Shear Strength of Soils and Failure Criteria

This chapter describes the shear strength characteristics of soils and related failure criteria. Soils fail either in tension or in shear; however, in the majority of soil mechanics problems, only failure in shear requires consideration. The shear strength along any plane is mobilized by cohesion and by the angle of internal friction, collectively referred to as shear strength parameters . If, at a point along a specified plane, the shear stress becomes equal to the peak shear strength along that plane, then the soil will fail at that point and large shear strains will result. A part of the soil on one side of this plane, called the failure plane, will slide relative to the other side, bringing about collapse of the soil structure. A failure criterion is a mathematical relationship between the state of peak stress and the shear strength parameters. When combined with the principles of solid mechanics, a failure criterion can be used in different types of stability problems to predict (collapse) loads that cause the failure. The post peak strength behaviour of the soil may be predicted by using the critical state models that are now common for specific types of soils.