Doctoral Dissertations

Date of Award

5-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

Dayakar Penumadu

Committee Members

Eric C. Drumm, Richard T. Williams, Randall W. Gentry

Abstract

A new method for identifying the microfabric (geometric arrangement of platelets) of cohesive soil using X-ray diffraction technique without the need for a pole figure device is introduced in this study. This method is based on identifying the occurrence of basal and prism peaks in a given X-ray diffraction pattern and their relative intensities observed in the pattern. Five different specimen preparation techniques are used to prepare the Kaolin clay specimens with different particle orientations to identify their microfabrics using X-ray diffraction analysis, and the results are assessed in conjunction with high magnification SEM (Scanning Electron Microscope) images. The proposed method does not require impregnation of clay with alternate pore fluids/epoxy, and is able to identify the undisturbed microfabric of clay sample at its natural water content. This method is further used for evaluating the variation in microfabric of slurry consolidated specimens of Kaolin clay due to triaxial shearing under compression and extension loading conditions. Based on the lubricated end triaxial tests on the clay specimens with two distinct microfabrics (flocculated and dispersed) and the X-ray diffraction analysis of these specimens before and after shearing, this study shows that the extension shearing may cause a significant re-orientation of particles within the specimen unlike the effect of compression shearing.

The impact of micro fabric on shear strength behavior of Kaolin clay is studied for the variation of confining pressure, stress history, and loading/boundary conditions by perfonning a series of triaxial experiments using lubricated ends. The importance of using appropriate drainage conditions for the lubricated ends in triaxial device is discussed based on the experimental observations during drained and undrained triaxial tests performed at three different drainage conditions. Good drainage conditions of lubricated end platens offered significantly higher values of shear strength, effective friction angle, volumetric deformation, and axial strain at failure conditions in comparison to the corresponding values obtained for the poor and intermediate drainage conditions. Using improved drainage conditions, a series of triaxial compression and extension tests are performed on normally consolidated (NC) and heavily overconsolidated (HOC, overconsolidation ratio =10) specimens of Kaolin clay with two extreme micro fabrics ( dispersed and flocculated) under drained and undrained conditions. Three different values of confining pressure (207, 276, and 345 kPa) are used to evaluate the effect of confining pressure on shear behavior of dispersed and flocculated specimens of Kaolin clay. A comparison between the drained and undrained shear response is presented for the variation in the loading conditions (compression and extension) and the stress history (NC and HOC) of soil. This study also includes a discussion on the occurrence of strain localization within the clay specimens tested using lubricated ends, which is believed to be a successful method of reducing non-uniformity in stress-strain and deformation behavior of soil based on the importance of lubricated ends over frictional ends. The impact of microfabric, confining pressure, loading conditions, stress history, and drainage conditions on the pattern of strain localization within the solid cylindrical specimen of Kaolin clay is discussed based on the digital image analysis of its deformation profile.

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