When soil is loaded shearing stresses are induced in it. When the shearing reach a limiting value shear deformation takes place, leading to the failure of the soil mass. The failure may be in the form of sinking, of a forting or movement of an edge of soil behind a retaining wall forcing it to move out or the slide in an earth embankment.
The shear strength of a soil can redefined as the resistance to deformation to continuous shear displacement of soil particles or on masses upon the action of a shear stress.
It can also be defined as the resisting shear stress a soil offers along the failure plan at the time of failure.
All stability analysis in soil mechanics involves a basic knowledge of the shearing properties and shearing resistance of the soil. The method used to determine the shear characteristics in the laboratory must be understood in detail to permit an intelligent application of laboratory results to likely conditions.
The shearing resistance of a soil is constituted basically of the following components.
- The structural resistance to displacement of the soil because of the interlocking of the particles.
- The frictional resistance to thin locations between the individual soil particles at their contact points.
- Cohesion or adhesion between the surfaces of the soil particles.
The shear strength in cohesion less soil results from inter granular friction alone while in all other soils it results both from internal friction as well as cohesion. However, plastic clay does not possess internal friction.
Effect of Structural Resistance on Shearing Strength of Soil
Shear strength of soils varies because of structural arrangement. Arrangement of a soil is not only a function of grain size or soil type hut also a function of manner in which load is applied. Same soil will exhibit remarkable different shear strengths at different void ratio or at different rate of loading. In a simple way, it may be stated that shear strength is a function of effective area.
Effect of Friction on Shearing Strength of Soil
Internal friction between the soil particles resist the shearing of soil mass. Internal friction is the important property of coarse grained soils in sand. Fine-grained soils do not exhibit any friction.
As per these two properties, fine-grained soils are also called as friction less cohesive soils and coarse-grained soils as cohesion less friction soils. Strength of cohesion less soils comes mostly from inter granular friction alone and cohesive soils give shearing resistance due to cohesive force alone while in other soils it comes from both cohesion and internal friction.
Effect of Cohesion on Shearing Strength of Soil
It is force of attraction between the particles binding them together. Cohesion is the important property of fine grained soils like clay. Coarse grained soils do no exhibit any cohesion.