Mechanics of unsaturated soils: discussion of fundamental principles

An unsaturated soil is not a special type of soil, rather a state of the soil. All soils can be partially saturated with water. A constitutive model should therefore represent the soil behaviour over all possible values of pore pressures and stresses. There are many concepts related to constitutive models for unsaturated soils, e.g., suction, net stress, effective stress, apparent consolidation, loading-collapse yield surface, suction-increase yield surface, etc. These concepts seem to make unsaturated soil modelling very special. This paper gives a critical review of these concepts, in an attempt to clarify some common confusion in constitutive modelling of unsaturated soils