Abstract

The exploration, characterisation, and monitoring of the subsurface is relevant to a wide range of applications, from environmental monitoring and in-situ characterisation to infrastructure construction (e.g., directional drilling and tunnelling) and resource withdrawal. Bio-inspiration offers promising solutions to overcome two of the most important challenges in the development of autonomous subsurface exploration probes: exploration range (related to drag forces) and steerability (i.e., ability to control direction of movement). The work uses a previously proposed bio-inspired intruder shape to study the relationship between tip orientation and the resulting forces (lateral and vertical); and explores the idea that these forces can be used to steer the probe, i.e., control the direction of motion of the probe as it advances. Numerical results with the discrete element method and experimental results with a large-scale test bed show a direct relation between tip orientation and the steering (lift and lateral) forces – supporting the case for steering systems based on intruder tip rotation. In the results, lateral and vertical forces also show a strong relation with tip orientation, however, for the tip shape tested, the vertical forces are limited to the neutral to upwards range (i.e., towards the free surface). Experimental results evidence the significant effect of intruder deformation (bending) and/or path deviation in the penetration forces acting on the intruder.

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