Based on the constitutive framework of unsaturated soils established by the continuum porous medium soil mechanics, an elastoplastic model for unsaturated soils in triaxial stress state is formulated.

It has long been recognised that the latter two parameters increase substantially whenever the state of stress is triaxial compression (confined concrete).

The volumetric strain induced by shear stress is shrinking at first, then dilating later under triaxial compression test conditions, of which the value is determined by density and stress state.