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1. Plant Root Circumnutation-Inspired Penetration in Sand and Clay

   Anilkumar, Riya; Martinez, Alejandro (June 2024, Scipedia)

   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 (F_z) 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. F_z 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 F_z can be reduced by around 50%. Thus, CIM penetration can produce significant reductions in F_z 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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2. Experimental investigation of circumnutation-inspired penetration in sand

   https://doi.org/10.1088/1748-3190/ad8c89  

   Anilkumar, Riya; Martinez, Alejandro (November 2024, Bioinspiration & Biomimetics)

   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 ( $\omega$). These probes have bent tips with a given bent angle ( $\alpha$) and bent length ( $L_1$). The variation of the mobilized vertical force ( $F_z$), torque ( $T_z$.), 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 ( $D_R$). The results show that the soil penetration resistance does not vary with $v$, but it increases as $\alpha$, $L_1$, and $D_R$are increased. $F_z$decays exponentially with increasing $\omega R/v$, $T_z$initially increases and then plateaus, while total work ( $W_T$) 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 of $\omega R/v$on the resultant force direction and soil disturbance. The results show that CI penetration within a specific range of $\omega R/v$leads to small increases in $W_T$(i.e., $\text{⩽}$25%), yet mobilizes $F_z$magnitudes 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. 

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3. Modeling the self-penetration process of a bio-inspired probe in granular soils

   https://doi.org/10.1088/1748-3190/abf46e  

   Chen, Yuyan; Khosravi, Ali; Martinez, Alejandro; DeJong, Jason (June 2021, Bioinspiration & Biomimetics)

   Abstract Soil penetration is an energy-intensive process that is common in both nature and civil infrastructure applications. Many human construction activities involve soil penetration that is typically accomplished through impact-driving, pushing against a reaction mass, excavating, or vibrating using large equipment. This paper presents a numerical investigation into the self-penetration process of a probe that uses an ‘anchor–tip’ burrowing strategy with the goal of extending the mechanics-based understanding of burrower–soil interactions at the physical dimensions and stress levels relevant for civil infrastructure applications. Self-penetration is defined here as the ability of a probe to generate enough anchorage forces to overcome the soil penetration resistance and advance the probe tip to greater depths. 3D Discrete element modeling simulations are employed to understand the self-penetration process of an idealized probe in noncohesive soil along with the interactions between the probe’s anchor and tip. The results indicate that self-penetration conditions improve with simulated soil depth, and favorable probe configurations for self-penetration include shorter anchor–tip distances, anchors with greater length and expansion magnitudes, and anchors with a greater friction coefficient. The results shed light on the scaling of burrowing forces across a range of soil depths relevant to civil infrastructure applications and provide design guidance for future self-penetrating probes. 

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4. A numerical study on the multi-cycle self-burrowing of a dual-anchor probe in shallow coarse-grained soils of varying density

   https://doi.org/10.1007/s11440-023-02088-9  

   Chen, Yuyan; Zhang, Ningning; Fuentes, Raul; Martinez, Alejandro (October 2023, Acta Geotechnica)

   Abstract Development of self-burrowing probes that can penetrate soils without the aid of external reaction force from drill rigs and trucks would facilitate site characterization activities and deployment of sensors underneath existing structures and in locations with limited access (e.g., toe of dams, extraterrestrial bodies). Successful deployment of self-burrowing probes in the field will require several cycles of expansion, penetration, and contraction motions due to the geometric constraints and the increase in soil strength with depth. This study explores the multi-cycle performance of a dual-anchor self-burrowing probe in granular assemblies of varying density using discrete element modeling simulations. The simulated probe consists of an expandable top shaft, expandable bottom shaft, and a conical tip. The expansion of the shafts are force-controlled, the shaft contraction and tip advancement are displacement-controlled, and the horizontal tip oscillation is employed to reduce the penetration resistance. The performance of the self-burrowing probe in terms of self-burrowing distance is greater in the medium dense specimen than in the dense and loose specimens due to the high magnitude of anchorage force in comparison with penetration resistance. For all three soil densities, most of the mechanical work is done by tip oscillation; however, this accounts for a greater percentage of the total work in the denser specimen. Additionally, while tip oscillation aids in enabling self-burrowing to greater depths, it also produces a greater work demand. The results presented here can help evaluate the effects of soil density on probe prototypes and estimate the work requited for self-burrowing. 

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5. A numerical investigation on the effect of rotation on the cone penetration test

   https://doi.org/10.1139/cgj-2023-0413  

   Yang, Xiaotong; Zhang, Ningning; Wang, Rui; Martinez, Alejandro; Chen, Yuyan; Fuentes, Raul; Zhang, Jian-Min (November 2024, Canadian Geotechnical Journal)

   The cone penetration test (CPT) is one of the most popular in situ soil characterization tools. However, the test is often difficult to conduct in soils with high penetration resistance. To resolve the problem, a rotary CPT device has recently been adopted in practice by rotating the rod to increase the penetrability, particularly in deep dense sand. This study investigates the underlying mechanism of the rotation effects from a micromechanical perspective using models based on the discrete element method. With rotation, the cone penetration resistance ( q_c) decreases by up to 50%, while the cone torque resistance ( t_c) increases gradually. These results are also used to successfully assess existing theoretical solutions. The mechanical work required during penetration is observed to keep rising as the rotational velocity increases. Microscopic variables including particle displacement and velocity field show that rotation reduces the volume of disturbed soil during penetration and drives particles to rotate horizontally, while contact force chain and contact fabric indicate that rotation increases the number of radial and tangential contacts and the corresponding contact forces, forming a lateral stable structure around the shaft, which can reduce the force transmitted to the particles below the cone, thus decreasing the vertical penetration resistance. 

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