Black cotton soil is unsuitable for foundations because of its:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2016
Detailed Explanation
Black cotton soil is a dark expansive clay, rich in the clay mineral montmorillonite, found widely in central and western India. It forms from basalt (Deccan trap) and is good for growing cotton — hence the name.
Why “swelling and shrinkage” Is Correct
Montmorillonite absorbs water between its layers. So black cotton soil swells a lot when wet and shrinks, with wide deep cracks, when dry. These volume changes push foundations up and let them drop, cracking walls and floors. This seasonal movement, not simply its strength, is what makes it unsuitable.
| Remedy for black cotton soil | Idea |
|---|---|
| Under-reamed piles | Anchor the building below the active zone |
| Remove and replace | Replace shallow BC soil with non-expansive fill |
| CNS layer | Cohesive non-swelling soil cushion under the footing |
| Lime stabilisation | Lime reduces plasticity and swelling |
Why the Other Options Are Wrong
- A. black colour: Colour has no engineering significance.
- B. low bearing capacity: Its bearing capacity is modest, but the main problem is volume change.
- D. cohesive particles: Many good foundation soils are cohesive; cohesion itself is not the problem.
Key Point
Black cotton soil = expansive clay (montmorillonite) → swell–shrink movements.
Key Concepts for Students
- Free swell index is used to grade expansiveness.
- Under-reamed piles (IS 2911 Part 3) are the standard foundation in BC soil.
- Keep water away from foundations (aprons, drainage) on expansive soils.
Practice more: this is question 34 in Shallow and Deep Foundation MCQs – Part 3. Try the whole set and check every answer instantly.