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How to Construct the Perfect Sand Castle

By Ross Pomeroy

A bright day on the beach. Water soothingly smacks the shore. The sun's rays aren't beating, but caressing. You're right in the middle of building an epic sandcastle. But then it happens. First the castle's outer wall goes. Then a turret collapses. Soon the once magnificent castle that rivaled the best of the Middle Ages has been transformed into a heaping pile of dirt. Hermit crabs click their claws. It sounds like laughter. Children cry.

Thankfully, this chaos can now become a thing of the past, because researchers have shown us how to construct the perfect sandcastle.

In a fun, enlightening, and freely accessible study appearing in Scientific Reports, an international group of scientists from France, Iran, and the Netherlands explored the stability of wet sand columns to find the maximum height of sandcastles.

In undertaking the study, the researchers noted that the scientific data on sandcastles was woefully out-of-date. The only estimate available indicated that the maximum height of a sandcastle could be no more than 20 centimeters. This has been invalidated by numerous observations from family vacations to the beach.

To find the actual maximum height of sandcastles the researchers started off by calculating that the optimum strength of wet sand is achieved with low water content, around 1%. Knowing this, they built their own cylindrical sand castle columns using PVC pipe. Each column had varying heights and diameters.

Their experiments yielded a result indicating that the maximum height of a sandcastle column increases as a 2/3 power of the radius of the base. This means that a sand column with a 2 cm radius could be 27 cm tall, a column with a 7 cm radius could be 60 cm tall, and a column with a 20 cm radius could theoretically be 2.5 meters tall! That's a big sandcastle.

What's more, the researchers posited that underwater sandcastles using hydrophobic sand could be far taller and more spectacular. This is because the sandcastle's effective weight is reduced by a factor of three compared to castles on land.

The study's results are of interest to civil engineers and soil mechanics. They can also be used to help prevent potentially dangerous landslides in coastal areas, as well as on your very own sandcastle.

Source: Pakpour, M., Habibi, M., Møller, P. & Bonn, D. How to construct the perfect sandcastle. Sci. Rep. 2, 549; DOI:10.1038/srep00549 (2012).

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