Many years ago, when I had been to my ancestral village as a kid, the people there excitedly showed us around places, telling us the history behind each and every place-from the temple to the fort. Near one of the cannons located on the edge of a hillock was a big sign that read: “DANGER! QUICKSAND AHEAD”. The guide accompanying us saw my eyes moving in that direction and started describing how dangerous that place was. Apparently a monster that had been living inside for ages, pulls in any animal or people going that way and eats them.
Lots of myths surround the very idea of a quicksand. Many movies also showcase quicksand, where the villain slowly sinks and dies. What is this quicksand? Why does it pull things in? Can people sink in a quicksand and die?
Firstly how does quicksand come into existence? A quicksand is composed of water, clay, salt and of course, as the name suggests lots of sand. You would have seen a house made of cards. A quicksand also has a similar structure, with sand for cards, water in the gaps and clay gluing the cards. When water saturates a region with loose sand and this sand gets agitated (due to flowing underground water or due to earthquakes, wherein shaking ground increases the pressure in the ground water, which then liquefies the sand) the water gets trapped between the agitated sand and liquefies the sand. That’s why one finds quicksand mainly near the river beds and marshy places.
What happens when you disturb a house of card? It collapses of course. That’s exactly what happens to a quicksand into which, say, the hero of our movie falls. So what happens next? Nope….he is not going to sink and die (heroes don’t die in our Indian films). In a series of events, rapidly following the collapse of the structure called the liquefaction of the quicksand, the water and sand separate out into different layers. And the clay particles come together and get packed nicely due to the salts. The densely packed (nearly double of what it was in the undisturbed quicksand) wet sand and the clay is what traps the hero in. Would he sink further? No he can’t, because the sand below and around him is a lot denser than his body weight. So he is just trapped in and can rest assured that he won’t die by sinking.
How do we get our hero out? Definitely not by throwing a rope to him and pulling him out. That would be nearly impossible. The researchers who have studied quicksand behavior have estimated that a force equivalent to lifting a mid sized family car is what is required to pull just a leg out of the quicksand. So if you try pulling him out with that force you will get our hero back…..but minus his limbs.
The only way out is to rebuild the house-of-cards structure again around the trapped hero. All that needs to be done is to reintroduce the water (now in a separate layer) into the sand and the clay and decrease their viscosity. It is easier said than done. This again needs lots of force estimated around to introduce water at a speed of 1 cm s-1, say, a pressure of 106 pascals (Pa) is needed (Ref 2). But persistently making small circles around his trapped body parts, our hero can help rebuild the house of cards and then someone can pull him out.
Now you know for sure that, as shown in the movies, people cant die in quicksand by sinking in. Nor are there any quicksand monsters (by the way I had brushed off that story even as a kid ).
References:
1. http://science.howstuffworks.com/quicksand1.htm (How Quicksand Works )
2. Khaldoun, Bonn et al., Nature 437; pp 635
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