Abstract: In this paper, the displacement, stress and stress level, which increase with loading, in saturated clay foundations are calculated under various conditions by using Duncan-Chang nonlinear elasticity model and the finite element method. Consequently, the natural rules of developing, spreading and joining of plastic zones （or shear failure zones） in soft clay foundations are studied. It is assumed that the clays are normally consolidated under overburden pressure and undrained during the increasing of superimposed load. At the end of this paper, results of a loading test on the site are analysed and the method of calculation presented in this paper has been verified.