Plant Root-Inspired Soil Penetration in Sands Using Circumnutations for Geotechnical Site Characterization

Subsurface exploration is vital for characterizing the soil engineering properties at project sites. Current in situ testing methods often face challenges in providing necessary reaction forces in sites with limited accessibility or with stiff surficial layers (i.e., desiccated or gravelly crusts). This paper presents two investigations on the circumnutation-inspired penetration strategy to decrease the vertical penetration forces. Both investigations were performed on dry sands, one consisting of experimental tests using a robotic arm and the second involving discrete element modeling (DEM) simulations. In plant-inspired circumnutative penetration, the penetrometer’s tip follows a helical path. The bio-inspired probes used in this study have conical tips that are bent at an angle and the entire probes are rotated at different angular velocities while they are advanced into the soil at a vertical velocity. The cumulative total work required to push the probes (i.e., the sum of the rotational and vertical work) was calculated and compared with that involved in quasi-static penetration (i.e., in cone penetration tests). Both the experimental and simulation results show a dependence of the vertical force and torque mobilized during penetration on the relative velocity of the probe tip, defined as the ratio of the tip’s tangential to vertical velocities. Namely, the vertical penetration force decreases as the relative velocity is increased, leading to a decrease in vertical work. The torque remains relatively constant for the circumnutation tests, but the rotational work increases with relative velocity. The total work of circumnutative penetration is similar or slightly smaller than that required for quasi-static penetration for small magnitudes of relative velocities for both experimental investigations and simulations. These findings show that circumnutative penetration can mobilize significantly smaller lower penetration resistances than quasi-static penetration while requiring a similar or slightly smaller amount of work. The reduction in penetration force would allow smaller equipment to be used in site investigation soundings.