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The Physics of Quicksand

Article #85 • Written by Alan Bellows

Next time you're traipsing around in a wet, sandy, unfamiliar area, you had best be on your guard for the exotic material known as quicksand. If you're not careful, you could lose your life. Or much more likely, a shoe.

Quicksand is one of the staple environmental hazards in cartoonish low-budget movies, where it has been known to quickly swallow unwitting persons as they wander through jungles and deserts. It is usually represented as a deep pool of sandy goo which blends in with its surroundings, lying in wait to slowly suck in anyone who attempts to traverse it. Barring any nearby vines or branch-wielding comrades, the victim will slowly sink until completely submerged. Unsurprisingly, real quicksand is pretty tame in comparison to Hollywood's depiction, but traditional quicksand does have an evil cousin that would pose a grave threat to anyone who might stumble upon it... if it exists.

Traditional quicksand is created when water seeps up from an underground source and saturates an area of sand, silt, clay, or any other grainy soil. Normal sand can support extreme amounts of weight because friction between the grains of sand creates a force chain, distributing the load across a large area. But once there is sufficient moisture, the sand and water becomes a suspension where the sand particles are floating within the water. This significantly reduces the friction between the grains of sand, compromising its ability to support weight. Because the water seeps in from the bottom, the top layer of sand is often dry, causing it to appear to be normal sand.

In reality, quicksand is very rarely more than a few feet deep, making it more of a messy nuisance than a life-threatening hazard. Exhaustion is the biggest risk, considering the amount of energy it can take to untangle oneself from the waterlogged soil. When limbs are moved around under the surface, the movement creates a vacuum pressure, greatly increasing the amount of effort necessary to move. The same vacuum effect is what causes certain consistencies of mud to pull the shoes off of your feet.

In the rare instance where quicksand is deep enough that a person could become submerged in it, one would have to try pretty hard to go under. The reason for this is because the human body is a lot more buoyant in quicksand than it is in water, causing a person to float like a cork on the surface.

That's not to say that there are no dangers to becoming mired in the saturated soil. A person's buoyancy will be decreased if they are carrying a heavy pack-- possibly enough to cause one to sink-- so a quick-release backpack is a good idea in areas where quicksand is known to occur. And if a person flails around in quicksand, the sucking effect caused by the moving limbs can cause a person to sink deeper into the goo. There is also the risk of heat exhaustion, animal attacks, rising tide, and other such elemental hazards which may strike as one is working themselves free... not to mention the extremely high risk that sand will become lodged in disagreeable places.

There is another, more sinister flavor of quicksand called dry quicksand which is potentially a lot more dangerous, though there are no confirmed natural occurrences of the phenomenon. Dry quicksand is created when grains of sand form a very loose structure which can barely hold it own weight, like a house of cards. In the lab, it is created by causing air to flow through the sand, but it can theoretically be caused by the gradual buildup of very fine sand after it has been blown into the air. If an object of sufficient weight is placed on the dry quicksand, it will immediately sink, and the delicate structure will rapidly collapse in on itself, burying the object in the process. When this happens, the energy released by the collapse causes a jet of sand particles to shoot high into the air.

A deep, naturally occurring area of dry quicksand would be a formidable hazard, because it would cause anyone who stepped on it to sink and become buried very rapidly. No dry quicksand has ever been officially observed outside of the laboratory, but there are reports of travelers, vehicles, and even whole caravans suddenly vanishing into the sandy earth. These reports have always been viewed as mere folklore, but perhaps there is more to the stories than we realize. Science does not completely dismiss the possibility of naturally occurring dry quicksand; in fact, during the planning of the Apollo moon missions, scientists added large plates to the ends of the Lunar Module legs to help support the craft in case the astronauts found dry quicksand on the moon... but the precaution proved unnecessary, since no such soil was encountered.

Article written by Alan Bellows, published on 05 January 2006. Alan is the founder/designer/head writer/managing editor of Damn Interesting.

Article design and artwork by Alan Bellows.
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36 Comments
Vishal
Posted 06 January 2006 at 03:55 am

Very intresting, how about a few pointers on what to do when a quicksand is encountered? Or how to be prepared for one?


JustAnotherName
Posted 06 January 2006 at 05:53 am

I had heard that you should not move and flail around; keep calm and either someone comes by to help you or put a bullet through you. Unload the back pack and toss it, except for your water and a morsel of food that will not attract a viscious animal.

But, as the article indicates you will probably lose a shoe. If you are in those fishing boots (I guess rivers teaming with fish could be around quicksand but I don't know) that may be a bigger problem. If you sink low enough for those to start filling, then you might be in trouble.

My failsafe: always carry a cell phone. I guess the only issue with that is "Can you hear me? Hello? HELLO?"


Marius
Posted 06 January 2006 at 06:41 am

I've never encountered quicksand, but once on a canoe trip down the Rio Grande I found out that the difference between dried river bank mud, and wet river bank mud is not readily apparant until you step onto it. Then you either hear it crunch underfoot, or you sink knee-deep into it. I'm just lucky that a) I was wearing jungle boots so I didn't lose my footwear, and b) my girlfriend was there to help pull me out.


ballaerina
Posted 06 January 2006 at 07:46 am

In marshy areas like South Carolina and Florida, the wet sandy mud is called pluff mud. The name is probably due to the fact that it's like fluffy mud. I've lost sandals numerous times to it while walking near a marsh, and sometimes boats can get lodged in it also. It's a nuisance, but I'd be surprised to hear of anyone dying because of it.


beanaroo
Posted 06 January 2006 at 11:10 am

On a motorcycle expedition to southern Utah we got our bike stuck in some quick sand. The bike only sunk about a foot and a half in, but even that little bit was enough to get it pretty well stuck. Lucky for us some other bikers came along an hour or so later and were able to help us out. It took 8 grown men to pull the bike out of the quick sand. It can be some pretty scary stuff when your out in the middle of no where with no help in sight.


superfart
Posted 06 January 2006 at 12:41 pm

I was on a hot air balloon safari in sub-saharan Africa last June. I saw some hippos so I ordered the balloonist to land as I love hippos. I approached the hippo with a big stick with a dab of feces on it and rubbed it in his ear.


humpedbigfatcow
Posted 06 January 2006 at 12:46 pm

One time I tried to make love to quick sand. It didn't do too well. I died.


flipsnake
Posted 06 January 2006 at 01:29 pm

ROFLMFAO superfart, that's the funniest thing I ever heard.


ExistenzKampf
Posted 06 January 2006 at 02:00 pm

superfart said: "I was on a hot air balloon safari in sub-saharan Africa last June. I saw some hippos so I ordered the balloonist to land as I love hippos. I approached the hippo with a big stick with a dab of feces on it and rubbed it in his ear."

Rofflesuace!


MeasureMan
Posted 06 January 2006 at 04:39 pm

While working for the local zoo, I was sent to Africa with a video crew to film water fowl in their natural habitat. We spent weeks on site without any luck. My camera man and I decided that the problem was that we were too visible and were frightening the wild life. As we discussed the problem, a great idea came to mind, and the next day we came back in a hippo suit……...........

Sorry Alan, I couldn't help myself.


Alan Bellows
Posted 06 January 2006 at 05:12 pm

MeasureMan said: "Sorry Alan, I couldn't help myself."

[shaking fist]Damn you, MeasureMan![/shaking fist]


Belanth
Posted 07 January 2006 at 01:51 am

Dry quicksand sounds somewhat similar to the description of how the 'prespice mass' explodes into spice in the book 'Dune'.


Furnace
Posted 07 January 2006 at 09:21 am

The Mythbusters did an interesting experiment with quicksand. Catch that episode if you can. I guess the dry quicksand is similar to very powdery snow that builds up quite high, but is so fine that you can blow a deep crater into it with just a deep breath. I suppose fine dust in large volumes (either from dust storms in desert and flatland areas, or possibly even volcanic ash) could collect in the same way... but be more of a threat because it doesn't just disappear from heat.


davida
Posted 07 January 2006 at 10:04 am

Ok...I've been scared of this my whole life...keeping my head down and avoiding even the smallest sandy areas in the back country. Now of course...I am in fear of the "other" dry quicksand...thanks...at least now I have a theory of what happened to Waldo...


gorgeousplanet
Posted 09 January 2006 at 12:13 am

Dry quicksand... wasn't that in the Fire Swamp? Next there will be an article about R.O.U.S.'s


mohdfadzlan
Posted 30 March 2006 at 03:38 am

Vishal said: "Very intresting, how about a few pointers on what to do when a quicksand is encountered? Or how to be prepared for one?"

Hurm.... From what I know, when you encounter a quicksand, you should lie back slowly. Try not to be too struggle and panicked. Relax and cool down. The best thing to do is to make slow movements and bring yourself to the surface, then just lie back. You'll float to a safe level.
Try visit the how stuff works website for further information.

The link: http://science.howstuffworks.com/quicksand2.htm


ur2sensitive
Posted 05 May 2006 at 04:25 am

One thing about dry quicksand we are leaving out completely...How about grain storage bins? You hear this story all the time about farm accidents involving some JimBo falling into the grain and disappearing in seconds. Certainly, the physix of the whole thing has a lot in common with the dry form of quicksand.


just_dave
Posted 03 June 2006 at 06:55 am

gorgeousplanet said: "Dry quicksand… wasn't that in the Fire Swamp? Next there will be an article about R.O.U.S.'s"

Lightningsand! Classic Morgenstern. My thought exactly!

ur2sensitive said: "One thing about dry quicksand we are leaving out completely…How about grain storage bins? You hear this story all the time about farm accidents involving some JimBo falling into the grain and disappearing in seconds. Certainly, the physix of the whole thing has a lot in common with the dry form of quicksand."

That does happen regularly, but I don't know that there is much similarity in the two situations. Corn kernels, soy beans, or wheat grain are much larger than your typical grain of sand, and have much smaller contact area from which a force chain can result. So a heavy weight placed at the top of a pile of grain can quickly displace the grain below and sink quickly. And those accidents seem to happen more often when the grain is being transferred out of a bin; usually an auger pulls the grain from the bottom of the bin as gravity keeps the grain flowing. Pockets can develop in the bin as grain is pulled out from the bottom; to break up the pockets and keep the grain flowing people will sometimes jump into the bin, and they can find themselves buried very quickly.

Growing up in farm country, I was always told if you fell into a wagon or bin of grain to not try to stand up; instead lie flat to distribute the weight over a greater area. Never had the chance to try that, nor saw it in action, but it seems like it'd be a better way to go about it.


CanInternet
Posted 04 June 2006 at 01:27 am

@Vishal

See the movie "Blazing Saddles" were you can see actual footage of the rescue of a valueable machine from quicksand.


another viewpoint
Posted 04 June 2006 at 06:05 am

CanInternet said: "@Vishal


See the movie "Blazing Saddles" were you can see actual footage of the rescue of a valueable machine from quicksand."

...it's not exactly ground and it's not exactly water...QUICKSAND! Then again, I get no kicks from Champagne.

I saw something on Discovery channel...there is a similar phenomena in water when excess air or other gasas are released from the sea floor. The density of water is reduced and therefore, once buoyant objects...sink. There was talk about this in the Bermuda Triangle too...but that wouldn't explain why aircraft have disappeared in the same area.


Hayley
Posted 04 June 2006 at 07:46 am

This is somewhat like liquefaction (the process, not the result) where soil becomes so saturated with water and "stacked" like a house of cards, so one gentle shake could send it tumbling. The soil is beyond its saturation point, but because of the way it is stacked, it does not become a liquid. Given an earthquake, or such, however, it will, and it poses a pretty big threat. One such example, from Japan, I think:

http://www.ce.washington.edu/~liquefaction/selectpiclique/nigata64/tiltedbuilding.jpg

As I understand it, they got the buildings righted again afterward, and since all the oversaturated soil was gone, there was no longer a threat.


Counter-Strike
Posted 04 June 2006 at 11:34 pm

The movie Kekexili (about Tibet) has the coolest quicksand death scene that I've ever seen.

What do you think about it? It that likely to happen?

http://www.myspace.com/basicaustin


WolfManDragon
Posted 22 June 2006 at 10:08 am

When I lived in Vegas, some friends and I would go off-road trucking. There were certain places in the desert that if the truck slowed below about 30 m.p.h., it was stuck. It was a layer of superfine dust on top of the clay, just a few inches deep. I don't think it was dry quicksand, but close. The only worry of it killing you, was if you couldn't get the truck unstuck and didn't have enough water to get back to a main road.


frenchsnake
Posted 11 July 2006 at 05:55 pm

I don't mean to sound ignorant, so please forgive the language major, but I've been wondering about something: why aren't beaches all quicksand? You don't have to dig very deep at all in most places to hit the wet sand, so with all that water around, it seems like beaches should be deathtraps. I admit I still don't quite understand the physics of quicksand...


smokefoot
Posted 19 July 2006 at 04:43 pm

Why aren't beaches quicksand? From what I understand from the Mythbusters show on quicksand, the water has to be coming up continuously - an Artesian spring which comes up through sand. Sand on a beach is simply wet, there is nothing forcing additional water between the sand grains.


Grib
Posted 28 November 2006 at 11:52 pm

In France, around the "Mt Saint Michel", quicksands are tragically famous. As said in the article there aren’t really deep, but still you can get stuck in it up to your knees.

So why are they deadly?

The bay of Mt Saint Michel is a beautiful place and it is really tempting to go for a walk when the tide is low. If you careless you can find yourself trap in the quicksand and this is when troubles start, because if, for sure, you are not about to sank in the sand and if nobody helps you to get out , it will be difficult to extract yourself of it. Then the tide is back and, as the people around say, it is coming “a la vitesse d’un cheval au gallop” (at the speed of a galloping horse) you will just drown.
Anyway you can visit the bay with a guide by feet or by horse and it is, somehow, a mystic experience due to presence of the Mont itself.


kclamken
Posted 07 May 2007 at 05:46 am

I'm going to get a little long winded here:

Beaches are not quicksand because the water migrates up into the sand after it has already been deposited by liquid force and consolidated due to wave action and the weight of the sand. If you have a water pocket in the depositional sequence it could theoretically become quicksand, but I have never seen anything like this.

Although, a mobile dune could migrate into tidal pools or such and probably create quicksand. This is much more likely than beach/river sands becoming quicksand for reasons I will get to later.

Quicksand forms is stream/river environments because there is large amounts of sand being transported by the water. Depending on the sands mineral composition, it can cause it to have some buoyancy. Most river sand is sub-angular mature quartz. It is mature, because most of the softer minerals have been eroded into silts and clays (feldspars in particularly). So anyway, quartz sand that gets deposited into a water "hole" can exhibit some buoyancy, causing it to become a loosely consolidated sludge. I suppose the current could probably undercut a bank or something and form quicksand at a deeper level also.

Note: some buoyancy, very minimal

In my opinion I don't think that a saturated sand within a few feet of the surface could turn into quicksand. Most alluvial sands have a dry density of about 115 pcf within a few feet of the surface. Maximum dry density of alluvial sands by the Standard Proctor Method is usually around 120pcf. That tells you several things, mostly that the sand is relatively consolidated to begin with, and the pores between the sand grains are most likely in a state of positive pore pressure, not allowing water to migrate into the sediment package.

In the case of water loss, the sands are going to remain compacted, but they are going to exhibit negative pore pressure, allowing water to migrate back into the package when it becomes available. The overburden of the existing sediment will keep the sand compacted though, even against the buoyancy of the sand grains.

Ok, about the sand dunes. Wind deposited (aeolian) sand is typically very mature, has well sorted grain sizes, and is rounded. Wind is a much more efficient erosional force than water, something on the order of about 100 times. It also is very good at sorting sand, because wind can only transport the smaller particles, leaving the gravels behind. It also tends to pick up some silt as it is transported and deposited. (silt is the same thing as sand, just smaller particle sizes)

I will state that aeolian sands are not entirely consistent everywhere. They can be beach or river sands that have been moved by wind, but in some instances they are actually glacial sediments (glaciofluvial outwash). Essentially, these blew off the moraines from the last ice age. This is what the sandhills in Nebraska are.

I would think that wind deposited sand would have a much greater tendency to form quicksand than alluvial sands because these sands are typically in place at about 85-90pcf dry density, with a max of about 115pcf dry density, they don't interlock well because of their rounded shape, and they have up to about a 15% silt content.

Note: dry densities are relative as is 15% silt content. Density would increase as depth increased due to consolidation of the sediment from overburden. The 15% is kind of an arbitrary number. It is what I'm used to seeing around here. The UCS would classify it as a cohesive soil instead of a sand at around 30%. The silt is an important factor here though because ambient moisture will cause it to form a sort of weak matrix with the sand grains in the first 5 or so feet of sediment. Increased contact with moisture or any type of overburden will cause the matrix to collapse, but it makes for larger pore sizes in said upper layer.

So aeolian sands have relatively more pore space than alluvial sands due to their small amount of consolidation. This would allow for more water to infiltrate the package, along with increased buoyancy due to smaller particle sizes. The same could be said about a sand dune migrating into a tidal pool or small water pocket along a river. As the sand was deposited in the water, it would not consolidate under it's own weight, allowing a very loose suspension to form, giving us quicksand.

This also implies some things about Peorian Loess that I had not previously though of, but thats for a different time and place.

Anyway, this was my first post. If I'm not supposed to be posting in this old of an entry, sorry. I really do enjoy this site, I have slowly been working my way through every article and I'm gonna have to pick up the book.


kclamken
Posted 07 May 2007 at 06:09 am

Also, there is a pretty major error in the How It Works page for quicksand.

It states "A moderate amount of water works to increase the friction between sand particles. This is what allows you to build sand castles.".

This is absolutely wrong. The effect that allows you to build sand castles is surface tension. As with most any soil/sand, a moderate amount of water acts as a lubricant allowing the particles to consolidate easier. This is the basis of the Proctor test, and describes the moisture/density relationship of a soil/sand with a parabolic curve indicating at what percent moisture maximum consolidation can be achieved.


Gila Monster
Posted 28 January 2008 at 12:17 pm

I've had two personal experiences with quicksand. The first was in south Alabama near a small pool of water found in the woods. I stepped onto a sandy beach and quickly sank up to my waist. Apparently the pool was fed by a spring which flowed under the sand. I laid back and stayed still and didn't sink any farther, and was lucky enough to have several friends with me. They pulled me out slowly with quite some effort, and I was barely able to keep my shoes on. I'm not sure how I would have gotten out had I been alone, but had the feeling that I could have laid flat and wiggled out given enough time.

The most recent was on a hike to the Keet Seel pueblo ruins in the Navajo National Monument in northern Arizona. The ruins are reputed to be the best preserved anywhere, probably because the site can only be accessed after a strenuous eight mile hike, most of which is along a sandy stream bed. The guides warn all visitors of the quicksand, which most often forms on the downstream side of boulders, since the surface water flows around the boulder but the subsurface water is pulled upwards and suspends the sand. The quicksand forms at other spots as well, particularly near the edge of the stream on the inside arcs, and we learned to anticipate it after a while. For the most part, the quicksand is no more than calf or knee deep, and is really just an annoyance.

In regards to dry quicksand, I remember my father telling me about a death at his chemical plant that often gave me nightmares as a child. An operator was filling a boxcar full of a very fine chemical powder, which my father likened to microbeads used for fiberglass repair. The powder was blown in through a large flexible hose, somewhat like an air conditioning duct. The boxcar was mostly full when the operator somehow fell in and sank to the bottom as if he were falling through air. The man apparently suffocated quickly, and was quite dead when they were finally able to fish him out.


ddonovan
Posted 29 December 2008 at 02:15 pm

Clearly this quicksand is all a Chinese conspiracy to steal American shoe technology through the center of the earth.


comamoto
Posted 28 February 2009 at 08:15 am

Artax, please!


MacAvity
Posted 04 January 2010 at 05:09 pm

My quicksand experiences are very similar to Gila Monster's. I was backpacking down Paria Canyon in northern Arizona, and repeatedly sank up to my knees. The areas of quicksand were small enough that I was able to get out with a bit of wading and squelching. Eventually I got pretty good at recognizing the good patches of sinking sand, and sometimes I would stand in it intentionally. Other times I would direct my hiking partner, who was either really dimwitted or else just pretending to be, to "Go stand over there by that rock so I can get a photo." Time after time, he would get stuck, he would yell at me, I would laugh. Muahahaha.


rikonjohn
Posted 07 July 2012 at 01:42 am

It's possible the dry quicksand phenomena is caused by soil or sand liquefaction following or during a seismic event. This is where the shaking caused by an earthquake causes the solid sand layers to behave more like a liquid. This is best seen by buildings partially sunk into the ground after an earthquake.

One of the phenomena accompanying this liquefaction is geysers of sand blowing upward as the surrounding sand settles, much like the graphic of an apple disappearing into the sand accompanying this article.

"Disappearing caravans" are possible under these conditions, I'd think. Dry quicksand would require an outflow of gas, such as methane (natural gas) to unsettle the sand particles enough to behave as a liquid - not just air. I can think of a condition or two where a large seismic event wouldn't even be necessary - just a breach of a methane pocket.

Liked this article!


rafgar
Posted 02 August 2012 at 08:54 am

I once fell into quicksand as a kid. I can assure you, from that experience, that there are varieties of 'wet' quicksand which are every bit as dangerous as those seen in cartoons. The ground in question was super saturated and it was at least five feet deep according to the guys who pulled me out. Apparently they had probed it with a 2x4 after they got me back on solid ground. As I was only about 3 1/2 feet tall at the time it would have proven more than sufficient to kill me. Unlike the movies, I sunk very quickly, barely slower than I would have in water. In about 2 seconds I was chest deep in the stuff and scrabbling at the ground I had just stepped off of trying in vain to pull myself out.


Donovan
Posted 23 May 2013 at 04:25 pm

Similar to quicksand,sinkholes in bogs and marshes are much more deadly.Being born and bred in Newfoundland and Labrador,I`ve encountered sinkholes countless times,counting the fact that they are in most marshes and bogs on both Provinces.Traditionally,a bog or marsh of NL is a humongous body of murky mud water,ridden with clay,peat,sand,net-like roots,etc,and usually is covered in a network of plants and grass intertwined with each other,forming say a ```sponge`` with the exception of trees.These marshes are home to Bake Apples,and of course sinkholes,except these are no run of the mill sinkholes....more like ``sink-pools``. Here is wiki`s definition of a sinkhole: http://en.wikipedia.org/wiki/Sinkhole .

Picture that you are on a dirt trail in a bog,and much like quicksand,the top of a sinkhole looks like normal solid ground(some are commonly noticeable).You then walk upon some ground,and suddenly you are submersed up to the groins.Unlike quicksand,a sinkhole will suck an object down almost instantaneously(depending on how solid the ground is. i.e if the ground looks solid,but in fact is water blended into the ground). Of course there are the instances where you can step on the ground without sinking,but with enough effort...you will...almost like playing in the mud.The big difference here is that in a marsh,the chances are solid ground vs soft is greatly overwhelmed do to a mass amount of water..Getting out is just as hard as quicksand and can be much more difficult considering that muck is much heavier than sand.If you don`t have anyone near by and are in a pool of muck.chances are you are screwed(unless the pool is made of up of more water than minerals). During the winter,traveling on a marsh is a brainless endeavour within itself. Although during the spring,summer,and fall seasons,it is safest to walk upon the spongy grass.

For example: Once upon a time while playing upon a marsh.A friend and I were traversing the marsh,during winter by the way.Although we knew the the dangers of a marsh,being kids we had a confidence of being impervious to such dangers.While running around on ice and snow,I so happened to walk upon a soft spot and all of a sudden I lost my footing. Within a second, my right leg was completely submersed in cold,thick muck. It just so happened that my left leg was on solid ground...so I fell to the side leaving myself halfway between safety and almost certain death.Lucky me,my friend was there to reach a hand and helped to pull out my leg. Of course I lost my boot and sock,but was relieved to be free from the clutches of the muck.


Click Your Poison Books
Posted 09 October 2014 at 12:46 pm

Brilliant! And just what I was looking for. The villain in my upcoming book was going to use a hidden pit of quicksand as his pulp-tastic method of disposing of the hero, but dry quicksand is so much more effective--and nefarious!

Thanks,
James Schannep


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