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Castles in the sky

Building sand castles at the beach may seem like an innocent pastime, but the physics going on inside those sandy walls could save lives if it was fully understood. Stephanie Northen reports

Have you unpacked your bucket and spade yet? Do memories of fierce fortresses that defied the tide get in the way of lesson planning, or are you haunted by visions of shell-covered towers from which cuttlefish Rapunzels dangled their seaweed hair? If you built just one sand castle during this year's holidays - and let's face it, most of us did - you'll be interested to know that the finest construction sand in the world is on Cancun beach in Mexico. That's the opinion of Peter Shannon, who has travelled 54,000 miles with a flowerpot this year. The 26-year-old computer science and marketing graduate from West Sussex won a competition to assess the top 20 beaches nominated on the Expedia travel website. Along with sea transparency, wave size and "creature content", he studied the colour, temperature and build-ability of the sand. That was where the flowerpot came in handy.

Peter would probably concede that his flowerpot sand castles don't represent the most scientific of tests. In fact, what happens inside those sandy walls and towers is sophisticated physics. (One aspect comes under the study of complex systems - a more accurate name for chaos theory - which tries to get to grips with phenomena such as the weather.) Particle interaction

Understanding the interactions between these particles of rock and mineral will help to predict avalanches, earthquakes, mud slides or the precise moment when a huge chunk of your castle will slide apologetically into the moat. Known as the mechanics of granular materials, it is a serious business at many of the world's universities. It is also relatively new and largely mysterious, according to Thomas C Halsey and Alex J Levine.

In 1998 Halsey and Levine, researchers working for Exxon, wrote to Physical Review presenting a theory on the stability of humid sandpiles - sand castles to you and me. They were disputing the findings of another group of US scientists based at the University of Notre Dame, which were published in Nature the year before. The science is complicated and has to do with locating the point of failure when a certain amount of liquid has been added to the sandpile. However, the importance of the work is clear enough.

Aside from improving our understanding of natural disasters, there are many industrial uses. Cereals, fertilisers, coal and ash are all granular materials that people sometimes want to pile up - in a silo, for example - or move through a pipe. Accessing the last dribbles of the world's gas and oil reserves depends on understanding how liquids move through rocks. And any factory that processes powders, from detergents to drugs, knows that it's a big deal if the stuff gets damp.

Wet strength

Dampness is, of course, the key to a successful bucket-and-spade session.

You can't work with dry sand because the grains act as individuals.

However, add water and they start to stick together as the liquid forms tiny elastic bridges between the grains. The strength of these links depends on many things, such as the size and shape of the particles, the amount of water present, and capillary action - that obliging natural force that gets sap up trees and blood around bodies, and helps mop up spilt milk. The narrower the spaces between the grains, the stronger the bond, because the water molecules are more attracted to the sand particles than to each other.

This was what Peter Shannon was investigating with his flowerpot. Some beaches make for good building, some don't. In fact, the experts don't use beach sand at all. The world's tallest sand castle was made entirely of riverbed sand. Constructed in 2001 in Almeerderzand in the Netherlands, it was as high as a 10-storey block of flats and took more than 50 people three weeks to build. Lorries brought 16,000 tonnes of sand to a beach where the team sculpted princesses, elves and giants round the base of fairy-tale turrets. (Professionals such as these make sand sculptures, not castles.) River sand is better than beach sand because it is less prone to erosion, they say. Their castle, which lasted for weeks, was so strong that bulldozers were needed to demolish it. Anique Koizenga, a sand-sculpture artist, says sand from the sea doesn't work, because it's very old and wind and water have made the grains round. Sand from rivers, however, is still young and the grains have edges. If you press it together in a wooden mould, it can get very strong.

Getting the measure of wet sand

In 1997, the first experiment to quantify the glueyness of wet sand was carried out. Physicists Peter Schiffer and Albert-Laszlo Barabasi and colleagues put tiny polystyrene balls in a container and a little oil to represent water. They made a hole in the bottom and studied the cone that formed as the polystyrene balls dribbled out through it. As they added more oil they got a bigger, steeper-sided cone, until suddenly a phenomenon they called "clumping" took over. Instead of moving individually, the particles were sticking together. This is the glueyness that allows a sand castle builder to start scooping out tunnels and modelling turrets. Their investigation made the cover of the journal Nature.

Space science

The results of another experiment involving sand and water were lost on board the ill-fated space shuttle Columbia, which exploded earlier this year. Stein Sture, professor of engineering at the University of Colorado at Boulder, wanted to investigate why soil sometimes liquifies during an earthquake, as happened in 1989 in Loma Prieta, California. During a 'quake, shockwaves compress the soil so fast that any water present in the soil gets trapped. As the pressure increases, the water bears more and more of the load, and the soil less and less. This seems to weaken the bonds holding the grains together, until eventually - and sometimes tragically - the ground beneath our feet starts to flow.

Dry sand experiments had been included on two previous shuttle trips, but this was to have been the first involving water. The idea was to repeatedly squeeze a column of saturated sand in a latex sleeve, mimicking what happens to water-filled soil in an earthquake. Cameras were to document how the column changed and the scientists had intended to study what had happened to the sand when it returned to Earth, hoping their findings would be of use to architects and civil engineers. Being able to study granular materials in space has implications for building not just on Earth, but eventually on the Moon or Mars.

Call in the diggers

Some of the sand sculptures created for the world championships held in Harrison Hot Springs, in Canada, every September look as if they would be quite at home on Mars. Hugely elaborate fantastic creatures and bizarre buildings take professionals days of back-breaking labour to create. The rules of the competition are strict: no contestant can work more than 100 hours, only sand and water can be used and no mechanical aids are allowed.

The temptation to ship in a small digger when faced with the need for a large pile of sand can be strong. Darrell Hardy, a Sussex seafront trader, soured tempers in Bognor Regis in 2001 when he asked judges at the annual seaside festival if he could use a 16-tonne digger in the sand-castle competition. After they refused, he turned up on the beach a few days before the event with a JCB digger, to make his point to the local media.

Diggers are just not in true bucket-and-spade spirit.

Certainly they would be anathema to Tim Bell, Bognor's marketing, tourism and events officer. One of his responsibilities is the Sands of Time festival, which took place this year in September. There were Punch-and-Judy shows, a leaping birdman, a crafts fair, a classic-car rally and the sand-castle competition. It is a traditional family event, taking place between two high tides, using only materials found on the beach, says Tim Bell. "As to the sand, well that's proper, genuine English Channel sand - we don't ship in truckloads of builders' stuff." Judges of the 100 or so entries include the mayor, though it can be a dangerous job, continues Tim Bell. "People complain if they think you haven't spent long enough looking at their child's creation. I'm not sure I would want to do it."

He confesses to being a life-long sand-castle builder, but of a particular sort, you understand. "The sort is a kind of helter-skelter," he explains.

"You build a tall cone and carve a groove round it. The idea is to get a tennis ball going down the groove, under a bridge and around the moat, and then to see how far down the beach it travels." He has been building these for the past 30 to 40 years around the coast of Britain with an assortment of children and grandchildren. Now that's the spirit.

NASA website:http:www.nasa.govSand sculpture world championships website:www.harrisand.orgExamples of award-winning sand castles:www.kirkrademaker.comsand

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