The moment force model for the soil shearing failure analysis is introduced
and the shearing failure process is analyzed. Then the moment model is used to explain the formation of the shear band under-cut broken angle of clay. Groundwater is the key factor which does more harm to the foundation pit project. How to analyze and study the effect of groundwater on the project is a difficult and important subject. Based on the destruction effect of groundwater
the change of interface with the variety of groundwater is analyzed; and the equation to calculate critical angle of planar slide under saturation condition is deduced.