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Quick sand condition- Equation, Physical model, Mechanism and Prevention

Quick sand condition or boiling quick sand condition is a condition occurs in a sand, when,  the effective stress is reduced due to upward flow of water. When the upward flow is increased, a stage is reached at which effective stress is reduced to zero. The condition so developed is known as quick sand condition.

How Quicksand Condition Occurs?

Figure 1. shows the physical model. The water flows from left tank to the right tank such that the flow through the soil in the right tank is in the upward direction.

Physical Model

The total stress at the bottom of soil sample is,

σ= ϒ sat x L

The upward pressure at the bottom of the soil sample = Water pressure from the left tank.

u= ϒw  x (h+L)

Effective stress, σ’ = σ- u

                                σ’  = ϒ sat x L – ϒw x (h+L)

                                  σ’= ϒ sat X L – ϒw x h – ϒw xL

                                  σ’= ϒ sub x L – ϒw x h

For quick condition,

Where, i is called hydraulic gradient.

Taking the specific gravity of soil as 2.65 and void ratio as 0.65, value of i becomes unity.

If I becomes unity, then i=iC

i.e. h/L =1

or, h=L

This indicates that when quick sand condition is achieved the head causing flow equals to the thickness of length of the specimen.

The shear strength of a cohesionless soi is given by:

τ= σ’ X tan ϕ

Where,        τ= Shear Strength

  σ’ = Effective stress

              ϕ= Angle of internal friction

When the effective normal stress      σ’= o,  τ =  0 .

If shear strength of a soil is zero, it behaves as liquid.

 In such situation the soil is said to be in quick condition.

If the critical hydraulic condition is exceeded, the soil particles move upwards, and the soil surface appears to be boiling.

During this stage, a violent and visible agitation of particles takes place. The discharge suddenly increases due to an increase in the coefficient of permeability in the process. If a weight is placed in the surface of the soil, it sinks down.

When a natural soil becomes quick, it cannot support the weight of a man or animal.

In contrary to common belief, the soil does not suck the victim beneath its surface. As a matter of fact, quick sand behaves as a liquid with a unit weight twice that of water.

Quicksand Condition in Daily life

A person can easily float in it with about one-third of its body out of quick sand. But quick sand is highly viscous; movement in it would require a great effort and energy.

 A person may die by drowning (suffocation) if he gets tired and its head sinks in to the quick sand.

If a person is caught in quick sand, he should keep his head above the soil surface and move slowly towards the bank. He should try to catch some tree on the bank and try to pull himself out of quick sand,

It is to be emphasized that quick sand is not a special type of sand. it is a condition which occurs in sand when the effective stress is zero.

 Any cohesionless soil can become quick when the upward seepage force is large enough to carry the soil particles upward. Due to presence of cohesion quick sand condition do not take place in clay soil.

Special condition of Quick sand

In summary, the following points are noteworthy.

  • Quick sand is not a special type of sand.
  • A cohesionless soil becomes quick when effective stress becomes zero.
  • For quick sand condition to take place prevailing hydraulic gradient should be about unity.
  • The permeability of the granular soil should be high enough for quick sand condition to take place.
  • A cohesive soil do not become quick even effective normal stress becomes zero because clay possesses some shear strength due to cohesion.

How to Avoid Quicksand Condition?

Quick sand condition can be  prevented by lowering the water table at the site before excavation or alternatively, that is a process of  increasing the length of upward flow.

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