What is Shear Strength of Soil ?

When a soil mass subjected to vertical stress and on any arbitrary plane in soil  mass normal and shear stress were developed which depends on the magnitude of vertical stress and orientation of arbitrary plane. If we go on increasing the value of vertical stress then the value of normal stress and shear stress also increases. Normal stress on the plane is always compressive and soil has good compressive strength but weak in shear and hence soil will fail in shear and the shear stress at which soil fail in shear is called ‘Shear strength of soil’ .

Strength strength of a soil is perhaps the most important of its engineering properties. This is because all stability analyses in the field of Geotechnical engineering, whether they relate to foundation, slopes of cuts or earth dams, involve a basic knowledge of this engineering property of the soil. ‘Shearing strength’ or merely ‘Shear strength’ may be defined as the resistance to shearing stresses and a consequent tendency for shear deformation. 

Test Used to Find the Shear Strength of Soil

Based on mode of application of shear force

• Stress controlled test
• Strain controlled test

Based on nature of soil samples

• shear test on undistributed samples
• Shear test on distributed samples

Based on drainage conditions

• Unconsolidated Undrained test ( UU Test )
• Consolidated Undrained test ( CU Test )
• Consolidated Drained test ( CD Test )

Based on Instruments used

• Unconfined compression test
• Direct shear test or Box shear test
• Triaxial compression test
• Vane shear test

Factors affecting shear strength of soil 

• Shape of particles :

The shearing strength of soil with angular particles having sharp edges is greater than that with rounded particle. Other parameters being identical.

• Gradation :

A well-graded sand exhibits greater shear strength than a uniform sand.

• Denseness :

The degree of interlocking increases with an increase in density. Consequently, the greater the denseness, the greater the shear strength of soil.

• Confining Pressure :

The shear strength of soil increases with an increase in confining pressure.

• Deviator Stress :

The angle of Φ decreases under very high stresses. As the maximum deviator stress is increased from 500 to 5000 kN/m², the value of  Φ decrease by about 10%. This is due to the crushing of particles.

• Loading :

The angle of shearing resistance of sand is independent of the rate of loading. The increase in the value of Φ from the slowest to the fastest possible rate of loading is only by about 1% to 2%.

• Type of Minerals :

If the sand contains mica, it will have a large void ratio and a lower value of Φ. However it makes no difference whether the sand is composed of quartz or feldspar material.

• Capillary Moisture :

The sand may have apparent cohesion due to capillary moisture. The apparent cohesion is destroyed as soon as sand becomes saturated.

Uses of Shear Strength Parameters 

• To compute load carrying capacity of pile/negative skin fraction in pile.
• For calculating earth pressure acting on retaining structures.
• Finding the safe bearing capacity of soil.
• To design supporting structures for open excavation.
• To check stability of slope (Infinite/Finite)