Shear Strength of Soil

Shear Strength of Soil

 Definition of Shear Strength of Soil

Shear strength of soil is the ability of soil to resist sliding or failure along a plane when subjected to shear stresses.

It represents the maximum shear stress that soil can sustain before failure occurs.

Shear strength is a fundamental parameter in geotechnical engineering, governing the stability of foundations, slopes, retaining walls, embankments, and earth dams.

 

 Mechanism of Shear Failure in Soil

Shear failure occurs when the applied shear stress exceeds the soil’s shear resistance, causing soil particles to slide along a failure surface.

Common Types of Shear Failure

        General Shear Failure

Occurs in dense sand and stiff clay; sudden failure with well-defined slip surfaces.

       Local Shear Failure

Occurs in medium-dense soils; gradual failure.

      Punching Shear Failure

Occurs in loose sand and soft clay; footing penetrates the soil without clear failure planes.

 Importance of Studying Shear Strength

The study of shear strength is essential because it is directly used in

·        Design of shallow and deep foundations

·        Slope stability analysis

·        Design of retaining walls

·        Bearing capacity calculations

·        Stability assessment of earth dams and embankments

·        ⚠ Underestimating shear strength may lead to structural collapse, while overestimation can cause unsafe designs.

 Factors Affecting Shear Strength of Soil

Shear strength depends on several factors, including

·        Soil type (clay, sand, gravel)

·        Soil density and compaction

·        Water content

·        Degree of saturation

·        Effective normal stress

·        Particle size and shape

·        Soil structure and fabric

        Stress history (normally consolidated or over consolidated soil)

 Shear Strength Parameters of Soil

According to the Mohr–Coulomb failure criterion, shear strength depends on two parameters:

 Represents bonding between soil particles

Cohesion (c)

·        Represents bonding between soil particles

·        Significant in clayey soils

·        Independent of normal stress

 Angle of Internal Friction (φ)

·        Represents resistance due to friction and interlocking

·        Dominant in sandy and gravelly soils

Mohr–Coulomb Equation

τ=c+σtan φ=c+σtanφ

Where:

·        τ= shear stress

·        c= cohesion

·        σ = effective normal stress

·        φ= angle of internal friction

 

 Laboratory Tests to Determine Shear Strength 

  Direct Shear Test

·        Simple and quick laboratory test

·        Failure plane is predetermined

·        Widely used in teaching laboratories

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 Triaxial Compression Test

·        Most accurate shear strength test

·        Allows control of drainage conditions

·        Suitable for advanced design applications

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 Unconfined Compression Test

·        Used for saturated clay soils

·        Provides undrained shear strength

·        No lateral confinement applied

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 Conclusion

Shear strength of soil is a key parameter in geotechnical engineering design. Understanding shear failure mechanisms, influencing factors, testing methods, and calculation procedures ensures safe, economical, and reliable engineering structures.

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