Types of Shear Failure in foundation

Types of Shear Failure

1. General shear failure:

• It occurs in dense sand or stiff clay.
• A heave on the sides is always observed in general shear failure.
• The failure surfaces extend up to the ground level.
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2. Local Shear Failure 

•  It occurs in medium-dense sand or on in clay of medium consistency.
• Failure surface extend to the ground surface after considerable vertical movement.
• A heave is observed only when there is a substantial vertical settlement

Some characteristics of local shear failure are:

• 1. Failure is not sudden and there is no tilting of footing.
• 2. Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed
• 3. Failure surface is not well defined
• 4. Failure is progressive
• 5. In the load-settlement curve, there is no well-defined peak
• 6. Failure is characterized by considerable settlement directly beneath the foundation
• 7. Significant compression of soil below the footing and partial development of plastic equilibrium is observed.
• 8. Well-defined wedge and slip surface only beneath the foundation.

Local shear Failure

Important Points

• In General Shear Failure  A well – defined failure pattern is observed.

3. Punching shear failure:

• It occurs in loose sand or soft clay.
• No heave is observed and failure surface does not extend upto the ground level.
• Only vertical movement of footing.

Criteria for General Shear Failure and Local Shear Failure

• 1. For a cohesionless soil, if  Φ is >36°, a general shear failure is likely to occur and, if  fΦ< 29°, local shear failure occurs.

• 2. If failure strain is less than 5%, general shear failure will occur and local shear failure occurs at a failure strain of 10–20%.

• 3. It relative density is greater than 70%, general shear failure would occur and if it is less than 35% local shear occurs.

• 4. If N > 30, GSF occurs and if N < 5, LSF occurs.

• 5. If e < 0.55, GSF occurs. If e > 0.75, LSF occurs.

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