A shallow foundation is defined as a foundation which:
Try it first: pick an option, then read the explanation.
Correct answer: B. has a depth of embedment less than its width
Asked in SSC JE (CWC & MES) 2011
Detailed Explanation
Foundations are classed as shallow or deep by geometry, not by strength or settlement. The comparison is between the depth of embedment Df and the width B.
Why “has a depth of embedment less than its width” Is Correct
By Terzaghi's definition a foundation is shallow when its depth of embedment is less than or equal to its width (Df ≤ B). Spread, strip, combined, strap and raft foundations are usually shallow. Option (b) states this definition.
| 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. has low bearing capacity: A shallow foundation on rock can have very high bearing capacity; capacity does not define it.
- C. is resting on the ground surface: Shallow foundations are usually placed some depth below the surface, not on it.
- D. causes less settlement: Settlement depends on soil and load; it does not define the type.
Key Point
Shallow: D_f ≤ B; deep: D_f > B.
Key Concepts for Students
- Minimum depth of shallow foundations is usually about 0.5 m to 1 m below ground.
- Rankine's formula gives a minimum depth based on φ and bearing pressure.
- Deep foundations carry load by end bearing and skin friction.
Practice more: this is question 44 in Shallow and Deep Foundation MCQs – Part 3. Try the whole set and check every answer instantly.