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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. Quicksand condition reaches when critical hydraulic gradient is approximately equal to unity. iv. Quicksand condition occurs more commonly in coarse grained sand or gravels than fine sand.

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. Quicksand condition reaches when critical hydraulic gradient is approximately equal to unity. iv. Quicksand condition occurs more commonly in coarse grained sand or gravels than fine sand. Correct Answer Both statements i and iii are true

Quick Sand is a type of Condition, it is not a type of Sand.

Explanation:

  • When water flows through the soil, it exerts drag forces called seepage forces on the individual grains of the soil.
  • The presence of seepage forces, which act in the direction of flow, will cause a change in the pore water pressure and effective stress in the soil.
  • In upward flow condition effective stress at a depth Z from the top, effective stress = Zγ' - iZγw. Thus, in this case, effective stress is reduced.

In upward flow condition, we can have a head difference for which the effective stress becomes zero,

i.e. Hγ' - iHγw = 0

⇒ i = γ'/γw = icr

on putting value of γ', icr = (G -1)/(1 + e)

icr is called the critical hydraulic gradient

When water flows in a soil mass, a seepage force acts on soil particles in the direction of flow of water. If this flow is in upward direction, then seepage force will act in upward direction, due to there is reduction in effective stress of soil. As the upward flow of water progress continuously in soil, a time comes, where effective stress in soil becomes zero, this called quick condition.

At the critical hydraulic gradient, a soil losses all its shearing strength and can not support any load. The soil becomes "quick" or 'alive' and boiling occurs. The popular name for this phenomenon is quick condition.

For fine sand and silt for which specific gravity is approximate 2.65 and void ratio (e) 0.65 then,

icr = (2.65 - 1)/(1 + 0.65) = 1

when the value of critical hydraulic gradient becomes unity then that condition is known as the boiling condition.

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