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Mechanics of Granular Materials

Science

The principal strength of particulate materials - whether it is coffee, soil beneath a house, or sand under your wheels on Mars - is intergranular friction and geometric interlocking. Billions of grains, ranging from large to microscopic, contribute to the total strength of the material. Moisture and air trapped within the soil can also affect its behavior if loading occurs faster than the entrapped fluid can escape. As the pore water pressure or air pressure increases, the effective or intergranular stresses or pressures decrease, weakening and softening the soil. When the external loading equals the internal pore pressure, the soil liquefies.

This is relevant to many fields, not the least being earthquakes which can loosen compacted soil and compact loosened soil. When this happens, buildings sink and buried structures float to the surface, as happened in the San Francisco Bay area in the 1989 Loma Prieta earthquake.sp
The packing of particles 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). Links to 749x271-pixel, 512K JPG. Illustrator version will be available presently. Credit: NASA/Marshall Space Flight Center.

Yet another example can be seen on the Moon in the scalloped walls of crater Copernicus. After the impact that formed the crater, gases trapped in the soil caused the lunar soil to lose strength and slide. The mechanics of soil under medium to high intergranular stresses are well known, but the behavior under low stresses is not. Studies under high stresses have been carried out on Earth for decades. But studies under low effective stresses have been hampered by the Earth's gravity. The weight of the soil has many effects on the test, including making it difficult to maintain a stable specimen at low stresses.

Terraced walls of Copernicus crater on the Moon (left) apparently were caused by soil fluidization after a meteor impact. Links to 465x538 pixel, 245K GIF image taken by Lunar Orbiter 2 in 1965. Credit: NASA/Goddard Space Flight Center.

Sojourner rover left its mark in the in Martian soil (right). The design of planetary rovers - and even terrestrial vehicles - may benefit from improved understanding of soil mechanics. Links to 2128x1090-pixel, 384K JPG panorama. Credit: Jet Propulsion Laboratory/NASA.

The weightless environment of space allows soil mechanics experiments at low effective stresses with very low confining pressures. In space, specimen weight is no longer a factor, and the stress across the specimen is constant. This allows correct measurements that can be applied to larger problems on Earth. Information to be examined by MGM includes load, deformation, and fluid pressure data gathered during testing, as well as changes in the soil structure, including the formation of shear bands and change in density.

Scientific investigations with MGM will span the STS-79 and -89 missions. STS-79 gathered useful scientific data and calibrated the MGM apparatus to understand its behavior. For STS-89, the software has been modified to expose the sand to a broader range of loading conditions.

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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack