  In unsaturated soil mechanics, the quest for an effective stress playing the
same role as Terzaghi's effective stress does for saturated soils has
introduced a long standing debate, dating back to the 1960s. Several
contributions have been proposed since the early work of Bishop. It is well
recognized to date that a single constitutive stress is not sufficient by
itself to catch the main features of the behaviour of unsaturated soils and it
is often combined with matric suction. In this paper, focus is given to a
largely used formulation for such a constitutive stress, based on the use of an
averaged pore pressure. In particular, this paper discusses on thermodynamics
bases the validity of the choice of the factor X weighting the fluid pressures
contribution to the constitutive stress. This factor is usually assumed to be
equal to the degree of saturation of water. In this work it is shown that the
choice of this natural candidate implies restrictive assumptions on the plastic
flow rule. As shown from experimental data obtained from a literature review,
this choice may not be pertinent for certain classes of materials, particularly
high plasticity clays.
