A raft foundation is provided if the area required for footings exceeds the plan area of the building by:
Try it first: pick an option, then read the explanation.
Correct answer: D. 50%
Asked in SSC JE (previous year paper)
Detailed Explanation
As loads rise or soil gets weaker, individual footings get bigger. At some point they almost touch each other, and it becomes simpler and cheaper to cast one slab under the whole building.
Why “50%” Is Correct
The common rule is: if the total area of the footings needed would be more than about 50% of the plan area of the building, provide a raft (mat) instead. The raft also reduces differential settlement and bridges over soft spots.
| Foundation | When it is used |
|---|---|
| Isolated (spread) footing | One column on reasonably good soil |
| Strip (continuous) footing | A wall, or several columns in one row |
| Combined footing | Two or more columns on one footing (rectangular or trapezoidal), e.g. a column near the property line |
| Strap (cantilever) footing | Two footings joined by a strap beam to balance an eccentric outer column |
| Raft (mat) foundation | Footings would cover more than about 50% of the plan area; weak soil; differential settlement must be limited |
| Pile / well / caisson | Deep foundations: good bearing stratum lies deep, or loads are very heavy |
Why the Other Options Are Wrong
- A. 10%: At 10% coverage, isolated footings are clearly more economical.
- B. 20%: At 20% coverage, isolated footings still work well.
- C. 30%: At 30% coverage, separate footings are still usually economical.
Key Point
Footings > 50% of plan area → raft.
Key Concepts for Students
- Rafts are designed as inverted flat slabs or beam-and-slab floors.
- A raft on clay can be compensated (floating) to limit settlement.
- Piled rafts combine a raft with piles to reduce settlement.
Practice more: this is question 28 in Shallow and Deep Foundation MCQs – Part 2. Try the whole set and check every answer instantly.