Quicksand
Quicksand is a colloid consisting of fine granular material such as sand, silt or clay and water.
Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight.
Quicksand can form in standing water or in upward flowing water (as from an artesian spring). In the case of upward flowing water, forces oppose the force of gravity and suspend the soil particles.
The saturated sediment may appear quite solid until a sudden change in pressure or shock initiates liquefaction. This causes the sand to form a suspension and lose strength. The cushioning of water gives quicksand, and other liquefied sediments, a spongy, fluid-like texture. Objects in liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced soil/water mix and the submerged object floats due to its buoyancy.
Soil liquefaction occurs in (partially) saturated soil when it is shaken by an earthquake or similar forces. The movement combined with an increase in pore pressure (of groundwater) leads to the loss of particle cohesion, causing buildings or other objects on that surface to sink.
Quicksand condition occurs in upward flow when the critical hydraulic gradient comes in the vicinity of unity.
Properties
Quicksand is a shear thinning non-Newtonian fluid: when undisturbed, it often appears to be solid ("gel" form), but a less than 1% change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form).
After an initial disturbance—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to decrease suddenly.
Someone stepping on it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of 0.01/s would require the same amount of force as needed to lift a car.
It is impossible for a human to sink entirely into quicksand due to the higher density of the fluid. Quicksand has a density of about 2 grams per milliliter, whereas the density of the human body is only about 1 gram per milliliter. At that level of density, sinking beyond about waist height in quicksand is impossible.
Even objects with a higher density than quicksand will float on it if stationary. Aluminum, for example, has a density of about 2.7 grams per milliliter, but a piece of aluminum will float on top of quicksand until motion causes the sand to liquefy.
Continued or panicked movement, however, may cause a person to sink further in the quicksand. Since this increasingly impairs movement, it can lead to a situation where other factors such as weather exposure, dehydration, hypothermia, tides or predators may harm a trapped person.
Quicksand may be escaped by slow movement of the legs in order to increase viscosity of the fluid, and rotation of the body so as to float in the supine position (lying horizontally with the face and torso facing up).
Dry quicksand
Is loose sand whose bulk density is reduced by blowing air through it and which yields easily to weight or pressure. It acts similarly to normal quicksand, but it does not contain any water and does not operate on the same principle. Dry quicksand is an example of a granular material.
Grain entrapment, or grain engulfment,
Occurs when a person becomes submerged in grain and cannot get out without assistance. It most frequently occurs in grain bins and other storage facilities such as silos or grain elevators, or in grain transportation vehicles, but has also been known to occur around any large quantity of grain, even freestanding piles outdoors. Usually, unstable grain collapses suddenly, wholly or partially burying workers who may be within it. Entrapment occurs when victims are partially submerged but cannot remove themselves; engulfment occurs when they are completely buried within the grain. Engulfment has a very high fatality rate.
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While quicksand can occur in almost any location where water is present, there are certain locations where it's more prevalent. Places where quicksand is most likely to occur include:
Riverbanks
Beaches
Lake shorelines
Near underground springs
Marshes
The next time you're at the beach, notice the difference in the sand as you stand on different parts of the beach that have varying levels of moisture. If you stand on the driest part of the beach, the sand holds you up just fine. The friction between the sand particles creates a stable surface to stand on.
If you move closer to the water, you'll notice that the sand that is moderately wet is even more tightly packed than the dry sand. A moderate amount of water creates the capillary attraction that allows sand particles to clump together. This is what allows you to build sand castles.
But beach sand could easily become quicksand if enough water were thrust up through it. If an excessive amount of water flows through the sand, it forces the sand particles apart. This separation of particles causes the ground to loosen, and any mass on the sand will begin to sink through it. In the next section, you will find out how to save yourself if you happen to fall into a pit of quicksand.
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Questions:
- Do you think this is a good example of Quick sand? Give reason for your answer.
- How does quicksand form in saturated loose sand?
- Take a hand full of sand and estimate the grain size and colour.:
- Describe stream rock formation on your left.:
- Describe rock face across the stream.:
- Take picture at stream and upload when logging.