Experimental investigation of circumnutation-inspired penetration in sand.

Abstract

Probes that penetrate soil are used in fields such as geotechnical engineering, agriculture, and ecology to classify soils and characterize their propertiesin situ. Conventional tools such as the Cone Penetration Test (CPT) often face challenges due to the lack of reaction force needed to penetrate stiff or dense soil layers, necessitating the use of large drill rigs. This paper investigates more efficient means of penetrating soil by taking inspiration from a plant-root motion known as circumnutation. Experimental penetration tests on sands are performed with circumnutation-inspired (CI) probes that advance at a constant vertical velocity (v) while simultaneously rotating at a constant angular velocity (ω). These probes have bent tips with a given bent angle (α) and bent length (L1). The variation of the mobilized vertical force (Fz), torque (Tz.), and the mechanical work components with the ratio of tangential to vertical velocity (ωR/ν, whereRis the distance of the tip of the probe from the vertical axis of rotation) is investigated along with the effects of probe geometry, vertical velocity, and soil relative density (DR). The results show that the soil penetration resistance does not vary withv, but it increases asα,L1, andDRare increased.Fzdecays exponentially with increasingωR/v,Tzinitially increases and then plateaus, while total work (WT) shows little magnitude changes initially but later increases monotonically. The mechanisms leading to these trends are identified as the changes in the probe projected areas and mobilized normal stresses due to differences in probe geometry and the effects ofωR/von the resultant force direction and soil disturbance. The results show that CI penetration within a specific range ofωR/vleads to small increases inWT(i.e.,⩽25%), yet mobilizesFzmagnitudes that are 50%-80% lower than that mobilized during non-rotational penetration (i.e., CPT). This indicates that CI penetration can be adopted forin situcharacterization or sensor placement with smaller vertical forces, allowing for use of lighter rigs.