effect of near-surface hydrology on soil strength and mobility

Geological Society, London, Special Publications

doi: 10.1144/SP362.17p301-320.

2012, v.362;Geological Society, London, Special Publications Jody D. Priddy, Ernest S. Berney IV and John F. Peters mobilityEffect of near-surface hydrology on soil strength and

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Effect of near-surface hydrology on soil strength and mobility

JODY D. PRIDDY*, ERNEST S. BERNEY IV & JOHN F. PETERS

US Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg,

MS 39180, USA

*Corresponding author (e-mail: [email protected])

Abstract: History has repeatedly demonstrated the potentially negative influence of near-surfacehydrology on military mobility. Increased moisture and saturation in soil results in a transition fromsolid to somewhat liquid states. As soil approaches the liquid state, the shear strength available forsupporting traffic of ground vehicles or aircraft diminishes. Historical engagements elucidate theimportance for armies to recognize soil conditions that could compromise manoeuvre. SinceWorld War II, the US Army has pursued research aimed at equipping soldiers with the tools andknowledge needed to account for the impact of near-surface hydrology on mobility. Significantportions of the research have been focused on characterizing soil trafficability as a controllingfactor in ground vehicle mobility and on developing methods for rapidly assessing soil conditionsto ensure adequate bearing capacity for expediently constructed roads and airfields. In contrast,hydrological conditions can also produce extremely dry soil with potential for surface layers tobreak down under ground vehicle and aircraft traffic loadings, resulting in a propensity forextreme dust generation, an entirely different problem for military mobility that the research hasalso been addressing. Mobility problems associated with these adverse soil conditions have notbeen eliminated, but the research has produced significant advancements.

Just after midday on 18 June 1815, near Waterloo inBelgium, a French army commanded by EmperorNapoleon finally pressed an attack that had beendelayed for several hours by ground too wet tofacilitate the movement of supporting artillery.The delay would prove disastrous, as it allowedtime for Prussian forces to join the opposingBritish troops led by the Duke of Wellington.Thus, Napoleon’s last great military thrust died onthe fields of Waterloo, as much the result of the vag-aries of the weather as the tenaciousness of coalitiontroops. With some irony, many of the victoriousBritish troops had suffered defeat at the Battle ofNew Orleans in the southern USA the precedingJanuary, in part because of the limited manoeuvrecaused by the cypress swamps that limited flankingmovements and also the reduced effectiveness ofartillery caused by the sodden ground. Bothbattles, which are arguably two of the most signifi-cant engagements of the 19th century, illustratethat the difference between success and failure laynot as much in the inability of the attacking forceto manoeuvre in wet conditions, but in the inabilityof commanders to recognize that those conditionswould compromise what should have been superiormilitary advantage. That lesson had been taught inancient battles, such as at Agincourt in medievalFrance. It was experienced again on more modernbattlefields, such as those of the Western Front inBelgium and France during World War I and atKursk in the Soviet Union during World War II.

An army that cannot manoeuvre cannot launch asuccessful attack.

The anticipation of meteorological, geologicaland hydrological conditions that might limitmanoeuvre is therefore a critical part of militaryplanning. This paper considers work that has beenperformed by the US Army, primarily since WorldWar II, to make rational assessment of conditionsand to improve the use of that information in theoperational environment to predict mobility. Inthis paper, the technical aspects of the problem arebriefly discussed, followed by a description of howconflicts beginning in World War II have drivenefforts to improve mobility planning and mitigateproblems associated with ground that is either toowet or too dry. In addition, modern warfare hascreated a need for expedient road and airfield con-struction, which requires methods to monitor soilmoisture, density and strength. The quest for suchprocedures parallels methods to evaluate mobilityconditions and will also be discussed.

The soil mechanics of mud and dust

Mud is a general term for a saturated soil when it haslost significant shear strength and behaves more likea fluid. Wood (2006) gives a very readable accountof the effect of mud on warfare, including the every-day plight of the common soldier. For the footsoldier, mobility is lost in wet soil when thesuction that must be overcome to pull one’s foot

From: Rose, E. P. F. & Mather, J. D. (eds) Military Aspects of Hydrogeology.Geological Society, London, Special Publications, 362, 301–320, DOI: 10.1144/SP362.17# The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

at Indiana University Libraries on July 1, 2014http://sp.lyellcollection.org/Downloaded from


from the mud exceeds the bearing capacity support-ing the other foot. For vehicles of modern warfare, itis the loss of traction and bearing capacity thatdegrades mobility. However, as important asstrength might be, other factors also bear on themobility assessment for the modern army, includingterrain and vegetation cover, off-road mobility, andtype and quality of roads. For aircraft, which wouldseem to be immune from mobility problems,landing zones must be maintained in all weather.Mechanized warfare has also brought about the‘dry soil’ problem of dust, which can limit visibilityand damage equipment. Both problems requireknowledge of hydrological conditions, soil types,and the ability to anticipate the effects of precipi-tation. Part of the difficulty in making assessmentsis the specialized knowledge needed to gather infor-mation and to make assessments on that informationonce it is gathered. Considerable effort has gone intodevising mobility assessment tools that can bereliably employed by non-specialists.

For military mobility, the effects of hydrologicalprocesses on soil moisture content in the uppermetre of the ground are important, largely becauseof the impact of soil moisture on soil strength. Avolume of soil is generally characterized as consist-ing of solid material (grains) and void space (voids)that contains air and water. At the low levels ofstress of relevance to mobility, volume changesare the result of reducing the volume of the voids,with the solid grains being relatively incompressi-ble. The moisture (or water) content of soil isdefined as the mass of water divided by the massof the solids, usually expressed as a percentage.The degree to which the void space is occupied bywater is referred to as its degree of saturation,which is defined as the volume of water dividedby the total volume of voids. The extent to whichsoil behaves as either wet or dry is controlled bythe degree of saturation rather than the moisturecontent. Increased moisture content to the point ofcomplete saturation typically results in decreasedshear strength and reduced soil bearing capacity,and this translates into a decreased ability of soilto support traffic of vehicles due to increasedvehicle sinkage and decreased traction capacity.Increased moisture content also results in a greateramount of free water within the ground, and this,together with decreased shear strength, translatesinto decreased vehicle traction potential. Freewater impacts vehicle traction, as is never moreevident than in vehicle manoeuvres on relativelyfirm ground during or shortly after rainfall events.Figure 1 illustrates the impact of soil moisturecontent and strength on the sinkage and traction per-formance of ground vehicles.

Soil derives its shear strength from friction,which is a general term describing the experimental

observation that the shear strength is proportional tothe confining stress (e.g. Lambe & Whitman 1969).In fact, the frictional resistance is derived from twosources: the Coulomb friction at grain contacts andthe tendency for granular material to change volumewhen sheared. Many relationships have been pro-posed to describe the relationships among strength,intergranular friction and volume change, althoughfor the present discussion a simple additive relation-ship suffices (see Fig. 2). If a soil is expanding (dilat-ing), work must be done against the confining stress,thus increasing strength. If the soil is contracting,the volume change detracts from the strength. Therelationship between shear stress and volumechange is illustrated in Figure 3, which also illus-trates the important fact that when stress reversalsare applied, the soil responds by contracting.

Whether the strength of the soil is obtained fromintergranular friction or from dilatancy would be ofmostly academic interest if the soil were dry, withthe exception of a few cases when the soil is

Fig. 1. Effect of soil moisture content and strength onvehicle sinkage and traction (notional illustration).The charts illustrate typical vehicle–soil interactionbehaviour for variations in vehicle sinkage (upper chart)and net traction (lower chart) v. soil strength andmoisture content. When the sinkage reaches a criticalmagnitude, the net traction becomes zero, resulting invehicle immobilization.

J. D. PRIDDY ET AL.302



confined (e.g. the soil surrounding a tunnel). Thetendency for soil to change volume when shearedhas profound effects when the soil is saturated.Water is virtually incompressible, constraining satu-rated soil to shear at a constant volume unless thesoil is very permeable or the load is applied veryslowly. For nearly impermeable soil, or for soilloaded quickly, the water that is trapped in thepore space inhibits volume change. The change inconfining stress required to maintain the constantvolume condition is reflected in the pressure in thewater through Terzaghi’s effective stress principle;that is, the change in pressure in the water is equalto the negative of the confining stress carried bythe solid grains. Therefore, when a saturated soil issheared in an undrained state, the pressure carriedby the water (pore water pressure) goes through atransition from increasing to decreasing that reflectsthe volume-change tendency of the granular solid.The degree to which the load is carried by thewater pressure is important, because water providesno shear resistance. Recalling Figure 3, particularlyimportant is the tendency for cyclic loading todensify soil. In the undrained state, this contractivebehaviour increases pore pressure, thus reducingconfining pressure to the point that the soil losesall strength and essentially flows like a fluid.

The important result of the previous discussion isthat the trafficability of a saturated soil, regardless ofits strength, will degrade under repeated loading byfoot or vehicle. The net result of loading is to causean increase in pore pressure, which reduces the shear

Fig. 2. Additive effect of friction f and dilatancy i on soil shear strength (notional illustration). Other symbols: s,normal stress; t, shear stress; g, shear strain; d, normal strain. The diagram on the left-hand side of the figure illustratesan infinitesimal cube of soil under a combined compression and shear loading with the resulting normal and shearstrain. The charts on the right-hand side of the figure illustrate typical soil behaviour for variations in shear stress v.shear strain (upper chart) and normal strain v. shear strain (lower chart) for this loading condition.

Fig. 3. Effect of stress reversal on volume change in soil(notional illustration). Symbols: t, shear stress; g, shearstrain. The charts illustrate typical soil behaviour forvariations in shear stress v. shear strain (upper chart) andvolume change v. shear strain (lower chart) for the sameloading condition shown in Figure 2 when stressreversals are applied for one unload cycle and onereload cycle.

EFFECT OF HYDROLOGY ON MOBILITY 303



strength, ultimately leading to a complete loss ofability to carry load or provide traction.

The usual state of surficial soils is to be unsatu-rated, usually by desiccation. Unsaturated soilderives additional strength from capillary forcesand intergranular cementation by soluble materialsdeposited from pore fluid as it evaporates. Capillaryforces are derived from surface tension between thewater surface in the voids and the solid grains, andincrease the forces between soil grains. More impor-tantly, the air within the pore space of a non-saturated soil is compressible, allowing volumechange without causing significant pore pressurechange. Thus, although traffic can break down inter-granular bonds, turning the soil to dust, it does notform mud. However, water need not occupy 100%of the pore space for the soil to behave as thoughit were saturated. In general, as water is added tosoil, resistance derived from capillary stress isreduced and the tendency to induce pore pressureis increased.

For fine-grained soils, the interplay betweenmechanical response and moisture content ischaracterized by the Atterberg limits, based on aprocedure to quantitatively classify the plasticityof clays that was adopted into soil mechanics byKarl Terzaghi and standardized by Arthur Casa-grande (1932). The ‘liquidity index’ scales themoisture content between the ‘liquid limit’, wherethe soil’s behaviour demonstrates a transition fromfluid to solid, and the ‘plastic limit’, where the soilexperiences a transition from plastic to solid behav-iour as characterized by its malleability. When theliquidity is near 1.0, the soil behaves as a liquid,whereas at a value of zero, it is effectively solid.

The ability to judge the moisture conditions inthe field requires a knowledge of soil mechanicsand the complexities of soil–water interaction thatexceeds the abilities both of soldiers performingreconnaissance and war planners. In the past 65years, the US Army has continually improved itsability to assess, plan for, and possibly mitigateadverse field conditions through a process that hasbeen shaped by the experience of various conflictssince the end of World War II.

The ground vehicle mobility problem

The ability of soil to support traffic of groundvehicles rose to the forefront of concern for Allied

forces near the end of World War II. After experien-cing significant difficulties with vehicles becomingimmobilized in various problematic soil conditionsin Europe, Africa and SE Asia, and with a pendinginvasion of the Japanese mainland, which containedmany rice paddies and other problematic soil con-ditions, the US Army launched significant researchaimed at quantifying the ability of soil to supporttraffic of vehicles (Knight 1957a). The Researchand Development Division of the US War Depart-ment General Staff assigned the research task tothe US Army Corps of Engineers (USACE). Theresearch was initiated in the summer of 1945 whenthe Office of the Chief of Engineers tasked the USArmy Engineer Waterways Experiment Station(WES)1 to study the problem, giving them abouttwo months to deliver the results. The Japanese sur-rendered and World War II came to an end prior tothe end of the initial trafficability study, but the USArmy committed to studying soil trafficability witheven greater resolve in a long-term USACE projectentitled ‘Trafficability of Soils as Related to theMobility of Military Vehicles’ to be conducted byWES (Anon. 1947–74; Fatherree 2006). The traffic-ability of soils project continued for about threedecades and led to USACE incorporating vehiclemobility into its ongoing research mission areasthat continue to this day.

Soil trafficability characterization

One of the significant results of the US Army’s soiltrafficability research was the development ofequipment and techniques for characterizing thestate of the ground as it relates to ground vehiclemobility. Various types of equipment were evalu-ated over the years for quantifying soil trafficability,including vane shear, direct shear, unconfined com-pression and triaxial shear test devices, but the mosteffective equipment evaluated was based on staticcone penetration testing (Anon. 1947–74). Theprincipal trafficability equipment was developedby WES and includes a handheld cone penetrom-eter, a piston-type soil sampler, and remouldingequipment (Anon. 1947–74; Knight 1957a; Anon.1994 & update in preparation). Figure 4 showsphotographs of soil trafficability equipment.

The trafficability cone penetrometer consistsof a circular cylindrical shaft (usually 18–36 in(46–91 cm) in length) with a 308 right circularcone mounted on one end and a calibrated

1The laboratories of the US Army Corps of Engineers Waterways Experiment Station (WES) were integrated with three

other Corps laboratories in 1999 to create the US Army Engineer Research and Development Center (ERDC). WES is

no longer an organization, but it is still the name of the campus in Vicksburg, MS where the headquarters and four

laboratories of the ERDC reside. References to WES imply the work was performed prior to the formation of the

ERDC.




load-measuring device on the other end. The shaftcurrently used for all soil conditions is 3/8 in(9.5 mm) diameter steel (a 5/8 in (15.9 mm) alu-minium shaft was previously also used). For low-strength soils, a cone with a 0.5 in2 (3.2 cm2) basearea is used, whereas a cone with 0.2 in2 (1.3 cm2)base area is typically used in high-strength soils.The output measurement is the average of pressurereadings in pounds-per-square-inch taken at speci-fied depths of penetration of the base of the coneinto the soil. The resulting average penetrationresistance is referred to as the cone index (CI) soilstrength. The depths of penetration used for traffic-ability assessments are usually those taken at thetop, mid-height and bottom of the critical layer.The pressure readings are the result of the pen-etration force divided by the base area of the conewith a standard penetration rate of 72 in/min(1.83 m/min).

The critical layer is a layer of soil lying near thenatural terrain surface that exerts the greatest influ-ence on soil trafficability. The depth of the criticallayer is dependent upon vehicle characteristics, thesoil type and the nature of the CI strength profilewith depth. A 6 in (15 cm) layer of soil is typicallyused, but sometimes a 12 in (30 cm) layer is used.WES developed criteria for estimating normal criti-cal layer depths for different types of vehicles. Thecritical layer criterion has varied over the yearsbased on the cumulation of additional vehicle per-formance data, and the most recent changes weremade in the 1990s. The critical layer for most

common military vehicles is the 6–12 in (15–30 cm) layer in fine-grained soils and the 0–6 in(0–15 cm) layer in coarse-grained soils.

The trafficability soil sampler is a piston-typesampling device that is used to obtain an undis-turbed sample in soft soil. It has a circular cylindri-cal tube with 1 7/8 in (4.76 cm) inside diameter thatis sharpened on the open end. The piston within thetube retracts during penetration into the soil suchthat a partial vacuum is maintained above thesample, preserving the soil’s in situ structure. It isused to collect soil samples from successive 6 in(15 cm) layers within the ground with samples typi-cally taken from the 0–6 (0–15), 6–12 (15–30) and12–18 in (30–46 cm) layers at a minimum. Thesamples are primarily used to conduct remouldingtests, but they can also be used to collect in situmoisture and density samples.

The remoulding equipment consists of a circularcylindrical tube mounted on a steel base and a drophammer. The sample tube has a 1 7/8 in (4.76 cm)inside diameter, and the open end mates tightlywith the open end on the trafficability soilsampler. The drop hammer weighs 2 1/2 lb(1.1 kg) and has 12 in (30 cm) of drop travel. Inuse, soil samples c. 6 in (15 cm) in height areinserted into the tube using the sampler. For fine-grained soils, CI measurements are taken in thecentre of the sample before and after 100 blows ofthe drop hammer. The CI measurements are basedon readings taken in the sample at depths of 0,1 (2.5), 2 (5), 3 (7.6) and 4 in (10 cm). The output

Fig. 4. Photographs of soil trafficability equipment for rapid field evaluation of soil strength. Approximate heightdimensions (as shown): remoulding drop hammer assembly, 0.6 m; cone penetrometer, 0.7 m; soil sampler,1.1 m. Courtesy of the US Army Engineer Research and Development Center, Vicksburg, MS.




measurement is the ratio of the average CI measuredafter 100 blows over the average CI measuredbefore blows, and the resulting ratio is referred toas the remould index (RI). When densificationoccurs, RI measurements can exceed unity, inwhich case the potential strength gain is ignoredfor trafficability assessments.

Figure 5 illustrates how measurements are madewith the trafficability equipment. Soil trafficabilityis assessed using the results of the cone penetrom-eter and remoulding equipment measurements.The average CI measurement resulting from numer-ous cone penetrometer readings distributed aroundthe area of interest provides the initial soil strengthmeasurement for fine-grained soils or the final soilstrength measurement for coarse-grained soils.The average remould index measurement resultingfrom various remoulding tests distributed aroundthe area of interest provides an indication of thepotential for strength loss under vehicle traffic. Forfine-grained soils, the final soil strength is deter-mined by multiplying the average CI measurementby the average RI measurement to determine therating cone index (RCI) soil strength for the layerof interest.

Cone penetrometer-based soil state measure-ments are ideally suited for trafficability assess-ments primarily due to the relative ease withwhich numerous soil strength measurements can

be conducted to characterize an area of interest.Soils are highly variable in the top metre of soildue largely to deposition processes. Spatial areasof relatively low size (e.g. 12 × 36 ft (3.7 ×11 m) vehicle lanes) with seemingly uniformcharacteristics and a flat, level surface can havewidely variable constituent layering, moisture pro-files and shear strength zones. High spatial variabil-ity in the soil characteristics related to bearingcapacity can result in wide variations in vehicle per-formance, with differential sinkages of severalcentimetres occurring easily in relatively shortvehicle lanes. The higher the moisture content andthe lower the soil strength, the higher the variabilitytends to be in terms of its impact on trafficability.

High variability in soil strength leads to arequirement for quantifying the soil state based onnumerous samples to develop a representativeaverage soil strength that relates well to vehicle per-formance. Cone penetrometer measurements canaccount for not only the spatial variability over thearea of vehicle lanes but also the three-dimensionalspatial variability that is captured by the penetrationresistance profiles with depth. Other soil statemeasurement devices such as vane shear or triaxialtests equipment do not offer nearly as effective sol-utions for capturing the soil variability on vehicleperformance. Far fewer samples can be assessed inthe same timeframe as with a cone penetrometer.

Fig. 5. Illustrations showing example measurement procedures for soil trafficability equipment. From Anon. (1994 andupdate in preparation), courtesy of the US Army Engineer Research and Development Center, Vicksburg, MS.




Moreover, many of the other soil test measurementalternatives do not offer the potential for immediateassessment of the soil state as is the case with conepenetrometer measurements.

The CI and RCI soil strengths as determinedusing the WES trafficability equipment provide arelative indicator for the state of the soil in termsof its potential to support the traffic of vehicles(Freitag 1968). These cone penetration-based soilstrengths are primarily related to the soil type andthe moisture content in the soil, as influenced bythe hydrological processes in the ground (Knight1957a; Smith 1964). When the soil moisturecontent is higher than the liquid limit, CI and RCIsoil strengths will be relatively low, and when thesoil moisture content is lower than the plasticlimit, CI and RCI soil strengths will be relativelyhigh. Therefore, the cone penetration-based soilstrengths provide a relative indicator of theamount of moisture present in the soil andvice versa.

Forecasting soil trafficability

Measurements with the soil trafficability equipmentare very effective when they can be carried out as inthe cases of secure route reconnaissance missions,testing for vehicle mobility performance evalu-ations and comparisons, and research for thedevelopment of mobility modelling capabilities.However, trafficability measurements can rarelybe conducted to support broader strategic militaryplanning or operations in tactical areas of interest,which span vast areas of terrain. Therefore,methods to infer soil type and moisture content aretypically employed to estimate CI and RCI soilstrengths for assessing the ability of militaryvehicles to perform manoeuvres that ultimatelyimpact broader mission capabilities. Soil type cannormally be assumed to be static over long time-scales of the order of decades or centuries dependingon geological processes in an area of interest, butmoisture content can vary significantly due tovarious hydrological processes in relatively shorttimescales of the order of days or even hours. There-fore, the dynamic nature of soil moisture contentmakes it the key parameter of interest when attempt-ing to forecast potential variations in the state of theground (and ultimately vehicle performance) forareas where ground truth measurements are notpractical or possible.

The US Army, like many other agencies aroundthe globe, has developed methods to predict vari-ations in soil moisture content for consideration ofimpacts on various applications. In the case ofground vehicle mobility, the US Army realizedthe need to forecast the state of the ground in theabsence of physical measurements during the

planning stages for the ‘Trafficability of Soils asRelated to the Mobility of Military Vehicles’project. This realization prompted the project plan-ners to make it an equally important yet separatephase of the overall project along with the phaseto quantify the ability of soil to support traffic ofvehicles (Anon. 1951–71). Responsibility forinvestigating both of these phases of the overallproject was assigned to WES. Research on forecast-ing the state of the ground for trafficability purposeswas initiated by WES in the summer of 1948, whichallowed three years for research in the other phase ofthe project to identify trafficability measurementequipment and procedures as well as the criticalsoil parameters related to vehicle mobility.

To predict temporal variations in soil moisturecontent requires appropriate consideration ofvarious contributing processes in the hydrologicalcycle such as precipitation, evaporation, transpira-tion, surface runoff and hydraulic conductivitywithin different layers of the ground. The degreeto which the contributing processes must beaccounted for depends upon the particular appli-cation and the temporal and spatial resolutionsdesired in the forecast. Seasonal weather patternscan be used to estimate the likelihood of certainhydrological conditions that affect soil moisturecontent, and this information can be used to quantifythe likelihood of occurrence of various soil moisturestates over time. Data from available meteorologicalobservation stations can be used with numericalmodels for predicting the various incrementalphases of moisture migration due to hydrologicalprocesses at the ground surface and within theground subsurface layers down to the water table.In addition, various sensing technologies based onsatellite, aerial and ground platforms can be usedto estimate soil moisture conditions at specificpoints in time with potential for frequent or near-continuous sampling over time. Regardless of themethods used to estimate variations in soil moisture,a key requirement for soil trafficability predictionsis to predict soil strength based on the moisturecontent.

During nearly three decades of research onmethods of forecasting trafficability of soils, WES,with significant contributions from the ForestService of the US Department of Agriculture, devel-oped methods for predicting soil moisture contentand strength for use in soil trafficability predictions.The composite of the forecasting research resultswas ultimately packaged into an automated modelfor predicting soil moisture and soil strength,referred to as SMSP, in 1973 (Smith & Meyer1973). This model was modified and improvedover time based on the accumulation of additionaldata and the development of improved method-ologies with major updates in 1988 (Updated




SMSP) (Kennedy et al. 1988) and 1997 (SMSP II)(Sullivan et al. 1997). A key component of this mod-elling capability is a set of relationships that predictCI and RCI soil strengths based on moisture contentin soil layers critical to trafficability. The relation-ships are based on field and laboratory data collectedat over 1000 sites in the USA, Panama, Costa Ricaand Puerto Rico (Morris 1994; Sullivan et al.1997), and they predict somewhat conservativecone penetrometer-based soil strengths for allmajor Unified Soil Classification System (USCS)soil types (Anon. 1953 [revised 1960]). The USCSis a soil classification system in which soils areclassified according to their performance as con-struction materials for engineering purposes basedon soil particle size distribution and the effect ofsoil moisture content on plasticity behaviour.Figure 6 shows an example of some of the predictionrelationships for RCI soil strengths. With capabili-ties to measure or forecast soil strength for traffic-ability predictions, the only remaining requirementfor understanding vehicle mobility in soil is thecapability to predict vehicle performance onvarious soil conditions.

Predicting ground vehicle mobility

During the three-decade project on soil trafficabil-ity, WES also developed methods to characterizesoil trafficability in terms of its impact on vehicleperformance. The primary vehicle performanceconsideration was the lower limit of a vehicle’soperating capability in soil, where immobilizations

occur. The lower limit of performance was quanti-fied as the minimum soil strength on which avehicle could consistently make a specifiednumber of passes without getting immobilized,and this performance metric was referred to as thevehicle cone index (VCI) because it represents thecone index (CI or RCI) required for a particularvehicle to effectively conduct manoeuvres. EarlyVCI performance was based on the limiting soilstrength for a convoy of 50 identical vehicles to suc-cessfully cross an area of soil with all of the vehiclesdriving on the same path (i.e. 50 vehicle passesthrough the same tyre or track ruts). However, thefocus later shifted towards identifying the limitingsoil strength for a single vehicle to successfullycross an area of soil (i.e. one vehicle pass), whichrepresents the absolute lower limit of vehicle per-formance. Once VCI performance measurementshad been collected for a wide range of vehicletypes, weights and critical dimensions (e.g. tyreand track size and shape), relationships were devel-oped for predicting VCI performance for anywheeled or tracked vehicle (Priddy 1995).Figure 7 shows examples of VCI performancerelationships.

Predictions of limiting soil strength based onVCI allow military planners to understand wherevehicles can and cannot conduct manoeuvres in tac-tical areas of interest. For areas where vehicles canoperate, the next logical consideration is how well.To address this consideration, the US Armyexpanded the research areas assigned to USACEbeyond the bounds of the original soil trafficability

Fig. 6. Example relations for variation in rating cone index (RCI) soil strength with gravimetric moisture content(Sullivan et al. 1997). The curve labels are USCS soil types (Anon. 1953 [revised 1960]). Unit conversions:1 psi ¼ 6.9 kPa. Courtesy of the US Army Engineer Research and Development Center, Vicksburg, MS.




project by initiating numerous other projectsfocused on the fundamentals of vehicle mobilityperformance. The USACE projects for mobility per-formance in soil were conducted by WES using abroad array of different wheeled and trackedvehicles and single wheel and track dynamometers,and the projects focused on developing techniquesfor quantifying the fundamental vehicle–soil inter-action performance quantities that affect theability of vehicles to develop productive motion insoil (Fatherree 2006). The quantities includedmetrics such as net traction, motion resistance, slipand sinkage, and they provide an understanding of

all the forces and speed-limiting effects acting onthe tyres and tracks of vehicles at the vehicle–soilinterface. WES developed measurement and predic-tion methodologies to determine these fundamentalquantities as a function of soil trafficability charac-teristics such as soil strength (CI or RCI), soil typeand soil slipperiness resulting from rainfall effects(Priddy 1995). Examples of these relationships areshown in Figure 8.

The composite of the prediction methodologiesfor limiting soil strengths and for fundamentalvehicle performance quantities was ultimately pack-aged into an automated model for predicting

Fig. 7. Example relations for predicting vehicle cone index (VCI) performance (Priddy 1995). The mobility index(MI) is a parameter that accounts for the influence of vehicle characteristics on VCI performance. The deflectioncorrection factor (DCF) is a parameter that accounts for the effect of tyre deflection (i.e. inflation pressure) on VCIperformance. The wheeled vehicle performance relation is applicable for both radial and bias ply tyre construction asdemonstrated on the upper chart. Unit conversions: 1 psi ¼ 6.9 kPa. Courtesy of the US Army Engineer Researchand Development Center, Vicksburg, MS.




mission effectiveness for military vehicles in termsof maximum speed capabilities. The model wasdeveloped based on an initiative by the US ArmyMateriel Command (AMC) to integrate the bestmobility performance prediction methodologiesfor ground vehicles developed by the three USArmy organizations involved in vehicle mobilityresearch and development (Anon. 1973; Fatherree2006). The organizations included WES, the USArmy Tank Automotive Command (TACOM) andthe US Army Engineer Cold Regions Researchand Engineering Laboratory (CRREL, now one ofthe seven laboratories that comprise the ERDC),and the resulting model provided comprehensivemobility predictions for mission-based speed per-formance for ground vehicle operations on soilsand on roads for various seasonal terrain conditions.The model also included all of the majorspeed-limiting factors for vehicle manoeuvres oncross-country terrain and on roads such as obstacleand vegetation override and avoidance manoeuvres,linear feature gap crossings with and withoutshallow and deep water (still and flowing), longi-tudinal and side slopes, driver visibility and reaction

times, driver vibration limits in rough terrain, driveracceleration limits crossing discrete obstacles, andpath curvature on roads and trails. The original auto-mated model was developed in 1971 and wasreferred to as the AMC-71 (Anon. 1973), whichwas updated in 1975 to incorporate variousimprovements (Army Mobility Model, AMM-75)(Anon. 1975; Fatherree 2006). Soon thereafter, themodel was further improved and became endorsedfor use beyond the limits of the USA when it wassanctioned for use by the North Atlantic TreatyOrganization (NATO) in 1978 and became knownas the NATO Reference Mobility Model (NRMM)(Haley et al. 1979), which was later upgraded in1992 to include additional improvements (NRMMII) (Ahlvin & Haley 1992).

The mobility modelling capabilities representedwithin the AMC-71, AMM-75 and different ver-sions of the NRMM have been used extensivelyby the US Army for various applications involvingground vehicle design and performance predictions.The mobility models have been used to supportground vehicle acquisition at various phases ofthe process to include requirements definition,

Fig. 8. Example relations for predicting fundamental vehicle–soil interaction performance quantities (Priddy 1995).Drawbar pull and motion resistance coefficients are fraction of gross vehicle weight. Sinkage coefficient is fractionof tyre diameter. The excess soil strength (RCIX) parameter equals RCI soil strength minus one-pass vehicle coneindex (VCI1). The tyre numeric is a dimensionless parameter that accounts for the influence of tyre characteristics andsoil strength on sinkage performance. The soil descriptions include symbols for USCS soil types (Anon. 1953 [revised1960]). Unit conversions: 1 psi ¼ 6.9 kPa. Courtesy of the US Army Engineer Research and Development Center,Vicksburg, MS.




specification of key performance parameters, andtest and evaluation. In particular, the mobilitymodels had significant impact on the design of theM1 Abrams Tank, the High Mobility Multi-purposeWheeled Vehicle (HMMWV), the M2/M3 BradleyFighting Vehicle Systems, and various other groundvehicle platforms (Fatherree 2006). The capabilityto incorporate results from physical testing withdemonstrator testbeds, pre-production units, pro-duction units and so on as input data into themodels has been used to facilitate even greater accu-racy for projecting mission effectiveness of militaryvehicles operating in areas of interest that canencompass vast areas of terrain. The mobility mod-elling capabilities have also been embedded invarious battle command and control systems fortactical-level decision support requiring terrain traf-ficability analysis and in various types of wargam-ing modelling and simulations for strategic-levelforce effectiveness assessments.

As summarized above, by committing itself to along-term study of soil trafficability in the USACEproject entitled ‘Trafficability of Soils as Relatedto the Mobility of Military Vehicles’ conducted byWES, the US Army developed methodologies topredict the ability of soil to support traffic ofvehicles and methodologies to forecast the state ofthe ground in terms of trafficability characteristics.By the conclusion of the soil trafficability projectand based on the results of numerous other projectson mobility performance in soil for a wide array ofvehicles and single wheel and track dynamometers,methodologies had also been developed for predict-ing other fundamental vehicle performance quan-tities that control the speed performance ofwheeled and tracked vehicles in soil. The initialgoal of the trafficability research was to allow theprediction of areas of terrain where immobilizationsmight occur so that they could be avoided in militaryoperations, but the goals were broadened over timeto include prediction of mission effectiveness formilitary vehicles in terms of maximum speed capa-bilities. The resulting (and continuously evolving)mobility modelling capabilities have been used inthe design of military vehicles with demonstratedmanoeuvre dominance on the battlefield and havebeen embedded in battle command and controlsystems and force-on-force wargaming simulations.The soil moisture and strength forecasting method-ologies in combination with the vehicle perform-ance prediction methodologies describe the criticallink between the hydrological cycle and groundvehicle mobility.

Vehicle designs for problem soils

Since the advent of the automobile, there have beennumerous attempts to develop ground vehicle

concepts that could overcome the most extreme soilconditions resulting from the natural hydrologicalcycle and human activity. In the 1960s, the USArmy investigated various ground vehicle concepts,which were largely designed for operations in extre-mely soft soils, through mobility performancetesting conducted by WES. Some of the conceptvehicles had unusual propulsion assemblies such asthe Marsh Screw Amphibian (Knight et al. 1964),which had an Archimedean Screw running gear, theXM759 Logistical Carrier (Schreiner & Rula 1968),which had a low-pressure pneumatic-tyred tracksystem, and the Lockheed Terrastar (Robinson &Rush 1968), which had major/minor tri-wheeledrunning gear assemblies (Fig. 9). Some of theconcept vehicles involved modifications to existingstandard military vehicles such as a modified M1514 × 4 Military Utility Tactical Truck (MUTT)(Nuttall et al. 1966), which was retrofitted withlarge, high-flotation tyres (Fig. 10). In one particularcase, concept vehicles, two wheeled and one tracked,were designed under the Mobility Exercise A(MEXA) Program (Schreiner 1971) with principalemphasis in the vehicle designs placed on soil traffic-ability concepts and the potential benefits of articula-tion (Fig. 11).

The performance capabilities of these conceptvehicles, ranging in gross vehicle weight (GVW)from 3500 lb (1588 kg) to 19 700 lb (8936 kg),demonstrated that they could operate as desired onvery soft soils. As shown in Table 1, the measuredone-pass VCI capabilities for the concepts rangedfrom c. 0 psi for the Marsh Screw Amphibian andXM759 Logistical Carrier to 12 psi (82.7 kPa) forthe MEXA 8 × 8 platform, with most of the conceptsable to operate on RCI soil strengths below 10 psi(68.9 kPa). The Marsh Screw Amphibian andXM759 Logistical Carrier concept vehicles alsodemonstrated fifty-pass VCI capabilities of about5 psi (34.4 kPa) and 2 psi (13.8 kPa), respectively.These VCI capabilities are far better than those oftypical military vehicles in use today, and some ofthe concept vehicles could even operate on soil con-ditions that most people would find difficult orimpossible to walk on. (To put these values intoperspective, a standing man imposes a contactpressure of approximately 8 psi (55.2 kPa), a valuethat doubles when walking. A standing horse has anaverage contact pressure of 25 psi (172 kPa) thatincreases to 500 psi (3447 kPa) when galloping.)By comparison, the one-pass VCI capabilitiesfor the HMMWV are c. 18 psi (124 kPa) for theoriginal platform (7500 lb (3402 kg) GVW) and24 psi (165 kPa) for the up-armoured platform(12 200 lb (5534 kg) GVW), and the one-pass VCIcapabilities for typical, modern-day four-wheelerall-terrain vehicle (ATV) platforms (800–1100 lb(363–499 kg) GVW) are c. 4–8 psi (27.6–55.2 kPa).




Although the concept vehicles demonstrated impres-sive capabilities for operations on soft soil, none ofthem ever garnered much use by the US military dueto limited effectiveness on other types of terrain,limited speed capabilities or other types of dura-bility or mechanical issues. So the ingenuity of mili-tary developers has proven that the effects ofnear-surface hydrology on soil trafficability can belargely defeated, but the practicality of resultingground vehicle designs has thus far been limited.

The road and airfield construction problem

Up to now, the discussion has focused on vehicletrafficking on naturally occurring soils, but a large

volume of vehicle and aircraft movement occurson improved or manufactured soil surfaces. Com-pacted soil layers for both road and airfieldwearing surfaces and bearing layers require strictcontrol on moisture to achieve the necessarystrength to support the increased structuralloading. In natural soils, seasonal changes in satur-ation due to environmental changes cause soils tostiffen or soften. This change in strength is oftenaccompanied by volume changes in the soil, suchas shrinkage or swelling, due to capillary pressurechanges within the grain matrix. However, in man-ufactured soils, strength is obtained by compactingsoil, thereby removing air and pressing solidgrains of soil into a tighter matrix. To ensure thatconditions for mechanical compaction are optimal,

Fig. 9. Images showing concept vehicles with unusual propulsion assemblies that were designed for soft soils: MarshScrew Amphibian with Archimedean Screw running gear from Knight et al. (1964), XM759 Logistical Carrier withlow-pressure pneumatic-tyred track system from Schreiner & Rula (1968), and Lockheed Terrastar with major/minortri-wheeled running gear assemblies from Robinson & Rush (1968). Courtesy of the US Army Engineer Research andDevelopment Center, Vicksburg, MS.

Fig. 10. Images showing concept vehicle based on the M151 Military Utility Tactical Truck (MUTT): modified M151concept vehicle with 36 × 20-14R Terra tyres from Nuttall et al. (1966) compared with a standard M151 with 7.00 × 16NDCC bias ply tyres. Courtesy of the US Army Engineer Research and Development Center, Vicksburg, MS.




tight control on moisture content and as-compacteddensity must be maintained during construction toensure adequate soil strength and stiffness for load-carrying capacity. The difficulty, especially formilitary applications, is to create diagnostic testscapable of monitoring construction that are reliablein the hands of non-specialists.

California bearing ratio

The development of new criteria to address manufac-tured roads began in the mid-1920s in California.With the advent of increased motor vehicle trafficat that time, there was a need for construction of a

network of highways that could support heavy carand truck traffic. However, it was soon noted that pro-blems were occurring frequently in the constructedroad beds. Highways were routinely failing attraffic levels far below the design requirements.Road specifications at that time only called forcertain soil gradations to be used but did not requirecontrols in moisture or density. The need for bettercontrols during fill placement led to the developmentof the California bearing ratio (CBR) test by O.J.Porter in 1929. This test uses a 3 in2 (19.4 cm2) areacylindrical piston pushed into soil compacted at aconstant energy but at varying moisture contents or,alternatively, saturations. The purpose of the test

Fig. 11. Images showing concept vehicles developed under the Mobility Exercise A (MEXA) Program. From Schreiner(1971) courtesy of the US Army Engineer Research and Development Center, Vicksburg, MS.

Table 1. One-pass and fifty-pass performance capabilities of concept vehicles in soft soil

Vehicle Running gear GVW(lb [kg])

VCI1(psi [kPa])

VCI50(psi [kPa])

Marsh ScrewAmphibian

Archimedean Screw 4000 [1814] ≈0 ≈5 [34.5]

XM759 LogisticalCarrier

Low-pressurepneumatic-tyred tracksystem

13 000 [5897] 0 ≈2 [13.8]

Lockheed Terrastar Major/minor tri-wheeledassemblies (major mode)

3600 [1633] 8 [55.2] 21 [145]

Standard M151MUTT

7.00 × 16 NDCC bias plytyres

≈3500 [1588] 20 [138] ≈46 [317]

Modified M151MUTT

36 × 20-14R Terra tyres ≈3500 [1588] 8 [55.2] 23 [159]

MEXA 10 × 10 42 × 40-16A Terra tyres 18 000 [8165] 9 [62.1] 18 [124]MEXA 8 × 8 48 × 31-16A Terra tyres 19 000 [8618] 12 [82.7] 23 [159]MEXA Track 20-in wide flexible tracks 19 700 [8936] 7 [48.3] ≈21 [145]

GVW, gross vehicle weight. VCI, vehicle cone index (subscript represents the specific number of passes). From Knight et al. (1964),Schreiner & Rula (1968), Robinson & Rush (1968), Nuttall et al. (1966) and Schreiner (1971).




was to develop an index that related moisture anddensity to stiffness, stiffness being the field require-ment that would provide a solid surface for vehicletrafficking. Thus, an engineer could create a CBRcurve in the laboratory using compaction energysimilar to the field equipment of the day and choosethe moisture content range at which an acceptableCBR value would exist. The CBR is plotted asstress v. penetration, not quite modulus and notquite strength, but a relative measure between thetwo properties. This alleviated a majority of roaddesign issues and caught the attention of the US mili-tary in the early 1940s at the start of World War II.

World War II directed the emphasis in militaryconstruction towards the rapid construction of air-fields, which also greatly increased the need forhigher construction standards. Prior to World WarII, the heaviest planes in regular use had grossweights of 25 000 lb (11 340 kg) with wheel loadsof 12 500 lb (5670 kg), and accepted highwaypaving methods served well enough. Indeed, aslate as 1939 both the US Army Air Corps and Quar-termaster Corps assumed that all aircraft except forheavy bombers could operate from sod (unsurfaced)fields, leaving no criteria for the construction ofrunways, taxiways and aprons to carry the muchlarger loaded bombers, with wheel loads of37 000 lb (16 783 kg), that soon were to come intouse (Fatherree 2006).

To address this design problem, the US WarDepartment called in the designers of the CBR in1942 to instruct the US military on its developmentand use (Porter 1942). The WES was then immedi-ately tasked to conduct field tests correlating CBRvalue with the increased compaction efforts beingused by heavier, modern compaction equipment.This led to a 1945 study introducing the CBR tothe US military as a design tool for soil pavementdesign for aircraft up to 70 000 lb (31 751 kg) inanticipation of a pending invasion of Japan nearthe end of World War II (Anon. 1945). To this day,65 years later, the CBR is still the US military stan-dard for the design of all pavement layers, while thecommercial industry has since moved away fromCBR to a mechanistic-empirical design approach.

As successful as the CBR became, a strong trendemerged towards developing a new array of field pen-etration cones designed to capture the CBR of boththe natural and manufactured soil to supply infor-mation to the US military’s pavement design pro-cedures. Although the CBR provides a good toolfor design, its implementation in the field forquality control is cumbersome at best. A soldiermust level an area or dig a pit depending on thedepth of interest and then align a vehicle-mountedCBR apparatus using the truck’s frame for reactionover the hole and perform the test. The developmentof an alternative to the CBR involved a progressionof development.

Aerial cone penetrometer

WES conducted a suite of field tests on an aerialcone penetrometer developed by A.A. Warlam atNew York University in 1953. WES was directedby the Office of the Chief of Engineers in 1956 toperform a complete evaluation after preliminarystudies by the US Air Force and Army. Techniciansat WES used descriptions and specifications fromprevious tests to design an improved version of thepenetrometer, consisting of an aluminium tube1.5 in (3.8 cm) in diameter and 30.5 in (77.5 cm)in length. The penetrometer could be droppedfrom a height or shot from a compressed air gun.A spike at the bottom of the device exerted pressureon a spring attached to a firing pin as the penetrom-eter entered the soil. If the soil resistance to pen-etration exceeded a specified amount of force, thespring would be compressed sufficiently to engagethe firing pin to detonate a cartridge. The resistancerequired to engage the firing pin was related to soilstrength, and therefore trafficability, thus providinga means to evaluate trafficability by dropping thedevice from aeroplanes or helicopters over combatzones, or even by firing them with grenade launchersahead of advancing troops.

The principal conclusion of the WES study wasthat considerably more investigations were required(Knight 1957b). Discouraging results were obtainedfrom further investigations that employed a com-pressed air gun method in tests on undisturbedsamples at four sites near Vicksburg and Meridian,Mississippi (Blackmon et al. 1963), leading toan eight-year hiatus in penetrometer research.Research was resumed in 1968 on a new design ofpenetrometer for possible use in the Vietnam War.The compressed air gun was supplanted as a launch-ing device in favour of dropping the device fromtowers, with added vanes attached for stabilization.The modified device was tested from up to 241 ft(73.5 m) using a parachute tower at Fort Benning,Georgia. Later tests from 1000 ft (304.8 m) aboveground were performed near Raymond, Mississippi,using a television transmission tower with anoutdoor platform. Unfortunately, in all tests fromthat height, winds blew the devices out of the testzone (Kennedy 1970). After much effort and expen-diture, the device proved useless (Fatheree 2006).

It was not until the early 2000s that the aerial conewas revisited by the US Army. This time research bythe US Army Engineer Research and DevelopmentCenter (ERDC) applied modern soil models andadvanced numerical analyses to predict soil proper-ties based on the deceleration response of an air-dropped probe as it entered a soil (Johnson 2003).Instrumented probes were shot into soil bins contain-ing sand, and the output response evaluated.Although this approach showed promise, the com-plexities of real soils, especially clay-based soils,




and variability in moisture content, created dataoutputs beyond the current state of the art in analysis.

Dynamic cone penetrometer

A limiting factor in replacing the CBR with a logis-tically simpler cone penetration test is that theassociated measurement devices operate at differentends of the soil-strength spectrum. Principally, com-parisons between CBR and CI show that a handheldtrafficability cone penetrometer at its highestreading, which can only be achieved throughextreme physical effort, correlates to a CBR ofc. 18. A CBR of 10 is about as low as one woulddesire for an improved bearing layer in a pavementsystem, let alone a structural layer. CBR values onthe order of 40–100 are desired in structurallayers and, as such, it is impossible to drivemanual cones into these soils to take a reading.Therefore, a more robust cone penetrometer wasdesigned to provide a stiffness measurement formanufactured soil surfaces; this was known as thedynamic cone penetrometer (DCP). The DCP hasbeen extensively used in South Africa, since itsdevelopment there in 1956, and in other countriessuch as the UK, the USA and Australia.

The DCP was originally developed as an in situpavement evaluation technique for evaluating pave-ment layer strength (Scala 1956) and is also knownas the Scala penetrometer. Compared to the CBRtest, the DCP device has the relative advantage ofportability, simplicity, cost-effectiveness and theability to provide rapid measurement of in situstrength of pavement layers and subgrades. TheDCP has also been proven to be useful for pavementdesign and quality control. The DCP is distin-guished from the trafficability cone penetrometerby the manner in which it is driven into the soil.Rather than being statically pushed into the soil byhand, a 17.6 lb (8 kg) sliding hammer droppedfrom a height of 22.6 in (57.4 cm) dynamicallydrives the DCP into the soil. Material strength ismeasured by the penetration (usually in millimetresor inches) per hammer blow.

Modifications to the device by WES (Websteret al. 1992; Amini 2003) made it better suited forautomation and extended its use to weaker soils.The number of blows required to penetrate a speci-fied distance measured in millimetres is recorded,much like the measured response of a modern-day pile-driving operation. An algorithm developedby WES relates the incremental penetrationv. number of blows recorded with a small correctionfor clayey v. non-clayey soils and returns anestimate of the CBR value with depth. The DCPcan penetrate up to 1 m into the ground, giving agood measure of the available bearing strength ofthe natural ground or constructed layers. The DCPis now a military and commercial standard for

evaluating the CBR of in situ and constructed pave-ment layers.

Because the DCP is ideally suited for relativelystiffer soils, the cross-over potential between thetrafficability cone penetrometer and the DCP isminimal. On a soft soil with a CBR value of c. 1or 2, which translates to a CI of c. 50 or 100 psi(345 or 689 kPa), respectively, a DCP would basi-cally sink deep into the ground with one or twohammer blows, rendering the tool impractical forthese soft soil conditions. Likewise, the trafficabiltycone penetrometer can only achieve a CBR of c. 18or less, meaning it is impossible to correlate withDCP measurements in the stiffer soils required forroad and airfield pavements. Therefore, each ofthese cone penetrometers provides a niche capa-bility for evaluating near-surface soil conditions indifferent strength regimes. Both are sensitive tothe soil’s classification, density and saturation, allof which influence the soil’s stiffness.

Rapid soil analysis kit

The correlation between soil strength and soil moist-ure content or saturation has already been empha-sized and demonstrated in earlier sections, and thisdependence holds true for the trafficability conepenetrometer, CBR, aerial cone penetrometer andDCP measurements. However, soil strength is alsoheavily dependent on the engineering soil classifi-cation, which defines the relative percentages ofcoarse and fine particles and the plasticity of thefine particles. The presence of coarser materials ina soil mixture usually increases the strength of thesoil over those that contain only fine soil. Sincethe US military’s adoption of the USCS engineeringsoil classification standard (Anon. 1953 [revised1960]), field techniques to estimate soil classifi-cation have relied on qualitative visual-manual tech-niques (e.g. dry strength, dilatancy and toughness)or sensory-based means to identify coarse and finematerial. While an experienced soil expert canmake good estimates of soil type, it is difficult toestimate the soil strength likely to occur withchanges in moisture.

Recent research at ERDC has provided a meansfor an inexperienced soldier to rapidly assess the soilclassification in a quantifiable manner using a set ofsmall-scale laboratory equipment called the RapidSoil Analysis Kit (RSAK) (Fig. 12) to develop quan-titative measures of grain size properties (Berney &Wahl 2008). Using no more than a 300 g field sample,the RSAK involves microwave drying of the soil toan oven-dried state, pulverizing and then sievingthe dried mixture, and conducting the plastic limittest for classification purposes. Accompanying soft-ware predicts both the soil’s liquid limit, helpingdetermine a USCS classification, and a suite ofmoisture-density and CBR curves for varying field




moisture contents. As such, these quantifiablemeasures can be used to better predict the potentialstrength of roadbeds and airfields during seasonalchanges in the constructed layers. US militarydesign guides provide a conservative estimate forsoil strength based on the assumption that long-termuse of a roadbed will include trafficking during wetseasons. Wet season soil strength is the most appro-priate strength to use for factor of safety designssince soil strength is at its lowest during wetseasons. However, in modern military scenarios ofexpedient construction and real-time battlefieldassessment, an estimate of soil strength in itspresent condition is the most critical measure.

In modern conflicts, soldiers and equipmentmight only be deployed for hours or weeks ratherthan months to years. Within the last five years,research at ERDC has begun to address the needfor a more accurate predictive tool for field strength(Anderton et al. 2008). The engine behind the pre-dictive capabilities housed within the rapid assess-ment tools is a series of statistically derived

relationships developed from an extensive databaseof moisture-density-strength properties amassed byERDC. These soil properties have been derivedfrom 60 years of ERDC performance studies onroad and airfield construction. Borrowing on thenotion that soil strength is tied closely to soilclassification, moisture content and grain size distri-bution, the regression software fits all of these dataand allows the user to predict not only theoptimum moisture content and maximum drydensity of a soil, but also the bearing strength atany given moisture content.

The capability to predict moisture, density andstrength of both in situ soil and compacted soilallows the soldier to more effectively plan vehicleroutes and design contingency structures for air-craft. The introduction of a rapid assessment kitcoupled with a push for faster, simpler tools toevaluate ground conditions provides much neededmethodologies for assessing soil strength. Asadvances in remote sensing begin to provide amore detailed picture of the soil conditions of

Fig. 12. Soldiers training on use of the Rapid Soil Analysis Kit (RSAK). From Berney & Wahl (2008) courtesy of theUS Army Engineer Research and Development Center, Vicksburg, MS.




interest, the advantage these predictive toolsprovide will only increase.

Dust, the dry-side problem

Thus far, mobility problems resulting from toomuch moisture in the soil have been discussed, butsignificant problems can also occur when there istoo little moisture in the soil. Dust is the suspensionin air of fine soil particles due to the disturbance ofloose surface materials from winds generated byatmospheric pressure changes and soldier, vehicleor aircraft movement. Dust problems are exacer-bated due to certain climatic factors, such as lowrainfall and high temperatures, like those experi-enced in arid and semi-arid regions. The dust bowlthat occurred in the Great Plains region of NorthAmerica during the 1930s was an excellentexample of how climatic factors and man-madeactivity can combine to produce a dust-fuelled cata-strophe. Dirt roads that are not properly maintained,by keeping a moist compacted surface, becomedriving hazards due to poor traction and visibilityfor vehicles in a convoy. Without maintenance, anoverall degradation of the road occurs because thesmaller soil particles necessary for proper bondingof the surface are loosened from the compactedsoil matrix during drying and tyre shear, and thesubsequent loosened roadway surface materials areeasily moved by winds and vehicle traffic.

Emergence of the dust problem

As noted previously, dry soils do not turn to mud,but can break down under repeated traffic loadingto form dust. Dust generated on unsurfaced tempor-ary airfields constructed in Normandy soon after theAllied landings of June 1944 is a well-knownexample (Rose & Pareyn 1998): the dust causedexcessive engine wear on types of aircraft notfitted with appropriate filters, and attracted artilleryfire to airfields when these were operational.Although dust is less of a trafficability problem, itcan be a severe mobility problem. The 1960s sawthe start of the Vietnam War in SE Asia and withit, the first occurrence of dust becoming elevatedto a critical area of research for the US military. Itmay seem odd today that a tropical, jungle environ-ment would have problems with dust. However,these jungle soils consisted of predominantly fine-grained materials such as silty sands and claysthat, when dried due to weathering, are susceptibleto becoming airborne.

For the US military, this extended campaigninvolved the use of large numbers of helicopters,ground vehicles and cargo transport aircraft tocarry the vast number of personnel and supplies

required to sustain such a remote operation. Thesevehicles produced a significant dust signature fromhelicopter rotor-wash, propeller- and jet-drivendust clouds from aircraft manoeuvres, and heavilytrafficked dirt roads. The presence of so much dustsignificantly decreased the logistical capabilities ofthe soldier. Helicopter rotor blades required repla-cing after only 200–300 h of operation instead ofthe 1100 h expected life, and their engines neededto be replaced after only one-third to one-half thenormal life. Dust clouds around military installa-tions provided the enemy with recognizable signa-tures of strategic operations and impaired visibilityof both airborne and ground personnel. In additionto the obvious safety and health hazards, lowmorale within the units was common due to thecontinuous exposure to extreme dust conditions(Styron & Eaves 1973).

Evolution of dust control solutions

The US Army had begun addressing the issue ofdust control as early as 1946, immediately followingWorld War II. Its study was a companion to the USmilitary’s broader ongoing soil-stabilization pro-gramme. Several materials were studied at thetime that could provide a dust-free, waterprooflayer when blended with the soil, but no sufficientproduct or technique emerged from these earlytests. As a consequence, by the start of theVietnam War, no dust control technology was avail-able to the soldiers other than their own imagination.From 1966 to 1974, WES pursued a research pro-gramme to identify a viable dust control solutionfor use in SE Asia. To further complicate things,the solution had to be a sprayed application ratherthan blended to reduce the logistics of bringing infurther construction equipment to the theatre. Theresearch returned only one solution, a petroleumsolvent treated asphalt (a cutback) known as Pene-prime, a particularly non-environmentally friendlysolution.

Dust re-emerged as an even bigger threat in thesummer of 1990 at the start of the Persian GulfWar. In this conflict, US forces were deployed toan entirely different climatic zone from Vietnam:a dry desert with the potential for producing evengreater dust issues for vehicle and personnel logis-tics than before. Not only was the military footprintresponsible for dust, but now the desert climateitself had the potential to generate dust, fromsimply blowing fine sand on a windy day to largedust storms that could engulf a base camp orconvoy. Water was at a premium in this locale, lim-iting dust suppression to the use of emulsifiedasphalts, an offshoot of the Peneprime designedduring the Vietnam era. Although any water thatcould be found would keep dust from helicopter




rotor-wash down, it quickly evaporated in the highheat and wind. The continuous dust presencealso proved a health hazard to the soldiers, who suf-fered a wide range of breathing ailments from theconstant inhalation of the fine particles. Fortunately,the Gulf War was short-lived, ending just sevenmonths from the first shot fired, but strategic plan-ners knew that the US presence in the MiddleEast would be far from over, leading to the USArmy’s next major series of dust control studiesundertaken by WES to address emerging dustcontrol suppressants for field use in desert envi-ronments (Grau 1993).

Contemporary dust control technology

The push for viable dust control solutions continueseven today, with hardly a slowdown in research overthe past 20 years. Current military action in theMiddle East has continued to place soldiers in thepresence of severe dust since 2001. Unfortunately,due to the political volatility in the region, actionin this extreme dust environment will likely con-tinue for many more years into the future. Relatedhealth problems from long-term exposure to dustcontinue to be documented. For example, fromMarch to August 2003, 19 US military personnelin Iraq developed pneumonia severe enough towarrant medical evacuation and mechanical venti-lation. Two of these died (Shorr et al. 2004).

Complicating matters, even more dust is nowbeing generated as advances in military technologyproduce larger aircraft capable of functioning onunpaved airfields and armoured transport vehicleswith large tyre pressures that tear through dirtroads with ease. The US Army’s C-17 Globemas-ter cargo aircraft is powered by four jet engines,each capable of over 40 000 lb (178 kN) ofthrust, on a 585 000 lb (265 351 kg) aircraft, andit has the capability to operate on semi-prepareddirt runways. The amount of dust generated intheatre from the presence of such a vehicle isenormous, not to mention that the C-17 is nowthe preferred cargo transport used in nearly allmilitary deployments. Heavy-lift helicopters haveexpanded in size so that large, single-rotor helicop-ters like the CH-53 Sea Stallion and twin-rotorCH-47 Sea Knight can generate extreme down-washes that are a danger to not only the pilot(who loses ground visibility quickly), but also tothe soldier on the ground trying to retrieve cargoor signal the helicopter to land. Photographs illus-trating examples of modern military dust problemsare shown in Figure 13. The need for effective dustcontrol has reached a critical point in the military’smission, and effective solutions are now beginningto emerge from USACE research.

Although asphalts and diesel fuel sufficed as dustcontrol options in earlier military operations, newenvironmental concerns have demanded safer

Fig. 13. Images showing examples of dust problems in military operations. The helicopter photographs are from flighttesting on untreated soil helipads by Rushing et al. (2006). The aircraft photographs are from flight testing onsemi-prepared, unstabilized soil runways. Courtesy of the US Army Engineer Research and Development Center,Vicksburg, MS.




alternatives. Current ERDC dust control research hasshown that products that cure and solidify, such asacrylic polymers and copolymers, are not ideal,because they can form thin, hardened surface cruststhat can be broken to produce pieces of foreignobject damage (FOD) during rotor wash (Rushinget al. 2006). FOD can injure soldiers on the ground,damage rotor blades on aircraft, and ruin turbineengines when sucked in. Preferred alternatives arenon-curing synthetic fluids that create a reworkablebinder in the soil. These fluids are insoluble inwater and are sprayed directly onto the soil surfacewithout mixing, producing a tacky, flexible surface(Tingle et al. 2004; Rushing et al. 2007). Throughjoint participation from the US Army and MarineCorps, research conducted at ERDC has produced aDust Abatement Handbook for military operations(Rushing & Tingle 2006) that has been adopted byboth services as their primary reference for dustcontrol. This recent work has already been madeavailable to the US military for ongoing MiddleEast operations and has laid the groundwork toquickly identify product solutions for new operatingenvironments when necessary.

Conclusion

Near-surface hydrological conditions have alwaysaffected military operations. Modern mechanizedwarfare has increased the problem. Vehicle designhas done little to obviate the problem of wet soil.The conditions that impeded the foot soldier alsoimpede the modern mechanized army. From anunderstanding of soil mechanics, any hydrologicalor meteorological condition that saturates a soilwill eventually reduce mobility. Even a soil thatappears to have significant strength quickly turnsto mud if it becomes saturated. The only mitigatingfactor is improved drainage, which is possible forairfields and roads, but generally not possible forexpedient landing zones or in the open fieldregime required for manoeuvring armoured forces.At the other end of the spectrum, dry soil breaksdown to dust and can lead to loss of mobility, par-ticularly in the use of aircraft. Although loss ofmobility under wet or dry conditions is far frombeing solved, the ability to predict, and thereforeplan, for the effects of near-surface hydrology onsoil conditions and mobility has evolved signifi-cantly since World War II.

The information presented herein, unless otherwise noted,was obtained from research of the US Army Corps ofEngineers, US Army Engineer Research and DevelopmentCenter (ERDC). Permission to publish this informationwas granted by the Director, Geotechnical and StructuresLaboratory, ERDC.

References

Ahlvin, R. B. & Haley, P. W. 1992. NATO ReferenceMobility Model edition II, NRMM II user’s guide.Technical Report No. GL-92-19. US Army EngineerWaterways Experiment Station, Vicksburg, MS.

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effect of near-surface hydrology on soil strength and mobility

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