The Science of Sandcastles

Three cameras on the space shuttle will document how the column deforms. After the experiment is returned to Earth, scientists will use Computed Tomography scans (CT scans) to study the internal structure of the sand column. Then they will inject epoxy to harden the sand, preserving internal patterns for further analysis under the microscope.

"This will be the first time that we've had a window into this important process," Sture says.

Earlier flights of the MGM device onboard shuttles Atlantis (STS-79) and Endeavour (STS-89) revealed surprising things about dry sand. Lacking real data for low-pressure soils, scientists had assumed that trends seen at higher pressures would simply continue to lower pressures as well. But MGM-I and II showed otherwise.

more Computed Tomography scans (CT scans) of the sand column after it's returned to Earth reveal important clues about the behaviour of the sand under compression.

"We found, for example, strength properties that are nearly twice what we would have normally thought," says Sture, which means that at low pressures a layer of sand can support twice as much weight as previously thought possible. Yet if you reduce the pressure a little more so that it approaches zero, that strength evaporates completely. Puzzling!

Maybe similar surprises await MGM-III. No one knows.

Sture notes that "understanding this soil liquefaction process will help engineers decide when a site is safe for construction, and perhaps lead to designs for building foundations that help prevent liquefaction from occurring."

The practical benefits of these experiments will reach beyond soils. Grain in a silo is also a granular material, as are bulk cereals, many fertilizers, and coal and ash. In all these cases, knowing how to coax the material into smoothly flowing or staying in place would be a good thing.

It's something to ponder the next time you're building a sandcastle: inside the moat lies some far-reaching physics.

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