Abstract: On the basis of experiment resuls, a simplified geomaterial behavior model is given. By using elasto-plastic theory the analytical solution to the expanson of cavities is presented. The solution takes the strain-softening and shear dilation into consideration Through volume-balance equation. the limit expansion pressure and plastic zone are presented in this paper as well. The effect of softening and shear dilation on expansion are examined, It is shown that the results in this paper are in good agreement with other availabe data.