Soil Mechanics Experiment Yields Unique Results

May 27, 1998

A soil mechanics experiment flown twice on the Space Shuttle is yielding new, unique data about the internal fabric of soil and powders under very low confining pressures.

Preliminary results from the Mechanics of Granular Materials (MGM) experiment were presented Tuesday at the American Geophysical Union's annual spring meeting in Boston.

Under low confining pressures, granular materials can behave like liquids. Indeed, this can happen during earthquakes when a shock wave causes an effect called soil liquefaction. When soil liquefies, homes can sink halfway into the ground without any structural collapse and underground structures can surface because of the buoyancy. Earthquake examples are Loma Prieta (in the San Francisco area) in 1989, Niigata, Japan, and Fairbanks, Alaska, in 1964, and the San Fernando Valley in 1971.

Understanding the behavior of granular materials under very low confining pressures can be applied to a variety of engineering areas, including powder processes (like pharmaceutical and cosmetics manufacture), to control of erosion, and designing vehicles for use off-road - on Earth and on other planets.

However, at the low confining pressures needed for these studies, the soil specimens cannot retain their initial shape or size because of the influence of gravity. Their grain structure collapses too quickly for effective measurements to be made. The solution was to go to space where an experiment could be run for at least two hours as the column of a special sand used as a standard in civil engineering was slowly compressed while pressures were measured.

Nine MGM samples were flown on two Shuttle missions, STS-79 in September 1996 and STS-89 in January 1998. On STS-79, three dense sand samples were processed, each at a different pressure. On STS-89, six looser specimens were processed.

After each flight, the samples were frozen by impregnating them with epoxy resin for computed tomography. In turn, these images show unusual density patterns (above and below) formed by sand grains being dislocated during the experiments.

"The flight data show very high friction angles in the range of 58 to 78 degrees," said Dr. Nicholas Costes, the project scientist and co-investigator at NASA/Marshall. "These are much higher than what we could achieve under the best conditions on the ground." The data also showed dilatancy angles (another measure used in engineering) that were higher than what could be reached in ground tests.

"In addition, the CT images and the volume models we built from the images revealed unique shear band formations and other soil fabric features that we have never seen before," Costes said.

Analyses of the nine MGM specimens continue. Costes and Dr. Stein Sture, the principal investigator at the University of Colorado at Boulder, anticipate that the results of further work will find their way into civil engineering standards and textbooks.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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