The mechanical behavior of a deformed, porous, saturated or partially saturated skeleton with fluid is important in many engineering applications (soil mechanics, oil industry, ...). In this article the porous, saturated and unsaturated medium is examined as a two-phase or three-phase mixture, respectively. We formulate the balance equations for each phase separately. Then, using the dissipated inequalities, written for a mixture as a totality, the nonlinear constitutive law is deduced. In the second part of the article we deduce Darcy's law and the consolidation equation by this medium.