The effective stress principle

The behaviour of soil at the macroscopic scale depends on the behaviour of its components and on the interactions between them at the microscopic scale. Here, assumptions are made only at the microscopic scale and, by use of a suitable averaging procedure, consequences at the macroscopic scale are derived. At the microscopic scale, the soil is assumed to be made of bulk phases and interfaces, both having very general constitutive relations. At the microscopic scale, the incompressibility of all bulk phases, incorporated by use of a thermodynamic theory of constrained materials, implies that a given thermokinetic process experienced by the soil does not depend on an arbitrary and uniform variation of the spherical part of the stress tensor field at all points of the soil. At the macroscopic scale, that thermokinetic process appears not to be influenced by certain variations of the macroscopic stresses and exchanges of momentum of the bulk phases. An extension to the case that one bulk phase is compressible is also presented. Finally, versions of the effective stress principle commonly used in soil mechanics for saturated and unsaturated soils are derived as particular cases.