If you’re vacationing by the beach this summer, here’s a challenge for you: What’s the tallest sandcastle you can build?
A team of international scientists set about trying to answer that very question four years ago. Although they knew a sandcastle’s height depended on its stability, they wanted to know how factors like what it’s made from and the way it’s built could be used to create a taller structure.
It turns out the secret to building a spectacularly tall sandcastle may not be such a mystery after all, but actually common sense. The wider the base of your sandcastle the more stable it will be, says Daniel Bonn. Bonn, who led the study, is a physics professor at the University of Amsterdam in the Netherlands.
But Bonn had other sandcastle-building tips to offer. In order to create a sturdy structure, it’s important to compact the sand as much as you can, he says. The mix you build with matters, too. The sand-to-water-ratio has to be just right. Only 1 to 3 percent of your mixture by volume should be water while the rest is sand.
“Adding a little bit of liquid helps form bridges between the sand grains,” explains Maryam Pakpour, a physicist who worked with Bonn. The bridges, called capillary bridges, create an attractive force between the individual grains, causing them to stick together. “The result is a much more stable structure,” she says.
But the scientists’ research wasn’t all sand and sun. “We spent a lot of time at the beach and it was good fun,” says Bonn. “But in the end, it was really important to understand the mechanical properties of sand.”
It might seem silly to study sandcastles, but understanding how water changes the way sand particles interact with one another has implications that stretch beyond a day at the beach. For example, one of the first steps of constructing roads and tunnels involves laying down wet sand and waiting for it to compact. But that can take a long time—up to 1 or 2 years! Because engineers now have a better understanding of how sand compacts depending on the amount of water, they can reduce that waiting time to weeks.
Knowing how the properties of granular materials like sand change when wet is useful in other areas too. It helps geologists understand landslides and pharmaceutical companies compact fine powders while making medicines.
While you probably won’t build a highway on your next beach vacation, you might build a sandcastle that’s 8 feet tall. That’s how high Bonn and his team managed to go with a tower that had a base radius of almost 8 inches. So get your shovel and pail, and start building!