The unsaturated soils upon wetting may swell or collapse depending the soil types or soil’s void ratio. The particle structure of the unsaturated soil is considered to be in sub-stable and super stable states depending on its void ratio. Both the meta-stable and super stable states are maintained by the suction. The sub-stable and super stable structure becomes unstable upon wetting because of the suction reduction. There may exist a stable state that neither swelling nor collapsing will happen upon wetting. Employing the concept of stable state void ratio

a general formula is obtained which can describe both the swelling and collapsing behavior of the unsaturated soil.