Did MythBusters definitively disprove death by killer quicksand?

Factual Questions
1

I saw the episode tonight for the second time on so called killer quicksand but I’m not quite convinced that its busted. Potential objections include.

I. Attempting to recreate natural conditions in an artificial situation is frought with many areas of potential error. Yes, they used really fine grain sand, but how about other considerations such as geology and the possibility of “suction” created from underground flow (be it water or quicksand) that might exist in nature.

II. I’m not sure that the really fine artificial sand possesses the same potential for hydrogen bonding/ viscosity issues that might be present in real muck or quicksand.

III. I’ve also read about so called depositional or “dry” quicksand espeically noted in desert areas like Algeria, Western Sahara ect. This type of dynamic was not tested.

So lets say they are correct and you really can float in quicksand. Could you “swim to the top” if you were dropped from a distance and went under the sand?

2

I’ll guess that I’m one of the few people who has actually spent many hours in quicksand. It’s very comfortable.

When I was at the U of I studying civil engineering one of the geotech grad students built a quicksand tank - huge tank that would be filled with sand and water with the right pressure to produce the “quick condition”, where the sand loses the ability to support a load. It was used for Engineering Open House, a yearly event.

Quicksand is effectively a very dense fluid (sand is around 2.7 times as dense as water) - you float like a cork in the stuff. With a 250 pound Marine putting all of his weight on my shoulders and me pushing on the walls of the tank we just managed to get my chin under. As soon as he let go I shot right back up. If you were dropped in from a height it might not be very comfortable (hitting a dense fluid) but there’s nothing that would keep you underneath - quicksand does not “suck you under” and it’s not sticky (like molasses). You’d pop right back to the surface.

If you fall into quicksand just swim slowly to shore.

As far as subterranean streams being able to pull you under, sure that could happen but that’s not death by quicksand, it’s death by undertow which happens at the beach all the time to unwary swimmers.

Dunno about “dry quicksand” - unless you mean a pit of fine powder? The quick condition is determined by a relation between the density of the sand, the hydraulic head and the depth of the material. Also not sure about “hydrogen bonding/viscosity”, can you elaborate?

You can have quicksand with pretty normal sand, it doesn’t have to be some special carefully-graded size particles (we are reminded of this in the Bay Area every time there’s a big earthquake).

3

So how do you make a quicksand tank? Can I make one at home, or does it require serious engineering skills?

It sounds like fun.

4

It’s very simple in concept - take a waterproof tank and build some kind of nozzle system into the bottom (such as a big bottom plate with lots of holes drilled in it) so that you can pump water in under pressure. Have an overflow hole towards the top so that water is recycled rather than spilling all over the floor. Pour sand in and add enough water to completely cover it.

Now turn on the water to the bottom of the tank. You’ll have to fiddle around to find the right pressure but what will happen at the magic point is that the sand will stop acting as a mass of damp sand and the individual grains will just float around as the water bubbles up through the sand. The sand will lose all strength to support a load until you turn the water pressure off.

IIRC the “quick condition” occurs when the hydraulic gradient (hydraulic head at the bottom divided by depth of sand) equals the specific gravity of the sand but it’s been a few years since I did that stuff.

A tank big enough for a person to float in is a major undertaking - ours was about 4 feet on a side and 8 feet tall and welded out of solid steel with a reinforced window at the front so everyone could see the sand swirling around. Full it weighed many tons.

But you don’t have to go that far - we had a little “auxiliary” tank which was maybe 2-3 feet on a side and a foot or so deep. We’d keep the water pressure off and have people put on a set of rubber boots, then step into the tank. They’d be standing on wet sand. Turn the water on and suddenly the solid sand gives way and their feet drop to the bottom of the tank. Turn the water off and the sand becomes solid again, kids could actually lean over backwards and they’d be held at an angle since their feet were trapped by the solid mass of sand.

You could even go smaller - take a big dishpan and pour a thick layer of sand in, then fill with water to just cover the sand. Sit a can of beans on it to represent a building. Now use an earthquake to generate hydrostatic pressure - smack the side of the container with your hand. Do it right and the sand will momentarily lose strength and your “building” will sink underground a bit.

I’ll see if I can find some archival photos of the thing.

5
      • The main factor here is that quicksand is considerably heavier than water, and a human will float in water. Water with sand in it will only make them more bouyant.
  • Being in farm country, I have heard of people falling into grain silos and “drowning”, because the grain is considerably less-dense than water (so you sink) but the grain is not breathable. Newer towers have no access doors big enough for a person near the silo tops, just to protect against this sort of accident happening–but there are some facilities with areas where it can still happen. New grain silos and facilities only have human-entry doors along their bottoms, so you can’t get in unless it’s almost totally empty anyway.

…The last particular incident was a number of years back, where a guy went inside a grain tower to do -something- (I don’t know much about the details of the industry) and somehow the access door got closed, and someone outside began unloading grain into the silo (where it is dropped in from the center top). The grain is so light that you sink in it, it is not possible to “swim” to its surface–and he suffocated.
~

6

I know it’s not quicksand exactly, but what do you call the properties of sand just at the water’s edge? You know, when you stand still on it, your body weight squeezes the water out and it become firm and dry. But if you wiggle your toes or just very slightly shift your weight from foot to foot, it liquidizes and you gradually sink into it. It seems to be acting a bit like cornflour and water.

I remember a field trip with our biology teacher to the seashore in high school. We stood at the water’s edge for about half an hour as he pointed out the features around us. Then he turned around and told us all to follow him, but as one, we were stuck, up to our knees and firmly rooted in the sand! He paused, looked around, said “Oh you dickheads” (first time I ever heard a teacher swear!), and set about unplugging us one by one…

7

Quicksand is a naturally-occurring fluidized bed, in which the upward welling of the water works against gravity to suspend the individual sand particles. They’re pretty free to move around without friction, which is why you can easily fall into it. But the same forces of welling up also push *you[/] up, which is why you won’t “drown” in quicksand.

The sand acts very weird, almost “alive”, which is what the “quick” is “quicksand” means. It’s the same archaic sense of the word as in the phrase “the quick and the dead” (meaning “the living and the dead”), or in “cut to the quick” (meaning a cut that goes through the dead outer layers of skin to the inner, bleeding portion).
So the Al Jaffee joke (“Is this quicksand?” “No, it’s slow sand. What’s your hurry?”) doesn’t really apply.

As you can imagine, the conditions that allow quicksand aren’t that common – you need an upwarxd rush of water from someplace, like an artesian well, or ocean waves. Hence the appearance of quicksand near the shore.

There are plenty of other situations in which you can get caught in mud, bogs, etc. that aren’t “quicksand”, but which resembles quicksand in the way it grabs you , so people call it “quicksand”, even though it isn’t. (Just as those hallucinations seen in the desert in cartoons aren’t really “mirages”. But the incorrect name sticks.) I got my shoes caught in very liquid clay once as a kid. Trying to pull one foot out only pushed the other deeper. People get caught in thick mud and sucking bogs the same way. In these situations you don’t float, and the recommended solution for extricating yourself from quicksand doesn’t work exactly as sold.

If you rapidly shake a mixture of sand and water, even without an upwelling, it “fluidizes”, and heavy objects on top will easily sink into the mixc, then get stuck there when the vibration stops. This can happen during earthquakes. Reportedly, houses built on wet sand can get completely buried this way.

Similarly, if you build a quicksand tank like they did on Mythbusters and some joker shuts off the water pressure while you’re in there, you will get stuck, and the recommended solution won’t get you out. In fact, you might end up with a problem breathing. You need someione to turn the water back on to get you out. This may be one reason Home Quicksand Tanks never really took off.

By the way, for lots of interesting references on the weird physical properties of beach sand (why does it “dry out” when you walk on it?) see the ever-recommended-by-me Flying Circus of Physics by Jearl D. Walker.