ABSTRACT
This chapter illustrates the effective stress. It demonstrates the interparticle stress generated by the external forces other than pore water pressure. Soil skeleton stress is a measurement for the internal force of the soil skeleton. The internal force of the soil skeleton induced by external loading is real. The concept of a stress state variable is broadly applicable in solving shear strength problems and establishing constitutive models, while effective stress is the specific stress variable that controls the strength and deformation of soil. The effective stress is determined by the equilibrium equation of the soil skeleton; so are the stress state variable. Saturated and unsaturated soils have a uniform effective stress equation. The equation for saturated soils is the special case of that for unsaturated soils. Fredlund has made great contributions in the investigation of unsaturated soil mechanics.