Shear strength is a soil property which is its ability to resist movement between grains along a plane. When there is shearing along a plane in a soil mass such as a slope failure or movement of backfill behind a retaining wall, stress is developed to resist particle-to-particle sliding along the failure plane. This stress is referred as soil shear strength. When the stress is lower than the maximum value, this stress is known as mobilised shear strength. As this stress increases with denser particle packing, a maximum value is achieved at some point before the stress starts to decrease. The maximum stress attained is referred as the shear strength at failure. Since these stresses are produced when there is particle movement within a soil mass, they must have a strong influence on the soil mechanical behaviour. In other words, shear strengths must be the controlling factor for slope failures, settlement, bearing capacity and movement of retaining walls. Normally, the term shear strength is used to refer to the shear strength at failure. Therefore, shear strength can be defined as the maximum shear stress that a soil can mobilise along a plane to resist movement along the plane. The SI unit for shear strength is kN/m2.