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William E. Willoughby, Randolph A. Jones, George L. Mason, Sally A. Shoop, James H. Lever U.S. Army Engineer Research and Development Center, CEERD-GM-M, 3909 Halls Ferry Road, Vicksburg, MS, USA 39180-6199

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  1. 1. APPLICATION OF HISTORICAL MOBILITY TESTING TO SENSOR-BASED ROBOTIC PERFORMANCE William E. Willoughby, Randolph A. Jones, George L. Mason, Sally A. Shoop, James H. Lever U.S. Army Engineer Research and Development Center, CEERD-GM-M, 3909 Halls Ferry Road, Vicksburg, MS, USA 39180-6199 ABSTRACTThe USA Engineer Research and Development Center (ERDC) has conducted on-/off-road experimental field testingwith full-sized and scale-model military vehicles for more than fifty years. Some 4000 acres of local terrain areavailable for tailored field evaluations or verification/validation of future robotic designs in a variety of climaticregimes. Field testing and data collection procedures, as well as techniques for quantifying terrain in engineering terms,have been developed and refined into algorithms and models for predicting vehicle-terrain interactions and resultingforces or speeds of military-sized vehicles. Based on recent experiments with Matilda, Talon, and Pacbot, thesepredictive capabilities appear to be relevant to most robotic systems currently in development. Utilization of currenttesting capabilities with sensor-based vehicle drivers, or use of the procedures for terrain quantification from sensordata, would immediately apply some fifty years of historical knowledge to the development, refinement, andimplementation of future robotic systems. Additionally, translation of sensor-collected terrain data into engineeringterms would allow assessment of robotic performance for a priori deployment of the actual system and ensure maximumsystem performance in the theater of operation.Keywords: mobility testing, mobility history, vehicle modeling, vehicle terrain interaction, terrain characterization,sensor development 1. INTRODUCTION The US Army Engineer Research and DevelopmentCenter (ERDC) has conducted on-/off-roadmobility/trafficability research including experimental fieldtesting with full-sized and scale-model military vehicles formore than fifty years. The Mobility/Trafficability Section wascreated in the mid-1940’s from the Soils Division of the U SArmy Corps of Engineers’ (USACE) Waterways ExperimentStation (WES) in Vicksburg, Mississippi, to address militaryvehicle shortcomings apparent to combat engineers in WorldWar II operations. Initially, actual field testing of vehicles in avariety of soil conditions was the focus of the group, andresults were encouraging. However, mobility planners soonrealized special facilities were required for laboratoryparametric testing of scale-model vehicles, so in 1957 some24,000 square feet of unique laboratory facilities were added(shown in Figure 1.) that defined the state of the art inmobility testing at that time. The main operational area Figure 1. SMALL-SCALE TEST FACILITY. Theenclosed the principal testing apparatus: a cantilevered facility consistsed of a remotely controlledstructure supporting a carriage to which scaled vehicle dynamometer carriage (shown) that rides on overheadrunning gear could be attached. Beneath the carriage, a line of rails and could accommodate wheels up to 32 in. inmetal cars filled with processed soil formed a test lane through diameter and tracks up to 4+ ft in overall length.
  2. 2. which the models traveled. Apparatuses for measuring soil strength, stress and strain factors, wheel or track torque, andother instrumented mobility data adjoined the edifice. The main building also contained a stationary soil processingplant, a mobile soil processor, additional soilcars, and a track system for moving the carsinto/out of position. Pressure cells placed atspecific positions and angles in the soil carsprovided important data on soil reactions totraffic of the models attached to the carriage. Results were very encouraging, and alarger facility (shown in Figure 2) was addedto allow full-scale testing of vehicles in largeconcrete soil bins that were 5-ft-deep by 20-ft-wide and nearly 200-ft-long. A variety ofsoil data collection devices, such as the hand-held cone penetrometer developed at WES,the bevameter developed by the LandLocomotion Laboratory at the DetroitArsenal, and the torque shear vanesuccessfully used by British trafficabilityengineers, were fully mechanized andinstrumented for data collection. Comparisonsof laboratory tests with actual field test dataconcluded that wheel or track models of one-fourth to one-third the size of those on actualmilitary vehicles could be effectively Figure 2. LARGE-SCALE TEST FACILITY. Equipment containedevaluated and data extrapolated for design of in this laboratory permited the simultaneous testing of single wheels upfull-sized vehicles. The large-scale facility to 72 in. in diameter, single tracks up to 48 in. long, and full-sizeactually allowed validation of the scaled data vehicles.from the vehicle models. The WES assumed another area of field research in 1954, when the USACE Office of the Chief of Engineers(OCE) charged the Trafficability Section with the task of developing a methodology for predicting trafficability insnow-covered terrain. In the summer of 1954 WES began field investigations on the Greenland Icecap, which continuedin 1955 and 1957. These test programs established criteria for trafficability projections in artic conditions andrecommended the adoption of the cone penetrometer as the most practical instrument for field tests. However, therelatively firm artic snow of Greenland produced few vehicle immobilizations, and the data indicated testing wasrequired in softer snows of subartic areas and during springtime thaws. Accordingly, testing was initiated in Colorado in1954, Canada and Michigan in 1955, and Colorado in 1958. Results of this testing indicated that artic snows generallysupported wheeled vehicles, but subartic and thaw-conditions produced “NOGO’ conditions for all conventionalwheeled military vehicles used in the experiments. Further, these tests determined that the first vehicle pass in subarticsnow was the most difficult. The success of the snow program led the USACE to embark on a consolidated research program regarding theeffects of winter conditions on military operations. The USACE had previously been responsible for two researchorganizations involved in cold regions research: the Snow, Ice and Permafrost Research Establishment (SIPRE) atWilmette, Illinois, and the Artic Construction and Frost Effects Laboratory (ACFCL) in Boston, Massachusetts.Several winter engineering cooperative programs with Dartmouth College led Dartmouth to offer leased land near thecollege in Hanover, New Hampshire, to USACE, and in June 1960 the SIPRE, ACFCL, and Dartmouth chartered theCold Regions Research and Engineering Laboratory (CRREL) as the Corps’ laboratory responsible for cold regionsresearch which continues today. In 1960, OCE provided more impetus to the encouraging mobility research area by assigning a long-termprogram to WES to determine the effects of the dimensions of pneumatic tires on the performance of military vehicles.
  3. 3. The primary objective of the program was to determine the behavior of various soil types when subjected to traffic by anumber of tires with various sizes, proportions, and inflation pressures. Hopefully, testing would lead to development ofprocedures for selecting proper tire sizes and inflations for specified soil conditions. Almost concurrently, hostilities inSoutheast Asia were drawing interest from the US military, and in early 1962, the US Army Materiel Command (AMC)requested that WES become the managing agency for the Mobility Environmental Research Study (MERS). The MERSprogram required an extensive study of the environment of Thailand, especially as it pertained to the design andemployment of materiel and materiel systems. Thailand was an ally of the United States, and operations in that countrycould be conducted unimpeded and with the support of a friendly government. Project planners noted that militaryenvironments in Thailand were very similar to those of Vietnam so that analogous comparisons could be made thatwould assist military operations later. As the US became more involved in Southeast Asia, the MERS program gained significant momentum, andother agencies including the Land Locomotion Laboratory, Army Transportation Board, and the Advanced ResearchProjects Agency joined in the study. Some 2400 sampling sites were carefully characterized using soil sampling, terrainsurveys, hydrographic mapping, vegetation analysis, aerial photography, and extensive field experiments withconventional and experimental vehicles yielded a wealth of correlated data over an extended time period. Additionally,engineering tests and procedures were developed and refined to allow characterization of terrain sites for mobilityanalysis, and concurrent experiments wereconducted stateside and in tropical areas toverify correlations of vehicle performance andterrain characterization. Additionally, fieldexperiments began stateside to developvehicles for operations in the marshy areas ofvery low trafficability, or in riverineenvironments. The WES and other agenciesbegan evaluations of experimental amphibiousvehicles such as the Marsh Screw Amphibian,Riverine Utility Craft, XM759 LogisticalCarrier, and others specifically underdevelopment to operate in Southeast Asianterrains. Stateside there was also interest insending astronauts to the moon. The WES soillaboratories were engaged by contractors forthe National Aeronautics and SpaceAdministration (NASA) to use the laboratorytest facilities with various sands and othermaterials to simulate lunar soils in order to Figure 3. LUNAR ROVER VEHICLE (LRV) WHEEL.. This is onedevelop wheels for a lunar roving vehicle. The of three wheels tested for NASA for the moon landings and is theBoeing-GM wheel, shown in Figure 3, was design selected for the landings. The mesh portion is made of woveneventually selected based on its superior piano wire and the chevrons are of titanium. The inner ribs are merelywoven wire-design and in 1971 the Lunar to protect the outer portion of the wheel from hitting the axle of theRover Vehicle successfully landed on the wheel in case of a sever jolt.moon and operated with a WES-validatedwheel design. 2. MOBILITY MODEL DEVELOPMENT By 1970 researchers had reached the conclusion that knowledge derived from previous WES studies, as well asexperimental and theoretical developments from other mobility research programs at other Army agencies, could beused to make limited predictions of vehicle performance in other areas of the world. Based on the premise that those
  4. 4. essential factors necessary to evaluate vehicle performance in a given environment could be quantified, a cross-countryprediction model was structured whose three primary elements of vehicle, terrain, and driver could be logicallystructured to take advantage of all the WES-derived vehicle/terrain interactions measured previously. The programneeded a proponent, and in 1971 The Army Materiel Command (AMC) stepped forward and requested that the threeArmy laboratories engaged at that time in mobility research (WES, the US Army Tank-Automotive Command(TACOM), and CRREL) cooperate to achieve a common goal. Thus, the groundwork was laid for development of afirst-generation computerized ground mobility model [1] to be known as the AMC-71 Mobility Model, or simply AMC-71. The physics-based, sum-of-forces model recognized the three primary elements in the mobility equation; thevehicle, the terrain, and the driver. It was noted that each required independent analysis and quantification, but thatinter-relationships were required in order to predict vehicle performance by summing tractive and resistive forcesrequired in a given terrain. Data bases for the most common military vehicles were developed, including specificationsof geometric and mechanical characteristics that could be easily computerized. Terrain modules consisted of relativelysmall, homogeneous “patches” characterized by some thirteen measurements reflecting soil type and strength, slope,surface roughness, obstacle geometries, and vegetation [2] derived from previous studies, especially the MERS researchin Southeast Asia. Linear features that crossed the terrain, such as streams or walls, were characterized as to width,depth, velocity, type, strength and material properties, and other elements. All of the terrain features were thencombined on detailed terrain ‘maps’ to present an overall mathematical interpretation of any area. Driver informationsuch as reaction time, acceleration-limited ride and shock performance, braking, and visibility were also incorporated. Amethodology was developed to depict vehicle performance for specific travel paths and for general cross-countrymovement. Thus, if areas anywhere in the world could be characterized using available data, vehicle performance couldbe accurately predicted. If the direction of movement over the terrain was known, then a more accurate traverse or trailspeed could be made that incorporated vehicle direction and interaction with each successive terrain unit encounteredthrough typical accelerations or decelerations that occurred as the terrains were challenged. Following development, a three-year validation program was initiated to refine the model’s relationships for amilitary jeep, a 2-1/2-ton truck, an M113 personnel carrier, and M48/M60 tanks [3]. Test sites included Fort Sill,Oklahoma; Eglin AFB, Florida; Yuma Proving Ground, Arizona; Houghton, Michigan; and Fort Knox, Kentucky.Results indicated that the initial model was about seventy percent accurate, and shortcomings were in quantification ofterrain surface roughness, obstacle override, and vegetation influences, and it was noted that driver performanceobviously exerted a tremendous influence on vehicle performance. This first-generation model also was successfullyused in ‘Project Wheels’ which resulted in identification of equivalent capabilities among the Army’s vehicle systems,and the subsequent elimination of several duplicate systems. This program demonstrated the power of this newtechnology and resulted in significant cost savings due to the vehicle systems eliminated with no reduction inperformance capability. By 1975 more refined quantification of terrain areas and improved vehicle-terrain interaction relationships led tothe second-generation product designated Army Mobility Model-74, or AMC -74 [4]. This version described each aerialunit through twenty-two mathematically–independent terrain factors rather than the previous thirteen. Ten classificationfactors for linear features and an additional nine for roads further enhanced the model. Whereas AMC-71 assumed allrunning gears of a vehicle were powered, geometrically identical, and equally loaded, AMC-74 could simulate vehiclesand vehicle combinations having various configurations of powered, braked, and towed wheels and tracks with variousloads. This version also contained equations that allowed simulations of travel across slippery soils, muskeg, and snowin addition to the fine- and coarse-grained soils covered by AMC-71. Additionally, AMC-74 provided more detailedand accurate quantification of driver behavior, one of the shortcomings of the earlier model. It had been recognized by the North Atlantic Treaty Organization (NATO) as early as 1976 that a need existedfor a ‘standard’ mobility model for member countries to use for comparing overall vehicle performances in terms ofmobility, armor protection, and fire power. In 1977, the model was offered as the solution for mobility comparisonswith recommendations for improvements in certain submodels. After a period of research and revision by the membercountries, the improved model soon found acceptance abroad. In 1978, NATO adopted AMC-74 and its subsequentrefinements for use by member countries as an ‘initial reference’ model for comparing current vehicles among membercountries, and also as a tool for planning and designing new vehicles. By 1979, the now internationally-sanctioned
  5. 5. mobility model became known as the NATO Reference Mobility Model, or NRMM [5]. Research and developmentshave continued and by 1992, the second version, NRMM II [6], was adopted and refinements continue to this day. 3. USE OF NRMM FOR VEHICLE PROCUREMENT AND COMPARISON During the late 1970’s and early 1980’s, the US embarked on development and acquisition of a new fleet ofvehicles to replace the aging jeeps, trucks, armored personnel carriers, and tanks of the 50’s and 60’s. Although NRMMwas not directly tailored for ‘building’ a vehicle per se, the USA Training and Doctrine Command (TRADOC) andUSA TACOM soon found that NRMM could be used to specify requirements for expected vehicle performances fornew vehicles or upgrades of existing vehicles. Three representative terrain scenarios developed previously to showcaseNRMM (central Germany, arid areas of the Middle East, and areas of interest in Korea) soon gained acceptance forevaluation of expected performances in these and analogous areas of the world. Studies began by TRADOC to deriveattributes for future vehicles using these terrains, and TACOM used NRMM mobility specifications and testingprocedures to derive required vehicleperformances within the contracts forprocurement. Figure 4 shows a PalletizedLoading System during Product QualificationTesting performed by ERDC. Most of the USvehicles still in use today were procuredthrough this process. Additionally, terrainstatistics from the study areas (percent slopeoccurrences, obstacle descriptions, vegetationstem sizes and spacings, soil strengths duringthe various seasons, etc) could be used basedon their occurrences within the terrain tospecify operational scenarios for vehicles inthese terrains [7]. Likewise, a reverse-engineering process has been used to deriverequired vehicle attributes for operations inspecific areas, generally leading to trade-off Figure 4. PALLETIZED LOADING SYSTEM. Tested in 1989 atanalyses of performance versus cost. ERDC for Production Qualification Testing. The PLS is undergoingUtilization of NRMM in this process over drawbar pull testing on a clay soil.some 25 years led to the US possessing thefinest fleet of military vehicles in the world today. 4. APPLICATION OF NRMM TO ROBOTICS AND SENSORS Currently, cooperative research at the ERDC and TACOM is focused on extension of NRMM to include smalleror mini-robotic vehicles while understanding that NRMM was developed for vehicles of 500-600 lbs and larger(generally vehicles that could be operated by a man onboard). Whereas the physics-based logic is appropriate forsmaller (and larger) vehicles, some of the model’s internal empirical relations may be extended to enhance modelaccuracy. For example, the effects on very small vehicle performance of small vegetation (trees less than 2-3 inches indiameter, tall grass, etc) or small obstacles (stones instead of boulders, stumps, small ditches or walls, etc) is not fullyquantified. Preliminary research indicates additional terrain classes may be required in order to make accuratepredictions or reverse-engineer robotic requirements, unless most future robotic vehicles will be expected to avoid allpotential immobilizing terrain elements, which does not appear logical. Obviously, inexpensive, disposable or tele-operated robots for use in mine detection or other dangerous tasks may not require extended analysis prior to use.However, if we assume that some 40 percent of the Future Combat System (FCS) vehicles will be robotic orautonomous, it would seem logical that the technology base that has been responsible for mobility research for some 50years would be consulted regarding an engineering approach to vehicle development rather than leaving suchdevelopment solely to a winning contractor who has little historical background in the field. In this respect,
  6. 6. modifications made to NRMM will only enhance future vehicle designs and should bolster a research area dedicated tomaximizing robotic and autonomous vehicle performances. In the pursuit of this goal, the ERDC has been developing a new physics-based terrain mechanics model that canbe used in multibody dynamic environments. The terrain mechanics model builds off the historical information used inNRMM but creates a more accurate and robust mobility prediction engine as shown in Figure 5. The heart of thismodel is the Vehicle Terrain Interaction (VTI) that can be used for onboard maneuver decision logic. The terrainmechanics model was developed for the real-time simulation environment and primarilyfor implementation into a multibodydynamics engine that drives motion-basedplatforms that can also link into the Semi-Automated Force (SAF) simulations such asthe OneSAF Objective System. The fidelityof the VTI is such that it can also serve as themobility engine for onboard maneuverdecision logic. When the VTI is coupled withthe onboard vehicle controller, it offers therealism of mobility performance predictionsthat can assist in maneuver decision logic forpath selection. This will enhance the abilityto select the best path from the “GO” pathcorridor. The VTI is designed to evaluate themobility potential at the wheel or tireinterface by describing the traction potentialof each terrain node under the traction Figure 5. HIGH FIDELITY VEHICLE TERRAIN INTERACTIONelement. This level of traction analysis is MODEL. HMMWV crossing a gap. The simulation shows tractioncritical for obstacle negotiation, gap crossing, and pressure distrubitions under the wheels and the rear of the vehicleor border-line “NOGO” situations. in contact with the terrain. Historically, the principal weakness in the various physics-based vehicle performance modeling systems hasbeen an inability to map terrain of sufficient area for practical military applications at the fidelity required to accuratelyassess vehicle movement at reasonable cost. The area of greatest research interest for such applications is developmentof procedures to evaluate and collect terrain data in areas of interest via remote and/or onboard sensors while the vehicleis in an operational context. The costs associated with development of terrain data sets of the quality and fidelity neededfor accurate vehicle predictions by NRMM in 1970’s dollars were on the order of $100,000-150,000 for developmentand validation of a 1:50000 quadrangle map sheet (about 20 km X 20 km) for use in tactical operations. Those costs areobviously substantially higher today, and the addition of characteristics to predict very small vehicle performanceswould increase this cost. There appears to be little interest among vehicle developers or the military in providing thefunds required to produce such data sets, and perhaps some underlying political pressure not to “map’ specific areas ofthe world in order not to single out potential areas of operation by US forces or unduly rile suspect countries. However,the model’s utility is based on supplying it with engineering physical properties of the terrain in order to obtainmeaningful predictions. Thus, the suggestion to research the area of processing static and onboard sensor informationalong the route traveled by a scout vehicle in order to develop relatively accurate assessments of the terrains beingchallenged and the logic required to negotiate the mission in the area. Based on the many years of expertise garnered from mobility research on a myriad of conventional andunconventional vehicles, the ERDC has expertise in the production of ‘typical’ high-fidelity terrain information,especially that sensitive to mobility and maneuver, which would be useful for overall system performance evaluation.Although much more basic and applied research is still required to create actual ‘mapped’ areas from sensor dataanywhere in the world while ‘traveling’ in the terrain, the logic required to accomplish this task is currently a proposedarea of ERDC research starting in 2007. Statistical extrapolation of ‘measured path’ tractive and resistive data tosurrounding terrains obviously will require more research in order to develop a capability to ‘map’ operational areaswith sufficient accuracy to take advantage of the available vehicle models. However, the potential of a capability for
  7. 7. defining a mission, identifying the characteristics of the operational environment, identifying sensitive environmentaleffects that will affect vehicle movement, and then developing the methodology to quantify the sensitive parameters viaonboard or remote sensing techniques seems well worth the research effort! How would we expect all of this to work in a sensor environment? The sensor technology could be used as asubstitute for human ‘sensors’ for remotely controlled or autonomous vehicle operation. In this manner, sensed datacould be translated into soil physical properties (type, strength, moisture condition, etc.) via remote means, or ‘look-ahead’ schemes could be developed to assess terrain impediments before the vehicle encountered them, or sensed datacould provide ‘near-area’ awareness to provide levels of autonomous capabilities (terrain information or location, othervehicle positions, relevant mission characteristics, etc). The sensor technology also could be used to enhance orsupplement the human capabilities for legacy vehicle tasks, such as providing enhanced wide-area assessment of actualor impending terrain conditions, enemy locations or other information for tactical operations, or related scout orreconnaissance missions. Based on the lengthy experience described earlier, the ERDC’s contribution would center onincreasing the fidelity and decreasing the resources required in obtaining the various information necessary to assess thephysical impediments to vehicle movement relative to the mission. The ERDC would also offer a specialty inquantifying vehicle system missions, identifying sensitive parameters associated with operational areas and missionrequirements, and quantifying the impact of the fidelity of sensitive vehicle movement parameters on mission success. One mission concept that could be advanced if onboard sensor technology could be used for terraincharacterization is the concept of “collect, use, and disseminate”. The process of planning missions is based on usingthe best available terrain information to determine the best “GO” corridor for the mission and adjust the missiononboard as it unfolds. For autonomous operations, the mission adjustment is based on maneuver logic using real-timeinformation from onboard sensors either stand-off or tactile. The accuracy of the mission adjustment is based on theamount of terrain information available and the onboard maneuver logic. More terrain information allows for higherfidelity onboard terrain mechanic models, which in turn will assist the maneuver decision logic in making a moreaccurate assessment of the “GO” path selection. Once the best path is chosen from all possible “GO” paths, the localterrain information can be disseminated and used for developing mobility data layers that can update maneuver supportinformation. The biggest problem in this scenario is quantifying the performance pay-off. We know that more terraininformation will result in better maneuver decisions but by how much is yet to be determined. It may only be a 10percent improvement over current abilities, but that 10 percent may be the difference in a successful mission versus amission failure. 5. SUMMARYWith a background of more than fifty years in mobility engineering research, the ERDC offers a unique and valuableresource for accurately evaluating and predicting the performances of actual or concept vehicles in various strategicareas of the world. Possibilities currently being developed for accessing and collecting important terrain information viaonboard or tactile sensors in concert with scout or reconnaissance robots should indicate that the ERDC’s pastexperience in terrain and vehicle quantification, whether by on-the-ground surveys or sensor–collected data, shouldmake it a valuable member of any maneuver support vehicle- or sensor-development team, whether government orindustry. 6. REFERENCES[1] Rula, A. A. and Nuttall, C. J. Jr. 1971 (May), “An Analysis of Ground Mobility Models (ANAMOB)”, TechnicalReport M-71-4, US Army Engineer Waterways Experiment Station, Vicksburg, MS[2] Shamburger, J.H., Grabau, W.E., Vol. I-Vol. VIII 1967-1968, Mobility Environmental Research Study, “AQuantitative Method for Describing Terrain for Ground Mobility”, US Army Engineer Waterways Experiment Station,Vicksburg, MS
  8. 8. [3] Schreiner, B.G. and Willoughby, W. E. 1976 (Mar), “Validation of the AMC-71 Mobility Model”, TechnicalReport M-76-5, US Army Engineer Waterways Experiment Station, Vicksburg, MS[4] Jurkat, M. P., Nuttall, C. J., Jr., and Haley, P. W., 1975 (May), “The AMC’74 Mobility Model”, Technical Report11921 (LL-149), US Army Tank Automotive Command, Warren, MI[5] Haley, P. W., Jurkat, M. P., and Brady, P. M., Jr. 1979 (Oct), “NATO Reference Mobility Model, Edition I, UsersGuide”, Volume I, Operational Modules and Volume II, Obstacle Module, Technical Report 12503, US Army Tank-Automotive Research and Development Command, Warren, MI[6] Ahlvin, R. B. and Haley, P. W. 1992 (Dec), “NATO Reference Mobility Model , Edition II, NRMM II User’sGuide”, Technical Report GL-92-19, US Army Corps of Engineers Geotechnical Laboratory, Vicksburg, MS[7] Willoughby, W. E., Jones, R. A., (in publication), “US Army Wheeled Versus Tracked Vehicle MobilityPerformance Test Program”, Vol I-IV, US Army Engineer Waterways Experiment Station, Vicksburg, MS 7. ACKNOWLEDGMENTThe tests described and the resulting data presented herein, unless otherwise noted, were obtained from research underthe Terrain Mechanics Modeling research program of the US Army Corps of Engineers by the Engineer ResearchDevelopment Center, Geotechnical and Structures Laboratory. Permission was granted by the Director, Geotechnicaland Structures Laboratory to publish this information.