Failure of soil slope is a developing process from local soil failure to overall failure of slope. And the extension of plastic zone and the change of lateral displacement increment of soil slope exhibit the process perfectly. Based on the actual failure of slope and numerical simulation of finite element method

the abruptness of lateral displacements increment at characteristic location and connectivity of plastic zone are suggested to be the instability criterion of slope. The failure behavior and stability of soil slope is analyzed by using the criterion and large deformation finite element method based on shear strength reduction. Moreover

the failure behavior and the stability of clay slope with weak layer are analyzed and the numerical results of large deformation finite element method are compared with that of small deformation finite element method. The results show that the failure behavior of clay slope with weak layer varied with the soil property of weak layer. And the safety factors of slope stability are coincident relatively

but the changing points of slope failure mechanism varying with soil property of weak layer are different.