Fundamentals of unsaturated soil modelling: Have we got it right?

An unsaturated soil is not a special type of soil, rather a state of the soil. All soils can be unsaturated with water. There are many concepts specifically related to constitutive models for unsaturated soils, e.g., suction, net stress, apparent consolidation, loading-collapse yield surface, suction-increase yield surface, etc. These concepts are not always well understood and also seem to make unsaturated soils 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