chapter 8 soil compaction section i. soil properties affected by

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CHAPTER 8

S o i l C o m p a c t i o n

Soil compaction is one of the most criticalcomponents in the construction of roads, air-fields, embankments, and foundations. Thedurability and stability of a structure are re-lated to the achievement of proper soilcompaction. Structural failure of roads andairfields and the damage caused by founda-tion settlement can often be traced back to thefailure to achieve proper soil compaction.

Compaction is the process of mechanicallydensifying a soil. Densification is ac-complished by pressing the soil particlestogether into a close state of contact with airbeing expelled from the soil mass in theprocess. Compaction, as used here, impliesdynamic compaction or densification by theapplication of moving loads to the soil mass.This is in contrast to the consolidation processfor fine-grained soil in which the soil isgradually made more dense as a result of theapplication of a static load. With relation tocompaction, the density of a soil is normallyexpressed in terms of dry density or dry unitweight. The common unit of measurement ispcf. Occasionally, the wet density or wet unitweight is used.

Section I. Soil PropertiesAffected by Compaction

ADVANTAGES OF SOIL COMPACTIONCertain advantages resulting from soil

compaction have made it a standard proce-dure in the construction of earth structures,

such as embankments, subgrades, and basesfor road and airfield pavements. No otherconstruction process that is applied to naturalsoils produces so marked a change in theirphysical properties at so low a cost as compac-tion (when it is properly controlled to producethe desired results). Principal soil propertiesaffected by compaction include—

Settlement.Shearing resistance.Movement of water.Volume change.

Compaction does not improve the desirableproperties of all soils to the same degree. Incertain cases, the engineer must carefullyconsider the effect of compaction on theseproperties. For example, with certain soilsthe desire to hold volume change to a mini-mum may be more important than just anincrease in shearing resistance.

SETTLEMENTA principal advantage resulting from the

compaction of soils used in embankments isthat it reduces settlement that might becaused by consolidation of the soil within thebody of the embankment. This is true be-cause compaction and consolidation bothbring about a closer arrangement of soil par-ticles.

Densification by compaction prevents laterconsolidation and settlement of an embank-ment. This does not necessarily mean thatthe embankment will be free of settlement; its

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weight may cause consolidation of compres-sible soil layers that form the embankmentfoundation.

SHEARING RESISTANCEIncreasing density by compaction usually

increases shearing resistance. This effect ishighly desirable in that it may allow the use ofa thinner pavement structure over a com-pacted subgrade or the use of steeper sideslopes for an embankment than would other-wise be possible. For the same density, thehighest strengths are frequently obtained byusing greater compactive efforts with watercontents somewhat below OMC. Large-scaleexperiments have indicated that the uncon-fined compressive strength of a clayey sandcould be doubled by compaction, within therange of practical field compaction proce-dures.

MOVEMENT OF WATERWhen soil particles are forced together by

compaction, both the number of voids con-tained in the soil mass and the size of theindividual void spaces are reduced. Thischange in voids has an obvious effect on themovement of water through the soil. One ef-fect is to reduce the permeability, thusreducing the seepage of water. Similarly, ifthe compaction is accomplished with propermoisture control, the movement of capillarywater is minimized. This reduces the ten-dency for the soil to take up water and sufferlater reductions in shearing resistance.

VOLUME CHANGEChange in volume (shrinkage and swelling)

is an important soil property, which is criticalwhen soils are used as subgrades for roadsand airfield pavements. Volume change isgenerally not a great concern in relation tocompaction except for clay soils where com-paction does have a marked influence. Forthese soils, the greater the density, thegreater the potential volume change due toswelling, unless the soil is restrained. An ex-pansive clay soil should be compacted at amoisture content at which swelling will notexceed 3 percent. Although the conditions

corresponding to a minimum swell and mini-mum shrinkage may not be exactly the same,soils in which volume change is a factorgenerally may be compacted so that these ef-fects are minimized. The effect of swelling onbearing capacity is important and isevaluated by the standard method used bythe US Army Corps of Engineers in preparingsamples for the CBR test.

Section II. DesignConsiderations

MOISTURE-DENSITY RELATIONSHIPSNearly all soils exhibit a similar relation-

ship between moisture content and drydensity when subjected to a given compactiveeffort (see Figure 8-1). For each soil, a maxi-mum dry density develops at an OMC for thecompactive effort used. The OMC at whichmaximum density is obtained is the moisturecontent at which the soil becomes sufficientlyworkable under a given compactive effort tocause the soil particles to become so closelypacked that most of the air is expelled. Formost soils (except cohesionless sands), whenthe moisture content is less than optimum,the soil is more difficult to compact. Beyondoptimum, most soils are not as dense under agiven effort because the water interferes withthe close packing of the soil particles. Beyondoptimum and for the stated conditions, the aircontent of most soils remains essentially thesame, even though the moisture content is in-creased.

The moisture-density relationship shownin Figure 8-1 is indicative of the workability ofthe soil over a range of water contents for thecompactive effort used. The relationship isvalid for laboratory and field compaction.The maximum dry density is frequentlyvisualized as corresponding to 100 percentcompaction for the given soil under the givencompactive effort.

The curve on Figure 8-1 is valid only for onecompactive effort, as established in thelaboratory. The standardized laboratorycompactive effort is the compactive effort(CE) 55 compaction procedure, which has

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been adopted by the US Army Corp of En-gineers. Detailed procedures for performingthe CE 55 compaction test are given in TM5-530. The maximum dry density (ydmax) atthe 100 percent compaction mark is usuallytermed the CE 55 maximum dry density, andthe corresponding moisture content is the op-timum moisture content. Table 8-1, page 8-4,shows the relationship between the US ArmyCorps of Engineers compaction tests andtheir civilian counterparts. Many times thenames of these tests are used interchange-ably in publications.

Figure 8-1 shows the zero air-voids curvefor the soil involved. This curve is obtained byplotting the dry densities corresponding tocomplete saturation at different moisturecontents. The zero air-voids curve representstheoretical maximum densities for givenwater contents. These densities are practi-cally unattainable because removing all the

air contained in the voids of the soil by com-paction alone is not possible. Typically, atmoisture contents beyond optimum for anycompactive effort, the actual compactioncurve closely parallels the zero air-voidscurve. Any values of the dry density curvethat plot to the right of the zero air-voidscurve are in error. The specific calculationnecessary to plot the zero air-voids curve arein TM 5-530.

Compaction Characteristicsof Various Soils

The nature of a soil itself has a great effecton its response to a given compactive effort!Soils that are extremely light in weight, suchas diatomaceous earths and some volcanicsoils, may have maximum densities under agiven compactive effort as low as 60 pcf.Under the same compactive effort, the maxi-mum density of a clay may be in the range of90 to 100 pcf, while that of a well-graded,

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coarse granular soil may be as high as 135 pcf.Moisture-density relationships for sevendifferent soils are shown in Figure 8-2.Compacted dry-unit weights of the soilgroups of the Unified Soil Classification Sys-tem are given in Table 5-2, page 5-8. Dry-unit weights given in column 14 are based oncompaction at OMC for the CE 55 compactiveeffort.

The curves of Figure 8-2 indicate that soilswith moisture contents somewhat less thanoptimum react differently to compaction.Moisture content is less critical for heavyclays (CH) than for the slightly plastic, clayeysands (SM) and silty sands (SC). Heavy claysmay be compacted through a relatively widerange of moisture contents below optimumwith comparatively small change in dry den-sity. However, if heavy clays are compactedwetter than the OMC (plus 2 percent), the soilbecomes similar in texture to peanut butterand nearly unworkable. The relatively clean,poorly graded sands also are relatively unaf-fected by changes in moisture. On the otherhand, granular soils that have better gradingand higher densities under the same cormpac-tive effort react sharply to slight changes in

moisture, producing sizable changes in drydensity.

There is no generally accepted and univer-sally applicable relationship between theOMC under a given compactive effort and theAtterberg limit tests described in Chapter 4.OMC varies from about 12 to 25 percent forfine-grained soils and from 7 to 12 percent forwell-graded granular soils. For some claysoils, the OMC and the PL will be ap-proximately the same.

Other Factors That Influence DensityIn addition to those factors previously dis-

cussed, several others influence soil density,to a smaller degree. For example, tempera-ture is a factor in the compaction of soils thathave a high clay content; both density andOMC may be altered by a great change intemperature. Some clay soils are sensitive tomanipulation; that is, the more they areworked, the lower the density for a given com-pactive effort. Manipulation has little effecton the degree of compaction of silty or cleansands. Curing, or drying, of a soil followingcompaction may increase the strength of

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subgrade and base materials, particularly ifcohesive soils are involved.

Addition of Water to SoilOften water must be added to soils being in-

corporated in embankments, subgrades, andbases to obtain the desired degree of compac-tion and to achieve uniformity. The soil canbe watered in the borrow pit or in place. Afterthe water is added, it must be thoroughlyand uniformly mixed with the soil. Even ifadditional water is not needed, mixing maystill be desirable to ensure uniformity. In

processing granular materials, the bestresults are generally obtained by sprinklingand mixing in place. Any good mixing equip-ment should be satisfactory. The more friablesandy and silty soils are easily mixed withwater. They may be handled by sprinklingand mixing, either on the grade or in the pit.Mixing can be done with motor graders,rotary mixers, and commercial harrows to adepth of 8 inches or more without difficulty.

If time is available, water may also beadded to these soils by diking or ponding the

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pit and flooding until the desired depth ofpenetration has taken place. This methodusually requires several days to accomplishuniform moisture distribution. Mediumclayey soils can be worked in the pit or in placeas conditions dictate. The best results are ob-tained by sprinkling and mixing withcultivators and rotary mixers. These soilscan be worked in lifts up to 8 inches or morewithout great difficulty. Heavy clay soilspresent many difficulties and should never beused as fill in an embankment foundation.They should be left alone without disturbancesince usually no compactive effort or equip-ment is capable of increasing the in-placecondition with reference to consolidation andshear strength.

The length of the section being rolled mayhave a great effect on densities in hot weatherwhen water evaporates quickly. When thiscondition occurs, quick handling of the soilmay mean the difference between obtainingadequate density with a few passes and re-quiring extra effort to add and mix water.

Handling of Wet SoilsWhen the moisture content of the soil to be

compacted greatly exceeds that necessary forthe desired density, some water must beremoved. In some cases, the use of exces-sively wet soils is possible without detrimen-tal effects. These soils (coarse aggregates)are called free-draining soils, and their maxi-mum dry density is unaffected by moisturecontent over a broad range of moisture. Mostoften, these soils must be dried; this can be aslow and costly process. The soil is usuallydried by manipulating and exposing it toaeration and to the rays of the sun.Manipulation is most often done with cul-tivators, plows, graders, and rotary mixers.Rotary mixers, with the tail-hood sectionraised, permit good aeration and are very ef-fective in drying excessively wet soils. Anexcellent method that may be useful whenboth wet and dry soils are available is simplyto mix them together.

Variation of Compactive EffortFor each compactive effort used in compact-

ing a given soil, there is a corresponding OMC

and maximum density. If the compactive ef-fort is increased, the maximum density isincreased and the OMC is decreased. Thisfact is illustrated in Figure 8-3. It showsmoisture-density relationships for two dif-ferent soils, each of which was compactedusing two different compactive efforts in thelaboratory. When the same soil is compactedunder several different compactive efforts, arelationship between density and compactiveeffort may be developed for that soil.

This information is of particular interest tothe engineer who is preparing specificationsfor compaction and to the inspector who mustinterpret the density test results made in thefield during compaction. The relationship be-tween compactive effort and density is notlinear. A considerably greater increase incompactive effort will be required to increasethe density of a clay soil from 90 to 95 percentof CE 55 maximum density than is required toeffect the same changes in the density of asand. The effect of variation in the compac-tive effort is as significant in the field rollingprocess as it is in the laboratory compactionprocedure. In the field, the compactive effortis a function of the weight of the roller and thenumber of passes for the width and depth ofthe area of soil that is being rolled. Increas-ing the weight of the roller or the number ofpasses generally increases the compactive ef-fort. Other factors that may be of consequenceinclude—

Lift thickness.Contact pressure. Size and length of the tamping feet(in the case of sheepsfoot rollers).Frequency and amplitude (in the caseof vibratory compactors).

To achieve the best results, laboratory andfield compaction must be carefully correlated.

COMPACTION SPECIFICATIONSTo prevent detrimental settlement under

traffic, a definite degree of compaction of theunderlying soil is needed. The degree dependson the wheel load and the depth below thesurface. For other airfield construction andmost road construction in the theater of oper-ations, greater settlement can be accepted,

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although the amount of maintenance willgenerally increase. In these cases, the mini-mum compaction requirements of Table 8-2,page 8-8, should be met. However. strengthcan possibly decrease with increased compac-tion. particularly with cohesive materials.As a result, normally a 5 percent compactionrange is established for density and a 4 per-cent range for moisture. Commonly, this“window” of density and moisture ranges isplotted directly on the GE 55 compactioncurve and is referred to as the specificationsblock. Figure 8-4, page 8-8, shows a density

range of 90 to 95 percent compaction and amoisture range of 12 to 16 percent.

CBR Design ProcedureThe concept of the CBR analysis was intro-

duced in Chapter 6. In the followingprocedures, the CBR analytical process willbe applied to develop soil compactionspecifications. Figure 8-5, page 8-10, outlinesthe CBR design process. The first step is tolook at the CE 55 compaction curve on a DDForm 2463, page 1. If it is U-shaped, the soilis classified as “free draining” for CBR

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analysis and the left-hand column of the flow-chart should be used through the designprocess. If it is bell-shaped, use the swell datagraphically displayed on a DD Form 1211.Soils that, when saturated, increase involume more than 3 percent at any initialmoisture content are classified as swellingsoils. If the percentage of swelling is ‹ 3 per-cent, the soil is considered nonswelling.

Regardless of the CBR classification of thesoil, the density value from the peak of the CE55 moisture density curve is ydmax. Thenext step is to determine the design moisturecontent range. For nonswelling soils, theOMC is used. When the OMC is used, thedesign moisture content range is + 2 percent.For swelling and free-draining soils, the min-imum moisture content (MMC) is used. TheMMC is determined differently for swellingsoils than it is for free-draining soils. TheMMC for swelling soils is determined by find-ing the point at which the 3 percent swelloccurs. The soil moisture content that cor-responds to the 3 percent swell is the MMC.Free-draining soils exhibit an increase indensity in response to increased soil moistureup to a certain moisture content, at whichpoint no further increase in density isachieved by increasing moisture. The mois-ture content that corresponds to ydmax is theMMC. For both swelling and free-drainingCBR soil classes, the design moisture-contentrange is MMC + 4 percent.

For swelling and free-draining soils, thefinal step in determining design compactionrequirements is to determine the densityrange. Free-draining soils are compacted to100-105 percent ydmax. Swelling soils arecompacted to 90-95 percent dmax.

Compaction requirement determinationsfor nonswelling soils require several additionalsteps. Once the OMC and design moisturecontent range have been determined, look at aDD Form 1207 for the PI of the soil. If PI > 5,the soil is cohesive and is compacted to 90-95percent ydmax. If the PI < 5, refer to the CBRFamily of Curves on page 3 of DD Form 2463.If the CBR values are insistently above 20,compact the soil to 100-105 percent ydmax. Ifthe CBR values are not above 20, compact thesoil to 95-100 percent

Once you have determined the design den-sity range and the moisture content range,you have the tools necessary to specify the re-quirements for and manage the compactionoperations. However, placing a particularsoil in a construction project is determined byits gradation. Atterberg limits, and designCBR value. Appendix A contains a discussionof the CBR design process.

A detailed discussion of placing soilsand aggregates in an aggregate surface or aflexible pavement design is in FM 5-430(for theater-of-operations construction),TM 5-822-2 (for permanent airfield design),and TM 5-822-5 (for permanent road design).

Subgrade CompactionIn fill sections, the subgrade is the top layer

of the embankment, which is compacted tothe required density and brought to thedesired grade and section. For subgrades,plastic soils should be compacted at moisturecontents that are close to optimum. Moisturecontents cannot always be carefully con-trolled during military construction, butcertain practical limits must be recognized.Generally, plastic soils cannot be compactedsatisfactorily at moisture contents more than10 percent above or below optimum. Muchbetter results are obtained if the moisturecontent is controlled to within 2 percent of op-timum. For cohesionless soils, moisturecontrol is not as important, but some sandstend to bulk at low moisture content. Com-paction should not be attempted until thissituation is corrected. Normally, cohesion-less soils are compacted at moisture contentsthat approach 100 percent saturation.

In cut sections, particularly when flexiblepavements are being built to carry heavywheel loads, subgrade soils that gain strengthwith compaction should be compacted to thegeneral requirements given earlier. Thismay make it necessary to remove the soil,replace it, and compact it in layers to obtainthe required densities at greater depths. Inmost construction in the theater of opera-tions, subgrade soil in cut sections should bescarified to a depth of about 6 inches andrecompacted. This is commonly referred to asa scarify/compact in-place (SCIP) operation.dmax.

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This procedure is generally desirable in theinterest of uniformity.

Expansive Clays. As indicated previously,soils that have a high clay content (partic-ularly (CH), (MH), and (OH)) may expand indetrimental amounts if compacted to a highdensity at a low moisture content and then ex-posed to water. Such soils are not desirable assubgrades and are difficult to compact. If theyhave to be used, they must be compacted tothe maximum density obtainable using theMMC that will result in a minimum amountof swelling. Swelling soils, if placed at mois-ture contents less than the MMC, can beexpected to swell more than 3 percent. Soilvolume increases of up to 3 percent generallydo not adversely affect theater-of-operationsstructures. This method requires detailedtesting and careful control of compaction. Insome cases, a base of sufficient thicknessshould be constructed to ensure against theharmful effects of expansion.

Clays and Organic Soils. Certain clay soilsand organic soils lose strength whenremolded. This is particularly true of some(CH) and (OH) soils. They have highstrengths in their undisturbed condition, butscarifying, reworking, and compacting themin cut areas may reduce their shearingstrengths, even though they are compacted todesign densities. Because of these qualities,they should be removed from the constructionsite.

Silts. When some silts and very fine sands(predominantly (ML) and (SC) soils) are com-pacted in the presence of a high water table,they will pump water to the surface and be-come “quick”, resulting in a loss of shearingstrength. These soils cannot be properly com-pacted unless they are dried. If they can becompacted at the proper moisture content.their shearing resistance is reasonably high.Every effort should be made to lower thewater table to reduce the potential of havingtoo much water present. If trouble occurswith these soils in localized areas, the soilscan be removed and replaced with moresuitable ones. If removal, or drainage andlater drying, cannot be accomplished, thesesoils should not be disturbed by attempting to

compact them. Instead, they should be left intheir natural state and additional covermaterial used to prevent the subgrade frombeing overstressed.

When these soils are encountered, theirsensitivity may be detected by performing un-confined compression tests on the un-disturbed soil and on the remolded soil com-pacted to the design density at the designmoisture content. If the undisturbed value ishigher, do not attempt to compact the soil;manage construction operations to producethe least possible disturbance of the soil.Base the pavement design on the bearingvalue of the undisturbed soil.

Base CompactionSelected soils that are used in base con-

struction must be compacted to the generalrequirements given earlier. The thickness oflayers must be within limits that will ensureproper compaction. This limit is generallyfrom 4 to 8 inches, depending on the materialand the method of construction.

Smooth-wheeled or vibratory rollers arerecommended for compacting hard, angularmaterials with a limited amount of fines orstone screenings. Pneumatic-tired rollers arerecommended for softer materials that maybreak down (degrade) under a steel roller.

Maintenance of Soil DensitySoil densities obtained by compaction

during construction may be changed duringthe life of the structure. Such considerationsare of great concern to the engineer engagedin the construction of semipermanent instal-lations, although they should be kept in mindduring the construction of any facility to en-sure satisfactory performance. The twoprincipal factors that tend to change the soildensity are—

Climate.Traffic.

As far as embankments are concerned, nor-mal embankments retain their degree ofcompaction unless subjected to unusual con-ditions and except in their outer portions,which are subjected to seasonal wetting and


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drying and frost action. Subgrades and basesare subject to more severe climatic changesand traffic than are embankments. Climaticchanges may bring about seasonal or per-manent changes in soil moisture andaccompanying changes in density, which maydistort the pavement surface. High-volume-change soils are particularly susceptible andshould be compacted to meet conditions ofminimum swelling and shrinkage. Granularsoils retain much of their compaction underexposure to climatic conditions. Other soilsmay be somewhat affected, particularly inareas of severe seasonal changes, such as—

Semiarid regions (where long, hot,dry periods may occur).Humid regions (where deep freezingoccurs).

Frost action may change the density of acompacted soil, particularly if it is fine-grained. Heavy traffic, particularly forsubgrades and bases of airfields, may bringabout an increase in density over that ob-tained during construction. This increase indensity may cause the rutting of a flexiblepavement or the subsidence of a rigid pave-ment. The protection that a subgrade soilreceives after construction is complete has animportant effect on the permanence of com-paction. The use of good shoulders, themaintenance of tight joints in a concretepavement, and adequate drainage all con-tribute toward maintaining the degree ofcompaction achieved during construction.

Section III. ConstructionProcedures

GENERAL CONSIDERATIONSThe general construction process of a

rolled-earth embankment requires that thefill be built in relatively thin layers or “lifts,”each of which is rolled until a satisfactory de-gree of compaction is obtained. The subgradein a fill section is usually the top lift in thecompacted fill, while the subgrade in a cutsection is usually compacted in in-place soil.Soil bases are normally compacted to a highdegree of density. Compaction requirements frequently stipulate a certain minimum density.

For military construction, this is generally aspecified minimum percentage of CE 55 max-imum density for the soil concerned. Themoisture content of the soil is maintained ator near optimum, within the practical limitsof field construction operations (normally + 2percent of the OMC). Principal types ofequipment used in field compaction aresheepsfoot, smooth steel-wheeled, vibratory,and pneumatic-tired rollers.

SELECTION OF MATERIALSSoils used in fills generally come from cut

sections of the road or airfield concerned,provided that this material is suitable. If thematerial excavated from cut sections is notsuitable, or if there is not enough of it, thensome material is obtained from other sources.Except for highly organic soils, nearly any soilcan be used in fills. However, some soils aremore difficult to compact than others andsome require flatter side slopes for stability.Certain soils require elaborate protectivedevices to maintain the fill in its original con-dition. When time is available, theseconsiderations and others may make it ad-vantageous to thoroughly investigateconstruction efforts, compaction charac-teristics, and shear strengths of soils to beused in major fills. Under expedient condi-tions, the military engineer must simplymake the best possible use of the soils athand.

In general terms, the coarse-grained soils ofthe USCS are desirable for fill construction,ranging from excellent to fair. The fine-grained soils are less desirable, being moredifficult to compact and requiring more care-ful control of the construction process. Table5-2, page 5-8, and Table 5-3, respectively con-tain more specific information concerning thesuitability of these soils.

DUMPING AND SPREADINGSince most fills are built up of thin lifts to

the desired height, the soil for each lift mustbe spread in a uniform layer of the desiredthickness. In typical operations, the soil isbrought in, dumped, and spread by scraperunits. The scrapers must be adjusted carefully


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to accomplish this objective. Materials mayalso be brought in by trucks or wagons anddumped at properly spaced locations so that auniform layer may be easily spread by bladegraders or bulldozers. Working alone,bulldozers may form very short and shallowfills. End dumping of soil material to form afill without compaction is rarely permitted inmodern embankment construction exceptwhen a fill is being built over very weak soils,as in a swamp. The bottom layers may thenbe end dumped until sufficient material hasbeen placed to allow hauling and compactingequipment to operate satisfactorily. The bestthickness of the layer to be used with a givensoil and a given equipment cannot be deter-mined exactly in advance. It is best deter-mined by trial during the early stages of roll-ing on a project. No lift, however, will have athickness less than twice the diameter of thelargest size particle in the lift. As stated pre-viously, compacted lifts will normally rangefrom 4 to 8 inches in depth (see Table 8-3, page8-13).

COMPACTION OF EMBANKMENTSIf the fill consists of cohesive or plastic soils,

the embankment generally must be built upof uniform layers (usually 4 to 6 inches incompacted thickness), with the moisture con-tent carefully controlled. Rolling should bedone with the sheepsfoot or tamping-footrollers. Bonding of a layer to the one placedon top of it is aided by the thin layer of loosematerial left on the surface of the rolled layerby the roller feet. Rubber-tired or smooth-wheeled rollers may be used to provide asmooth, dense, final surface. Rubber-tiredconstruction equipment may provide sup-plemental compaction if it is properly routedover the area.

If the fill material is clean sand or sandygravel, the moisture range at which compac-tion is possible is generally greater. Becauseof their rapid draining characteristics, thesesoils may be compacted effectively at or aboveOMC. Vibratory equipment may be used.Soils may be effectively compacted by com-bined saturation and the vibratory effects ofcrawler tractors, particularly when tractorsare operated at fairly high speeds so thatvibration is increased.

For adequate compaction, sands andgravels that have silt and clay fines requireeffective control of moisture. Certain soils ofthe (GM) and (SM) groups have especiallygreat need for close control. Pneumatic-tiredrollers are best for compacting these soils, al-though vibratory rollers may be usedeffectively.

Large rock is sometimes used in fills, par-ticularly in the lower portion. In some cases,the entire fill may be composed of rock layerswith the voids filled with smaller rocks or soiland only a cushion layer of soil for the sub-grade. The thickness of such rock layersshould not be more than 24 inches with thediameter of the largest rock fragment beingnot greater than 90 percent of the lift thick-ness. Compaction of this type of fill is difficultbut may generally be done by vibration fromthe passage of tack-type equipment over thefill area or possibly 50-ton pneumatic-tiredrollers.

Finishing in embankment construction in-cludes all the operations necessary tocomplete the earthwork. Included amongthese operations are the trimming of the sideand ditch slopes, where necessary, and thefine grading needed to bring the embankmentsection to final grade and cross section. Mostof these are not separate operations per-formed after the completion of otheroperations but are carried along as the workprogresses. The tool used most often infinishing operations is the motor grader,while scraper and dozer units may be used ifthe finish tolerances are not too strict. Theprovision of adequate drainage facilities is anessential part of the work at all stages of con-struction, temporary and final.

DENSITY DETERMINATIONSDensity determinations are made in the

field by measuring the wet weight of a knownvolume of compacted soil. The sample to beweighed is taken from a roughly cylindricalhole that is dug in the compacted layer. Thevolume of the hole may be determined by oneof several methods. including the use of—

Heavy oil of known specific gravity.Rubber balloon density apparatus.


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Calibrated sand.Nuclear densimeter.

When the wet weight and the volume areknown, the unit wet weight may then be cal-culated, as described in FM 5-430.

In very arid regions, or when working withsoils that lose strength when remolded, theadequacy of compaction should be judged byperforming the in-place CBR test on the com-pacted soil of a subgrade or base. The CBRthus obtained can then be compared with thedesign CBR, provided that the design wasbased on CBR tests on unsoaked samples. Ifthe design was based on soaked samples, theresults of field in-place CBR tests must be cor-related with the results of laboratory testsperformed on undisturbed mold samples ofthe in-place soil subjected to soaking.Methods of determining the in-place CBR of asoil are described in TM 5-530.

FIELD CONTROL OF COMPACTIONAs stated in previous paragraphs, specifica-

tions for adequate compaction of soiI used inmilitary construction generally require theattainment of a certain minimum density infield rolling. This requirement is most oftenstated in terms of a specified percentagerange of CE 55 maximum density. Withmany soils, the close control of moisture con-tent is necessary to achieve the stated densitywith the available equipment. Careful con-trol of the entire compaction process isnecessary if the required density is to beachieved with ease and economy. Controlgenerally takes the form of field checks ofmoisture and density to—

Determine if the specified density isbeing achieved.Control the rolling process.Permit adjustments in the field, as re-quired.

The following discussion assumes that thelaboratory compaction curve is available forthe soil being compacted so that the maxi-mum density and OMC are known. It is alsoassumed that laboratory-compacted soil andfield-compacted soil are similar and that the

required density can be achieved in the fieldwith the equipment available.

Determination of Moisture ContentIt may be necessary to check the moisture

content of the soil during field rolling for tworeasons. First, since the specified density isin terms of dry unit weight and the densitymeasured directly in the field is generally thewet unit weight, the moisture content mustbe known so that the dry unit weight can becalculated. Second, the moisture content ofsome soils must be maintained close to op-timum if satisfactory densities are to beobtained. Adjustment of the field moisturecontent can only be done if the moisture con-tent is known. The determination of densityand moisture content is often done in oneoverall test procedure; these determinationsare described here separately for con-venience.

Field Examination. Experienced engineerswho have become familiar with the soils en-countered on a particular project canfrequently judge moisture content accuratelyby visual and manual examination. Friableor slightly plastic soils usually containenough moisture at optimum to permit theforming of a strong cast by compressing it inthe hand. As noted, some clay soils haveOMCs that are close to their PLs; thus, a PLor “thread” test conducted in the field may behighly informative.

Field Drying. The moisture content of a soilis best and most accurately determined bydrying the soil in an oven at a controlledtemperature. Methods of determining themoisture content in this fashion are describedin TM 5-530.

The moisture content of the soil may also bedetermined by air drying the soil in the sun.Frequent turning of the soil speeds up thedrying process. From a practical standpoint,this method is generally too slow to be ofmuch value in the control of field rolling.

Several quick methods may be used todetermine approximate moisture contentsunder expedient conditions. For example,


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the sample may be placed in a frying pan anddried over a hot plate or a field stove. Thetemperature is difficult to control in this pro-cedure, and organic materials may be burned,thus causing a slight to moderate error in theresults. On large-scale projects where manysamples are involved, this quick method maybe used to speed up determinations by com-paring the results obtained from this methodwith comparable results obtained by oven-drying.

Another quick method that may be useful isto mix the damp soil with enough denaturedgrain alcohol to form a slurry in a perforatedmetal cup, ignite the alcohol, and permit it toburn off. The alcohol method, if carefullydone, produces results roughly equivalent tothose obtained by careful laboratory drying.For best results, the process of saturating thesoil with alcohol and burning it off completelyshould be repeated three times. This methodis not reliable with clay soils. Safetymeasures must be observed when using thismethod. The burning must be done outside orin a well-ventilated room and at a safe dis-tance from the alcohol supply and otherflammable materials. The metal cup gets ex-tremely hot, arid it should be allowed to coolbefore handling.

“Speedy” Moisture-Content Test. The“speedy” moisture test kit provided with thesoil test set provides a very rapid moisture-content determination and can be highly ac-curate if the test is performed properly. Caremust be exercised to ensure that the reagentused has not lost its strength. The reagentmust be very finely powdered (like portlandcement) and must not have been exposed towater or high humidity before it is used. Thespecific test procedures are contained in thetest set.

Nuclear Denimeter. This device providesreal-time in-place moisture content and den-sity of a soil. Accuracy is high if the test isperformed properly and if the device has beencalibrated with the specific material beingtested. Operators must be certified, andproper safety precautions must be taken to

ensure that the operator does not receive amedically significant dose of radiation duringthe operation of this device. There are strin-gent safety and monitoring procedures thatmust be followed. The method of determiningthe moisture content of a soil in this fashion isdescribed in the operator’s manual.

Determination of Water to Be AddedIf the moisture content of the soil is less

than optimum, the amount of water to beadded for efficient compaction is generallycomputed in gallons per square yards. Thecomputation is based on the dry weight of soilcontained in a compacted layer. For example,assume that the soil is to be placed in 6-inch,compacted layers at a dry weight of 120 pcf.The moisture content of the soil is determinedto be 5 percent while the OMC is 12 percent.Assume that the strip to be compacted is 40feet wide. Compute the amount of water thatmust be added per 100-foot station to bringthe soi1 to optimum moisture. The followingformula applies:

Substituting in the above formula from theconditions given:

If either drying conditions or rain conditionsexist at the time work is in progress, it may beadvisable to either add to or reduce this quan-tity by up to 10 percent.

COMPACTION EQUIPMENTEquipment normally available to the

military engineer for the compaction of soilsincludes the following types of rollers:

Pneumatic-tired.Sheepsfoot.


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Tamping-foot.Smooth steel-wheeled.Vibratory.

Pneumatic-Tired RollerThese heavy pneumatic-tired rollers are

designed so that the weight can be varied toapply the desired compactive effort. Rollerswith capacities up to 50 tons usually have tworows of wheels, each with four wheels andtires designed for 90 psi inflation. They canbe obtained with tires designed for inflationpressures up to 150 psi. As a rule, the higherthe tire pressure the greater the contact pres-sures and, consequently, the greater thecompactive effort obtained. Informationavailable from projects indicates that largerubber-tired compactors are capable of com-pacting clay layers effectively up to about 6inches compacted depth and coarse granularor sand layers slightly deeper. Often it isused especially for final compaction (proofrolling) of the upper 6 inches of subgrade, forsubbases, and for base courses. These rollersare very good for obtaining a high degree ofcompaction. When a large rubber-tired rolleris to be used, care should be exercised to en-sure that the moisture content of cohesivematerials is low enough so that excessive porepressures do not occur. Weaving or springingof the soil under the roller indicates that porepressures are developing.

Since this roller does not aerate the soil asmuch as the sheepsfoot, the moisture contentat the start of compaction should be ap-proximately the optimum. In a soil that hasthe proper moisture content and lift thick-ness, tire contact pressure and the number ofpasses are the important variables affectingthe degree of compaction obtained by rubber-tired rollers. Generally, the tire contactpressure can be assumed to be approximatelyequal to the inflation pressure.

Variants of the pneumatic-tired roller in-clue the pneumatic roller and the self-propelled pneumatic-tired roller.

Pneumatic RollerAs used in this manual, the term “pneu-

matic roller” applies to a small rubber-tired

roller, usually a “wobble wheel. ” Thepneumatic roller is suitable for granularmaterials; however, it is not recommended forfine-grained clay soils except as necessary forsealing the surface after a sheepsfoot rollerhas “walked out. ” It compacts from the topdown and is used for finishing all types ofmaterials, following immediately behind theblade and water truck.

Self-Propelled, Pneumatic-Tired RollerThe self-propelled, pneumatic-tired roller

has nine wheels (see Figure 8-6). It is verymaneuverable, making it excellent for use inconfined spaces. It corn pacts from the topdown. Like the towed models, the self-propelled, pneumatic-tired roller can be usedfor compaction of most soil materials. It isalso suitable for the initial compaction ofbituminous pavement.

For a given number of passes of a rubber-tired roller, higher densities are obtainedwith the higher tire pressures. However, cau-tion and good judgment must be used and thetire pressure adjusted in the field dependingon the nature of the soil being compacted. Forcompaction to occur under a rubber-tiredroller, permanent deformation has to occur.If more than slight pumping or spring occursunder the tires, the roller weight and tirepressure are too high and should be loweredimmediately. Continued rolling under theseconditions causes a decrease in strength eventhough a slight increase in density may occur,For any given tire pressure, the degree of


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compaction increases with additional passes,although the increase may be negligible aftersix to eight passes.

Sheepsfoot RollerThis roller compacts all fine-grained

materials, including materials that will breakdown or degrade under the roller feet, but itwill not compact cohesion less granularmaterials. The number of passes necessaryfor this type of roller to obtain the requireddensities must be determined for each type ofsoil encountered. The roller compacts fromthe bottom up and is used especially for plas-tic materials. The lift thickness forsheepsfoot rollers is limited to 6 inches incompacted depth. Penetration of’ the rollerfeet must be obtained at the start of rollingoperations This roller “walks out” as it com-pletes its compactive effort, leaving the top 1to 2 inches uncompacted.

The roller may tend to “walk out” beforeproper compaction is obtained. To preventthis, the soil may be scarified lightly behindthe roller during the first two or three passes,and additional weight may be added to theroller.

A uniform density can usually be obtainedthroughout the full depth of the lift if thematerial is loose and workable enough toallow the roller feet to penetrate the layer onthe initial passes. This produces compactionfrom the bottom up; therefore, material thatbecomes compacted by the wheels of equip-ment during pulverizing, wetting, blending,and mixing should be thoroughly loosenedbefore compaction operations are begun.This also ensures uniformity of the mixture.The same amount of rolling generallyproduces increased densities as the depth ofthe lift is decreased. If the required densitiesare not being obtained, it is often necessary tochange to a thinner lift to ensure that thespecified density is obtained.

In a soil that has the proper moisture con-tent and lift thickness, foot contact pressureand the number of passes are the importantvariables affecting the degree of compaction

obtained by sheepsfoot rollers. The minimumfoot contact pressure for proper compaction is250 psi. Most available sheepsfoot rollers areequipped with feet having a contact area of 5to 8 square inches. The foot pressure can bechanged by varying the weight of the roller(varying the amount of ballast in the drum),or in special cases, by welding larger platesonto the faces of the feet. For the most effi-cient operation of the roller, the contactpressure should be close to the maximum atwhich the roller will “walk out” satisfactorily,as indicated in Figure 8-7.

The desirable foot contact pressure variesfor different soils, depending on the bearingcapacity of the soil; therefore, the proper ad-justments have to be made in the field basedon observations of the roller. If the feet of theroller tend to “walk out,” too quickly (for ex-ample, after two passes), then bridging mayoccur and the bottom of the lift does not getsufficient compaction. This indicates that theroller is too light or the feet too large, and theweight should be increased. However, if the


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roller shows no tendency to “walk out” withinthe required number of passes, then the in-dications are that the roller is to (heavy andthe pressure on the roller feet is exceeding thebearing capacity of the soil. After making theproper adjustments in foot pressure (bychangingroller size), the only other variableis the repetition of passes. Tests have shownthat density increases progressively withanincrease in the number of passes.

Tamping-Foot RollerA tamping-foot roller is a modification of

the sheepsfoot roller. The tamping feet aretrapezoidal pads attached to a drum. Tamp-ing-foot rollers are normally self-propelled,and the drum may be capable of vibrating.The tamping-foot roller is suitable for usewith a wide range of soil types.

Steel-Wheeled RollerThe steel-wheeled roller is much less ver-

satile than the pneumatic roller. Althoughextensively used, it is normally operated inconjunction with one of the other three typesof compaction rollers. It is used for compact-ing granular materials in thin lifts. Probablyits most effective use in subgrade work is inthe final finish of a surface. following immedi-ately behind the blade, forming a dense andwatertight surface. Figure 8-8 shows a two-axle tandem (5- to 8-ton) roller.

Self-Propelled, Smooth-DrumVibratory Roller

The self-propelled, smooth-drum vibratoryroller compacts with a vibratory action thatrearranges the soil particles into a densermass (see Figure 8-9). The best results are ob-tained on cohesionless sands arid gravels.Vibratory rollers are relatively light butdevelop high dynamic force through an ec-centric weight arrangement. Compactionefficiency is impacted by the ground speed ofthe roller and the frequency and amplitude ofthe vibrating drum.

Other EquipmentOther construction equipment may be

useful in certain instances, particularly

crawler-type tractor units and loaded haulingunits, including rubber-tired scrapers.Crawler tractors are practical compactingunits, especially for rock and cohesionlessgravels and sands. The material should bespread in thin layers (about 3 or 4 inchesthick) and is usually compacted by vibration.

COMPACTOR SELECTIONTable 8-3, page 8-13, gives information con-

cerning compaction equipment and compactiveefforts recommended for use with each of thegroups of the USCS.

Normally, there is more than one type ofcompactor suitable for use on a project’s


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type(s) of soil. When selecting a compactor,use the following criteria:

Availability.Efficiency.

AvailabilityAscertain the types of compactors that are

available and operationally ready. On majorconstruction projects or when deployed, itmay be necessary to lease compaction equip-ment. The rationale for leasing compactionequipment is based on the role it plays indetermining overall project duration and con-struction quality. Uncompacted lifts cannotbe built on until they are compacted. Sub-stituting less efficient types of compactionequipment decreases productivity and mayreduce project quality if desired dry densitiesare not achieved.

EfficiencyDecide how many passes of each type of

compactor are required to achieve thespecified desired dry density. Determiningthe most efficient compactor is best done on atest strip. A test strip is an area that is locatedadjacent to the project and used to evaluatecompactors and construction procedures. Thecompactive effort of each type of compactorcan be determined on the test strip andplotted graphically. Figure 8-10 compares thefollowing types of compactors:

Vibratory (vibrating drum) roller.Tamping-foot roller.Pneumatic-tired roller.

In this example, a dry density of 129 to 137pcf is desired. The vibrating roller was themost efficient, achieving densities within thespecified density range in three passes. Thetamping foot compactor also compacted thesoil to the desired density in three passes.However, the density achieved (130 pcf) is soclose to the lower limit of the desired densityrange that any variation in the soil may causethe achieved density to drop below 129pcf. The pneumatic-tired roller was theleast efficient and did not densify the soilmaterial to densities within the specifieddensity range.

Once the type(s) of compactor is selected,optimum lift thicknesses can be determined.Table 8-3, page 8-13, provides information onaverage optimum lift thicknesses, but this in-formation must be verified. Again, the teststrip is a way to determine optimum lift thick-ness without interfering with otheroperations occurring on the actual project.

In actual operation, it is likely that more thanone type of compactor will be operating on theproject to maintain peak productivity and tocontinue operations when the primary com-pactors require maintenance or repair. Test-strip data helps to maintain control of projectquality while providing the flexibility to allowconstruction at maximum productivity.

Section IV. Quality Control

PURPOSEPoor construction procedures can in-

validate a good pavement or embankment


FM 5-410

design. Therefore, quality control of construc-tion procedures is as important to the finalproduct as is proper design. The purpose ofquality control is to ensure that the soilis being placed at the proper density andmoisture content to provide adequate bearingstrength (CBR) in the fill. This is ac-complished by taking samples or testing ateach stage of construction. The test resultsare compared to limiting values or specifica-tions, and the compaction should be acceptedor reworked based on the results of the den-sity and moisture content tests. A quality-control plan should be developed for eachproject to ensure that high standards areachieved. For permanent construction, statisti-cal quality-control plans provide the mostreliable check on the quality of compaction.

QUALITY-CONTROL PLANGenerally, a quality-control plan consists of

breaking the total job down into lots with eachlot consisting of “X” units of work. Each lot isconsidered a separate job, and each job will beaccepted or rejected depending on the testresults representing this lot. By handling thecontrol procedure in this way, the project en-gineer is able to determine the quality of thejob on a lot-by-lot basis. This benefits the en-gineering construction unit and projectengineer by identifying the lots that will beaccepted and the lots that will be rejected. Asthis type of information is accumulated fromlot to lot, a better picture of the quality of theentire project is obtained.

The following essential items should beconsidered in a quality-control plan:

Lot size.Random sampling.Test tolerance.Penalty system.

Lot SizeThere are two methods of defining a lot size

(unit of work). A lot size may be defined as anoperational time period or as a quantity ofproduction. One advantage that the quantity-of-production method has over theoperational-time-period method is that theengineering construction unit will probably

have plant and equipment breakdowns andother problems that would require thatproduction be stopped for certain periods oftime. This halt in production could cause dif-ficulties in recording production time. On theother hand, there are always records thatwould show the amount of materials thathave been produced. Therefore, the betterway to describe a lot is to specify that a lot willbe expressed in units of quantity of produc-tion By using this method, each lot willcontain the same amount of materials, estab-lishing each one with the same relativeimportance. Factors such as the size of thejob and the operational capacity usuallygovern the size of a production lot. Typical lotsizes are 2,000 square yards for subbase con-struction and 1,200 square yards forstabilized subgrade construction. To statisti-cally evaluate a lot, at least four samplesshould be obtained and tested properly.

Random SamplingFor a statistical analysis to be acceptable,

the data used for this analysis must be ob-tained from random sampling. Randomsampling means that every sample within thelot has an equal chance of being selected.There are two common types of random sam-pling. One type consists of dividing the lotinto a number of equal size sublets; one ran-dom sample is then taken from each of thesublets. The second method consists of takingthe random samples from the entire lot. Thesublet method has one big advantage, espe-cially when testing during production, in thatthe time between testing is spaced somewhat;when taking random samples from the lot,all tests might occur within a short time.The sublet method is recommended whentaking random samples. It is also recom-mended that all tests be conducted onsamples obtained from in-place material. Byconducting tests in this manner, obtainingadditional samples for testing would not bea problem.

Test ToleranceA specification tolerance for test results

should be developed for various tests with


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consideration given to a tolerance that couldbe met in the field and a tolerance narrowenough so that the quality of the finishedproduct is satisfactory. For instance, thespecifications for a base course would usuallystate that the material must be compacted toat least 100 percent CE 55 maximum density.However, because of natural variation inmaterial, the 100 percent requirement cannotalways be met. Field data indicates that theaverage density is 95 percent and the stand-ard deviation is 3.5. Therefore, it appears thatthe specification should require 95 percentdensity and a standard deviation of 3.5, al-though there is a good possibility that thematerial will further densify under traffic.

Penalty SystemAfter the project is completed, the job

should be rated based on the results of thestatistical quality-control plan for thatproject. A satisfactory job, meeting all of thespecification tolerances, should be considered100 percent satisfactory. On the other hand,those jobs that are not 100 percent satisfac-tory should be rated as such. Any job that iscompletely unsatisfactory should be removedand reconstructed satisfactorily.

THEATER-OF-OPERATIONSQUALITY CONTROL

In the theater of operations, quality controlis usually simplified to a set pattern. This isnot as reliable as statistical testing but is ade-quate for the temporary nature oftheater-of-operations construction. There isno way to ensure that all areas of a project arechecked; however, guidelines for planningquality control are as follows:

Use a “test strip” to determine the ap-proximate number of passes neededto attain proper densities.Test every lift as soon as compactionis completed.Test every roller lane.Test obvious weak spots.Test roads and airfields every 250linear feet, staggering tests about thecenterline.Test parking lots and storage areasevery 250 square yards.

Test trenches every 50 linear feet.Remove all oversized materials.Remove any pockets of organic orunsuitable soil material.Increase the distance between testsas construction progresses, if initialchecks are satisfactory.

CORRECTIVE ACTIONSWhen the density and/or moisture of a soil

does not meet specifications, corrective actionmust be taken. The appropriate correctiveaction depends on the specific problem situa-tion. There are four fundamental problemsituations:

Overcompaction.Undercompaction.Too wet.Too dry.

It is possible to have a situation where oneor more of these problems occur at the sametime, such as when the soil is too dry and alsounder compacted. The specification block thatwas plotted on the moisture density curve(CE 55) is an excellent tool for determining ifa problem exists and what the problem is.

OvercompactionOvercompaction occurs when the material

is densified in excess of the specified densityrange. An overcompacted material may bestronger than required, which indicates—

Wasted construction effort (but notrequiring corrective action to the mate-rial).Sheared material (which no longermeets the design CBR criteria).

In the latter case, scarify the overcompactedlift and recompact to the specified density.Laboratory analysis of overcompacted soils(to include CBR analysis) is required before acorrective action decision can be made.

UndercompactionUndercompaction may indicate—

A missed roller pass.A change in soil type.Insufficient roller weight.


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A change in operating frequency oramplitude (if vibratory rollers are in use).A defective roller drum.The use of an improper type of com-paction equipment.

Corrective action is based on a sequential ap-proach. Initially, apply additional compactiveeffort to the problem area. If undercompact-ing is a frequent problem or develops afrequent pattern, look beyond a missed rollerpass as the cause of the problem.

Too WetSoils that are too wet when compacted are

susceptible to shearing and strength loss,Corrective action for a soil compacted too wetis to—

Scarify.

Aerate.Retest the moisture content.Recompact, if moisture content iswithin the specified range.Retest for both moisture and density.

Too DrySoils that are too dry when compacted do not

achieve the specified degree of densification asdo properly moistened soils. Corrective actionfor a soil compacted too dry is to-

Scarify.Add water.Mix thoroughly.Retest the moisture content.Recompact, if moisture content iswithin the specified range.Retest for both moisture and density.


chapter 8 soil compaction section i. soil properties affected by

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