Quicksand is a colloid hydrogel consisting of fine granular matter (such as sand or silt), clay, and salt water.
Water circulation underground can focus in an area with the optimal mixture of fine sands and other materials such as clay. The water moves up and then down slowly in a convection-like manner throughout a column of sand, and the sand remains a generally solid mass. The water lubricates the sand particles and renders them unable to support significant weight. Since water does not usually go up to the surface of the sand, the sand on top appears solid and can support leaves and other small debris, making quicksand difficult to distinguish from the surrounding environment.
Quicksand is a non-Newtonian fluid: when undisturbed, it often appears to be solid (“gel” form), but a minor (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 increase 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 .01 m/s would require the same amount of force as “that needed to lift a medium-sized car.”
Because of the higher density of the quicksand, it would be impossible for a human or animal to completely sink in the quicksand, though natural hazards present around the quicksand would lead people to believe that quicksand is dangerous. In actuality the quicksand is harmless on its own, but because it greatly impedes human locomotion, the quicksand would allow harsher elements like solar radiation, dehydration, or tides to harm a trapped person.
If you ever find yourself in a pit of quicksand, don’t worry — it’s not going to swallow you whole, and it’s not as hard to escape from as you might think.
The human body has a density of 62.4 pounds per cubic foot (1 g/cm3) and is able to float on water. Quicksand is denser than water — it has a density of about 125 pounds per cubic foot (2 g/cm3) — which means you can float more easily on quicksand than on water. The key is to not panic. Most people who drown in quicksand, or any liquid for that matter, are usually those who panic and begin flailing their arms and legs.
It may be possible to drown in quicksand if you were to fall in over your head and couldn’t get your head back above the surface, although it’s rare for quicksand to be that deep. Most likely, if you fall in, you will float to the surface. However, thesand-to-water ratio of quicksand can vary, causing some quicksand to be less buoyant
Links and Sources: Wikipedia, HowStuffsWork