Which of the following types of foundation is suitable for cohesive soil, and distributes the load over a wider area compared to other foundation types?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 06/06/2024 (Morning)
Detailed Explanation
A raft or mat foundation is one thick reinforced concrete slab (often stiffened with beams) under the entire building, carrying all the columns and walls together.
Why “Raft foundation” Is Correct
Because it covers the whole plan area, a raft spreads the load over the largest possible area, so the pressure on the soil is low. It also ties all the columns together and reduces differential settlement. It is used on weak or compressible (cohesive) soil, or when individual footings would cover more than about half of the plan area.
| 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. Pier foundation: A pier carries a concentrated load down to a deeper stratum; it does not spread load widely.
- B. Strip foundation: A strip footing spreads load only along a wall or a row of columns.
- D. Pile foundation: Piles transfer load to depth, not over a wide area at the surface.
Key Point
Raft = one slab under the whole building; lowest bearing pressure, least differential settlement.
Key Concepts for Students
- Raft is chosen when footings would cover more than about 50% of the plan area.
- A floating (compensated) raft removes soil whose weight nearly equals the building's weight.
- Rafts can bridge over local soft pockets.
Practice more: this is question 4 in Shallow and Deep Foundation MCQs – Part 1. Try the whole set and check every answer instantly.