| This dissertation examines the constitutive behavior of sand and describes the development of a constitutive model based on this examination. Emphasis is placed on the determination of conditions that lead to unstable strain softening response in undrained shearing of loose sand and modeling the constitutive behavior of sand exhibiting such response. The yield surface was identified an element of the model that significantly influences the predicted response of such materials and is therefore investigated in detail.; Based on the analysis of results of tests on very loose Ottawa sand and following some suggestions in previous literature, it was assumed that the stress ratio at the peak of the capped yield surface of loose sand can be approximated by the stress ratio Mp measured at the peak of the undrained effective stress path (UESP). Effects of various factors on the yielding behavior of loose sand were therefore investigated by studying the variation of Mp measured from the UESPs of different sands. Variations of the stress ratio Mp with factors such as density, mean normal stress, intermediate principal stress, direction of loading, anisotropic consolidation, and soil fabric were examined and formulated. These formulations were used to construct yield surfaces that were functions of the factors mentioned above. Model predictions using these yield surfaces showed that the effects of these factors on the behavior of sand can be accounted for using these yield surfaces. Formulation of the variation of Mp also made it possible to investigate quantitatively the effects of these factors on the susceptibility of loose sand to collapse and flow failure.; The critical state constitutive model that was developed in this study is capable of predicting the behavior of sand over a wide range of void ratios and confining pressures and subjected to different loading conditions. A unique set of model parameters including a unique ultimate (steady) state line is used in all model predictions. The significant differences in the response of sand to loading in different directions and modes of shearing are modeled using these unique parameters. The two types of directional properties resulting from stress-induced anisotropy and inherent anisotropy are characterized by two separate and independent parameters. The ways in which the predicted behavior is influenced by the two types of anisotropy are also different from each other. Formulation of the hardening rule is based on “state hardening” such that hardening of the material is related to the current “state” rather than the “strain” experienced by the soil from the start of shearing. |
