Effective stresses in unsaturated soils and their applicability in the theory of critical state soil mechanics

Cricital state soil mechanics, which expresses relationships between deformation and effective stresses, has not yet been established for unsaturated soils. Effective stresses in unsaturated soils are not only complicated functions of the applied stress and the pore-water pressure, but are also hysteretic. Furthermore, they cannot yet be determined independently, because present theories are inadequate, and they must be obtained by the less satisfying method of comparing the deformation of unsaturated and saturated soils subjected to similar stress paths. The paper discusses the further measurements that need to be made to validate the extension of critical state soil mechanics to unsaturated soils, and concludes that unless the effect of hysteresis proves to be small, the theory may be too difficult, if not impossible, to apply quantitatively.