Traffic loads combined with moisture variations are the most harmful for road structures. Even if the pavement is assumed to have an elastic behaviour, the partially saturated subsoil can present irreversible strains that endanger the integrity of the road structure. It is proposed to define a proper stress framework for modelling the mechanical behaviour of subgrade layers submitted to both moisture changes and mechanical loads. The major consequences of the application of the effective stress concept to unsaturated subsoils are investigated. Several experimental data sets are re-examined in the light of a generalized effective stress, intended to describing the mechanical behaviour of porous media. The critical state lines at different saturation states tend to converge remarkably toward a unique saturated line in the deviatoric stress versus mean effective stress plane. The effective stress interpretation is also applied to isotropic paths and compared with traditional net stress conception. The accent is finally laid on the key feature for constitutive frameworks based on a unified stress, namely the sufficiency of a unique mechanical yield surface.