In the field construction

the expansive soil cut slope is very stable

but during or after a rainfall

it is destroyed. After the destroyed part is dug

the slope is destroyed again when a rainfall comes again. It happens again and again according to the above course. So the paper gives a water expansive model of expansive soils. The model memorizes the max water content in the history under some certain stress

and it has stored much energy. When the stress is removed

the expansive soil is deformed only according to the stress-strain curve of the general soils. But when a rainfall comes

the model begins to show its nature of expanse. The paper gives the stress-strain curve and the elastic modulus formula

which is different to the one of the before elastoplastic hardening model

and the difference is obvious; and it is obtained after removing the strain-strain curve of the general elastoplastic hardening model to the left some distance.