Investigation of fabric anisotropic effects on granular soil behavior

In the past few decades, within the framework of critical state soil mechanics (CSSM), understanding on soil behavior has become much more developed and extensive. The following are examples of current knowledge: soil behavior is both density and confining pressure dependent; the drainage condition has influence on the soil response; the loading path, characterized not only by the magnitude but also the direction of the load applied, is another influential factor affecting the mechanical behavior of soil. In practical applications, recent geotechnical problems, such as liquefaction analysis and bearing capacity of sandy grounds, have required a comprehensive understanding of the stress-strain-strength relation of sand. Substantial knowledge on the mechanical behavior of granular materials has been gained mostly from the results of well-controlled laboratory testing on homogeneous specimens subjected to uniform stress and strain states. Previous experimental investigations provide a fundamental understanding and give a clear picture of soils, when subjected to various loadings. It is known that the importance of the fabric anisotropy has long been recognized. However, previous studies on the effects of fabric anisotropy were mostly carried out in a qualitative manner. No extensive and unified studies have been made to quantify the fabric anisotropy of granular soils and thus its influence on mechanical behavior. In this dissertation, a systematic and comprehensive investigation is presented. Firstly, the anisotropic sand behavior under rotational shear is studied. Secondly, quantification of the fabric anisotropy of granular soils is made using the method of electrical property measurements, as well as the approach using an image analysis on the microstructures of thin sections. The inherent fabric anisotropies for both the dry deposited and moist tamped specimens are identified and further quantified. A series of the conventional laboratory tests are then performed under various loading conditions, using both the dry deposited and moist tamped specimens. The relations between the mechanical behavior and the fabric anisotropy are identified. A constitutive model accounting for the effects of different inherent fabric anisotropies is calibrated based on the experimental test results and observations. The laboratory tests are simulated using this model and a satisfactory performance of this model is obtained