The two criteria for determining the allowable bearing capacity of a foundation are:
Try it first: pick an option, then read the explanation.
Correct answer: A. Shear failure and settlement
Asked in SSC JE 02/03/2017 (Morning)
Detailed Explanation
A foundation can fail in two ways: the soil can break (shear failure, the footing plunges or tilts), or it can settle too much even without breaking, damaging the building.
Why “Shear failure and settlement” Is Correct
So the allowable bearing capacity is found from two checks:
- Shear failure criterion — net ultimate bearing capacity divided by a factor of safety.
- Settlement criterion — the pressure that causes the permissible settlement.
The lower of the two values governs.
| Term | Meaning |
|---|---|
| Gross ultimate bearing capacity, qu | Total pressure at the base of the footing that causes shear failure |
| Net ultimate bearing capacity, qnu | qu − γDf (pressure in excess of the overburden removed) |
| Net safe bearing capacity, qns | qnu ÷ factor of safety (usually 2.5 to 3) — safe against shear only |
| Safe bearing pressure (settlement) | Net pressure at which settlement just reaches the permissible value |
| Allowable bearing pressure, qa | Lower of the two: safe against shear failure AND settlement within limits |
Why the Other Options Are Wrong
- B. Bond failure and shear failure: Bond failure concerns reinforcement in concrete, not soil.
- C. Tensile failure and settlement: Soil does not fail in tension under foundations.
- D. Tensile failure and compression failure: Neither tension nor compression failure is a design criterion for soil under a footing.
Key Point
Allowable bearing capacity = min(shear criterion, settlement criterion).
Key Concepts for Students
- Narrow footings on sand: usually shear governs.
- Wide footings and rafts: usually settlement governs.
- Differential settlement often matters more than total settlement.
Practice more: this is question 37 in Shallow and Deep Foundation MCQs – Part 3. Try the whole set and check every answer instantly.