  A thermodynamically consistent extension of the constitutive equations of
saturated soils to unsaturated conditions is often worked out through the use a
unique 'effective' interstitial pressure, accounting equivalently for the
pressures of the saturating fluids acting separately on the internal solid
walls of the pore network. The natural candidate for this effective
interstitial pressure is the space averaged interstitial pressure. In contrast
experimental observations have revealed that, at least, a pair of stress state
variables was needed for a suitable framework to describe
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 invasion of the soil by the liquid
water phase through the retention curve; two effective stresses, which are
required to describe the soil deformation at water saturation held constant.
However a simple assumption related to the plastic flow rule leads to the final
need of only a Bishop-like effective stress to formulate the stress-strain
constitutive equation describing the soil deformation, while the retention
properties still involve the suction and possibly the deformation. Commonly
accepted models for unsaturated soils, that is the Barcelona Basic Model and
any approach based on the use of an effective averaged interstitial pressure,
appear as special extreme cases of the thermodynamic formulation proposed here.
