Multiscale analysis of rotational penetration in shallow dry sand and implications for self-burrowing robot design

In nature, seeds of some flowering plants such as Erodium and Pelargonium can bury themselves into the ground effectively for germination. Jung et al. (Phys Fluids 29:041702. https://doi.org/10.1063/1.4979998, 2017) hypothesized that rotation induced by the hygroscopic coiling and uncoiling movement of the awn reduces the penetration resistance. Rotational penetration was also studied in geotechnical engineering, as it is relevant to the rotary installation of piles. However, there are limited fundamental explanations of the effect of rotation on the reduction of penetration resistance. In this study, shallow rotational penetration in dry sand is studied using the discrete element method (DEM); the directly available particle-scale data and the derived meso-scale data were analyzed to reveal the underlying mechanism of the rotational effect on penetration. A series of rotational penetration tests with different rotational speeds were conducted. It was confirmed that the penetration resistance at the cone decreases with rotational speed. Analysis of the particle–cone contact data shows that rotation does not only result in the inclination of the contact forces, but also significantly reduces their magnitude and the overall contact number. The force chain network, displacement fields and particle trajectories visualize the rotational effects at the particle-scale; and the evolution of the principal stresses of the soil provides a meso-scale explanation. The new multi-scale data tested the “force chain breakage” hypothesis and challenged the assumptions previously used in developing analytical models. Insights were also provided to power consumption and implications on the design of a self-burrowing robot, which could take advantage of the rotational effect on penetration.