It is defined as:
“The maximum resisting force applied by the soil particles against the deformation due to the relative sliding of the soil particles immediately prior to the failure of the soil mass is called as the shear strength of soil mass”
Soil is a special type of material which is composed of discrete particles which are allowed to move freely with respect to one another. The mineral skeleton of the soil seems to be deformable due to inter particle rearrangement. When we gradually apply a compression load to a soil mass it will not work due to the relative movement of the soil particle with respect to one another. This occurs along a surface known as shear surface.
So the structural strength of the soil is very important. It is the function of its shear strength. The ability of the soil mass to bear the structural load is due to shear strength.