Methods for Achieving and Measuring Soil Compaction_tcm45-341155

7/23/2019 Methods for Achieving and Measuring Soil Compaction_tcm45-341155

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o perf o rm well, concrete stru c t u res must bebuilt on firm soil. Soil consists of solids andvoids that are filled with either air or water. It is

the voids that compress when a soil is loaded,and the fewer the voids, the less chance there is that ex-c e s s i ve settlement or sliding failures will occur. Pro p e rcompaction improves a loose soil by forcing out air andreducing the volume of voids; this strengthens the soiland minimizes potential settlement, rutting, sliding orother pro b l e m s. Poor compaction or no compactionmay cause a concrete foundation, floor slab or pave-ment to fail. The failure may take place immediately orit may occur we e k s, months, or even years after con-struction (Figure 1).

Compaction is achieved by applying a pre s s u reon the surface or by vibrating the soil mass. Di f f e re n t

compaction methods are needed for different types ofsoils and the amount of compaction re q u i red for dif-f e rent soils must be established using standard testingp ro c e d u re s.

Measuring compaction

To find out how well a soil has been compacted wemust measure the dry unit weight or dry density inpounds per cubic foot. Dry density is a measure of theweight of solid material present in a cubic foot of soil.

The higher the dry density, the stronger and less com-p ressible the soil will be. One method for determ i n i n gd ry density invo l ves digging a hole in the compacted

soil, finding the volume of the hole and determining thed ry weight of the soil re m oved. The volume of the holeequals the sum of the volumes of soil solids, water andair and the dry weight of the soil equals the weight of soils o l i d s. Dry weight of the soil, divided by the volume ofthe hole is dry density.

A m e rican Society for Testing and Ma t e rials (ASTM)s t a n d a rds describe seve ral methods for measuring thed ry density of an in-place soil. In the rubber balloonmethod (ASTM D 2167), water is used to find the vo l u m eof the hole and in the sand-cone method (ASTM D 1556)sand is used to find the vo l u m e. There is also a methodfor determining the in-place density of soils by nuclear

methods (ASTM D 2922).The rubber balloon method re q u i res a level test loca-

tion. A hole is dug, and all of the soil dug from the holeis carefully collected in a sealable plastic bag. The vo l-ume of the hole is measured, using a calibrated ru b b e rdevice filled with water (Fi g u re 2). The contents of theplastic bag are taken to a labora t o ry where they areweighed, ove n - d ried and re weighed. By dividing thesewet and dry weights by the volume of the hole as mea-s u red with the rubber balloon, the wet and dry densities

Methods for achieving andmeasur ing soil compaction

Soil properties determine type of compaction needed

BY NORBERT 0. SCHMIDTUNIVERSITY OF M ISSOURI-ROLLAROLLA, M ISSOURI

AND

CHARLES 0. RIGGSCENTRAL M INE EQUIPMENT COMPANYST. LOUIS, M ISSOURI

Figure 1. An example of bad fillpractices and resulting settlement. Theoriginal ground surface was not strippedof organic matt er and debris. The init iallift was placed too thick and too dry,causing it to collapse and consolidatewhen wet ted by groundwater seepage.Fill layers have irregular thickness,resulting in inadequate compact ion ofthe t hicker portions.


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a re obtained.In the sand-cone method, a hole is also dug and the

re m oved soil is sealed in a plastic bag. Dry sand that hasa carefully measured unit weight is allowed to run intothe hole from a pre weighed bottle. After the exact

amount that fills the hole has run out, the bottle is re-t u rned to the labora t o ry for re weighing the re m a i n i n gsand. The difference in weight is the amount of sand re-q u i red to fill the hole. The volume of the hole is calcu-lated by dividing the weight sand used to fill it by the unitweight of the sand. The rest of the method is the sameas that for the rubber balloon method.

The rubber balloon method tends to be faster, butw h e re there are sharp, angular particles in the soil, themethod is of little use, since the balloons are often punc-t u red and must be replaced and the water in the systemre c h a rg e d .

How much compact ion is needed?For the results of a field density test to have meaning,

a re f e rence point is needed. There are two tests that pro-vide a re f e rence point for granular soils. One (ASTM D4524) determines the minimum index density of a co-hesionless (granular) soil and the other (ASTM D 4253)d e t e rmines the maximum index density of the soil.These are labora t o ry tests that determine how loose orh ow dense a given granular soil can be made in the lab-o ra t o ry. These become the re f e rence points. Field den-

sities can then be specified to fall within a desired ra n g es o m e w h e re between the two re f e rence points.

To determine how much compaction is re q u i red forc o h e s i ve soils, results of labora t o ry compaction testsa re needed. Because the compacted density of clays andother cohesive soils is ve ry sensitive to the water contentat which they are compacted, these soils must have justthe right amount of moisture to compact well. Thism o i s t u re content is called the optimum moisture con-

tent. Optimum moisture content va ries with the type ofsoil and the compactive effort used.

M oisture-density relat ionships

L a b o ra t o ry compaction methods are used to deter-mine the relationship between the moisture content anddensity of soils. A man named Proctor developed the

methods as an aid in determining the amount of com-paction that a contractor could reasonably expect toa c h i e ve in the field. In the standard Proctor test (ASTM D698), samples of the soil are mixed at seve ral differe n twater contents in the labora t o ry, allowed to stand for atleast 16 hours, and then compacted using a standard-i zed pro c e d u re. After the wet weight is obtained, the soilis dried and the dry weight determined. For differe n t

Figure 2. A rubber balloon device can be used to measurethe volume of soil removed from a compacted fill. Drydensity of the soil is t hen calculated by dividing t he dryweight of the soil by the volume.

Figure 3. A typical five-point Proct or density curve for aclayey silt. The zero air voids line at t he right representsthe maximum density that could be achieved if the soil wascompletely saturated. If any field density test data fallsabove or t o the right of a correct ly calculat ed zero air voidsline t he data are suspect and should be rechecked.


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m o i s t u re contents, the dry density is calculated and ag raphical plot of dry density versus moisture content ismade (Fi g u re 3). The highest dry density on the graph iscalled the maximum Proctor dry density and the mois-t u re content corresponding to the peak of the curve iscalled the optimum moisture content.

Di f f e rent curves will be obtained for different soils, butin general, the more plastic the clay, the lower will bethe maximum dry density and the higher the optimumm o i s t u re content. Two layers of soil in a borrow pit, onesandier than the other, will have different Proctor maxi-mum dry densities and different optimum moisturec o n t e n t s.

Many years after the Proctor test had been put intou s e, a modified version of the test was developed to cor-

relate better with compactive efforts comparable tothose obtained with heavy rollers under favo rable work-ing conditions. In the modified Proctor test a heaviercompaction hammer is dropped from a greater height.The number of blows per layer remains the same as forthe standard test but more layers are used. The test re-sults are similar except that the density is about 5pounds per cubic foot greater and the optimum mois-t u re content is 2 or 3 percent less than that obtained us-

ing the standard Proctor test (Fi g u re 4).If field tests on compacted cohesive soil indicate thatfield compaction pro c e d u res are providing densities ofabout 95 percent of the maximum density according tothe standard Proctor method, or about 90 percent of themaximum density according to the modified Pro c t o rmethod, compaction is re l a t i vely good. Obtaining 100p e rcent of even the standard Proctor maximum densitymay be practically impossible for some cohesive soils.The density that is re q u i red depends upon the loads thatwill be placed on the fill.

Figure 4. M aximum dry densit y is higher and the optimummoisture content is lower when the modified Proctormethod instead of t he standard Proct or method is used tocompact a soil.

Figure 5. Surface t ension in the water pulls moist sand orsoil particles together and may make the soil more difficultto compact.

Figure 6. Vibration compacts loose granular soils. Thecompacted soil is stronger and less likely to settle in

A. LOOSE STRUCTURE B. VIBRATION AND CONSOLIDATION C. COMPACTED STRUCTURE


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Compact ing clean granular materials

Clean granular materials (sands and gra vels) are fre e -d raining so that water can enter or leave the voids withre l a t i ve ease. If voids in the sand are completely filledwith water or are completely dry there are no forc e sholding the sand particles together. Vi b ration causes thep a rticles to bounce around and roll or slide into a densec o n f i g u ration. Howe ve r, if the voids are only part i a l l yfilled with water, surface tension in the water pulls the

p a rticles together as shown in Fi g u re 5. When this hap-p e n s, sand particles dont move as freely and much morec o m p a c t i ve effort is needed to reduce the void content.Fi g u re 6a shows a loose stru c t u re, Fi g u re 6b shows howv i b ration allows densification, and Fi g u re 6c shows theresultant dense configura t i o n .

Dumping sand or gra vel from the bed of a truck orf rom a scraper places the granular material in a re l a t i ve-ly loose condition, particularly if the sand contains onlya small amount of surface moisture. This loose-dumpedm a t e rial must be compacted if it is to have adequates t rength and not settle exc e s s i vely under load. Left un-compacted, it is especially likely to settle if it gets we t t e r

after a stru c t u re is built on it.Smooth-wheeled or grid-wheeled vibra t o ry rollers are

specially designed to consolidate granular soils to a highd e n s i t y, compacting ve ry efficiently to shallow depths.T h e re are also excellent flat plate vibra t o r s, with a gaso-line engine mounted to a unit that causes a flat skid plateto vibra t e. These will do an excellent job on sands andsmall gra ve l s, compacting to a depth or lift thickness ofabout 6 inches.

One of the more successful methods for compactinga deep natural sand deposit is to dri ve piles into thesand, perhaps using a vibra t o ry hammer, and then topull them out again. An air or steam hammer also de-

velops sufficient vibration to be quite effective, at leastfor a short distance around the pile. You can tell that it ise f f e c t i ve because a cone-shaped depression forms at theg round surface in the sand around the pile for a distanceequal to about 3 pile diameters. When 1-foot-diameterpiles are dri ven, they must be spaced about 3 to 5 feeta p a rt to be effective.

Another ve ry effective device for compacting clean,f re e - d raining sands and gra vels is a patented vibra t i n gp ro b e. It resembles a standard internal concrete vibra t o rbut is much larger and more powe rful. The probe pro-vides large capacity water jets which act dow n w a rd ands i d e w a y s, flooding the soil and breaking the surface ten-sion. This allows the sand particles more freedom to set-tle into a compact configuration as the granular part i c l e sa re vibra t e d .

Compact ing clay soils wit h a sheepsfoot roller

Sheepsfoot rollers are commonly used to compactc l a y s. The original sheepsfoot compaction was just whatthe name implied. Ancients had found that paths usedby sheep on clay soils we re ve ry firm, having been we l lcompacted by the feet of the sheep. To d a y s sheepsfoot

roller is a large drum that can be filled with water tomake it heavier. Attached to the drum are a number oft a p e red feet with square cross section. Ex p e rience hass h own that high plasticity, tough clays compact best un-der small feet, but silty cohesive soils re q u i re the use ofl a rge feet. The roller may be pulled by a dozer or a farmt ractor or it may be self-propelled. Examples of self-pro-

pelled types are the two-wheeled tandem and thre e -wheeled sheepsfoot ro l l e r s.

Clays dont drain fre e l y; it takes time and continuouse f f o rt to change their moisture content. Getting the ri g h tm o i s t u re content for compaction is accomplished eitherat the borrow pit or at the fill site. Water may be addedto the soil to increase the moisture content or the soilmay be aerated with a disc harrow to dry it. Neither ofthese pro c e d u res is simple or inexpensive. After the cor-rect moisture content has been obtained compaction

with a sheepsfoot roller becomes the simple act of forc-ing out the air vo i d s.

The first compacted layer is the most important one.

Compaction cant be accomplished on top of a spongyl a yer of soil. There must be a base to compact against.T h e re f o re its necessary to strip down to sound materi a lb e f o re beginning to compact a fill.

Prior to compacting a large area fill, the soil is bro u g h tto the site by hauling equipment and spread in about 8-to 10-inch lifts so that the finished compacted layer willbe 6 inches thick. Clay cannot be properly compactedinto lifts thicker than 6 inches. As previously mentioned,the clay should be brought to approximately optimum

Figure 7. During t he first pass of a sheepsfoot roller the feetpunch full depth into t he soil (upper drawing). With

subsequent passes, compaction st rengthens the soil andthe feet dont penetrat e as deeply (lower drawing). Whenthe roller walks out of the fill, adequate compaction isindicated.


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m o i s t u re content before rolling. To l e rances on moisturecontent are usually given in contract specifications.

The sheepsfoot roller has a kneading action. Du ri n gthe first pass of the roller across the soil, the feet gener-ally punch full depth into the loose material. As furt h e rpasses of the roller work the soil and knead out the airvo i d s, the soil becomes stronger so that the feet do notp e n e t rate the soil to a great depth, and the roller is saidto be walking out of the fill. Fi g u re 7 illustrates this. Us u-

ally the specification will re q u i re that the fill be placedat a moisture content within about 2 or 3 percent of theoptimum moisture content as determined by the Pro c-tor density test, and compacted to a percentage (usually90 or 95 percent) of the Proctor maximum dry density.

Tests confirm field observat ions

Being within specifications for density is not goodenough unless the moisture content is also at the corre c tl e vel. Soils having the same density but differing in mois-t u re content will also have differing strength, compre s s-ibility and permeability chara c t e ri s t i c s. They will alsoh a ve different tendencies to swell or shrink with changes

in water content. It has been found that staying near theoptimum moisture content minimizes these pro b l e m s.

Mo i s t u re tests are re l a t i vely inexpensive to perf o rm. Ifa fill is known to have the correct moisture content fro ms e ve ral tests, if each layer of the fill has re c e i ved uniformc o m p a c t i ve effort, and if a few in-place density tests in-dicate that the specified density has been achieved, thenit may be inferred that the untested areas of the fill alsomeet compaction specifications. To make such a deci-sion re q u i res continuous inspection of fill placementand observation of all spreading and compacting pro-c e d u res by an experienced technologist.

Tests for both in-place density and moisture content

will be done on the soil in the field. Mo i s t u re content anddensity tests re q u i re ove rnight drying of the soil, butwaiting for the results would delay the contractor un-n e c e s s a ri l y. After developing some experience on a pro-

ject, the inspector may use a simple field test to deter-mine whether the soil is at its optimum moisturecontent. A small sample of the soil is formed ands q u e ezed by both hands, and then broken apart be-t ween two fingers and the thumb. If the ball bre a k scleanly without crumbling it is an indication of near-optimum moisture content. If the contractor has theright water content in the soil and properly uses hiscompaction equipment, there is reasonably good assur-ance that compaction specifications will be met. De n s i-ty tests simply confirm the inspectors observa t i o n s.

If a cohesive soil is being compacted with a sheepsfootroller weighing 4000 pounds per foot of drum length,

with tamping feet at least 8 inches in length and a cro s ssection of about 7 to 10 square inches (all of this appearsin many specifications for this type of roller), and a re a-sonable number of passes are made to achieve com-paction, proper compaction should be achieved if thesoil is near the optimum moisture content. The U.S. Bu-

reau of Reclamation suggests 12 passes of a fully ballast-ed roller to achieve 95 percent of the standard Pro c t o rd e n s i t y. As few as 6 passes may do for some soils, butm o re are likely to be needed.

Other met hods for compact ing clays

Ru b b e r- t i red rollers are generally not as effective inclays as are sheepsfoot ro l l e r s. The practice of trying tocompact with hauling equipment, by using ru b b e r- t i re d

l a rge scraper pans to compact the fill while at the sametime hauling additional soil to the site just does not ac-complish proper compaction. Compaction occurs onlyunder the wide tires which have low ground pre s s u re form o b i l i t y, and then only for about 6 inches of depth. On epass of a scraper wheel is much less effective than a passwith a sheepsfoot ro l l e r. To achieve what 6 passes of thesheepsfoot roller will accomplish re q u i res at least 12p roperly routed passes of large loaded scra p e r s. It is vir-tually impossible to control individual operators so thatthey will follow the wheel paths of the previous ma-chines or correctly distribute the compaction effort .

Timely inspection is neededWhen a fill is totally in place and compacted, it is gen-

e rally too late to determine whether or not the fill hasbeen placed correctly according to the specifications. Inn a t u ral ground, we can drill holes and perf o rm tests tod e t e rmine the adequacy of the ground both at the bor-ing location and between the bori n g s. Assumptions ofconditions to be found between borings in natura lg round are made on the basis of understanding the nat-u ral geologic processes of soil deposits and we a t h e ri n g .In earth fills, howe ve r, no such assumptions can bem a d e. If assurances are re q u i red that a fill was placed ac-c o rding to the specifications, inspection during fill

placement is necessary to confirm :

1. the condition of the ground surface before the fill wasp l a c e d

2. the quality and loose lift thickness of the fill materi a l s

3. the correct compaction pro c e d u re

4. the behavior of the compaction equipment as it pro-g resses over the fill surf a c e

Visual inspection helps in detecting problems t hatmay occur during fill compaction opera t i o n s. If the fill

pumps or heaves at a particular location, the fill materi-al is probably too wet at this location. If dust is flying atanother location, the fill is probably too dry. These are a sshould be suspected of having low density fill, and den-sity tests should be made in these locations. Re m e m-bertests confirm the results of visual observa t i o n s. If afill tester is called upon only to make tests, without thebenefit of inspection of placement and compaction,then he can only attest to the quality of the test re s u l tand not to the quality of the fill.


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Compact ing soils in small areas and againstconcret e walls and foundations

Flat plate vibrators and vibra t o ry rollers come in manys i zes and the smaller ones are well adapted for vibra t i n gg ranular materials in close quarters or near concre t e

w a l l s. A ra m m e r-type machine that delivers ra p i d - i m-pact blows is also useful for compacting granular soilsin narrow tre n c h e s. The granular soil must be confined;o t h e rwise the rammer will push it to the sides ra t h e r

than compact it.Compacting small areas of cohesive soil presents ave ry difficult problem. Rammers and rammer platecompactors can be used to force the air out of clays butit is most important to have the moisture content asclose to optimum as possible. Water must be ve ry we l lmixed into the soil being compacted and the soil itselfmust be well broken so that clumps are small. Lift thick-ness should always be less than the 6-inch compactedthickness of sheepsfoot roller lifts. Vi b ra t o ry padfootrollers with drums as little as 24 inches wide have alsobeen used for compacting cohesive soils in trenches and

other areas with re s t ricted access. Again, moisture con-tent of the soil must be maintained at or near optimum.

With seve ral passes, heavy ru b b e r- t i red rollers cana c h i e ve compaction against concrete walls and againstp i p e, but caution must be exercised so that the com-paction will not cause lateral loads high enough to en-danger the concrete stru c t u re.

Ex t e n s i ve working of thin lifts may be needed to meetspecification density re q u i rements for cohesive soils in

h a rd - t o - reach are a s. It may be more practical and eco-nomical to avoid problems in hand compacting thesesoils by backfilling with clean granular material andcompacting using vibration as previously discussed.

P U B L I C AT I O N# C 8 5 0 6 8 1Copyright 1985, The Aberdeen Gro u p

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Methods for Achieving and Measuring Soil Compaction_tcm45-341155

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