ABSTRACT
Using the theory of mixtures with interfaces (TMI) the thermodynamic basis for two independent stress state variables in unsaturated soils is investigated. Overall balance of linear momentum of all the bulk phases and the interfaces leads to a definition of total stress for unsaturated soils. Consideration of constitutive equations within the realm of the second law of thermodynamics provides an expression for effective stress in unsaturated soils. In addition to including total stress, pore air and water pressures, the expression for effective stress also includes, explicitly, the properties of the interfaces. The inclusion of these interfacial properties explains the experimentally observed dependence of effective stress on stress paths and soil types in unsaturated soils. It is argued that it is preferable to consider two independent stress state variables rather than a single, but complex, effective stress in describing the behavior of unsaturated soils.
