Quicksand is such a state where the saturated sand loses the capacity to support eventually acquiring the character of a liquid. Quicksand is found at the mouths of large rivers or along the flat streams or beaches where the pools of water are partly filled with sand with an underlying layer of clay or any kind of dense material that prevents drainage. Quick Sand is a mixture of sand, mud, and vegetation.
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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.
How Quicksand Condition Occurs?
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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.
Quicksand Condition at Construction Site
The construction sites are very much susceptible to the quicksand type conditions:
The excavations in the granular materials behind the cofferdams alongside the rivers.
Any place where the artesian pressures exist i.e., where the head of water is greater than the usual static of the water pressure.
Behind the river embankments also in order to protect the floods.
How to Avoid Quicksand Condition?
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.
Quicksand is not a kind of sand, this is a hydraulic condition.
A sand soil becomes a ‘quicksand’ when this flow is upward under the hydraulic gradient that reduces the effective stress down to zero.
In a typical sand soil condition, the critical hydraulic gradient is of the ratio 1: 4.
The Quick Sand condition occurs mainly in fine sand conditions.
The High artesian pressure in coarse-type sand is one of the most important reasons for the development of the quicksand condition.