Chunmei He reported in 2009 via a dissertation at Case Western Reserve University (Ohio) that many of the essential materials needed for the construction of permanent lunar bases and test beds for future missions to Mars can be produced from the resources on the lunar surface. Utilization of in situ resources on the moon will reduce the need and cost to bring everything from the Earth. Therefore, it is essential to have a thorough understanding of the geotechnical behavior of lunar regolith. However, only a limited amount of information is available about the geotechnical properties of lunar soils. In addition, the amount of lunar regolith brought back to the earth is small. To execute the many small and large scale equipment tests planned for In-Situ Resource Utilization (ISRU), it is necessary to develop a simulant which is inexpensive and can be produced in large quantities.
This dissertation presents the methodology behind developing such a lunar-like geotechnical soil, GRC-3, and compares the properties of this soil with that of lunar regolith to provide insight into the material’s geotechnical properties. Results show that particle size distribution, specific gravity, relative density, friction angle and compressibility are similar to that of the lunar regolith. Additionally, the lunar regolith mare simulant JSC-1A and the lunar regolith highland simulant NU-LHT-2M were mechanically characterized and compared to those of the returned lunar regolith. The ASTM standard laboratory tests used in the studies of these lunar simulants were compared to the techniques used in the studies of lunar regolith.
The author concluded that based on the results of the analyses performed throughout this investigation it can be concluded that soil sample GRC-3 is a useful lunar regolith simulant for design excavation tools and interpret soil parameters form the cone penetrometer test. As GRC-3 is created from industrially produced silica sand from the Best Sand Corporation in Chardon, Ohio and Bonnie silt excavated from a site in Burlington, Colorado; it can be easily reproduced in large quantities for large scale laboratory testing at a relatively low cost. Based on the results of this study, the following additional conclusions can be drawn:
1) The particle size distribution of GRC-3 simulant falls between +1 standard deviation and -1 standard deviation of typical lunar soils. In medium range of the particle size, slightly exceeds the -1 standard deviation of typical lunar soil. GRC-3 is classified as a silty sand like lunar regolith.
2) The average specific gravity was found to be 2.633, which is lower than that of typical lunar regolith.
3) The maximum porosity of GRC-3 is 0.264 while the minimum porosity is0.423. The maximum void ratio of GRC-3 is 0.732 while the minimum void ratio is 0.358. The best way to achieve high compaction for the simulant is by vibration with a vertical surcharge. The range in mass density that can be achieved by GRC-3 is close to that estimated for lunar soils.
4) The peak angle of internal friction of GRC-3 is a little bit lower than that of typical lunar soils and it increases with density. The cohesion of GRC-3 is difficult to measure using a triaxial device. The variation of internal friction angle with bulk densityof lunar regolith has never been studied.
5) The compression and swelling index of GRC-3 are within the range for lunar soils provided by Lunar Sourcebook and a little bit lower than the recommended typical
values of lunar soils.
6) The shear, constrained, and elastic moduli of GRC-3 increase with confining pressure and density. Generally, the ratios of shear, constrained, and elastic moluli between dense and loose GRC-3 decreases as the increase of the confining pressure. The Poisson’s ratio of GRC-3 shows little difference with the increase of the confining pressure, but increases with density.
In summary, most geotechnical properties of GRC-3 are similar to that of lunar soils. Since it can be produced in large quantity at a low cost, it is acceptable as a
simulant for ISRU studies.
The author recommended future work in this area of study should include but not limited to:
• Since most of the testing procedures used to determine the geotechnical properties of lunar soils are different to those used to determine the geotechnical properties of lunar simulants because of the limited quantities of lunar samples, it is necessary to study how much the different techniques will affect the test results.
• It may be more useful to perform the UU triaxial tests using an internal vacuum as the confining pressure on the lunar simulants instead of the typical external air confining pressure. The confining pressure applied by the vacuum is better to mimic the environment on the Moon which is lacking of air. Additionally, the low confining pressures applied by the vacuum should also provide more reliable values of cohesion for these simulants.
• The cohesion of these lunar regolith simulants were too small to be measured using the conventional triaxial test under high confining pressure. It is necessary
to develop a reliable technique to effectively measure the small values of cohesion for these simulants.
• Volume change characteristics are key to understanding and predicting the mechanical behavior of the regolith simulant. This is a challenge as the volume change of the material under the applied normal load cannot physically be measured by the conventional triaxial equipment at CWRU due to limitations of the equipment.
• Further study should be conducted on the affect of the lunar dust as well as the electrostatic charges and magnetism of the particles on the excavators and vehicle systems since the dust causes material and contamination issues and astronaut toxicological issues.
• The gravity has a profound influence on the mechanical properties of soils while the gravity on the Moon is only 1/6th that on the Earth, so it is necessary to develop relevant simulation rules and scaling laws to properly relate the results of studies conducted on the Earth to those expected on the Moon.
• An investigation should be conducted on the minimum depth to which soil can be prepared in a testing bin without being influenced by bottom boundary conditions.
• A study should be conducted on how to properly store and process large quantities of lunar regolith simulants and how to mitigate the dust produced in the process of the simulants.
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