Flowing Sand in Space

NASA scientists are sending sand into Earth 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.

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November 17, 2000 -- 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?

Right: The liquid-like behavior of sand during the 1989 Loma Prieta earthquake in California damaged this bridge leading to the Moss Landing Marine Laboratory. Credit: J.C. Tinsley, U.S. Geological Survey

When STS-107 is launched next 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!

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The Mechanics of Granular Materials (MGM) experiment utilizes the microgravity of freefall in Earth 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 Center. "Our role here is to share our findings with others in academia, as well as engineering and civil construction."

Left: MGM may have applications on other worlds, too. The terraced walls of the Moon's Copernicus crater, shown here in an image captured by the Lunar Orbiter 2 in 1965, apparently were caused by soil fluidization after a meteorite impact.

From the beginning, the MGM project has received high accolades, scrutinized by seven science peer reviews along with reviews by 18 different academicians and four industrial researchers. Dr Robert Schrieffer, winner of the Nobel Prize in 1972, praised the MGM project as "world-class science" and "an appropriate effort for NASA."

"The important findings are that we have 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 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."

Above: How particles are packed can change radically during cyclic loading such as in an earthquake or when shaking a container to compact a powder. A large hole (1) is maintained by the particles sticking to each other. A small, counterclockwise strain (2) collapses the hole, and another large strain (3) forms more new holes which collapse when the strain reverses (4). (after T.L. Youd, "Packing Changes and Liquefaction Susceptibility," Journal of the Geotechnical Engineering Division,103: GT8, 918-922, 1977).

The tests on STS-107 will concentrate on water trapped within the soil and how that water affects soil behavior 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. Its 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.

Right: 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. Credit: Jet Propulsion Laboratory/NASA.

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 fertilizers.

The MGM hardware includes prism-shaped test cells with Lexan jackets pressurized and filled with water to confine and stabilize 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 1.3 kg (2.8 lbs.) of sand 7.5 cm in diameter and 15 cm tall (3 in. x 6 in.). Tungsten metal plates on three guide rods cap each end of the specimens. The sand is Ottawa F-75 banding sand, widely used in civil engineering experiments and evaluations. It is a natural quartz sand with fine grains (0.1 to 0.3 mm diameter).

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 center of three CCD cameras.

"The cameras are mounted 120 degrees apart giving us a view of 360 degrees," said Alshibli.

Right: Click on the image to view a larger 9-frame movie showing how one MGM specimen was compressed during shuttle flight STS-79. The speed of the movie is misleading; the complete sequence takes about an hour.

Specimens returning to Earth are examined to reveal the details of their structure. Computed Tomography (CT) scans 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 stabilize the sand column, then sawed into1 mm thick slabs for detailed inspection under an optical microscope.

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.