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In recent years many researchers have attempted to address the need for a comprehensive understanding of the three-dimensional mechanical behavior of frictional materials. Advances in testing methods have added the capability of studying various aspects of generalized stress–strain behavior in a controlled environment. Strain-controlled true triaxial undrained tests on normally consolidated kaolin clay are performed in this study using a fully automated flexible boundary experimental setup with a real-time feedback control system. The influence of the intermediate principal stress and principal stress rotation on the stress–strain–strength and pore pressure behavior is investigated considering the occurrence of strain localization within the specimen. The strength behavior observed in this study of kaolin clay is used to evaluate existing failure criteria for cohesive soil. Comparative laboratory tests, such as the lubricated end triaxial test on a solid cylindrical kaolin specimen and combined axial–torsional tests on a hollow cylindrical kaolin specimen, were also performed to evaluate the corresponding mechanical behavior in different loading systems. Despite using identical techniques for specimen preparation and a similar consolidation stress state, the soil behavior obtained from the three types of tests showed observable variations, demonstrating the importance of specimen shape and loading–boundary conditions.Key words: normally consolidated clay, stress-strain behavior, pore pressure, anisotropy, testing methods.

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GPR often encounters difficulty in visualizing the buried target when signals are weak and enveloped by noise, despite using the best of existing GPR data analysis tools. In this study, new method has been proposed based on modified S-transform to analyze weak signals of GPR data. The time-frequency analysis has been implemented to capture the change in electromagnetic waves reflected at the boundary of dielectric contrast. A series of experimental tests were conducted at different site conditions and with different materials using GSSI SIR 3000 GPR system with 400 MHz antenna. An improved 2-D vertical subsurface profile has been developed using time-frequency information to locate the target spatially as well as temporally. The new method has provided better results to improve the target visualization in case of noisy signals. However, it did not add any significant where conventional processing could do reasonable. The proposed method is convenient to use and capable of processing large number of scans effectively.

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This study explores the effect of stress state on small-strain modulus (Gmax) in the naturally occurring Ganga sand and silt using bender elements during a series of carefully performed undrained triaxial tests. Unlike previous studies with drained shearing or testing of dry samples, the undrained shearing is chosen here to decouple the effects of void ratio and stress state on the small-strain stiffness. The results from undrained shearing of anisotropically and isotropically consolidated medium dense sand specimens are compared by keeping the same relative density and matching the initial mean effective pressure. The measured Gmax values show stress path dependency – that is, it is a function of both mean effective stress and deviatoric stress. This behaviour of Gmax is captured well by a non-linear isotropic model following the energy conservation principle, which incorporates the effect of both mean effective stress and deviatoric stress. The stress–strain response of soil provides some interesting observations, such as the strain hardening occurs mostly at constant effective stress ratio and the stress ratio at phase transition varies with the effective confinement. Experiments with varying silt content in the sand indicate that higher fines content results in slightly lower Gmax values, mainly due to the lower values of mean effective stress and deviatoric stress during undrained shearing.