Simulation of Lunar Soil With Irregularly Shaped, Crushable Grains: Effects of Grain Shapes on the Mechanical Behaviors

One of the challenges to overcome in Moon mining operations, such as soil handling, drilling, excavation, and wheeled movement, is understanding the mechanical behaviors of lunar soil, which is composed of grains characterized by highly irregular shapes. The impracticality of performing mechanical experiments on lunar soil samples has made computational techniques useful for exploring the mechanical behaviors of lunar soil. This paper uses particle flow code and describes a procedure for simulating lunar soil grains with specific size, shape, and strength distributions. We adopt data from soil samples 64501 and 60501 retrieved in Apollo 16. Lunar soil samples are simulated as assemblies of different shapes of grains consisting of rigid spheres connected through parallel bonds. We classify grains into four categories based on their shape: agglutinate, breccia A, breccia B, and plagioclase. We simulate each grain based on available imaging studies on their shape characteristics. We reveal the significance of grain shape irregularity through angle-of-repose tests on samples with and without irregularly shaped agglutinates. Results show that the shape irregularity increases the angle of repose by 6 degrees. We repeat the test under different gravitational acceleration ranging from 0.1 to 25 m/s(2) and show that for values below about 10 m/s(2), the angle of repose is inversely related to the gravity but above10 m/s(2), remains independent of the gravity.We perform triaxial compression tests to investigate behaviors of simulated samples under confined loadings. The confinement varies from zero to 15kPa, corresponding to the lateral in situ stress at depths up to 250cm. The cohesion and friction angle derived from the triaxial tests are shown to agree with the lab and in situ measurements. This numerical practice and presented methodology pave the way for full-scale simulation of mining operations on the Moon surface. Plain Language Summary Utilizing space resources such as water, atmospheric constituents, volatiles, and minerals is critical for making the Moon habitable. To explore and mine these resources, we need to advance technologies whose main requirement is to understand mechanical behaviors of lunar soil. There are limited lunar soil samples retrieved in the Apollo missions, but due to their scarcity, performing destructible mechanical tests on them is impractical. Numerical modeling techniques are useful for simulating the lunar soil with a behavior similar to real samples. However, devising modeling approaches capable of simulating irregularly shaped grains of lunar soil has been a challenge in this field. We present a modeling approach here that can simulate lunar soil grains with irregular shapes. Following this approach and using available data on the soil samples 64501 and 60501 from Apollo 16, we also simulate the lunar soil sample and analyze some aspects of its mechanical behaviors under different gravity. These analyses provide helpful information for designing mining operations on the Moon surface, while the analyses on the effects of gravity make the approach applicable to Mars and other celestial bodies.