Science of sandcastles | South China Morning Post
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  • Mar 25, 2015
  • Updated: 8:06pm

Science of sandcastles

PUBLISHED : Saturday, 12 August, 2006, 12:00am
UPDATED : Saturday, 12 August, 2006, 12:00am

Water, sand, a bucket and a spade - you're set to build the castle of your dreams, complete with a moat and towers with delicate turrets. What could be more pleasant on a hot summer's day at the beach?

While most people just see it as fun, scientists are puzzled by the whole sandcastle-building experience. A team from Bournemouth University in the UK spent the summer of 2004 on the beach experimenting with various combinations of sand and water to figure out the consistency needed to create a strong structure.

They announced that the formula for the best sandy structures is: OW = 0.125 x S. Or to put it in normal English, you need one measure of water (OW) for every eight measures of sand (S) to create the perfect sandcastle-building material.

Checking the formula in other parts of the world, it seems that sand is different from beach to beach. As the size of the grains and the presence of different types of rocks and minerals create slight variations, you may need a little less water or a drop more if you plan to build your dream castle on Hung Shing Yeh this summer.

Sandcastle science in space

When you're sitting on the sand watching it flow from your bucket into a hole you've dug, it's easy to see that sand has some intriguing properties. It can be solid enough to sit on, but flow like a liquid at the same time. Add a little water and sand sticks together; add a lot, and it turns to mud.

Scientists say damp sand sticks together because water forms bridges between individual grains of sand, creating surface tension that acts to keep the grains together.

Add too little water and the sand remains grainy because there isn't enough tension to keep the grains together. If you add too much, the water fills the spaces between the grains, destroying the tension that's keeping the bridges intact, and the sand begins to flow like a liquid.

While this is easy to understand, scientists are puzzled by the way pressure acts on damp sand. At normal pressures, sand can be incredibly strong, supporting heavy structures easily. But a seemingly strong solid base can turn into a liquid mud when an earthquake passes through it.

It has been difficult trying to work out how pressure can make sand liquid-like because the sand's own weight creates stress on the grains, interfering with experiment results. Now scientists are planning to deal with the problem by studying the relationship in gravity-free spacecraft labs.

If they can figure out exactly how granular mixtures turn from a solid to a liquid, it could help us design stronger building foundations, work out how to keep grain moving efficiently in a grain store, and much more.

Anatomy of a castle

Like modern homes, all castles look slightly different from each other. But many have these features in common:

A moat or ditch filled with water surrounding the outer walls.

A drawbridge that allows friends to cross the moat into the stronghold.

A gatehouse filled with guards who check visitors coming over the drawbridge. Gatehouses often had strong wooden doors and metal grids that could be closed quickly if the enemy tried to enter the castle by launching a surprise attack.

The curtain, a thick wall surrounding the castle that was built to withstand battering rams and missiles such as rocks. Arrow slits, narrow spaces that allowed archers to fire arrows at the enemy outside, were cut into this massive wall.

Towers set in the curtain allowed those inside to keep watch in safety. In some castles, they were also used to house prisoners.

The keep was a building inside the walls that was considered the strongest place of the castle. If invaders breached the curtain, everyone would dive into the keep and keep fighting.

A well, ensuring there was enough water even if the castle was under siege for many months.

Storage barns for food, stables for horses and housing for animals like chickens and goats ensured there were enough provisions to withstand sieges.

Sandcastle experts recommend you start by smoothing a foundation, and then build outwards, starting with a keep, storage sheds, stables, towers, curtain and finally the moat.

Imagination, skill and patience could turn your sandcastle into a beach attraction in its own right. Enjoy your summer construction project.


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