils' effective strengths become more isotropic with increasing silt content. Lower possibilities of shear band initiation and development are observed in the contractive silt-clay soils with higher silt content