ABSTRACT
In general, soil is a multi-phase medium with three separate phases of solid (soil skeleton), liquid (pore water), and gas (pore air). The behavior of such a system should then be modelled by considering the interaction of all the phases. This requires an analysis of the coupled flow (of the pore fluid) and deformation (of the solid skeleton). it has been common to treat the saturated porous medium (i.e. soil) as a combination of separate phases of fluid and solid resulting in a simpler decoupled analysis. Depending on the motion of the pore fluid and the solid skeleton as well as the permeability of the porous medium, it is possible to obtain different formulations in the coupled flow and deformation problem. Most of these formulations are obtained by neglecting some or all of the inertial terms while others refer to drained and undrained extremes in the porous medium.
In this chapter, the basic principles and derivations of various governing equations (with varying degrees of idealizations) are presented using the basic laws of conservation of momentum and conservation of mass. These, in turn, yield equilibrium and mass balance equations, respectively.
