South Australia & NT Chapter

Mechanics of Unsaturated Soils

Professor Nasser Khalili


The effective stress principle is one of most fundamental contributions in the field of soil mechanics. It converts a multi–phase, multi–stress state porous medium to a mechanically equivalent, single-phase, single-stress state continuum, thus allowing the application of the principles of continuum solid mechanics to fluid-filled deformable porous media. Without the advent of the effective stress principle many of the significant achievements of solid mechanics such as elasto-plasticity, viscoplasticity, wave propagation and numerical methods could not be extended to soil engineering problems.

In this presentation, the application of the effective stress principle to unsaturated soils is critically reviewed, and the reasons underlying the difficulties in previous investigations of effective stress in unsaturated soils are highlighted. In particular, the validity of the relationship proposed by Khalili and Khabbaz (1998) for the determination of the effective stress parameter c 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 and unsaturated soils is also discussed, and the incremental form of the effective stress parameter is derived.

About Professor Khalili

Professor Khalili has extensive experience in the field of geotechnical engineering both as a consultant and as an academic/researcher for more than 25 years. Prior to joining The University of New South Wales in 1993, Professor Khalili was responsible for managing the geotechnical group in the Chicago Office of the consulting firm Dames & Moore. He is currently the President of the Australian Association for Computational Mechanics (AACM), a member of the Institution of Engineers Australia, and a member of Australian Geomechanics Society. He is also a core member of the International Technical Committee on Unsaturated Soil (TC106), representing the Australasian Region. Professor Khalili’s research interests lie primarily in the areas of mechanics of unsaturated soils, soil plasticity, and mechanics of multi phase multi porous media.

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