  A poromechanical model of partially saturated deformable porous media is
proposed based on a phase field approach at modeling the behavior of the
mixture of liquid water and wet air, which saturates the pore space, the phase
field being the saturation (ratio). While the standard retention curve is
expected still to provide the intrinsic retention properties of the porous
skeleton, depending on the porous texture, an enhanced description of surface
tension between the wetting (liquid water) and the non-wetting (wet air) fluid,
occupying the pore space, is stated considering a regularization of the phase
field model based on an additional contribution to the overall free energy
depending on the saturation gradient. The aim is to provide a more refined
description of surface tension interactions.
  An enhanced constitutive relation for the capillary pressure is established
together with a suitable generalization of Darcy's law, in which the gradient
of the capillary pressure is replaced by the gradient of the so-called
generalized chemical potential, which also accounts for the \lq\lq
force\rq\rq\, associated to the local free energy of the phase field model. A
micro-scale heuristic interpretation of the novel constitutive law of capillary
pressure is proposed, in order to compare the envisaged model with that one
endowed with the concept of average interfacial area.
  The considered poromechanical model is formulated within the framework of
strain gradient theory in order to account for possible effects, at laboratory
scale, of the micro-scale hydro-mechanical couplings between highly-localized
flows (fingering) and localized deformations of the skeleton (fracturing).
