Sandcastle Science

Even on the beach, you can't get away from science: observe the humble sandcastle. You may think it's child's play; but physics is lurking in its every crevice.

Summertime may be here (with apologies to our FirstScience visitors from south of the Equator), and the living may be easy for some. But not for the serious beach-visitor. There's so much sand, and so much to do with it. If you want some ideas, Wikipedia even has a whole article on "Sand art and play".

For myself, I've always been one for a bit of beach hydraulics: find a natural stream, and attempt to halt its flow with a nifty bit of sand engineering. A sand-dam, of course, is doomed to failure: if the water doesn't rise over it in the end, the flow will undermine the dam and wash it away.

Not so the mighty sandcastle. The ultimate form of sand-play can last far longer than the average holiday, up to a year or two. And it can take all your vacation to construct it. The world's biggest sandcastle was built last year, at Myrtle Beach in South Carolina. A team of seven "professional sandcastlers" took 10 days to mould 300 truckloads of sand into this colossal construction, whose pinnacle towered over 15 metres (49.5 feet) above the beach.

Secrets of the Sand

So what is the secret of a supersized sandcastle? According to research by the BBC for a television series called Coast, there's one key piece of science. For every eight parts of sand, you need one part of water.

Every child knows - as they say - that you can't make a sandcastle out of sand that's completely dry. It's the surface tension in the water between the sand grains that holds them together. Add too much water, though, and the whole thing goes mushy, because there's not enough air-water surface.

But BBC Focus magazine begs to differ. For their summer special issue, they have checked out the strength of damp sand - and found that "wet sand's strength is more or less constant for anywhere between one and 30 per cent water." That's because there's a trade-off between the strength of each bond between the grains - which lessens as the sand gets wetter - and the number of bonds, which increases as the sand becomes more saturated.

But sandcastles are not just for fun - they can be lethal. Read on to find out how....