The bearing capacity and deformation behavior of geostructures are highly dependent on the mechanical response of a thin layer of soil in contact with a structure that is technically called an interface. This paper presents a constitutive model for the mechanical behavior of sand-structure interfaces. To this goal, the critical state compatible bounding surface framework of Manzari & Dafalias is modified to capture the fundamental state dependent aspects of sand-structure interface behavior. The model has relatively few parameters which have clear physical meanings. The model predictions are compared with experimental results for various interface types, normal stress levels, and stiffness boundary conditions. Using a unique set of model parameters, it is shown that the model is capable of providing reasonable predictions for stress-displacements and normal displacement-tangential displacement behaviors of sand-structure interfaces.