Flowing Sand in Space

Scientists are sending sand into Earths orbit to learn more about how soil behaves during earthquakes. Their results will help engineers build safer structures on Earth and someday on other planets, too.

When an earthquake hits and your home or office begins to vibrate, it's too late to think about how strong is the ground under your feet. You depend on the civil engineers and the building designers to know that and design accordingly.

But in many cases soil doesn't act as you'd expect. Sometimes soil (like snow during an avalanche) acts as if it were a liquid. It flows! But how - and when?

With the launching of the STS-107 Space Shuttle this year, an experiment on board will try to answer that question. That same flight and experiment will mark one of the few times in the history of the Shuttle program that a particular project's experiment will have flown on three separate missions. Not bad for an experiment that has, as its main ingredient, cans of sand!

The Mechanics of Granular Materials (MGM) experiment utilises the microgravity of freefall in Earths orbit to study test cells of sand under conditions that cannot be duplicated on Earth. The first two highly successful experiments involving nine dry specimens flew aboard STS-79 (1996) and STS-89 (1998). The experiments on STS-107 will involve water-saturated sand resembling soil on Earth.

"We hope to duplicate the soil liquefaction that occurs on the ground during an earthquake," said Dr. Khalid Alshibli, MGM Project Scientist at NASA's Marshall Space Flight Centre."Our role here is to share our findings with others in academia, as well as engineering and civil construction."
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"The important findings are that we have a new knowledge about the properties of granular materials at very low stress levels -- properties that scientists and engineers have not really been aware of," said Professor Stein Sture, of the Department of Civil, Environmental and Architectural Engineering with the University of Colorado. Sture serves as the principal investigator on the MGM-III project.

"We found, for example, strength properties that are nearly twice what we would have normally thought," said Sture, which means that under some conditions a layer of sand can support twice as much weight as previously thought possible.

According to Dr. Alshibli, the strength of granular materials - whether it is coffee, soil beneath a house, or sand under the wheels of a Moon rover - is primarily caused by friction between the particles and interlocking between faces on individual particles. Billions of particles contribute to the overall strength of the material and any small change in conditions can have a large effect on that strength. "An example of this would be a vacuum-pack of coffee," said Alshibli. Before it is opened, it's solid and strong. "When you open it, the pressure is released and the grains shift freely."