When quick sand condition occurs?

Quicksand condition is the floatation of particles of cohesionless soil, like fine gravel and sand, due to vertical upward seepage flow. As sand boiling occurs, the bearing capacity and shear strength of the cohesionless soil decrease and the agitations of soil particles become apparent.

Quicksand condition is not a type of soil but a flow condition that occurs in cohesionless soils. Practically, boiling condition may occur when excavations are made below the water table and water is pumped out from the excavation pit to keep the area free from water.

Quicksand condition occurs when seepage pressure, which acts in the upward direction, overcomes the downward direction pressure due to submerged weight of soil, and the sand grains are forced apart. The result is that the soil has no capability to support a load.

The soil that experiences quicksand condition would lose shear strength and bearing capacity. The shear strength of cohesionless soil depends on the effective stress. The shear strength is given by:

The effective stress is given by the following expression:

The terminologies of equation 2 are explained and illustrated in fig. 2. Plugging components of equation 2 results in the following expressions:

So, equation 5 may be expressed as follows:

Substituting the value of submerged unit weight in terms of void ratio:

Taking G=2.67, and e=0.67, the result of equation 9 is equal to one.

Thus, the effective stress becomes zero for the soil with above values of G and e, when the hydraulic gradient ‘i’ is unity, i.e. head causing the flow is equal to the length of the specimen.

If the critical gradient exceeds, the soil moves upward, and the soil surface appears to be boiling. The quick condition is also known as boiling condition. During this stage, a violent and visible agitation of particles occurs. The discharge suddenly increases due to an increase in the coefficient of permeability occurred in the process. If a weight is placed on the surface of the soil, it sinks down. The soil behaves as a liquid having no shear strength.

Quicksand cannot support the weight of man or animal and it behaves like a liquid with a unit weight about twice that of water. A person can easily float in it with about one-third of his body out of quicksand. However, quicksand is highly viscous and movement in it would require a great effort and energy. A person may die by drowning (suffocation) if he gets tired and let his head fall into the quicksand in panic.

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

When there is some surcharge on the cohesionless soils, the head required to cause quicksand increases.

There are a number of construction sites which are susceptible to quicksand conditions:

We will further know about this typical sand situation, the condition of quicksand, its occurrence, and other amazing facts about this sand.

Quicksand condition is for the floatation of particles of the cohesion less soil, like the fine gravel and the sand particles, this is due to the vertical upward seepage flow. Here the sand boiling occurs, and the bearing capacity and the shear strength of this type of soil decreases, and the agitations of the soil particles are then apparent.

Quicksand condition is actually not a type of soil rather this is a flow condition of the soil that occurs in cohesion less soils. In this situation, the boiling occurs when the excavations are made which is below the water table and at this time the water is pumped out from the excavation pit to keep the area around there free from water.

Quicksand condition occurs when there is a seepage pressure, this acts in the upward direction, while overcoming the downward direction pressure, this is due to the submerged weight of the soil, and here the sand grains are forced out apart. This results in the formation of quicksand which cannot support a load on this sand.

When the critical gradient exceeds, the soil here moves upward gradually, and then the soil surface appears to be boiling. This quick condition in the sand is also known as the boiling condition. At this stage, a violent and visible agitation of these particles occurs. The discharge then suddenly increases for the increase in the coefficient of the permeability that occurred in the process. If there is a weight being placed on the surface of the soil, it eventually sinks. The soil for this phenomenon behaves like a liquid that has no shear strength.

Quicksand fails to support the weight of a man or an animal and thus it behaves like a liquid that has a unit weight that is about twice that of the water. A person can easily float in this with about one-third of his body out of the quicksand. However, the quicksand is very viscous and movement in this would require a high amount of energy. A person might end up dying by drowning, getting suffocated if he gets tired and then let his head fall into this quicksand in a fit of panic.

This is to be kept in mind, if a person is caught in this kind of quicksand conditions, he should keep his head high above the soil surface and then move slowly towards the bank. The person should try to catch some tree on the bank and then try to pull himself out of it.

The construction sites are very much susceptible to the quicksand type conditions:

Quick Sand condition is prevented by lowering the water table at the site before excavation or by alternatively, increasing the length of the upward flow of water. The Boiling condition is quite common at the time when a previous sand stratum that underlies the clay soil is in an artesian pressure condition.

Quick sand condition occurs when the upward seepage pressure in soil becomes equal to submerged unit weight of the soil. This results into effective stress equal to zero.

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.

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, the value of i becomes unity.

If I becomes unity, then i=iC

i.e. h/L =1

or, h=L

This indicates that when quicksand 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.

Contrary to common belief, the soil does not suck the victim beneath its surface. As a matter of fact, quicksand behaves like a liquid with a unit weight twice that of water.

Also Read: Underpinning Methods, Procedure, Use in Foundation Strengthening and Repair

A person can easily float in it with about one-third of its body out of quicksand. But quicksand 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 into the quick sand.

If a person is caught in quicksand, 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 that occurs in the 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 the presence of cohesion, quick sand condition does not take place in clay soil.

In summary, the following points are noteworthy.

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.