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Various aspects of the mechanical behavior of kaolin clay are discussed in light of experimental observations from a series of strain controlled true triaxial undrained tests performed on cubical kaolin clay specimens with flocculated and dispersed microfabric, using a fully automated flexible boundary experimental setup with real-time feedback control system. The laboratory procedures used to prepare flocculated and dispersed microfabric specimens are presented. Mercury intrusion porosimetry is used to evaluate the pore structure of these specimens. The influence of microfabric on the consolidation behavior of kaolin clay is evaluated based on the data obtained from [K.sub.0] consolidation during constant rate of strain tests and the isotropic consolidation during true triaxial tests. Undrained tests on kaolin clay show that the following vary with microfabric of specimen: The shear stiffness, excess pore pressure generated during shear, and strength and strain to failure. For both microfabrics, the observed strength behavior using cubical triaxial testing shows a similar pattern of variation with applied stress anisotropy; hence, only a marginal influence of fabric-induced anisotropy. DOI: 10.1061/(ASCE)1090-0241(2007)133:4(433) CE Database subject headings: Soil consolidation; Microstructure; Shear deformation; Three-dimensional analysis; Shear tests; Soil structure; Clays; Triaxial tests.

The influence of geometric arrangement of platelets (microfabric) on the mechanical behavior of kaolin clay is investigated using lubricated end triaxial testing on solid cylindrical specimens. A series of compression and extension tests under drained and undrained conditions were performed on clay specimens with different microfabric for overconsolidated ratio values of 1 and 10. The solid cylindrical specimens with dispersed and flocculated microfabric were produced in the laboratory using slurry consolidation technique under [K.sub.0] condition. Based on the experimental observations, it is evident that microfabric strongly affects the mechanical behavior of kaolin clay, such as its stress-strain relationship, effective stress ratio, shear strength, excess pore-pressure evolution, and volumetric response. The influence of confining pressure on clay specimens with dispersed and flocculated microfabric is also studied in this research. This study shows that the microfabric can change the basic nature of clay. For example the normally consolidated kaolin clay shows its dilative nature during shearing for dispersed microfabric and contractive nature for flocculated microfabric. DOI: 10.1061/(ASCE)1090-0241(2007)133:3(306) CE Database subject headings: Clays; Kaolin; Overconsolidated soils; Soil compression; Triaxial tests.

A series of combined axial-torsional tests were performed to study the 3D mechanical behavior of kaolin clay in an undrained condition. Using the digital image analysis technique, discrete local deformation of the surface of a hollow cylindrical specimen under loading was recorded. A linear interpolation method was used to generate a continuous deformation and strain field of the specimen based on the recorded discrete local deformations. Evolution of shear band was vividly visualized and recorded during the loading process for various inclinations of major principal stress. The theory of strain localization on continuous bifurcation was briefly reviewed and applied to the Mohr-Coulomb model, and a single hardening model incorporating the concept of loading-history-dependent plastic potential was developed by the writers. The largest critical plastic modulus and orientation of the shear bands were predicted by using the theoretical solution. Significant disagreement was observed between the experimental results and theoretical predictions related to the initial occurrence of strain localization and the inclination of fully developed shear bands. CE Database subject headings: Elastoplasticity; Kaolin; Clays; Torsion; Stress; Strain.

A combined axial-torsional testing system was developed to investigate the effect of rotation of principal stresses on the three-dimensional mechanical behavior of Kaolin clay. Uniform and reproducible cohesive specimens having a specimen shape of a hollow cylinder were obtained using a two-stage slurry consolidation technique. Precise stress paths (triaxial compression to pure torsional shear to triaxial extension), corresponding to a fixed rotation of the major principal stress axis, were achieved by using the proportional-integral-derivative (PID) feedback control technique. Kaolin clay specimens were tested under a variety of stress paths associated with a constant principal stress rotation angle ([beta]) under undrained conditions. Typical test results, such as effective friction angle, undrained shear strength, stress strain relationship, pore pressure evolution, and stress paths are presented as a function of [beta]. During shearing, the procedure to use advanced servo-hydraulic control (using PID algorithm in this study) to maintain a fixed [beta] value that involves updating specimen geometry in real-time is described. A new approach for data analysis and visualization is presented for providing a convenient way of incorporating the effect of major principal stress rotation angle considering the degradation of stiffness as a function of stress path in three dimensions. CE Database subject headings: Kaolin; Clays; Stress; Rotation; Anisotropy.

The influence of intermediate principal stress on the mechanical behavior of overconsolidated kaolin clay is investigated using three-dimensional true triaxial testing on cubical specimens. A flexible boundary, true triaxial setup with a real-time feedback control system was used to test soil specimens under stress and strain-control modes. Undrained tests on kaolin clay show that the following vary with intermediate principal stress: the stiffness at small strains, excess pore pressure generated during shear, and strength and strain to failure. Failure occurred at peak deviator stress followed by shear band formations and localized bulging. Prior theoretical formulations of bifurcation and undrained instability support these experimental observations. Analysis of data in the octahedral plane indicates that kaolin clay follows a nonassociative flow rule, which is described by a constant third stress invariant failure criterion with von Mises plastic potential surface. DOI: 10.1061/(ASCE) 1090-0241 (2004)130:3(284) CE Database subject headings: Bifurcations; Localization; Overconsolidated clays; Shear deformation; Strain; Three-dimensional analysis; Undrained shear tests; Yield surface.

The shape of aggregates has an important influence on the behavior of civil engineering materials. Digital imaging techniques provide unique opportunities for describing these features in an automated fashion. Shape analysis of fine and coarse aggregate particles is investigated in this study using Fourier transform of digital images. The geometric signature of each shape is extracted by measuring the distance between its centroid and the boundary at constant increment of angles, and Fourier transforms are used to evaluate its spectral information. The number of highest amplitude harmonics required for accurate profile regeneration is evaluated. Shape of a given aggregate particle is reconstructed using inverse Fourier transforms considering a limited number of significant harmonics. A parameter that can quantify the error between regenerated and original profiles is proposed. Using this value, two shape parameters are defined to describe the overall shape and the ruggedness of a particle. A procedure of quantitatively describing the roundness/angularity of aggregate shape is presented and extended to three dimensions using orthogonal views. CE Database subject headings: Digital techniques; Fourier analysis; Image analysis; Particles; Shape.

A series of one-eighth-scale strain controlled model pressuremeter tests were performed using an automated flexible wall calibration chamber testing system. Undrained model pressuremeter tests were performed with and without the internal pore-water pressure measuring piezometer needles at radial strain rates ranging from 1 to 8%/min in a controlled laboratory setup in beds of kaolin clay prepared with known stress histories. Reference strength and deformations properties were determined using one-dimensional consolidation, isotropically consolidated undrained compression (CIUC) and [K.sub.o] consolidated undrained compression (C[K.sub.o]UC) laboratory tests, and comparisons are made with the interpreted values from the pressuremeter test data. Interpretations of the undrained shear strength, initial shear modulus, shear stress-radial strain variation, limit pressure, and horizontal stress for each net pressuremeter curve were evaluated using various interpretation procedures, using the Marquardt-Levenberg algorithm. Based on the laboratory test data and the analytical interpretations, several conclusions were made regarding the effects of the rate of probe expansion on the deduced shear modulus, undrained shear strength, and limit pressure in the pressuremeter testing of clays.