GEOMECHANICAL TESTING OF REGOLITH SIMULANTS

Experimental tasks have begun to compare regolith grain shape and size to mechanical behavior observed on the moon and other small airless bodies. Grain size and shape distributions are compared to the Apollo lunar sample returns that were extensively characterized during the 1970s and whose records are now publically available in electronic format. Regolith grains are highly angular and non-spherical, which may influence larger scale geomechanical behaviors such as angle of repose and porosity.

Three main types of unconsolidated materials were used for the density and repose experiments: spherical glass beads, pumice grit, and JSC-1A (a lunar regolith simulant), each with grain sizes ranging from <40 µm to 1.4 mm. These represent the end members on the grain sphericity range. Density and angle of repose testing was performed on the various sediments, with preliminary results pointing to confirmation that grain shape does indeed have an effect on the geophysical behavior of the bulk material. In early testing, materials with the same grain size distribution have remarkably different constructional cone angles of repose. The pumice grit (44-74 µm) constructs a much higher angle of repose cone, with slopes reaching upwards of 80°, whereas the glass beads (45-90 µm) constructs a flatter cone, with angles of repose around 30°. Additionally, drainage pit slope angles vary widely dependent on the grain shape. These tests confirm that the microscale characteristic of grain shape has a macroscale geomechanical effect on angle of repose under Earth’s atmospheric pressure.

Larger particle size (>2 mm) experiments using dry, unfired clay shapes under Earth’s atmospheric pressure were found to best mimic the small-scale edge erosion that regolith particles would be subjected to under turbulent conditions. These experiments are two-dimensional (2D) in nature and are designed to determine the effect of grain shape on an evolving (mass wasting) angle of repose and porosity. Similar to the microscale observations, grain shape has a direct effect on the rotating angle of repose. Rolling drum angle of repose experiments show circles easily rolling over each other to reach a low angle of repose, whereas star-shapes achieve a high angle of repose due to their interlocking configurations.