This dissertation is comprised of six chapters. In the first chapter the motivation of this research, which was modeling the deformation behavior and strength characteristics of soils under internal erosion, is briefly explained. In the second chapter a micromechanis-based stress-strain model developed for prediction of sand-silt mixtures behavior is presented. The components of the micromechanics-based model are described and undrained behavior of six different types of sand-silt mixtures is predicted for several samples with different fines contents. The need for a more comprehensive compression model for sand-silt mixtures is identified at the end of this chapter. This desired compression model should be able to explicitly consider the fines content of the mixture and incorporates particle crushing effects as well. In the third chapter a new hypothesis of active and inactive void ratios in granular material and its application in modeling compressibility is examined and a compression model for sands is proposed. In the fourth chapter the concept of inactive void ratio is extended to sand-silt mixtures and a new model is developed for compression of these mixtures that can explicitly consider the fines content in its formulation. After the validity of the new hypothesis of active and inactive voids in granular material is verified in chapters 3 and 4, the model is further developed in chapter 5 to incorporate the effects of particle crushing on the compressibility of granular material. The sixth chapter is conclusion of this work and recommendations for future investigations.