The method employed to determine the plastic critical load and plastic zone is based on Mohr-Coulomb criterion and a hypothesis that the coefficient of the lateral earth pressure is unit along inclination. It is found that the settlement obtained by this method is larger than the actual situation. Considering change of coefficient of lateral earth pressure along inclination

two methods are constructed to determine the plastic critical load and plastic zone. One is based on Mohr-Coulomb criterion and the other is based on three generalized Mises criterions. The computational results show that even the normal stresses are reasonably cut down the plastic zone from the first proposed method is larger than that from the present employed method for ignoring the intermediate principle stress. The shapes of the plastic zone obtain from three different criterions adopted in proposed method are similar. The plastic critical load

gain from the generalized Mises criterion whose circle is circumscribe to the exterior angle of Mohr-Coulomb’s scalene hexagon

is almost equal to that got by the present employed method

but the corresponding plastic zone is smaller.