Plant Root Circumnutation-Inspired Penetration in Sand and Clay

Abstract

Site investigation (SI) and subsurface exploration are vital for characterizing soil properties. However, a common challenge is the lack of sufficient reaction force to penetrate through stiff crusts or deep layers, leading to refusal. To address this issue, rigs typically have large sizes that can make mobility and accessibility challenging and increase the carbon footprint of SI activities. This paper experimentally investigates a plant root-inspired strategy called circumnutation-inspired motion (CIM) to reduce the vertical penetration forces (𝐹𝑧) in comparison to quasi-static penetration used for example for Cone Penetration Testing (CPT). The CIM probes have a bent tip end and are rotated at a constant angular velocity (𝜔) while they are advanced at a constant vertical velocity (𝑣) in uniform specimens of clay and sand. 𝐹𝑧 for both soils decay exponentially by factors as high as 10 with increasing relative velocity, defined as the ratio of the tangential to the vertical velocity of the probe tip (𝜔𝑅/𝑣). Torques for both soils increase with initial increases in 𝜔𝑅/𝑣 which stabilize at higher velocities. While the cumulative total work, calculated for both clay and sand from the measured forces and torques, increases less than 25% for initial increases in 𝜔𝑅/𝑣 between 0 and 0.3𝜋, the 𝐹𝑧 can be reduced by around 50%. Thus, CIM penetration can produce significant reductions in 𝐹𝑧 in comparison to CPTs while limiting the additional energy consumed. CIM could be implemented to perform site investigation activities, such as obtaining samples or installing sensors, using smaller-sized, light-weight rigs

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