General, local and punching shear failure

Terzaghi and Vesic described three ways in which a soil fails below a footing. They differ in the shape of the failure surface and in the load–settlement curve.

 General shear                 Local shear                   Punching shear

      [== footing ==]              [== footing ==]               [== footing ==]
 heave  /\   ||   /\  heave   slight  /\  ||  /\  slight        |     ||     |
 ______/  \  ||  /  \______   ______/ \  || / \______  heave    |     ||     |  no heave
        \  \ || /  /                  \  \||/  /                |     ||     |
         `--.\/.--'                    `-.\/.-'                 |_____||_____|
 failure surface reaches        failure zone fades out         vertical shear only
 the ground surface             in the soil

Ascertaining the mode of failure

• From relative density (sand): $D_r > 70\%$ gives general shear, $D_r < 20\%$ gives punching shear, and in between local shear occurs. From the SPT: $N > 30$ (dense) gives general shear and $N < 10$ (loose) gives punching or local shear.
• From stress–strain behaviour (clay): use an unconfined or triaxial test. A well-defined peak at small strain (below about 5%) means general shear. A strain at failure of 20% or more, with no clear peak, indicates local or punching shear.
• From the plate load test: a defined breaking point in the load–settlement curve means general shear.
• If the soil is likely to fail in local shear, Terzaghi's design reduces the strength: $c' = \tfrac{2}{3}c$ and $\tan\phi' = \tfrac{2}{3}\tan\phi$, and the modified factors $N_c', N_q', N_\gamma'$ are used.