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Granular materials are typically used as fills in earth structures, including foundation, embankment, and slope. The majority of these granular materials are built and maintained in unsaturated conditions. It is still an open question to develop simple and suitable models to capture the behaviors of unsaturated granular materials. This study aimed to formulate a model for describing the behaviors of granular materials under unsaturated conditions. An exponential function based on a phenomenological model was used to reproduce the asymptotic relationship between stress ratio and deviatoric strain during shearing, and a quantitative relationship considering variable void ratios was adopted to describe the changes of degree of saturation during shearing. In this new nonlinear model, nonlinear stiffness, nonlinear stress dilatancy, and critical state were introduced to enhance the model. Compared with the model for saturated soils, the new model for unsaturated soils could capture the suction effect on the behavior of stiffness and critical state. Finally, the new nonlinear model was used to simulate the behavior of saturated and unsaturated Chongqing granular fill and Beijing sand under triaxial stress conditions. The comparisons of experimental and simulated results showed that this model can consider the suction effect in reproducing the stress–strain and volumetric behavior of unsaturated granular materials. [ABSTRACT FROM AUTHOR]