To accomplish a proper conception and further modeling of the mechanical behavior of soils, observations at the micro and macro scales need to be merged. The authors believe that the link between scales can be achieved using the effective stress concept. In this paper, we present a model that quantifies the air and water volumes contained within the pores of a solid when their pore pressure is varied. The macroscopic consequences of this are expressed in terms of a single stress that is used to formulate a simple elastoplastic constitutive model to predict volume strains and shear strength of soils.