  Constitutive equations of unsaturated soils are often derived in a
thermodynamically consistent framework through the use a unique 'effective'
interstitial pressure. This later is naturally chosen as the space averaged
interstitial pressure. However, experimental observations have revealed that
two stress state variables were needed to describe the stress-strain-strength
behaviour of unsaturated soils. The thermodynamics analysis presented here
shows that the most general approach to the behaviour of unsaturated soils
actually requires three stress state variables: the suction, which is required
to describe the retention properties of the soil and two effective stresses,
which are required to describe the soil deformation at water saturation held
constant. Actually, it is shown that a simple assumption related to internal
deformation leads to the need of a unique effective stress to formulate the
stress-strain constitutive equation describing the soil deformation. An
elastoplastic framework is then presented and it is finally shown that the
Barcelona Basic Model, a commonly accepted model for unsaturated soils, as well
as all models deriving from it, appear as special extreme cases of the
thermodynamic framework proposed here.
