Effective Stress in Unsaturated Soils: Review with New Evidence

The application of the effective stress principle to unsaturated soils is critically reviewed, and the reasons underlying the difficulties in previous investigations of the effective stress in unsaturated soils are highlighted. The validity of the relationship proposed by Khalili and Khabbaz in 1998 for the determination of the effective stress parameter, χ, is examined using an extensive array of experimental data. It is shown that quantitative predictions of shear strength and volume change in unsaturated soils can be made using the effective stress concept. The uniqueness of the critical state line in the deviatoric stress–effective mean stress plane for saturated as well as unsaturated soils is investigated, and the incremental form of the effective stress parameter is derived.