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Thursday, 26 January 2012

The odd truth about quicksand

It doesn't really exist. Well that was an easy blog post.

Oh, you want more? Okay well quicksand does exist, but it's not quite how it's normally portrayed. There are two types: wet and dry.

Wet quicksand is the type we've sort of heard of. It's made of particles of sand or mud suspended in water. Normally when sand becomes wet, water seeps into the gaps between grains, but with quicksand the water flows around the grains encasing them in a layer of water. It happens because of a nearby water source pushing water up through the sand and causing each grain to be suspended in water.

In the films, as soon as you step into quicksand it sucks you in and you're dragged under to die. In real life, quicksand isn't very deep and it won't suck you under. As it's mainly water, you can float in it without any problem. But as soon as you step onto quicksand, it will just part beneath you and you'll sink down quickly. It won't suck you down per se, but there is suction all around you which makes it more difficult to extract yourself, especially if you're wearing shoes. The flat surface that is the sole of your shoe will cause a vacuum to attempt to form under it when you lift it and then you're fighting against the suction of that vacuum.

It's rarely more than chest-deep so you won't necessarily die from the quicksand itself, but it can be rather hard to get out and that's what really kills you. There are regular reports of lone hikers getting stuck in a patch and not being able to get out. Then exposure to the elements, dehydration, etc will kill them.

In the video, note that guy is smart enough to remove his shoes before stepping into it. He'd have a lot harder of a time getting out if he didn't. Now the quicksand in the video is relatively easy to spot because of the wet surface, but often the surface will be covered with leaves or even a dried layer of sand so it can be very hard to spot.

That's wet quicksand and that's the kind we've sort-of heard of. But there's another kind which has long been a rumour, but has only recently been confirmed to exist: dry quicksand.

Dry quicksand is really scary. Instead of water flowing through it, it has had air. The airflow moving through the sand causes it to become more loosely packed. Ordinarily sand has a packing fraction of around 60% - that's the ratio of sand to air in the mixture. Because of sand's rounded shape, it can't be packed tightly. Consider a ball-pit at a kids playground; that's how sand is packed. In dry quicksand, the airflow through the sand has flowed around each grain separating it from the others. As the sand has fallen back, the packing fraction has been reduced making for a much looser sand mixture. In a recent experiment published in Nature, Dutch scientist Detlef Lohse and his mates built a box with a perforated base, filled it with fine-grain sand  and passed air through it. When the air was turned off, the packing fraction of the sand had been reduced to 40%.

Now reducing the packing fraction from 60 to 40% doesn't seem like a big difference, but look what happened when they dropped a ping pong ball onto the sand.

It sank down to a depth equal to five times its diameter before the sand underneath became compacted enough to support its weight. So if you were unlucky enough to find some of this stuff and step onto it, within a second or so, you'd be 10 metres down trying to discover a new method of breathing.

There had long been rumours of desert caravans vanishing into dry quicksand, but it was generally assumed to be bullshit. It wasn't until Dr Lohse's experiments that the existence of dry quicksand was confirmed.

During the Apollo lunar missions, there was a very real fear of something like dry quicksand. Previous lunar probes had confirmed a rocky, dusty surface on the Moon and there was concern about the packing fraction of the lunar dust. The Apollo sites were particularly chosen for being hard and rocky to reduce the risk of the Lunar Lander just vanishing below the surface into a pit of dust. The bottoms of the Lander legs were fitted with dish-like pads to help prevent this. Even though the Lander made it down okay, nobody knew for sure if it would be okay for the astronauts to step onto it. There was a chance the Neil Armstrong might have stepped off the Lander and disappeared into the dust.

Oh, Princess Bride, is there nothing you can't teach us?
Of course, NASA and  Dr Lohse could have saved themselves a lot of trouble if they'd just watched The Princess Bride. Look, there's the Lightning Sand (called Snow Sand in the book), the second trial of the Fire Swamp. That reminds me: I really must write an article about Cary Elwes and his odd connection with the Lord Lucan mystery.

Getting back to quicksand... It ain't like in the movies, but it's real enough. Wet quicksand can be dangerous, but dry quicksand is downright terrifying. I read a statistic somewhere that 3% of all movies made in the 1950s and 1960s featured quicksand as a plot element, but it's now sadly missing from modern films as too much of a cliché. Well, apart from The Princess Bride and the Chinese film Kekexili: Mountain Patrol (which is on my list of films I need to watch).


  1. Not exactly quicksand, but you might be interested in the phenomenon of Montezuma Well, Arizona, a waterfilled limestone sinkhole with a "false bottom" composed of fluidised sand about 55 feet down, which conceals another 69 feet of shaft.

    1. Neat. Liked that a lot. Thanks!

    2. Hate to admit to watching (and even enjoying, dammit) but Mad Max 3 has dry sand in it... though not quite as rapid as you say here. Wait, I don't hate to admit it. It may not have been as good as the first two but it was still pretty amusing and had a chainsaw in it. An amusing chainsaw at that.

  2. Aha! I was going to mention the Princess Bride connection(just finished the e-book during my commute). Amazing stuff about the dry quicksand actually existing. I think the problem with the beaches of wet quicksand in Britain is that if you get stuck then you're probably going to be underwater when the tide comes in. Quicker than you can say Chinese Cocklers.

    Graham - great stuff with that well link too - I think the "diving into a well full of thousands of free swimming leeches" is bad enough without it having a liquid sand bottom.