MGM fact sheet: science
Mechanics of Granular Materials
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|
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