Using Percussive, Dynamic, And Static Soil Penetrometers To Assess .
Earth and Space 2012 ASCE 2012 Using Percussive, Dynamic, and Static Soil Penetrometers to Assess Geotechnical Properties and the Depth to Ground Ice of the Mars and Lunar Analog Terrains on the Devon Island, Canadian Arctic K. Zacny1, M. Bualat2, P. Lee3, L. Alvarez4, T. Fong2, M. Deans2, L. VanGundy2, D. Lees2 1 Honeybee Robotics, Pasadena, CA 91101; email: zacny@honeybeerobotics.com NASA Ames Research Center, Moffett Field, CA 3 SETI Institute, Mountain View, CA 4 College of San Mateo, San Mateo, CA 2 ABSTRACT In the summer of 2011, we used three geotechnical instruments to assess the ground conditions in planetary analog sites on the Devon Island, Canadian High Arctic. The instruments included Percussive Cone Penetrometer (PCP) developed by Honeybee Robotics, and the two off the shelf instruments: Dynamic Cone Penetrometer (DCP), and the Static Cone Penetrometer (SCP). The three systems differed by the methods the rod was driven into the soil. SCP used a reaction force provided by the operator to drive the rod into the ground, the DCP used a drop hammer, and PCP used a high frequency hammer (percussive) system. The three instruments were evaluated based on their ability to be used by astronauts and be deployed autonomously on planetary robotic platforms (e.g. rovers, hoppers). The SCP, although simple to operate, was limited to soft soils and its data was unreliable. DCP required two people to operate, was heavy, and though the data was reliable, it took a few minutes to obtain it. The PCP has proven to be very reliable, fast, and the data was obtained and plotted in real time. Hence, PCP is recommended as the optimum geotechnical tool for planetary exploration for either a robotic system or an astronaut. The tests were performed at the Drill Hill site within the Haughton Crater. The site was covered by the polygonal features, the telltale signs of water activity (freeze-thaw cycle) beneath the surface. The measurements were taken at the polygons junctions, sides, and the center. It was found the polygon junctions are the weakest, followed by polygon sides, and finally polygon centers (the strongest). The depth to ground ice at these three locations was 65 cm in each case. INTRODUCTION Flawless operation of mobility systems, excavation, mining and In Situ Resource Utilization (ISRU) operations, regolith transport and many others depend on knowledge of geotechnical properties of soils. For example, knowing the soil strength and its density (and in turn fundamental soil parameters: friction angle and apparent or true cohesion) will drive the design of the wheels and excavation systems, and help to determine anticipated excavation energies, time, and forces. 284
Earth and Space 2012 ASCE 2012 The geotechnical tool can be used to assess in-situ soil strength prior to the deployment of larger vehicles or the placement of structures. It could also be used to evaluate the stability of slopes (berms or holes dug for nuclear reactors) as well as to evaluate landing zones after soil/rocket plume interaction, which could leave landed spacecraft unstable. Soil physical properties may also be used to help interpret surface geologic processes and to constrain the origins and formation processes of the soils. Geological examination of the near subsurface will increase our understanding of the formation and history of any planet or moon and, by extension, of the solar system. There are many historical instances indicating that the lack of geotechnical soil properties could have, or in fact had, severely affected or even ended missions. A few examples include: During one of its traverses, the Apollo 15 Lunar Roving Vehicle (LRV) became bogged down in the soil. The empty LRV weighed only 38 kg in lunar gravity, so the astronauts moved it to solve the problem (Kring, 2006). This solution would not be possible on a robotic mission. Soviet Lunokhod rover had to make a 90º change in course after encountering wheel sinkage up to 20 cm (normal wheel sinkage was 2 cm). Soils on the inside walls of craters and in particular at the base of slopes were very soft (Kring, 2006). The Mars Exploration Rover (MER) Spirit became stuck in a “sand trap”. It was unable to drive out and eventually the rover’s battery depleted and the mission came to an end. Luckily, this did not happen on the first day of the mission, but after 7 years. The Apollo 15 drill cores were very difficult to retrieve from 3 meter holes. This was again attributed to poor knowledge of lunar soil at greater depths. The Apollo 16 and 17 drill augers were later redesigned (Heiken et al. 1991). Figure 1. Left: MER Opportunity stuck in 2005; Right Tests at JPL. Learning to get the rover out of the sand dune. Photos courtesy NASA JPL. 285
Earth and Space 2012 ASCE 2012 Soil properties such as strength and density are important for several reasons. Soil strength determines excavation forces, energies and time and hence drives the design of the excavators, drills, scoops and various mining systems. Soil strength is also needed to design wheels and traction systems for planetary rovers. The first lunar soft lander was Surveyor 1, in 1966. It had three tasks, one of them was to determine lunar surface bearing strength. The successful Surveyor landers: 3, 5, 6, and 7 each also performed up to 16 soil bearing strength tests. During the Apollo program, astronauts on Apollo 14-16 used a Self-Recording Penetrometer (SRP) to measure geotechnical properties of lunar soil in 3, 6, and 11 locations, respectively (Carrier et al., 1991). The 1970 Soviet Lunokhod was the first robotic spacecraft to drive on another planetary body. One of its instruments included shear-vane geotechnical tool. During its 10 km traverse, the Lunokhod 1 performed over 500 geotechnical tests. In 1976 Lunokhod 2 traversed 37 km and performed over 740 geotechnical tests. Note – the robotic systems performed almost two orders of magnitude more tests than astronauts. Since 1976, there have been no geotechnical instruments deployed on any planetary body, which means the progress of exploring planetary regolith in the solar systems has virtually come to a halt. In this paper we are reporting on the geotechnical tests conducted on Devon Island, in the Canadian High Arctic using three geotechnical instruments Percussive, Dynamic, and Static Cone Penetrometers. The Percussive Cone Penetrometer (also called Percussive Dynamic Cone Penetrometer) has been developed by Honeybee Robotics (Zacny et al. 2010), while the Dynamic Cone Penetrometer and the Static Cone Penetrometer are off the shelf tools. DESCRIPTION OF GEOTECHNICAL TOOLS During the course of the field campaign, we used the Percussive Cone Penetrometer (also called Percussive Dynamic Cone Penetrometer), Dynamic Cone Penetrometer and the Static Cone Penetrometer. The three systems are shown in Figure 2 and described in the sections below as well as in Table 1. Static Cone Penetrometer (SCP). SCP is used to determine soil strength in terms of Cone Index which can be converted to bearing strength. The penetrometer is pushed into the soil, and the forces required to push it in are read-off the load cell and recorded. The penetrometer force is then divided by the cone projected area to obtain a Cone Index. The SCP is an off the shelf geotechnical tool and has proven itself to be quite reliable in soft soils and when deployed correctly. The SCP can quickly provide approximate strength of soil. The Apollo Self Recording Penetrometer was essentially a Static Cone Penetrometer. In order for the penetrometer to provide reliable data, it has to be inserted into the soil at a constant rate of 3 cm/second. However, this is difficult and sometimes even impossible to do when the system is man-deployed. When the soil strength varies from soft to hard the operator tends to lean on the penetrometer and as soon as 286
Earth and Space 2012 ASCE 2012 the soil strength changes, the operator can not react fast enough to reduce the push force. The result is that the load on the load cell fluctuates rapidly and hence it is difficult to record the correct values. Since the reaction force is provided by an operator, the system is limited to soft soils. This limitation also has to be taken into account when deploying from a robotic system having low mass and in low gravity environment (e.g. the Moon). In summary, the penetrometer is fast and easy to operate, but it is limited to soft grounds, and its data is unreliable. Dynamic Cone Penetrometer (DCP). DCP uses single hammer blows to advance a rod into the soil. The depth after each hammer blow is read off by an assistant and recorded. DCP has an 8 kg hammer for hard soils and a 4.6 kg hammer for weak soils. The DCP is used to determine a California Bearing Ratio (CBR), which can be converted to soil bearing strength and other soil properties (Zacny et al., 2010). The CBR ranges from less than 0.5 to 100% and the corresponding values of bearing strength range from 430 to 10,800 psf. DCP has good depth resolution and works well in soft and in hard soils. However, it is a heavy system, requires two operators (one operator hammers the rod while the other records the depth), is physically exhausting (the 8 kg hammer has to be lifted 20 or 30 times in every test), and a single test takes several minutes to complete. The DCP also uses disposable cones and hence after every measurement a new cone has to be attached to the end of the rod. In summary, DCP is a reliable system and works well in a range of soils, but it is not suitable for robotic or astronaut deployment. Percussive Cone Penetrometer (PCP). PCP is based on Honeybee Robotics Percussive Dynamic Cone Penetrometer or PDCP (Zacny et al., 2010). PCP uses percussive hammer to drive a rod into a soil. Penetration rate is measured in real time using laser range finder. The penetration rate in turn can be converted to the soil bearing strength (Zacny et al., 2010). During our tests, we used a conventional hammer drill in hammer-only mode to deliver blows at the frequency of 35 Hz. The energy per blow was 2.5 J. We found that PCP had a very good depth resolution, fast deployment time (place the rod on the ground and turn on the hammer), and the data is available in real time. We found that during the deployment in hard soils, the operator had to push harder to keep the rod advancing into the soil. It was believed that this variation in the push force would affect the measurement; however, this was not the case as shown in Figure 8. In addition, we found the PCP to be a great tool to determine the depth to ground ice or ice cemented ground. Because of the high strength of ice or icy-soil, the penetration ceased once the cone hit the ice, even when large thrust was applied. When driving a rod with a cone into hard compacted soil, the cone very often becomes an effective anchor (hence DCP uses disposable cones that are left in the ground once the rod is pulled out). When the PCP was difficult to pull out of the ground, we would use the rotary-only mode and essentially drill the cone/rod out of 287
Earth and Space 2012 ASCE 2012 288 the ground as if it was a drill bit. This approach worked every time. Hence, we recommend that the PCP should have the ability not only to hammer but also to rotate. An added benefit of this approach is that the same drive head for powering the PCP could be used to power a drill bit. Figure 2. Geotechnical tools used during the 2011 field campaign included (from left to right): Static Cone Penetrometer (SCP), Dynamic Cone Penetrometer (DCP), and Percussive Cone Penetrometer (PCP). Table 1. Description of Static Cone Penetrometer (SCP), Dynamic Cone Penetrometer (DCP), and Percussive Cone Penetrometer (PCP). Static/Soil Cone Penetrometer SCP Dynamic Cone Penetrometer DCP Percussive Cone Penetrometer PCP Apollo N/A N/A Human Human Human or Robotic Controlled penetration rate at 3cm/s Repeated impact with dead weight 10 mm, 30 Penetration force/displacement Trafficability, Excavation/Mining 20 mm, 60 Penetration depth after each impact Trafficability Excavation/Mining Controlled penetration rate or force with percussion 10 mm, 30 Penetration force/displacement Trafficability Excavation/Mining Depth of utility 74 cm 1 m 1 m Metrics CI (kPa) and G (kPa/mm) Density (g/cc), friction angle (degrees) and Cohesion (kPa) may also be estimated. (Rohani and Baladi, 1981) CBR (dimensionless) Bearing Strength (kPa) and Resilient/Dynamic Modulus (Pa/m) may also be estimated. (Kleyn, 1971; Harison, 1987) All aforementioned metrics Mission Deployment method Operation Cone Measured data Application
Earth and Space 2012 ASCE 2012 289 FIELD TESTING Although during the summer of 2011 we deployed the three geotechnical systems in four different sites, in this paper, we report results from only one site, the so called Drill Hill as shown in Figure 3. The Drill Hill is so named because it has been used to test planetary drill systems (Glass et al. 2006; Zacny et al. 2007). Drill Tent Station 5 Station 2, 3 Station 4 Station 1 Figure 3. The Drill Hill within the Haughton Crater was subdivided into 5 Stations. Several geotechnical tests were performed with each instrument at every Station. The data presented in this paper comes from Station 1 only. The Drill Hill lies within the Haughton Crater and is covered by the impact breccia (Lee 2007). Because of the presence of subsurface ice and temperature fluctuations (above freezing in summer and below freezing in winter) the Drill Hill has well defined polygons as shown in Figure 4. We performed testes at 5 different stations, but only the data from one station (Station 1) is reported here. Before commencing geotechnical tests, we performed measurements of the environmental conditions. These included Air Temperature: 14.7 C, Relative Humidity: 51%, Wind Speed: 2 m/s, Insolation: Sunny, no cloud cover. We also measured the temperature of the ground at the depth of 10 cm and found it to be 14 C. The volumetric moisture at that depth was measured to be approximately 15%.
Earth and Space 2012 ASCE 2012 290 In addition, we used a deep drill to determine the depth to ground ice at 65 cm. The site location was obtained using a GPS and found to be: 75 25’ 15.1” N, 089 45’ 36.9” W, while the altitude was: 165 m. Polygon Side Polygon Junction Polygon Center Figure 4. Station 1 at the Haughton Crater test site. The station included polygons as shown above. Tests were performed in the polygon junction, side and the center. Since the site contained well defined polygons, we decided to perform tests at the Polygon Junction, Side, and Center as shown in Figure 4. Initially we used Static Cone Penetrometer in all three locations. The force data is shown in Table 2. The force data was read off by the operator in real time and written down by the assistant. To convert from the force to the Cone Index (CI) the values in Table 2 need to be multiplied by 0.580. The units of the CI will then be kPa. The data suggest that the Polygon Junctions are the weakest, followed by the Polygon Sides, and the Polygon Center. The hard layer, beyond which we could not penetrate, was reached at the depth of 40-50 cm depth within polygon junctions and sides. Table 2. Static Cone Penetrometer data. Depth / Test 10 cm 20 cm 30 cm 40 cm 50 cm Polygon 3pt Junction 1 2 3 lbs lbs lbs 10 8 6 40 20 30 10 10 10 10 1 lbs 70 70 80 10 20 Polygon Side 2 3 lbs lbs 20 60 50 40 100 120 120 40 Polygon Center 1 2 3 lbs lbs lbs 190 200 200 Figure 5, Figure 6, and Figure 7 show CBR and Bearing Strength data obtained using the Dynamic Cone Penetrometer (DCP) at the Polygon Junction, Side, and Center, respectively. The data again illustrate that the strength of soil increased
Earth and Space 2012 ASCE 2012 291 from the Polygon Junction, to the Side, and finally to the Center. In addition, the data shows that at the depth of approximately 381 mm, the strength initially reaches a peak before dropping off, and then again increases at the depth of approximately 635 mm (i.e. depth of ice and ice cemented ground). It is believed that the layer directly above the ice contained liquid water which has previously thawed. Hence, it is possible that this layer formed mud, which in turn was weaker than the dry soil directly above it. The data from the Percussive Cone Penetrometer was plotted on the Depth vs. Time graph as shown in Figure 8. The “time” is an indication of soil strength (shorter time Æ softer soil). It can be seen that PCP reached the depth of 65 cm in different times depending on the location. At the Polygon Junction, the depth of 65 cm was reached in 15 seconds, at the Polygon Side the depth of 65 cm was reached in 22 seconds, while at the Polygon Center the depth of 65 cm was reached in 35 seconds. Note that during these tests, the operator would apply variable force on the hammer drill (an in turn the rod), and hence, we suspected that we will see a large variability of data in each location. However, the 3 lines for each location (Junction, Side, and Center) are quite close to each other. Note also all the depth data stopped at 65 cm because as we found during our drill tests, the ice was at that 65 cm depth. Hence, the PCP can be a viable tool for measuring depth to ice or ice cemented ground. CBR BEARING CAPACITY, psf 10.0 100.0 0 0 5 0 2000 4000 6000 8000 10000 12000 0 127 5 127 10 254 10 254 15 381 15 381 20 508 20 508 25 635 25 635 30 762 35 889 DEPTH, in 0 30 762 Based on approximate interrelationships of CBR and Bearing values (Design of Concrete Airport Pavement, Portland Cement Association, page 8, 1955) 35 889 40 40 0.1 1.0 10.0 1016 100.0 1016 0 14 28 42 56 69 83 BEARING CAPACITY, psi Figure 5. Dynamic Cone Penetrometer (DCP): CBR and Bearing Capacity values for Station 1 at the Polygon Junction. DEPTH, mm 1.0 DEPTH, mm DEPTH, in. 0.1
Earth and Space 2012 ASCE 2012 292 BEARING CAPACITY, psf 100.0 0 0 5 127 10 254 15 381 20 508 25 635 30 762 35 889 0 DEPTH, in 10.0 DEPTH, mm DEPTH, in. 1.0 2000 4000 1.0 10.0 10000 12000 5 127 10 254 15 381 20 508 25 635 762 Based on approximate interrelationships of CBR and Bearing values (Design of Concrete Airport Pavement, Portland Cement Association, page 8, 1955) 35 889 40 1016 0 0.1 8000 0 30 40 6000 0 DEPTH, mm CBR 0.1 14 28 42 56 69 83 BEARING CAPACITY, psi 1016 100.0 Figure 6. Dynamic Cone Penetrometer (DCP): CBR and Bearing Capacity values for Station 1 at the Polygon Side. CBR BEARING CAPACITY, psf 10.0 100.0 0 2000 4000 6000 8000 10000 12000 0 0 0 5 127 5 127 10 254 10 254 15 381 15 381 20 508 20 508 25 635 25 635 30 762 35 889 DEPTH, in 0 30 35 0.1 1.0 10.0 1016 100.0 889 40 1016 0 40 762 Based on approximate interrelationships of CBR and Bearing values (Design of Concrete Airport Pavement, Portland Cement Association, page 8, 1955) 14 28 42 56 69 83 BEARING CAPACITY, psi Figure 7. Dynamic Cone Penetrometer (DCP): CBR and Bearing Capacity values for Station 1 at the Polygon Center. Figure 8. PCP data shows large difference in penetration resistance (i.e. soil strength) for three different regions (polygon junction, side, and center). Although the PCP was manually deployed, the data for each location is similar. DEPTH, mm 1.0 DEPTH, mm DEPTH, in. 0.1
Earth and Space 2012 ASCE 2012 CONCLUSION In this paper we presented results from testing of three geotechnical instruments on the Devon Island during the summer of 2011. The primary purpose was to evaluate the three geotechnical instruments based on their ability to be used by astronauts and be deployed autonomously on planetary robotic platforms (e.g. rovers, hoppers). The instruments included Percussive Cone Penetrometer (PCP) developed by Honeybee Robotics, and the two off the shelf instruments: Dynamic Cone Penetrometer (DCP), and the Static Cone Penetrometer (SCP). The three systems differed by the methods the rod was driven into the soil. SCP used a reaction force provided by the operator to drive the rod into the ground, the DCP used a drop hammer, and PCP used a high frequency hammer (percussive) system. The SCP, although simple to operate, was limited to soft soils and its data was unreliable. DCP required two people to operate, it was heavy, and though the data was reliable, it took a few minutes to obtain it. The PCP has proven to be very reliable, fast, and the data was obtained and plotted in real time. Hence, PCP is recommended as the optimum geotechnical tool for planetary exploration for either a robotic system or an astronaut. The tests were performed at the Drill Hill site within the Haughton Crater. The site was covered by the polygonal features, the telltale signs of water activity (freeze-thaw cycle) beneath the surface. The measurements were taken at the polygons junctions, sides, and the center. It was found the polygon junctions are the weakest, followed by polygon sides, and finally polygon centers (strongest). The depth to ground ice at these three locations was 65 cm in each case. ACKWNOLEDGEMNTS The project was part of the "Robotic Follow-up Experiment" led by Principal Investigator Maria Bualat of NASA Ames Research Center and funded by NASA's Moon and Mars Analog Missions Activities (MMAMA) Program. REFERENCES Carrier, W. D. III, G. Olhoeft, W. Mendell (1991). Physical Properties of the Lunar Surface. In: G. Heiken, D. Vaniman, B. French (eds.). Lunar Sourcebook - A User's Guide to the Moon. Cambridge University Press. Glass, B.; Cannon, H.; Hanagud, S.; Lee, P.; Paulsen, G., Drilling Automation Tests at a Lunar/Mars Analog Site, Abstract no. 2300, 37th Annual Lunar and Planetary Science Conference, March 13-17, 2006, League City, Texas, Harison, A. (1987), Correlation between California Bearing Ratio and Dynamic Cone Penetrometer Strength Measurement of Soils, Proc. Instn Civ. Engrg, Part2 pp832-844. Heiken G.H., Vaniman, D.T., French B.M. 1991. Lunar Source Book. Cambridge University Press. 293
Earth and Space 2012 ASCE 2012 Kleyn, E.G. (1975), the Use of the Dynamic Cone Penetrometer (DCP), Transvaal Roads, Department, Report No. L2/74, Pretoria. Kring, D, 2006, Lunar Mobility Review, accessed 10 January 14, 2012, http://www.lpi.usra.edu/science/kring/lunar exploration/briefings/lunar mobility rev iew.pdf Lee, P. 1997. A unique Mars/Early Mars analog on Earth: The Haughton impact structure, Devon Island, Canadian Arctic. In Conf. on Early Mars: Geologic and hydrologic evolution, physical and chemical environments, and the implications for life. LPI Contrib. No. 916, p.50 [#3059] Rohani, B. and Baladi, G. Y. Correlation of Mobility Cone Index With Fundamental Engineering Properties of Soil. US Army Corps of Engineers, US Army Engineer Waterways Experiment Station. Vicksburg, Mississippi : US Army, 1981. Miscellaneous Paper SL-81-4. Zacny, K., J. Wilson, J. Craft, V. Asnani, H. Oravec, C. Creager, J. Johnson, and T. Fong, Robotic Lunar Geotechnical Tool, ASCE Earth and Space 2010, 15-17 March 2010, Honolulu HI. Zacny, G. Paulsen, K. Davis, and B. Glass, Drilling and automation for Mars Exploration – 3rd filed test on Devon Island. (2007). [abstract 1765] 38th Lunar and Planetary Science Conference. 294
Cone Penetrometer are off the shelf tools. DESCRIPTION OF GEOTECHNICAL TOOLS During the course of the field campaign, we used the Percussive Cone Penetrometer (also called Percussive Dynamic Cone Penetrometer), Dynamic Cone Penetrometer and the Static Cone Penetrometer. The three systems are shown in Figure 2 and described in the sections below .
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