For a very deep footing in loose sand, the type of soil failure will be:

Asked in SSC JE 05/06/2024 (Morning)

Detailed Explanation

A footing can fail in three ways, depending mainly on how dense (or stiff) the soil is and how deep the footing is:

General shear — dense sand, stiff clay.
Local shear — medium dense sand, medium clay.
Punching shear — loose sand, soft clay, and deep footings.

Why “punching shear failure” Is Correct

In loose sand the soil under the footing compresses a lot before it fails, so no continuous slip surface reaching the ground forms. For a deep footing the surrounding overburden also stops the soil from heaving. The footing simply sinks, shearing the soil vertically along its sides. This is punching shear failure.

The three modes of bearing capacity failure (Vesic). General shear: a continuous slip surface runs from the footing edge to the ground surface, which heaves; the load–settlement curve shows a clear peak. Local shear: slip surfaces form under the footing but do not reach the surface; heave is slight. Punching shear: the footing pushes straight down, shearing the soil vertically along its edges, with no heave. Typical relative density: above about 70%, about 35–70%, and below about 35% (some books quote 30%).

Why the Other Options Are Wrong

A. general shear failure: General shear needs dense soil and a shallow footing.
C. local shear failure: Local shear is typical of medium dense soil under shallow footings.
D. cracking shear failure: ‘Cracking shear failure’ is not one of the recognised failure modes.

Key Point

Loose soil + deep footing → punching shear. Dense soil + shallow footing → general shear.

Key Concepts for Students

Punching shear shows no clear peak load and no surface heave.
Terzaghi reduces c and tan φ to two-thirds for local shear.
Even dense sand can fail in punching if the footing is deep enough.

Practice more: this is question 7 in Shallow and Deep Foundation MCQs – Part 1. Try the whole set and check every answer instantly.