Jörg Renner, professor at the Ruhr-University Bochum, and Holger Steeb, member of the SimTech board of directors and professor at the Institute of Mechanics, have published a joint review article dealing with the topic of poroelasticity. Both develop the model based on the (linearized) mixture theory in great detail and discuss the role of the state variable.
The abstract of the article can be found here
"We review fundamental aspects of linear poro-elasticity. In contrast to most available text-books and review articles, our treatment of poro-elastic media is based on the continuum Mixture Theory. Kinematic state variables and dynamic variables are introduced and formally linearized before the fundamental constitutive relations, between pairs of these, are extensively discussed. The role of porosity in linear poro-elasticity is highlighted, and it is shown that porosity is one of the possible choices for one of the two kinematic state variables, and therefore, relations to alternative pairs of kinematic variables can be formulated. The treatment is concluded by the formulation of the governing set of partial differential equations that constitute the basis for analytical or numerical investigations of boundary value problems."
The article was published as an "Open Access" article and can be read and downloaded for example here.