What is Quicksand?

[miss-terious]

My 8 year old daughter asked me why people in films sink in sand. Of course she is referring to quicksand but I am ashamed to say I did not know the answer.

If someone could explain this, I would very much appreciate it.

[Laykie]

Quicksand is a mixture of sand and water, or sand and air, that looks solid, but becomes unstable when disturbed by any additional stress.

In normal sand, grains are packed tightly together to form a rigid mass, with about 25 to 30 percent of the voids between the grains filled with air or water.

Because many sand grains are elongate rather than spherical, loose packing of the grains can produce sand in which voids make up 30 to 70 percent of the mass. This arrangement is similar to a house of cards in that the space between the cards is significantly greater than the space occupied by the cards. The sand collapses, or becomes 'quick,' when additional force from loading, vibration or the upward migration of water overcomes the friction holding the grains together.

[HWKE]

I would like to add that, contrary to what you see in films, you will not drown in quicksand. The likelihood is that you would sink down to about your waist then stop. The difficulty is with extracting yourself from the quicksand. In order to remove yourself from the sand, water must be allowed to pass beneath you and disturb the sand sediment (the finer grains at the bottom), otherwise it would be like being in wet cement and you would be battling against a vacuum that would be too strong.
If you slowly wriggled your legs, you would gradually rise to safety.

[miss-terious]

Thank you Laykie and HWKE.

That's great!

[Brugel]

I think I read somewhere that the word 'quick' used to mean 'alive' and that quicksand is so called not because you sink quickly but because it seems to be alive.

[miss-terious]

Thanks Brugel.

I watched 'Survivor' where the guy dropped in to quicksand to show that it is possible to get back out. He started moving his legs and it slowly pushed him back up. Brilliant programme, the guy is a lunatic.