Flowing Sand in Space

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.

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.