Modes of Shear Failure of Soil - General, Local, Punching Shear Failure

Modes of shear failure

There are three modes of shear failure, i.e. General, local and Punching shear failures depending upon the compressibility of soil and depth of footing with respect to its breadth (i.e D/B Ratio)

1. General Shear Failure

• Characterized by well defined failure pattern, consisting of a wedge and slip surface and bulging (heaving) of soil surface adjacent to the footing
• Sudden collapse occurs, accompanied by tilting of the footing
• Occurs in denser or stiff soil
• Failure load is well defined

2. Local shear failure

Failure pattern consists of wedge and slip surface but is well defined only under the footing. Slight bulging of soil surface occurs. Tilting of footing is not expected.

• Large settlement occurs
• Ultimate load is not well defined
• Occurs in soil of high compressibility

3. Punching shear failure

• Failure pattern is not well defined
• No bulging of ground surface, no tilting of footing
• Failure take place immediately below footing and surrounding soil remains relatively unaffected
• Large settlements-ultimate load is not well defined
• Occurs in soil of very high compressibility
• It also occurs in the soil of low compressibility, if the foundation is located at considerable depth.