Soils consist of individual particles that can slide and roll over to each other easily. Shear strength value of a soil is almost equal to the maximum value of shear stress applied within the soil mass which can be easily mobilized within a soil mass without failure occurs.

The shear strength of a soil is main function or parameter of which the stresses are generally applied, to it as well as it has been observed that the manner in which these stresses are applied is very much common. The general concept it can be notified that the shear strength of soils is essential to determine the bearing capacity of any type of foundations, the lateral or side pressure which is exerted on retaining walls, and the stability of slopes of structures.

Mohr Circle of Stresses
In soil testing on laboratory, a cylindricalsamples are maximum time used in which there are radial and axial stresses act on the principal planes. The vertical planes is generally the minor principal plane whereas the horizontal plane is considered here as major principal plane. The radial stress is denoted(sr) is the minor principal stress is denoted(s3), and the axial stress is denoted by(sa) is the major principal stress denoted(s1).

Now to visualise the normal and shear stresses value which is acting on any plane within the soil sample, a graphical representation of different stresses called the Mohr circle is obtained by plotting the principal stresses value. The sign convention in the construction is to consider in case of compressive stresses as positive value and angles of more circles are measured from counter-clockwise direction with positive value.

Now in the necessary drawing a line inclined at angle θ with the horizontal portion through the pole of the Mohr circle so as to intersect the circle properly. The coordinates of the point of intersection are basically normal and shear stresses acting on the plane, which is inclined at angle θ within the soil sample.

Normal stress

Shear stress

The plane inclined at an angle of 450 with horizontal also acting on it the maximum shear stress value which is  equal to the  and the normal stress on this plane is equal to the  .
The  inclined plane with the maximum ratio of shear stress  value to normal stress value is inclined at an angle of  to the horizontal plane, where a is denoted the slope of the line tangent to the Mohr circles directly and which is passing through the origin of the mohr circles.