Normal Stress and Shear Stress
Any stress can be resolved into two co-planar components. One component acting normal to a given plane and the other is tangential to the plane.
The component of stress which is normal to a given plane is called normal stress.
The component of stress which is tangential to a given plane and contained within the plane is called shear stress or tangential stress.
Principal Planes and Principal Stresses
Through a particular point in a soil mass or any other stress medium, can pass hundreds of planes. Different quantum and combination of normal and shear stress may act on such planes.
The planes on which only normal stresses act and shear stresses remain absent are known as principal planes. The normal stresses acting on these planes are called principal stresses.
It can be shown that there exist three principal planes mutually orthogonal to each other. In the order decreasing the magnitude of the normal stresses, these planes are called the major, the intermediate and the min principal planes and the corresponding normal stresses on them are called major principal stress (σ1), intermediate principal stress (σ2), and the minor principal stress(σ3).
The intermediate principal stress (σ2) has very little influence on the shear strength of soil mass and hence neglected.