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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.

By Steve Price

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."

Copernicus Crater

MGM may have applications on other worlds, too. The terraced walls of the moons Copernicus crater show similar 'liquid sand' effects following a meteorite impact.

"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."

The tests on STS-107 will concentrate on water trapped within the soil and how that water affects soil behaviour when external loading changes faster than the entrapped fluid can escape. As the water pressure or air pressure increases on the particles, the intergranular stresses holding the soil together decrease and the soil weakens. When external loading equals the internal pressure, soil liquefaction occurs.

Under these conditions, the soil particles act as if they are not linked together and the entire mass flows like a liquid. It's important for civil engineers to understand how and when this happens. "When sand is under the ground water table, an earthquake can cause the sand to liquefy and behave like a fluid," said Alshibli.

The Shuttle microgravity studies of these properties are critical because the Earth's gravity-induced stresses complicate the analysis. The weightless environment allows scientists to conduct soil mechanics experiments with very low confining pressures. Understanding these phenomena is essential for improving building techniques for sites here on Earth as well as for future building sites on the Moon or Mars. Information obtained from these studies will also aid in storage, handling and processing of materials such as grains, powders and fertilisers.

Mars Sojourner
Credit: Jet Propulsion Laboratory/NASA

NASA's Sojourner rover left its mark in the Martian soil. The design of planetary rovers -- and even terrestrial vehicles - may benefit from improved understanding of soil mechanics.

The MGM hardware includes prism-shaped test cells, pressurised and filled with water to confine and stabilise sand specimens during launch and re-entry.

The sand is contained in a latex sleeve printed with a grid pattern allowing cameras to record changes in shape and position. The sleeved specimen is 2.8 lbs of sand, 7.5 cm in diameter and 15 cm tall. Tungsten metal plates on three rods cap each end of the specimens. The sand is a natural quartz with fine grains, widely used in civil engineering experiments and evaluations.

An electric stepper motor, moving the top plate, controls the compression and relaxation of the specimen. The test cell is attached to a test/observation platform mounted in the centre of three CCD cameras.

"The cameras are mounted 120 degrees apart giving us a view of 360 degrees," said Alshibli. Enabling detailed pictures to be taken for later analysis.

Specimens returning to Earth are examined to reveal the details of their structure. They are scanned to produce a series of "slice" images every 1 mm along the length of the specimen. From such data, scientists construct three-dimensional images that reveal complex patterns and show how the sand specimen has shifted internally. Finally the specimens are impregnated with epoxy to stabilise the sand column, then sawed into1 mm thick slabs for detailed inspection under an optical microscope.

From - Journal of the Geotechnical Engineering Division,103: GT8, 918-922, 1977

How particles are packed can change radically during events such as an earthquake or when shaking a container to compact a powder.

All this playing around in the sand might seem incongruous for serious scientists, but studies of such granular materials will certainly lead to better engineering here on Earth and, perhaps one day, on other planets as well.


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First Science 2014