SHEAR STRENGTH OF SOIL

Presentation byShreedhar S

Shear force is the force applied along or parallel to surface or cross section, instead of being applied perpendicular to the cross section.

Shear stress is the force divided by the area on which it acts.

Shear strength- the capacity to with stand the maximum shear stress with out occurrence of failure is the shear strength of the material.

Failure of soil rarely occurs due to tension, as if the soil is pulled in the both directions, particles will separate and it will immediately collapse, Hence the tensile load can hardly be applied to soil.

Due to compression, the particles becomes more and more closely packed, so that density of soil and strength increases, thus compression failure of soil is also rare.

Now remaining is shear failure, it is most common mode of failure for any soil, Hence in foundation design, in earth dams, in retaining structures shear strength is important.

Shear failure of soil

When there is a load on soil, shear stress with different intensities will develop along different planes.

A pair of planes perpendicular to each other will be the planes with maximum shear.

If this maximum applied shear stress is more than the shear strength, failure occur along one of the two planes of maximum shear.

An entire wedge of soil slides over another when shear failure of soil occurs.

Field situation of shear failure

Upstream side of earthen dam, especially during sudden draw down.

Earth behind retaining wall. Especially ‘surcharge’

Under foundation along planes of maximum shear.

Subgrade of road.

Concept of shear strength as an Engineering properties of soil

In the most of the problems in soil mechanics such as concerning the foundation of structure, the soil mass has to with stand shearing stresses, which are unlike in nature then compressive stresses.

Shearing stresses tends to displace part of the soil mass relative to rest of soil mass.

Shear strength of soil is the capacity of soil to resist shearing stress.

It can be defined as the maximum value of shear stress that can be mobilized with in a soil mass.

If this value of shear stress exceed shear strength the failure will occur in the soil because of movement of portion of the soil mass along that plane or surface.

The soil is then said to have failed in shear

The shear strength depends up on 1 type of soil 2 water content3 cohesion 4 internal friction 5 compaction

Cohesion The force of attraction between soil

particles is called as cohesion. The particles of soil at contact points

have bond of attraction with each other, the sum total of these small forces of attraction will cause cohesion of soil.

In some soil this force of cohesion is more prominent, these are called as cohesive soil (clay)

In some soils, the force of cohesion is negligible such soil is called as cohesionless soil(sand)

Internal friction When the soil particles move against one

another a force of friction comes to play in the opposite direction of forces.

These small forces of friction resisting the motion of particles against one another addup to give a force, which resist the movement of soil. This force is called as Internal friction of soil.

The soil having microscoping particles often have negligible internal friction ( clay)

Some soil have maximum shear resistant due to internal friction ( sand)

Principal planes In general a soil mass is subjected to

three dimensional stress system. However in many soil engineering

problems, the stress in third direction are not relevant, hence the stress system is simplified as two dimensional.

At every point of stressed body, there are three planes on which shear stresses are zero, these planes are called as principal planes.

The planes with maximum compressive stress(1) is called as major principal plane.

The plane with minimum compressive stress(3) is called as minor principal plane.