2D DEM analysis of the interactions between bio-inspired geo-probe and soil during inflation-deflation cycles

The cone penetration test and the standard penetration test are perhaps the most versatile techniques for investigating soil properties in-situ. Advancing the probe through the soil requires substantial downforce due to friction between the probe and the soil. Inspired by the ability of worms to conform their body to the most mechanically advantageous shape while tunneling, an innovative self-excavating geo-probe has been developed and deployed in a laboratory environment. Analogous to a worm's ability to expand its body, a soft balloon mounted behind a rigid cone is inflated or deflated by applying pressure or vacuum periodically to alter the cone penetration resistance. Laboratory experiments have shown promising results that the new geo-probe can penetrate the soil more efficiently. However, the interactions at balloon-soil-cone interfaces are complex and not fully understood. This numerical study focuses on the behavior of the geo-probe during balloon inflation and deflation. A two-dimensional discrete element model is developed to provide particle-level insight into the micromechanics of the interactions between the geo-probe and the soil. A baseline simulation is first conducted to study the failure mechanisms of the bulk soil during inflation and deflation. Additionally, the contact forces at balloon-soil and soil-cone interfaces are analyzed to show the variation of penetration resistance. The displacement field, shear strain field, and contact force chains are numerically investigated to gain a fundamental understanding of the tool-soil interaction. Then, the effect of balloon locations and overburden stresses on the behavior of the probe are studied by performing sensitivity analyses. This study provides a numerical technique to study the tool-soil interactions that was inaccessible in laboratory test. Implementation of the simulation results enables extrapolation of bench-scale testing to a wider range of stresses and boundary conditions.