How does Quick Sand Work?

george · « **on:** 07/10/2006 23:18:23 »

A film I saw recently contained the usual cliched quicksand situation where someone runs into a watery grave.

But what is quicksand, do you really drown or sink in it, and what's the best way to get out?

George

another_someone · « **Reply #1 on:** 08/10/2006 01:28:00 »

I am guessing a little here, but I will make the following experimental analogy, and judge that it has some relevance.

If you have a large tray of small coated lead beads (the coating is to protect the soft surface of the lead), and large rigid blocks of expanded foam, such that the lead beads, while being very small, are nonetheless heavier than the very large blocks of foam.

There is sufficient number of lead beads to easily cover the rigid foam blocks.

If you then agitate the mixture, the small lead, rather than falling to the bottom, will actually rise to the top (i.e. the lighter foam blocks will fall into the sea of lead beads). The reason for this is that because of the small size of the lead beads, they are more mobile. The beads and the foam blocks will both be moving about, but it is easier for the smaller lead beads to find their way past the large foam blocks than it is for the opposite to happen.

In terms of quicksand, there are two things you want to do:

Minimise the amount of agitation – move slowly and smoothly.
Lay flat on the surface to trap as much of the quicksand beneath you to prevent it from find a path around you.

George

gecko · « **Reply #2 on:** 08/10/2006 04:38:16 »

the cool part is it does work just like the movies. unfortunately, the deepest quicksand ever documented was only a few feet deep. it can trap car tires though, the suction is so strong.

daveshorts · « **Reply #3 on:** 11/10/2006 22:56:01 »

The story I heard about quicksand was that it was a type of fluidised bed, you get some forum of upward flowing water through sand that pushes the particles apart alowing them to flow around, acting like a liquid. This wouldn't be too dangerous on it's own as you would just be floating in sand water mix, and you should be able to swim out agian. The problem comes if the water stops flowing, then you are buried up to your chest in wet sand and would have to dig yourself out.

This has happened a few times in a river estuary where I grew up, you sometimes get layers of sand on a thin layer of waterproof clay, and then sand on the top. As the tide comes in, water can build up in the lower sand layer that wants to escape. If youwhen walking across break the mud, this water will well up, liquidising the sand, causing you to rapidly sink in. The sand then goes back to normal, as the tide races in around you... if you are lucky someone digs you out...

chris · « **Reply #4 on:** 13/10/2006 10:21:17 »

I was lucky enough to interview a guy called Daniel Bonn last year who published a paper in Nature on how quicksand works.

He told me that he was wandering around in the Iranian desert, as you do, and saw a sign saying "Danger: Quicksand" (presumably in local language!). Locals told him that camels routinely disappeared in the wet season, together with anyone who disagreed with the local regime. So he decided to take a look.

Taking some samples back to his lab in Europe he set about re-creating his own "homemade" quicksand to work out exactly how it works. It turns out that quicksand consists of sand, silt and salty water and the particles assemble like a house of cards suspended in the water.

But when you disturb the structure, by stepping on it, the house of cards collapses. All of the particles rapidly sink around the object that has fallen into the quicksand, and the water rises. This traps the inserted object in a very dense layer of sand and silt which is difficult to move.

So how do you get out? Well definitely not by being pulled free, because Daniel Bonn's measurements show that the force needed to extract you would be the same as that needed to pull the average family car. Instead, he suggests, you need to rebuild the house of cards by moving the trapped body parts in small circles to gently re-suspend the sunken particles.

And now for the really good news? The density of quicksand, he found, at 2g/cubic cm, is twice that of the average human (1g/cubic cm). So it's actually impossible to drown - you should just sink to about waist height! Sorry Hollywood.

"I never forget a face, but in your case I'll make an exception"
- Groucho Marx

another_someone · « **Reply #5 on:** 13/10/2006 17:40:55 »

Is all quick sand the same? Could there be different mechanisms at work for quick sand in different environments?

George

JJVIGGIANO · « **Reply #6 on:** 27/10/2006 22:33:23 »

Chris is right. The Mythbusters (a show in the U.S.) proved that quicksand is much denser than the human body, so no one could die in quite the same fashion as in hollywood. However, one could get stuck (as the water level dropped), with the remaining sand at about waist to chest level and die from dehydration and/or exposure to the elements