Dynamic Cone Penetration Test Gradient in Lunar Highlands Regolith Simulants

DYNAMIC CONE PENETRATION TEST GRADIENT IN LUNAR HIGHLANDS REGOLITH SIMULANT. K. Šļumba1,*, B.T. Scott1 and M.B. Jaksa1, 1Lunar Construction Group, Andy Thomas Centre for Space Resources, University of Adelaide, SA 5005, Australia, *[email protected] Introduction: Cone penetrometers are often proposed for space missions (e.g. Beagle 2, InSight, Philae) and even were used by Apollo astronauts. The cone penetrometer (CP) is a simple instrument that yields quantitative and qualitative information about the geotechnical properties of the material. Quantitative information is the penetration resistance from which density and layering can be inferred. Qualitative information from the CP includes identifying layering and inhomogeneities, as well as the existence of boulders, see [e.g. 1] for a deeper discussion. All penetrometers that have been used extraterrestrially so far either had difficulties with penetration or were designed to only penetrate less than 100 mm deep [e.g. 2–4]. Cone penetration can be divided into static and dynamic, where in the case of the dynamic cone penetration test (DCPT), the probe is hammered into the ground as opposed to pushing it at a steady rate. The DCPT can provide roughly similar data to those of the regular cone penetration test (CPT), and it comes with the major advantage that it doesn’t rely on a reactive force. In effect, the dynamic cone penetrometer (DCP) is often used at locations that are difficult to access and could be mounted even on a rover. Hence, the DCP was proposed for SurveyorBot rover by XArc [5]. In this work, analysis and results of a representative DCPT are demonstrated. Methods: DCPs typically used in civil engineering, geology, agriculture, and military applications are too large and heavy to be directly transferable to a lunar rover. A bespoke instrument, the Mini-DCP, was produced with 4 different cones (12.7, 20.0, 27.5, and 38.0 mm diameters) and with changeable hammer stroke energy, from 0.23 to 5.87 J. Previous experiments in uncontrolled density [e.g. 6] concluded that density needed to be controlled for such experiments. To overcome this limitation, a cubic metre regolith compaction chamber was constructed to precisely control the density of engineering grade lunar highlands regolith simulant (LHS-1E). In the compaction chamber, the regolith simulant was prepared at constant density, 400 mm deep. In total, 9 setups were prepared with densities from 1600 to 1900 kg/m3 corresponding to relative densities that varied from 40 to 80% respectively. A Humboldt HS5001SD nuclear density gauge was used to measure the density and to confirm that the density is constant with depth. A 500 mm2 Geomil digital cone penetrometer was used to perform between 2 and 4 CPTs in each density setup. The DCPT was carried out consistent with published standards [7] with 26 different instrument setups trialled, allowing cone tip diameter, hammer mass and drop height to be varied. Between 25 and 30 tests in each density setup were undertaken, as typically shown in Figure 1. Figure 1 Mini-DCP in the regolith compaction chamber. Although 26 different Mini-DCP setups were tested only one representative setup is analysed in this paper. The setup presented here penetrated a wide range of densities at an appropriate rate consistent with Sowers and Hedges criterion [8] which states that a proper DCPT requires between 4 and 30 hammer strikes per 45 mm penetration. For this setup, the Mini-DCP was equipped with a 20 mm diameter cone with a 30° apex angle, a 16 mm diameter and 500 mm long shaft, and a 477 g hammer dropped from 200 mm height. This cone is the same size and shape as one of the cones used by Apollo astronauts [2]. This setup could provide up to 0.938 J energy with each hammer strike, excluding energy losses due to hammering efficiency. Results: The number of Mini-DCP hammer strikes (N) was recorded after every 50 mm of penetration and average values with uncertainties are plotted for 8 different density (ρ) setups in Figure 2. This figure shows that N increases rapidly with ρ and linearly with depth (h). On average ρ uncertainty was 28 kg/m3. The test at the highest density tested, 1871 kg/m3 (82% relative density) shows slightly worse linear fit (R2 = 0.92) compared to the tests at lower densities. This can be explained by divergence from the Sowers and Hedges criterion [8], where large number of required hammer strikes caused stagnation from regolith compaction. Figure 2 Mini-DCP hammer strikes per 50 mm penetration for varying densities of LHS-1E. When measured with CP, the rate at which penetration resistance increases with h can be called CP gradient (GCP) [9,10]. Lucas et al. [10] found a relationship between GCP and ρ in LHS-1. In this work, similar analysis is performed for the Mini-DCP gradient in LHS-1E. For a comparison between both simulants see https://spaceresourcetech.com. The rate at which N changes with h in this work is called the Mini-DCP gradient (G). It was calculated for each measurement and plotted against the ρ in Figure 3; note the log scale on the vertical axis. It was found that there is a strong exponential relationship between G and ρ, which can be expressed by Eq. (1) and Eq. (2). 𝑮 = 1.14 ∙10!"# ∙ 𝑒#.#"%"∙𝝆 ("1")" Solving for ρ gives 𝝆 = 82.6 ∙ ln(8.77 ∙10(∙ 𝑮) ("2")" Where ρ is in units of kg/m3 and G is in units of N/50mm. Figure 3 Mini-DCP gradient versus LHS-1E density. Note log scale. Eq. (2) can be applied when the density of LHS1E is unknown and the Mini-DCP with corresponding setup can be used. The results of the other 25 MiniDCP setups, together with correlations to penetration resistance, will be shown in a journal paper “Dynamic Cone Penetration in Lunar Highlands Regolith Simulant”, currently in preparation. Conclusions: The relationship between a single Mini-DCP setup and LHS-1E density was found. The DCPT, similarly to the CPT, can be used to assess the density of materials, although calibration under controlled conditions is necessary for both methods. The findings of the DCP experiments can be directly used when developing payloads for lunar rovers and to interpret the penetration data. Acknowledgements: This work is a part of a PhD project that is supported by Andy Thomas Centre for Space Resources and a University of Adelaide Research Scholarship. Thanks to Honours students Parth Deodhar and Owen Ong Zi Xyan and Master’s students Mingze Guo and Yiqi Hao for their assistance in the laboratory, especially with the data collection. Thanks to the Exterres Lab manager Kevin Farries and Geotech lab manager Gary Bowman for their relentless help, guidance and recommendations. References: [1] Lutenegger, A.J. (2021) CRC Press, 370 p., [2] Carrier, W.D., III et al. (1991). Lunar Sourceb. Users Guide Moon, 475–594, [3] Spohn, T. et al. (2022). Space Sci. Rev., 218, 72, [4] Boehnhardt, H. et al. (2017). Philos. Trans. R. Soc. Math. Phys. Eng. Sci., 375, 20160248, [5] Ximenes, S. (2023). NASA SBIR STTR Contract T704-1837, [6] Šļumba, K. et al. (2024). Proc. ISTVS, 0356, [7] Standards Australia AS 1289.6.3.2 (1997)., [8] Sowers, G.F. and Hedges C.S. (1966). ASME, [9] Oravec, H.A. et al. (2010). J. Terramechanics, 47, 361–377, [10] Lucas, M.P. et al. (2024). Acta Astronaut., 224, 161–171. 0 50 100 150 200 250 050 100 150 200 250 300 Hammer+strikes+per+50+mm+penetration+ N Penetration+depth+ h ,+mm 1871+kg/m³+(82%) 1808+kg/m³+(72%) 1762+kg/m³+(64%) 1753+kg/m³+(63%) 1734+kg/m³+(59%) 1713+kg/m³+(56%) 1686+kg/m³+(51%) 1624+kg/m³+(39%) y"="1.14E-10e1.21E-02x R²"="0.98 0.01 0.1 1 1600 1700 1800 1900 Mini-DCP"Gradient" G Density" ρ ,"kg/m3