Allowable bearing capacity is defined as:
Try it first: pick an option, then read the explanation.
Asked in SSC JE (previous year paper)
Detailed Explanation
Several closely related terms are used for bearing capacity. They differ in whether they are gross or net, and in which failure they guard against.
Why “net loading intensity at which neither shear failure nor settlement failure occurs in the soil” Is Correct
The allowable bearing capacity (allowable bearing pressure) is the net loading intensity that is safe against both shear failure (with a factor of safety) and excessive settlement. It is the smaller of the net safe bearing capacity and the safe bearing pressure from settlement. That is option (d).
| 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
- A. net ultimate bearing capacity / FOS: q_nu/FOS is the net safe bearing capacity; it covers shear only, not settlement.
- B. net safe bearing capacity + γDf: Net safe + γD_f is the gross safe bearing capacity.
- C. net loading intensity at which the soil just fails in shear: The intensity at which soil just fails in shear is the net ultimate bearing capacity.
Key Point
Allowable = min(net safe bearing capacity, safe bearing pressure from settlement).
Key Concepts for Students
- On clay, settlement usually governs the allowable pressure.
- Permissible settlement: about 25 mm for isolated footings on sand (IS 1904).
- Wider footings on sand settle more at the same pressure.
Practice more: this is question 13 in Shallow and Deep Foundation MCQs – Part 1. Try the whole set and check every answer instantly.