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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, thiswindow
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.
Soil Compaction 8-15
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FM 5-410
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,
Soil Compaction 8-16
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FM 5-410
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. Thespeedy 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 operators 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.
Soil Compaction 8-17
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FM 5-410
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
Soil Compaction 8-18
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FM 5-410
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
Soil Compaction 8-19
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F M 5 - 4 1 0
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 projects
Soil Compaction 8-20
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FM 5-410
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
Soil Compaction 8-21
-
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
Soil Compaction 8-22
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FM 5-410
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.
Soil Compaction 8-23
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FM 5-410
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.
Soil Compaction 8-24