Stress State Variables for Unsaturated Soils

by Delwyn G. Fredlund, (M.ASCE), Prof.; Dept. of Civ. Engrg., Univ. of Saskatchewan, Saskatoon, Saskatchewan, Canada,
Norbert R. Morgenstern, (M.ASCE), Prof.; Dept. of Civ. Engrg., Univ. of Alberta, Edmonton, Alberta, Canada,

Serial Information: Journal of the Geotechnical Engineering Division, 1977, Vol. 103, Issue 5, Pg. 447-466

Document Type: Journal Paper


An unsaturated soil is visualized as a four-phase system, the fourth phase being the air-water interface commonly referred to as the contractile skin. Suitable stress state variables for an unsaturated soil are proposed on the basis of writing force equilibrium equations for each phase, within the context of multiphase continuum mechanics. The analysis indicates that any two of three possible normal stress variables can be used to define the stress state. Experimental null-type tests verified the proposed stress state variables for the soil structure and contractile skin of an unsaturated soil.

Subject Headings: Soil stress | Unsaturated soils

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