Shear strength of a soil is the maximum resistance to shearing stress at failure on the failure plane. Shear strength is composed of:

1. Internal friction which is the resistance due to friction between individual particles at their contact points as well as due to the interlocking of particles. This interlocking strength is indicated by parameter θ (angle of internal friction).
2. Cohesion which is the resistance due to the inter-particle force which tends to hold the particles together in a soil mass. The indicative parameter is called Cohesion intercept (C).

Coulomb has represented the shear strength of soil by the equation: S = C + σ * tan θ

Where, S = Shear strength of soil = Shear stress at failure, C = Cohesion intercept, σ = Total normal stress on the failure plane, θ = Angle of internal friction or shearing resistance.

The graphical representation of the above equation gives a straight line called the Failure envelope. The parameters c and θ are not constant for a given type of soil but depend on its degree of saturation, drainage conditions and the condition of laboratory testing.

In the direct shear test, the sample is sheared along the horizontal plane. This indicates that the failure plane is horizontal. The normal stress on this plane is the external vertical load divided by the corrected area of the soil sample. The shear stress at failure is the external lateral load divided by the corrected area of the soil sample.

The purpose of direct shear test is to get the ultimate shear resistance, peak shear resistance, cohesion, angle of shearing resistance and stress-strain characteristics of the soils. Shear parameters are used in the design of earthen dams and embankments. These are used in calculating the bearing capacity of soil-foundation systems. These parameters help in estimating the earth pressures behind the retaining walls. The values of these parameters are also used in checking the stability to natural slopes, cuts andfills.