Prediction of Shear Strength for Lunar Subsurface Regolith with Varying Particle Size Distributions and Relative Densities

Future lunar mining missions are expected to involve deeper geological conditions. Understanding the mechanical behaviors of the lunar subsurface regolith is essential to operational safety. Recent findings from the Chang’e-4 and Chang’e-5 missions revealed a marked increase in particle size and relative density of lunar regolith with depth. In addition, the geostatic stress naturally increases with depth. These three variables pose significant challenges for accurately predicting the shear strength. Existing predictive models, such as the Alshibli model, fail to account for the distinct conditions of lunar subsurface regolith. To address this, consolidated drained triaxial tests were conducted on the CUMT-1 lunar regolith simulants. The influences of confining pressure, relative density, and particle size distribution on shear strength were systematically analyzed. A novel indicator, named inter-particle void ratio, was introduced to capture the combined effects of relative density and particle size distribution. Based on this indicator, a new empirical model was proposed for predicting peak shear strength under varying subsurface conditions. The results suggest that deeper lunar regolith may have significantly lower shear strength than previously estimated, primarily due to the combined effect of increased inter-particle void ratio and geostatic stress. This finding has important implications for the assessment of excavation efficiency, underground construction stability, and the overall safety of lunar subsurface infrastructure.