Abstract: The first section of the report presents failure shape of clay specimen and analyses of inducing sources by mohr-theory .A soft layer within the specimen can effect and control the strength and the failure shape of clay ;In the second section of the report the whole process of deformation of the clay specimen under external loads is analysed .The relationship between the deformation and strength is stated .The meaning of statical limit-equilibrium condition ,kinematical equilibrium condition and limit kinematical equilibrium is delineated .the differences under various equilibrium condition were discussed ;In the last section the use and the existing problems of the various failure criterias of soil are discussed .