Modeling Clam-inspired Burrowing in Dry Sand using Cavity Expansion Theory and DEM

The Atlantic razor clam exhibits exceptional penetration performance in wet sands by periodically expanding and contracting its shell and foot during burrowing. Essentially, this periodic penetration movement can be simplified as a cyclic alternation of cylinder expansion and cone penetration, which are analogous to the geotechnical pressuremeter test and the cone penetration test. The dynamic penetration movement of the razor clam was simplified as four major connective steps—cylinder expansion, cone penetration, cylinder contraction, and cylinder retraction—and the kinematics was parameterized based on the biological data. Using a simplified and idealized synthetic dry sand sample, we attempted to model the clam-inspired penetration process using two common geomechanics tools, an analytical model based on cavity expansion theory and a numerical discrete element method (DEM), and to showcase the advantages and limitations of these two approaches in the dynamic penetration modeling. In the analytical model, the four consecutive steps were assumed to be independent. The penetration resistance and energy consumption for each step were therefore roughly estimated using cavity expansion theory; in parallel, the independent cylinder expansion and cone penetration process as well as the clam-inspired coupled dynamic penetration process were also modeled using the DEM method. When the results were compared, it was found that the adopted cavity expansion theory solutions can predict the independent cylinder expansion and cone penetration behaviors, given that the parameters were carefully chosen and calibrated. However, it cannot capture the interference effect in a coupled dynamic penetration process. Specifically, the analytical model overestimates the cone penetration resistance of the coupled dynamic penetration process; the cylindrical shaft expansion causes stress release around the cone, leading to a reduction in the tip resistance as the cone continues to penetrate. The analytical model also underestimates the expansion pressure in the dynamic penetration process, which is attributed to the change in stress and fabric state caused by the cyclic expansion/contraction movement. Moreover, the clam-inspired dynamic penetration was found to reduce the energetic cost on the penetration of cone and shaft for about 36% and still maintain a slightly lower energy consumption with additional cost on the cyclic shaft expansion, compared with the pure cone penetration strategy. With a better future understanding of the highly effective and efficient self-burrowing behavior of natural burrowers, it is envisioned to develop self-burrowing robots for a spectrum of geotechnical applications.