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
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.
Credit: Jet Propulsion Laboratory/NASA
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
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.