Research Progress of Effective Stress and Effective Stress Equation

In 1920’s and 1930’s, Terzaghi presented the concepts of effective stress and the effective stress equation based on tests and engineering experience, which were concluded into the principle of effective stress by Jennings in 1961: firstly, effective stress equals total stress minus pore water pressure; secondly, the effective stress controls the strength and deformation of a soil. The principle of effective stress is considered the keystone of soil mechanics. Only with the concept of effective stress can we grasp the strength and deformation nature of a soil, establish the theory of strength and constitutive relationship of a soil, and thereby develop the theoretical system of Soil mechanics. Terzaghi’s principle of effective stress can only be applied to saturated soils. The application of this principle has achieved great success in soil mechanics and geotechnical engineering. However, since the physical meaning of the effective stress is still unclear, and the derivation of the effective stress equation lacks theoretical basis, there has always been controversial arguments about this theory since it was presented. The controversy focuses on what the effective stress is; whether the effective stress equation needs to be modified; and if there is the effective stress controlling shear strength and deformation for unsaturated soils. There are still no consensus on these issues among scholars till now. There are some main ideas about the essential nature of effective stress, such as inter-granular stress, skeleton stress, and equivalent stress that controls the shear strength and deformation of a soil. Lade and Boer gathered some of the effective stress equations for saturated. For unsaturated soils, because the effective stress equation has not been verified, some researchers believe that the strength and deformation of unsaturated soil cannot be characterized by a single effective stress. They must be characterized by different independent stress state variables [1].