The Science of Sandcastles

An upcoming shuttle mission will carry small columns of sand into space - and will return with valuable lessons for earthquake engineers, scientists, farmers and physicists.

Give a plastic bucket and a shovel to a child, then turn her loose on a beach full of sand. She'll happily toil the day away building the sandcastle to end all sandcastles. It's pure fun.

It's also serious physics.

Sandcastles are built from grains -billions of tiny sharp-edged particles that rub and tumble together. The strength of a sandcastle depends on how the grains interact. What happens when they're wet? How do they respond to a jolt? It's not only beachgoers who are interested; farmers, physicists and engineers want to know, too.

When kids work on a sandcastle, they begin by gathering water from the ocean to wet the sand. Not too much-just enough to make sand stick together without oozing. (Emergency planners: think of predicting a devastating mudslide.) Next they pack the damp sand into a bucket, and flip it over to create an extra-strong base for a tower (Engineers: think of designing compacted road foundations.)

Kids love to build the towers taller and taller-until a wall suddenly caves and the tower slides into the moat. (Farmers: think of grain in a silo sticking together, then suddenly collapsing and destroying the silo.) They might even decorate the castle by letting watery sand drip from their fingertips, solidifying in place to form odd-looking stalagmites. (Artists: don't forget, physics is beautiful.)

Photo by George Vetter, for "Cannon Beach Sand Castle Contest," an Oregon Local Legacies project A Sand Fortress - July 1980

Scientists mostly understand why sand on a beach behaves as it does. Damp sand sticks together because water forms little grain-to-grain bridges. Surface tension - the same force that lets some insects walk on the surface of a pond, acts like rubber bands between the grains. Adding water to damp sand fills spaces between the grains. The bridges vanish and the sand begins to flow more easily.

Sounds simple, but wet sand can still puzzle researchers.