How quicksand works

Thursday, January 10, 2008

The old movies depicting people walking through the forest or desert and disappearing into pools of quicksand were a cause of many nightmares in young and old alike. Quicksand seems to evoke in us a curious fear that might only be quelled by some understanding of what exactly it is and how it works.

Part of the fear lies in the fact that our ground is not always stable where we place our feet. Imagine, taking for granted the solidity of the Earth’s crust and then all of a sudden falling into a pool of goo. Well, that’s exactly what quicksand is – in a figurative sense.

In fact, quicksand has an illusion of being solid ground, but when it’s disturbed in some way it becomes unstable and “collapses” under pressure.

Quicksand is a mixture of water with sand, the water usually coming from some type of underground source. Contrary to the old movies that showed people sinking in a desert, quicksand is rarely found where there is a lack of water. It’s usually close to areas like beaches, ponds, swamps, marshes – anywhere where there is a convergence of land and water. The ground must be saturated with liquid to a point where it can no longer support weight.
Normally, sand grains are packed tightly and feel more like solid ground. The friction between the grains is large enough to give the sand solidity. This is because the force of weight pushing down on the sand is equal to the force of sand pushing up. That’s why we can stand on regular dry sand without being swallowed up. We can even stand on relatively wet sand because it becomes stronger with a bit of water added to it since the water acts as a force which pulls grains of sand closer to each other. But if you’ve ever stood at the shore and seen your feet disappear under the sand as waves came in, you can understand that the seemingly solid structure of sand when “liquefied” becomes unstable and disintegrates. Quicksand is a form of sand that becomes supersaturated with liquid, thereby forming a type of “suspension”. At this point, there is little friction holding the grains of sand together and hence it is almost impossible for it to support any weight on its surface. Besides water moving upward and disturbing the friction levels, sand can also "quicken" when there is vibration like an earthquake or other type of movement in the earth.
When the water pressure is that high and the friction is that low, the liquid starts to flow between the grains of sand and they can no longer hold weight. Anything of substance will inevitably sink on quicksand. Because of the amount of liquid in the sand, it can form a partial vacuum. If you’ve ever played with wet sand or mud, you can understand the feeling of having your foot or hand “sucked” deeper in as you moved it to pull it out. This is because the air is unable to replace your hand or foot when it is removed, and not because the quicksand is swallowing you up. Another interesting point is something picked up at a Scout meeting many years ago. A human body has a specific gravity that is denser than water and so, should you fall into quicksand, your body will float provided you aren’t panicking and thrashing about wildly (which can further upset the friction levels.) So take this sage advice from the Scouts and float on your back until your rescuers arrive. Since quicksand is usually only a few feet deep, chances are you’ll live to tell all about your adventure.

Author: Written by Michele Natarelli