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

Abstract

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°. We repeat the test under different gravitational acceleration ranging from 0.1 to 25 m/s2 and show that for values below about 10 m/s2, the angle of repose is inversely related to the gravity but above 10 m/s2, 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 15 kPa, corresponding to the lateral in situ stress at depths up to 250 cm. 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.

Survival on land: A dark-grown seedling searching for path

“…On the surface layer of the Moon, the average bulk density is 1500 kg/m 3 , the particle density is 3100 kg/m 3 , and the gravity is 1.62 m/s 2 . Despite the highly irregular shape of Lunar soil particles, it can still be assumed as assemblies of spherical particles in DEM simulations ( Khademian et al., 2019 ). The particle size was set to be the upper value of the Lunar soil, 0.500 mm due to the limitations of the DEM method and computation power.…”

“…The particle size was set to be the upper value of the Lunar soil, 0.500 mm due to the limitations of the DEM method and computation power. In simulating the mechanical properties of Lunar soil using the DEM, Khademian et al. (2019) also used a larger size range.…”

“…Although there is no water or organic matter in Lunar soil, its cohesion is high due to the interlocking among the angular particles. Therefore, Lunar soil has high shear strength, and thus the parallel bond model was used ( Khademian et al., 2019 ). PFC model parameters were adopted from Khademian et al.…”

“…The particle size was set to be the upper value of the Lunar soil, 0.500 mm due to the limitations of the DEM method and computation power. In simulating the mechanical properties of Lunar soil using the DEM, Khademian et al (2019) also used a larger size range. Although there is no water or organic matter in Lunar soil, its cohesion is high due to the interlocking among the angular particles.…”

“…Although there is no water or organic matter in Lunar soil, its cohesion is high due to the interlocking among the angular particles. Therefore, Lunar soil has high shear strength, and thus the parallel bond model was used (Khademian et al, 2019). PFC model parameters were adopted from Khademian et al (2019), and they were 3.2x10 12 Pa for particle and bond effective modulus, 1.0 for normal-toshear stiffness ratio, 2.2x10 9 Pa for bond tensile strength and cohesion, 0.42 for particle friction coefficient, and 0.28 for normal and shear viscous damping ratio.…”