The Science of Sandcastles

For example, when an earthquake strikes, wet soil underground sometimes "liquefies". Suddenly becoming more like quicksand than the sturdy walls of a sandcastle. This happened during the 1989 Loma Prieta earthquake in San Francisco. Vibrations liquefied water-soaked soil in the Marina District, causing buildings to sink until their third floors were at ground level.

This transition is rapid and poorly understood.

During an earthquake, shockwave's compress the soil faster than water can escape, raising the pressure of the water. As the water pressure increases, the water bears more and more of the load; the sand bears less and less. Ironically, this sudden compression reduces the pressure between individual sand grains-sometimes even beneath tons of rock and dirt.

"That much is understood," says Stein Sture, a professor of engineering at the University of Colorado-Boulder, "But how exactly do the grains interact as the pressures between them approach zero?"

"Studying this process in ground-based laboratories is difficult because the sand's own weight creates stress on the grains," he continued. If experimenters could remove that stress (and do so for a long time), they could more easily probe soil liquefaction .

That's why Sture is sending sand to space. He's the lead investigator for an experiment called Mechanics of Granular Materials-III-"MGM-III" for short-slated to fly onboard space shuttle Columbia (STS-107) later this year.

more An automobile lies crushed under the third story of this San Francisco apartment building after the 1989 Loma Prieta Earthquake

The experiment is deceptively simple: A column of water-saturated sand in a latex sleeve is repeatedly squeezed between two plates. (In sandcastle terms, the consistency of the sand is more like the watery sand dripping from your fingers than the damp sand packed to make a strong tower base.) A full cycle of "squeeze and release" takes about ten minutes. This compression mimics what happens to water-filled soil during an earthquake.