Quick sand is
Correct Answer: C. Flow condition occuring in cohesion less soil
📚 Detailed Explanation: Quick Sand Condition
Quick sand is a widely misunderstood concept. It is not a special type of sand or material — it is a hydraulic condition that can occur in any saturated, cohesionless soil (typically fine sand or silt) when upward water flow exerts enough pressure to neutralise the effective stress between soil particles.
Why C is correct: Quick sand occurs when the upward seepage gradient equals the critical hydraulic gradient (ic = Gs − 1 / 1 + e, approximately 1.0 for most sands). At this point, effective stress becomes zero and the soil behaves like a liquid — it has no shear strength. This can only happen in cohesionless soils (sand, silt) where there is no cohesion to provide additional strength. Cohesive soils (clay) develop pore pressure but their cohesion prevents the quick condition from occurring in the same way.
Quick Sand Condition Summary
| Aspect | Detail |
|---|---|
| What is it? | A hydraulic flow condition, not a material type |
| Soil type affected | Cohesionless soils (fine sand, silt) |
| Trigger | Upward seepage gradient ≥ critical gradient |
| Critical gradient ic | ≈ (Gs − 1)/(1 + e) ≈ 1.0 for sand |
| Effect | Effective stress = 0; soil loses shear strength |
Key Concepts for Students
- Quick sand is a condition, not a material — any fine cohesionless soil can become “quick” if the hydraulic gradient is high enough.
- Prevention: reduce the hydraulic gradient (cut-off walls, dewatering) or increase the overburden stress.
- This question belongs to soil mechanics / geotechnical engineering, not pure concrete technology, but appears in some aggregates chapters as fine aggregate behaviour context.