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Mechanics of Granular Materials

Second round of space experiments on STS-89 will help us
understand better the behavior of soils, powders, and other solid particles
under very low confining pressures

Anyone who has ripped open a vacuum packed pouch of coffee has experienced a fundamental aspect of granular mechanics: a singular shift in conditions can drastically change the properties of a bulk material. While the atmosphere presses on the pack, the grains push against one other, locking each other in place, creating a stiff "brick." Once pressures are released, the grain assembly becomes very weak and soft, and moves about freely, almost like a liquid.

During critical, unstable states - like liquefaction of saturated, loose sand during an earthquake - gravity acts as a "follower load" that makes the structure collapse. Even under laboratory conditions, this is too rapid to allow detailed study of intergranular forces and conditions. Further, gravity-induced stresses complicate the analysis.

To understand how granular materials behave under low stresses, NASA has sponsored the Mechanics of Granular Materials (MGM) experiment for flights aboard the U.S. Space Shuttle. In orbit, MGM uses the weightless environment of orbital flight to test soil under very low pressures. The results will further understanding of the behavior of granular materials and help in conceptual and analytical modeling. This will be applied to improving foundations for buildings, managing undeveloped land, and handling of powdered and granular materials in chemical, agricultural, and other industries.

Granular mechanics touches on many disciplines

Numerous technologies involve bulk solid-flow processes, including: storage, handling, processing and managing coarse grains materials and powders; the designs of silos, powder feeders, conveyors, and systems for processing coal, ash, limestone, cement, grain, pharmaceuticals, fertilizers.

Geologic processes are closely related. Wind and river transport processes are controlled by the properties of cohesionless granular assemblies. Liquefaction phenomena observed in unconsolidated and cohesionless soil deposits during earthquakes are governed by constitutive, dilatancy, and stability properties. When intergranular stresses or pressures become very low, as during earthquake-induced liquefaction, the soil-water composite momentarily acts like a viscous liquid, allowing buildings to sink and tilt, bridge piers to move, and buried structures to float.

car is crushedsubsurface liquefaction of sand

Above left: An automobile lies crushed under the third story of this apartment building in the Marina District after the Oct. 17, 1989, Loma Prieta earthquake. The ground levels are no longer visible because of structural failure and sinking due to liquefaction. Links to 768x512, 224K JPG. Credit: J.K. Nakata, U.S. Geological Survey.

Above right: Ground shaking triggered liquefaction in a subsurface layer of sand, producing differential lateral and vertical movement in a overlying carapace of unliquified sand and silt, which moved from right to left towards the Pajaro River. This mode of ground failure, termed "lateral spreading," is a principal causet of liquefaction-related earthquake damage caused by the Oct. 17, 1989, Loma Prieta earthquake. Links to 768x512, 224K JPG. Credit: S.D. Ellen, U.S. Geological Survey.

The first flight of MGM, on STS-79 (September 1996) was highly successful. One of the main findings is that the lower the confining pressure on the dense specimens, the higher the friction angle becomes (i.e., the specimens become stiffer). The second flight, scheduled for STS-89 in January 1998, will comprise twice as many experiment runs and expanded test conditions.

MGM traces its origins to studies to help design the wire mesh wheels for the Lunar Rover Vehicles driven by astronauts on the last three Apollo missions in 1971-72. Results from MGM may be applied in the future to advanced rovers for the exploration of Mars in addition to terrestrial needs.

Key personnel

  • Principal Investigator: Dr. Stein Sture, University of Colorado at Boulder.
  • Project Scientist: Dr. Khalid A. Alshibli, University of Alabama in Huntsville, Huntsville, Ala.
  • Co-investigator: Dr. Nicholas C. Costes, University of Colorado at Boulder.
  • Project Manager: Buddy V. Guynes, Marshall Space Flight Center, Huntsville, Ala.

 


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