Note on effective pressure

When a pore pressure Pp and a confining pressure Pc are applied to a porous solid, various physical properties are affected. When dealing with these properties, the concept of effective pressure is often used. On the basis of the examples of pore volume and of bulk volumetric strain for several elastic solids, we emphasize that (1) there is no unique law of effective pressure, (2) effective pressure may not be a very useful concept, unless it has the simple form Pe = Pc − Pp, and (3) when a property does not vary linearly with pore or confining pressure, effective pressure does not in general have any simple analytic expression, unless again it reduces to Pe = Pc − Pp.