  The principle of virtual power is used to derive the equilibrium field
equations of a porous solid saturated with a fluid, including second
density-gradient effects; the intention is the elucidation and extension of the
effective stress principle of Terzaghi and Fillunger. In the context of a first
density-gradient theory for a saturated solid we interpret the porewater
pressure as a Lagrange multiplier in the expression for the deformation energy,
assuring that the saturation constraint is verified. We prove that this
saturation pressure is distributed among the constituents according their
respective volume fraction (Delesse law) only if they are both true
density-preserving.
