Mechanics of granular materials at low effective stresses

Six displacement-controlled, conventional, drained axisymmetric triaxial experiments on cohesionless granular soil specimens were conducted at very low effective confining stresses in a SPACEHAB module on the Space Shuttle during the STS-79 and STS-89 missions. Three specimens were tested on each mission. The specimens measured 75 mm in diameter by 150 mm long. The average grain size of the subangular Ottawa quartz sand was 0.22 mm. The specimens' relative density for the STS-79 mission was 86.5% (+/- 0.8%); for the STS-89 mission it was 65.0% (+/- 1.0%). The confining stresses in the six experiments were 0.05, 0.52, and 1.30 kPa. Very high friction angles in the range of 47.6 to 70.0 degrees were observed. The dilatancy angles were also unusually high, in the range of 30 to 31 degrees. In spite of the very low effective stresses, high elastic moduli were observed.