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One-third or more of the earth’s surface is situated in arid or semi-arid regions where the potential evaporation exceeds the precipitation and soils exist in their unsaturated state. Unsaturated soils are also abundant in most parts of the world where there is seasonal groundwater table fluctuation. The variation in the degree of saturation gives rise to a gamut of variability in soil’s hydromechanical behavior. The co-existence of pore-air and water in the void spaces and their interaction with each other and the solid particles are the main reasons why such variability exists and why unsaturated soils are more complex than saturated or dry ones. A robust understanding of the hydromechanical properties of unsaturated soils is crucial for geotechnical engineers worldwide, as well as for those concerned with the interaction of structures with the ground. Some engineering problems associated with unsaturated soils include precipitation-induced shallow-depth landslides, settlement of soil in the vadose zone, drainage of roadway materials, and borehole stability. Proper understanding of unsaturated behavior requires considerations that go beyond those available for saturated soils. In pursuit of addressing this requirement, a plethora of research has been devoted to measuring, modeling, predicting, and interpreting unsaturated soil behavior. Many theories for characterizing the mechanical response, methodologies for laboratory testing, as well as equipment to determine the constitutive parameters have been developed. Instruments to study in-situ behavior of unsaturated soils have also been promoted and used in a few cases. The outcomes of past research include: the development and critical evaluation of various forms of the effective stress principle and its fundamental role in determining strength and deformation properties of unsaturated soils; identification of independent state variables; development of failure envelopes and yield surfaces; formulation of macromechanical and micromechanical constitutive relationships; and formulation of suction-induced stress as a component of the intergranular stress tensor. Despite the volume of work dedicated to the field of unsaturated soil mechanics, compared to two-phase (i.e., saturated) soils, relatively few advancements have been made in the development of characterization frameworks for unsaturated soils. In this doctoral research, a novel, 14-parameter, state-dependent bounding surface plasticity model that simulates the behavior of unsaturated granular soils is developed. In the development of this model, a critical state compatible hyperelastic formulation for saturated granular soils is selected as a base model and is enhanced and extended to predict unsaturated granular soil behavior. Accounting for deformation phenomena in unsaturated soils, the elastoplastic response has no purely elastic component. The hyperelasticity and assumption of no purely elastic deformation sets this model apart from existing ones. To handle the inherent hydro-mechanical coupling in unsaturated soils, a newer generation stress framework, consisting of the Bishop-type effective stress with a second stress variable, is used in conjunction with a soil-water characteristic curve function. Available unsaturated soil data for sands and silty sands were used to calibrate, validate, and assess the performance of the new model. Additional laboratory data, consisting of a suite of consolidated drained triaxial shear tests, were generated. The shear strength and volumetric behavior of a native mid-Atlantic transitional silty sand were investigated under varying values of matric suction, confining pressure, strain rate, and fines content. The experimental results are used to validate the predictive capabilities of the new bounding surface plasticity constitutive model for unsaturated granular soils. It is shown that with a set of parameter values, the model realistically simulates the main features that characterize the shear and volumetric behavior of unsaturated granular soils over a wide range of matric suction, density, and net confining pressure. In the literature, it was observed that multiple analytical expressions exist for effective stress, critical void ratio, and soil water characteristic curve. To see the effects that variations in these different analytical forms have on the model simulations of unsaturated soil behavior, a parametric investigation is performed using the constitutive model developed and the aforementioned in-house generated laboratory data. It is observed that, depending on the desired prediction accuracy, a variety of functions (with varying numbers of model parameters) could be implemented as part of the constitutive model.