What happens to sand in an earthquake?

15 October 2013

Interview with

Dave Ansell, Naked Scientists

What happens when you build a tunnel through certain typesSand Dune of ground and there's a lot of sand there? Guest Rodney Craig said was that it can liquefy the sand.  It goes from being a solid to something which is akin to being a liquid.  Dave Ansell brought his own demonstration of this to the studio.

Dave -   What I've got here is my own very little sand pit.  I have a box full of sand.  You can kind of push it around a bit but mostly, it's fairly solid and you can kind of put things on it and they don't tend to sink very fast.  You build a building on it and it's fairly stable.  The problem comes if that sand starts to get shaken so, if you're in an earthquake because at the moment, the sand isn't moving because all the hard particles are jammed against one another.  And so, the friction hold it altogether you have a nice solid foundation.  The problem is, if you shake it, the sand particles will bounce around a bit and as it starts bouncing around a bit, they can start bouncing past each other.  So, I will start to shake my sand pit.

Chris -   So, Dave has his little mini sand pit and you have a very large bolt standing up to resemble, a skyscraper or some kind of heavy building and then secreted in the sand is a light object.  You haven't told me what it is, but there's something hiding in there.  I'm going to give it a shake.

Dave -   Give it a shake.

Chris -   Okay, so the result is pretty clear.  What has popped up which was previously invisible under the sand is a Ping-Pong ball.  That's very light obviously and then the very heavy metal bolt has completely disappeared, just by shaking and agitating the sand.

Dave -   So, the sand starts to shake around, the particles move past each other and they can flow and start to behave like a liquid so anything denser than the sand will sink.  Anything less dense than the sand will float because of course a tunnel is full of air.  So, it's basically a great big kind of submarine.  If it's in a liquid, it will start to float upwards, so you start getting bends in your tunnels and if some bits are solid and other bits are bending due to this liquid, you can get kinks in your tunnels and it can cause huge problems.

Chris -   Has this actually happened anywhere?  I mean, theoretically, it's obvious this can happen, but has this actually happened to anybody?

Dave -   The liquification is a big problem.  So, various places in Japan that have built great big office blocks, the half in sand and half on something slightly more solid and the sand side has just liquefied and they'll just tip over like just a great big block twisting around.

Chris -   Why does the vibration do this?  What's the mechanism behind this because the sand particles are pretty rough and uneven and they jam together quite well I would think?

Dave -   As soon as you start shaking it, you're basically causing them to bounce off each other and they start behaving a bit like atoms in a liquid, so molecules in a liquid.  So, they're not locked in place and as soon as they can move past each other a bit, then they can change shape and they can flow.  If you have violence like an earthquake with the right kind of sand or gravels then it can just liquefy and flow around the place.

Chris -   So, it isn't just sand.  It could be other types of particles or other materials as well.

Dave -   Anything granular.  It'll depend on the size of the earthquake.  Also, if you got water in there, it works far better, something you may have done on the beach as a child or again, more recently, if you stand on a smooth piece of sand which is very wet, the water helps to lubricate it and it sort of causes the rock to float a bit, so it's less heavy than normal sand.  And so, if you jiggle your feet up and down, you can sink down into the sand.  It's the same what's called a thixotropic effect.

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