Average Soil Skeleton Stress for Unsaturated Soils and Discussion on Effective Stress

The principle of effective stress is the most fundamental and important principle in soil mechanics. It serves as the foundation of modern soil mechanics. This paper starts with a discussion of effective stress expressions for unsaturated soils. The relationship between the total stress and stress of each phase in unsaturated soils is developed based on three-phase equilibrium equations of a representative element volume (REV). The expression of average soil skeleton stress, defined as effective stress, for unsaturated soils is then derived. This expression is consistent with the effective stress equation in unsaturated soils derived based on deformation work in the energy conservation equation. The general philosophy for the definition of effective stress in unsaturated soils is pointed out, followed by the discussion of some fundamental problems of unsaturated soils, including effective stress, choice of stress state, capillary phenomenon, and so on. The limitations and application scope of the effective stress in unsaturated soils are also addressed. (C) 2015 American Society of Civil Engineers.