______ condition develops in a saturated thick layer of loose fine sandy soil when disturbed by vibration from pile driving in an adjoining area or by the pressure of flowing water.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 15/11/2022 (Shift-I)
Detailed Explanation
In a cohesionless soil, strength comes only from effective stress: τ = σ′ tan φ. If something makes the pore water pressure rise until it equals the total stress, σ′ becomes zero and so does the strength.
Safety against quick condition (upward flow): \(\text{FOS} = \dfrac{i_c}{i}\), with \(i_c = \dfrac{G-1}{1+e} \approx 1\).
Why “Quick sand” Is Correct
Loose saturated fine sand tends to contract when vibrated (by pile driving, machines or earthquakes); the water cannot escape quickly, pore pressure rises and effective stress falls to zero. Upward-flowing water can do the same. The sand then behaves like a liquid — the quick sand condition.
Why the Other Options Are Wrong
- A. Unstable: ‘Unstable’ is too general; the specific condition is quick sand.
- B. Failure: ‘Failure’ is a general term; the question asks for the named condition.
- D. Collapse: ‘Collapse’ refers to sudden volume reduction of dry, loose (collapsible) soils on wetting.
Key Point
Quick condition: σ′ = 0 in cohesionless soil → strength = 0. Under earthquakes it is called liquefaction.
Key Concepts for Students
- Clays do not usually go quick because of their cohesion.
- Fine and medium sands are most prone; gravels drain too quickly.
- FOS > 1 safe; FOS = 1 quick; FOS < 1 unsafe.
Practice more: this is question 16 in Shear Strength of Soils MCQs – Part 2. Try the whole set and check every answer instantly.