Through massive examples of multilayer slopes with different intensities, slopes and thicknesses of soil layer, the factors of safety and the locations of critical failure surfaces obtained by the strength reduction method and limit equilibrium method are compared. The differences between two methods are investigated. Failure mechanism of multilayer slopes is studied. The result of numerical simulation shows:. If the shear strength of the lower layer is larger than the upper one, when difference in shear strength between two layers increases to a certain degree, soil in the upper breaks. Meanwhile, the relative difference of strength reduction method (SRM) and limit equilibrium method (LEM) is in maximum, about 5%-7%. When thickness of upper soil layer increases, the factor of safety gradually decreases; when the upper layer locates below surface, critical failure surface throughs the toe of slope; the factor of safety remains unchanged.. If the shear strength of the upper layer is larger than the lower, when difference in shear strength between two layers increases to a certain degree, the factors of safety obtained by SRM tend to be stable. As to LEM, the factors of safety maintains increase, the relative difference of is 12% in maximum. When thickness of upper soil layer is increased, the factor of safety increases correspondingly. When thickness of upper soil layer is in the range of 9-12 m, failure mode is deep layer slide, while h=12 m, critical failure surface obtained by LEM just throughs the toe of slope; but with a very small difference in the factor of safety. When thickness of upper soil layer is in the range of 3-5 m, the relative difference of two method is in maximum, about 6%-10%. The distribution of failure zone indicates that multilayer slope is caused by tension and shear failure. As to the slope which the shear strength of the upper layer is larger, this composite failure mechanism is obvious. In comparison with single shear failure, the relative difference is about 10% in maximum.
