Analyses of Sampling Disturbance of the Lunar Surface in Direct Push Sampling Method

The study of direct push sampling characteristics forms the theoretical basis for deep sampling tool design. In this paper, a ground test of the direct push sampling was carried out to analyze three influencing factors: tube diameter; friction coefficient; and soil plug length. Using the discrete element method software, EDEM, a lunar subsurface direct push sampling model was established. We propose a vortex structure of the particle displacement field in the sampling tube and establish a connection between the three influencing factors and the structure disturbance of the soil plug. The EDEM simulation revealed the microscopic mechanism of penetration sampling disturbance. The results show that the soil plug in the compressive state had a small disturbance and a high degree of sequence retention and that the internal stress was changed compared to that in the in-situ state. The structure disturbance of the soil plug in the stick-slip state was large, while the parts of the soil plug that are away from the center line of the tube were in disorder. In the center line of the tube, the in-situ sequence state was gradually restored with the increase in tube diameter or decrease in soil plug length. Increasing the friction coefficient would change the soil plug from the compression state to the stick-slip or adhesion state. In case of the stick-slip state or adhesion, reducing the diameter of the tube or increasing the length of the soil plug would change the soil plug from the stick-slip state or adhesion state to the compression state.