Which of the following statements are true about quicksand? i. It is only a condition but not a type of sand. ii. It is a condition and a type of sand too. iii. Quick sand condition is reached when the critical hydraulic gradient is approximately equal to unity. iv. Quicksand condition occurs more commonly in coarse grained sand or gravels than in fine sand.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 29/10/2020 (Morning)
Detailed Explanation
Quicksand is the state in which upward seepage makes the effective stress in a cohesionless soil zero, so the soil has no strength and cannot support weight.
\[ i_c = \frac{G-1}{1+e} \approx 1 \]
Why “Both statements i and iii are true” Is Correct
i true / ii false: any cohesionless soil can become ‘quick’ under the right flow; there is no special kind of sand called quicksand.
iii true: for typical G ≈ 2.65 and e ≈ 0.65, ic ≈ 1.
iv false: in coarse sands and gravels the high permeability means very large flows are needed to build up the gradient, so quick conditions are much more common in fine sands and silts.
Why the Other Options Are Wrong
- A. Statements i, iii and iv are true: Statement iv is false: quick conditions are more common in fine sand.
- B. Statements ii, iii and iv are true: Statements ii and iv are false.
- C. Both statements ii and iii are true: Statement ii is false: quicksand is not a type of sand.
Key Point
Quicksand = condition (σ′ = 0) in fine cohesionless soil when i ≥ ic ≈ 1.
Key Concepts for Students
- Prevent it by lowering the upward gradient: longer seepage paths, filters, relief wells or surcharge.
- FOS against boiling = ic / i_exit.
- Clays do not go quick because cohesion remains.
Practice more: this is question 24 in Shear Strength of Soils MCQs – Part 2. Try the whole set and check every answer instantly.