grading contractor-compactor.pdf

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The how and what of so il compaction equipment

Soil compaction is such a basic part of almost every construction project that it often getsoverlooked and not given a second thought. But almost every manmade structure rests on afirm foundation of compacted soil. Compacting soil involves the increase of its in-place

density to specified depth by means of mechanically applied force. A repeated impact to thesoil by various weights and applied pressures reduces the soils void ratio and reduces itsoverall volume. Compaction is performed to avoid excessive settlement, minimize damagefrom frost heave, and minimize water seeps and structural instability. Soil compaction is aclassic case of out of sight, out of mind with the compaction effort almost always hiddenfrom view below grade or underneath a buildings foundation. It only becomes noticeablewhen it fails and its effects become apparent at the surface with cracking walls, tiltingstructures, and sinking buildings.

Why soil compaction is performed is obvious. This article examines how soil compaction isaccomplished and what equipment performs the compaction.

Compaction TechniquesThere are basically two ways to compact soil: the static application of heavy loads or therepeated dynamic application of smaller loads at high frequencies. Most compactionequipment utilizes both types of forces. These forces are in turn each divided into twoseparate techniques for applying these forces:

Static1. Pressure2. Kneading

Dynamic1. Impact

2. Vibration

Static force is simply the operating weight of the compaction machine applied as a dead loadto the soil underneath, relying on applied pressure and/or kneading of the soil to achievecompaction. Heavier machines have greater static loads. Compaction equipment operatingon relatively steep slopes applies proportionally less direct dead weight depending on theangle of the slope to the horizontal. Static load is measured as pressure, the weight of themachine over the area of the parts of the machine that are in direct contact with the soil(pneumatic tires, steel drum, padded foot, etc.). The applied pressure, especially when


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delivered to many small areas, has the effect of kneading the soil. Static force is simple toapply but does not usually affect the soil to a significant depth. A static sheepsfoot and astatic roller are examples of machines that primarily deliver pressure. Kneading of soil can beaccomplished with scrapers and pneumatic tires.

Dynamic force is created not by dead weight but by a live engine that moves a mass in a

reciprocating motion that delivers multiple impacts to the soil in a short period of time, relyingonvibration and impact to get the job done. In keeping with the laws of physics, the resultantforce is proportional the mass of the impact object times the square of its velocity. So whilethe weight of the impact mass doesnt change, the force it translates to the soil can begreatly increased by increasing the speed of its impact. The repeated shock waves passingthrough the soil cause compaction to occur at greater depths than static force application.Rammers and dropped weights are examples of machines that use heavy impact forcompaction. Vibratory sheepsfoot, vibratory roller, and vibratory plate are examples ofdynamic compaction machines utilizing vibration.

So what method works best on which soils? As there are two main methods of compaction,there are two broad categories of soils: granular soils and cohesive soils. Granular soilsconsist of large particles from fine sands to large cobbles but are divided into twosubcategories of sand and gravels. Cohesive soils consist of smaller particles of either claysor silts that are densely compacted and adhere together. Granular soils are best forfoundation support, embankment shell construction, and roadway pavement subgrades.Cohesive soils are typically used to construct embankment cores as well as low-permeabilityliners for liquid containment in ponds, impoundments, and landfills.Each type of compaction equipment is suitable for different kinds of soil:

Sheepsfoot rollers using both dynamic vibration and static pressure applied overmultiple small contact points are useful for compacting cohesive soils (100% to 50%clay).

Pneumatic wheels using static kneading action are used for mixed cohesive andgranular soils (75% sand/25% clay to 25% sand/75% clay).

Rollers using both dynamic vibration and static pressure applied over a single largecontact area can be used on granular soils (50% sand to 100% sand).

High-speed rammers and tamping plates using dynamic impact are used on eithercohesive or granular soils.

Measuring CompactionSo how much compaction is enough? That depends on the soil and its moisture content aswell as the compaction application and the acceptance criteria. Soil can be compacted toprovide a stable base for constructing a building foundation, a roadbed, a structuralembankment, or an impoundment liner. Each application has a preferred level ofcompaction, soil type, and moisture content.

Detailed information on a soils physical state can be derived from a relatively simple seriesof laboratory tests. Two of the most common are the Standard and Modified Proctor tests.

These tests determine the relationship between a soils density and moisture content and aset compaction effort. Each soil sample is subdivided into smaller test samples that arewetted to varying moisture contents. Each test sample is compacted into a standard moldmeasuring 57.6 cubic inches. In the Standard Proctor test, a hammer weight of 5.5 pounds isdropped on each of three lifts of soil from a height of 12 inches (resulting in compactiveenergy of 12,375 foot-pounds per cubic foot). The Modified Proctor increases this energy to56,250 foot-pounds per cubic foot by dropping a 10-pound hammer from a height of 18


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inches on each of five lifts of soil. The Proctor test was originally developed to control andmeasure the compaction of soils used as the foundation for highways and airfields. Thestandard test simulates the compaction of soil in the field with medium-sized compactionequipment, while the modified test approximates compaction with the heaviest compactionequipment.

The results of the compaction effort are measured by first determining the water content ofeach compacted sample and its dry unit weight. These values are plotted on a graph with thewater content as the x-axis and the dry unit weight as the y-axis. Three sample points areusually used to create a compaction curve that resembles an upside-down parabola. Thefirst sample is compacted at field water content. The second sample is compacted at a watercontent two percentage points wetter than the field sample. Depending on the outcome ofthe send test, a third sample is compacted at either two percentage points drier than the firstsample or two percentage points wetter than the second sample. The top of the resultantparabola curve represents the soils optimum water content. That is, the specific moisturecontent that results in the highest dry unit weight.

Soil is typically compacted to either an acceptable moisture/density zone based on shearstrength criteria (for embankment and foundation construction) or to an acceptable zonebased on hydraulic conductivity (for lining ponds and landfills). To determine these zones,soil samples are compacted and the results plotted for three different curves resulting frommodified, standard, and reduced compaction efforts. Along the upper right-hand side of thesecurves is plotted another curve representing the soils density and moisture characteristics ata zero air voids situation representing a completely saturated condition. This represents anideal upper bound condition and is calculated from the previously determined compactiondata. Acceptable zones are determined by the zero air voids curve, the lower bounds thatrepresent the moisture and density relationships that result in the soil having the minimumrequired shear strength of the maximum allowable hydraulic conductivity.

Field tests can also be performed on in-situ compacted soils. They can all be done relativelyfast with reasonable to high degrees of accuracy. Aside from applying a pocket penetrometer

to the soil with hand pressure, a field measurement of soil usually involves the removal of alarge sample with hand shovels. The resultant hole is then filled with material of knownvolume, such as sand, which is poured into the hole to measure its volume (also known asthe sand cone test). Meanwhile, the excavated soil is weighed and compared with thevolume of the hole it was extracted from to determine its in-place density. A more measuredvolume sample can be extracted with a Shelby tube. Direct measurement of a soils densitycan be made by using a nuclear gauge to measure the radiation from a gamma ray sourcedriven into the soil to a fixed depth.

Compaction EquipmentThere are four broad categories of soil compaction equipment: handheld rammers andtamping plates, vibratory and static sheepsfoot, vibratory and static rollers, and pneumatic

tire compactors.

Rammers and Tamping PlatesOften referred to as jumping jacks, handheld vibratory plate compactors and rammers areused for those situations that do not require production of large quantities of compacted soilfor soil compaction efforts confined to small areas. Like their cousin the jackhammer,tampers and rammers use centrifugal force generated by machine vibration induced into aflat contact plate to compact soils. They come in two types, one way or reversible. With aone-way tamping plate, the operator can only move forward with his machine as it performs


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compaction. A reversible tamper allows the operator to move back and forth over the area heis compacting.

Small compacted soil quantities are usuallyrequired for small repair and maintenance workdone to failed areas (or areas that were not

compacted correctly in the first place) of a lineror embankment, the compaction of soil inconfined areas (such as along the bottom of atrench), or small quantities of granular soils(such as sand, gravel, crushed aggregate, ormixed soils used as the base for small pavedareas). When working in confined spaces, suchas a trench bottom, where the operator cannoteasily make a full turn back to his stoppingpoint, reversible tampers and rammers are anecessity. Diesel rammers provide the powernecessary for heavy-duty compaction in areas

to small for heavy soil compaction equipment. Neither a vibratory plate nor a power rammeris suitable for use in wet clayey or otherwise cohesive soils, though vibratory tampers workwell on soil with low to moderate fines content.

Multiquip Inc. manufactures a complete line of rammers and vibratory plate tampers. Itsrammers can run either on diesel or a four-cycle gasoline engine. With contact shoes rangingfrom 5.9 inches to 1.2 inches, Multiquips MT series delivers ramming forces from 1,215 to3,500 pounds at up to 700 blows per minute. In size they start at the lightweight and easilyportable MTR40F to the MT84FA with its 3.5-horsepower engine. Even heavier areMultiquips rugged diesel models, the MT76D2 and the MT86D2, which can pound a 13-inchshoe with a force of 4,400 pounds. The company also offers a line of forward and reversibleplate compactors powered by 2.4-horsepower to 6.0-horsepower engines designed for theefficient compaction of sand, gravel, and cohesive soils. The reversible plate compactors

also come equipped with two eccentric weights, which increase the compaction force whileallowing for a smooth transition from forward to reverse.

In addition to its lines of planers, pavers, and vibratory rollers, Dynapac produces vibratoryplates and rammers. The companys LF series of single direction plate compactors delivers(at the light end) 2,000 pounds of force at a frequency of 5,520 revolutions per minute (rpm)to 4,725 pounds at 5,100 rpm. Vibratory jumping jack plate tampers, the LT series, aresuitable for small repair jobs and confined spaces. Rounding out Dynapacs product line areits LG series of gas-powered reversible plate compactors designed for compacting eitherasphalt or granular soils. For bigger jobs, there is the LP series of diesel-powered, walk-behind vibratory drum rollers.

Amman America produces four-stroke diesel rammers and two-stroke gasoline rammers forsmall compaction jobs. A wide variety (80 different types) of vibrating compactors is alsoavailable from Amman, allowing for site-specific compaction applications. In addition totraditional forward and reversible plate compactors, Amman offers remote-controlled models.Complementing this product line is a specialized line of rollers for trench compaction, boththe light AC series for trench compaction and the tandem roller AV series for basecompaction.

Photo: Dynapac

Dynapac's LG machines can compact both asphalt andgranular soils.


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Sheepsfoot and Padded RollersSheepsfoot rollers, so the story goes, wereborn out of the observation that the soilsadjacent to the gate of a sheep hold weredense and stiffly compacted as the result ofthousands of little sheep hooves pounding the

earth. With this observation came therealization that force concentrated into manysmall footprints could be more effective onsome soils than the same weight applied to abroad flat area impacted by a smooth drumroller. Sheepsfoot rollers are also known astamping rollers. They resemble a smooth drumroller with projecting club-shaped feet fixed tothe drums surface in a regular pattern. Theroller drums themselves are empty, and theadditional weight is provided by water or sandfill. The operating mass of a sheepsfoot rollercan vary between 5 and 8 tons. Though mostly

used on cohesive soils, they can be used onsands and gravels with sufficient fines andmoisture content. Sheepsfoot rollers are notused on very coarse soils and uniform gravels.

Certain distinctions should be made betweensheepsfoot rollers and the similar padded footrollers. The sheepsfoot contact points are pegshaped and more deeply penetrating thanthose of padded foot rollers. As such,sheepsfoot rollers are mostly used to tietogether via compaction and secondary

kneading separate layers of clay. Usually, theclay is placed in loose lifts as thick as thelength of the sheepsfoot itself; an 8-inch looselift is fairly common. Repeated passes by the sheepsfoot roller compacts the loose lift andbinds it to the lower, previously compacted lift, resulting in a newly compacted layer of about6 inches in thickness. Such an operation is typically performed to construct compacted clayliners for landfills and other impoundments. Sheepsfoot rollers come in both static andvibratory types (adding dynamic compaction along with the static pressure and kneading).

They also come in pull-behind models (typically hitched to a large dozer) or self-propelledride-on models with one or two rollers.

By contrast, padded foot compactors are of a shallower rectangular shape but can also beused to compact clays. However, a padded foots penetration is less than that of a

sheepsfoot but is also used in earth compaction primarily for sub-base construction. Whenused for sub-base construction, padded foot rollers more resemble smooth drum rollers inapplication. While the sheepsfoot roller is ideal for achieving low permeability, padded foottrollers are suitable for achieving strength compaction. Both are different than the smoothdrum compactor, which is used on dry materials such as granular soils used to constructroad base.

Caterpillars newest addition to its extensive line of soil compaction equipment is the 825Hsoil compactor. With an operating weight of 72,164 pounds and powered by a 453-net-

Photo: Sakai

Soil compactors from Sakai can handle jobs in all types ofsoil, rockfill, and soil cement.

Photo: Sakai

Features include high-force outputs, heavy-duty center hitch,and three breaking choices.


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horsepower C15 ACERT engine, the 825H is designed to meet aggressive Tier 3 emissionstandards. While adding a new engine and cooling system, it retains those features thatmade the old 825G a success (high-efficiency hydraulics, ease of use, durable framestructure, etc.). Its engine is designed to reduce emissions while ensuring constant power bymeans of an electronically configured transmission system that provides constanthorsepower throughout its operating range. This addition to its line of compaction equipment

complements Caterpillars established padded foot and smooth drum CP series and CSseries vibratory soil rollers. The CP series machines range in size from 10,190 pounds to37,050 pounds and are designed to operate on uneven or steep terrain. The CS seriesmachines are designed for site preparation and base compaction applications. Some ofthese models come with optional padded foot shell kit for differing applications.

Bomag provides static, padded foot compactors in the heavy weight class, the 35-ton BC772and the 29-ton BC672. Able to handle large earth compaction jobs such as damembankments, they are high-speed compactors driven by water-cooled diesel enginesprotected by complete underbody enclosure. The enclosed frames fully protect thehydrostatic drives, axles, and engine compartment, protecting them all from dirt penetration.One interesting feature is the special polygonal wheels with high-torsion-resistant padfeetshaped to maximize static compaction.

Smooth RollersFor large-quantity production of compacted granular soils, smooth rollers are the machinesof choice. Whether in single-drum or dual-drum configurations, smooth-drum rollers aresuitable for compaction of granular soil base layers. This includes building foundations androadway sub-bases, as well as the compaction of surface courses of asphalt paving. Likesheepsfoot drums, smooth-drum rollers are available in either tow-behind or self-propelledride-on models. Their operating weights range from 2 to 20 tons, and (again, like otherrollers) they are hollow with weight provided by sand or water fill. While useful for compactinggranular soils such as well-graded sands and gravels, they are not used for the compactionof cohesive silts and clays with low plasticity and/or high moisture content and are not aseffective on uniform sands or silty sands.

Similar to the smooth-drum roller is the not-so-smooth grid roller. Ranging in weight from 5 to12 tons, grid rollers have surfaces of spacedheavy bar stock constructed in a cylinder orraised surface Z-patterns that resemble basketweaving. This raised drum surface impressesgrooves into the soil while leaving raised knobsof earth in between. When used on wet clays,the soil tends to adhere to the lower portion ofthe surface pattern, filling it in and effectivelytransforming the grid drum into a smooth roller.More suitable soils include well-graded sands,soft rocks, and stones. In fact, one of theprimary uses of grid is the breaking up orrubble-ization of existing concrete pavementsurface and the pulverization of stone andrubble into finer particles.

Photo: Dynapac

Compacting soil may provide a stable base for a buildingfoundation, a roadbed, a structural embankment, or animpoundment liner.


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In addition to its heavy weight padded foot static compactors, Bomag manufactures a line oftandem vibratory smooth rollers, the BW series running from the lightweight BW120AD-4(5,732-pound operating weight) to the hefty BW284 (28,425 pounds). What makes thesemachines unique are the little things that make for a more efficient compaction effort: slanteddrum supporting improving operator visibility of the drum edges, central lifting point for easeof loading and off-loading, easy access to daily maintenance check points, bolt-on heavy-

duty articulation joints, and so on. All are designed for high-production soil compaction tasks.

The Case Corp. (aka CNH Global) manufactures a line of SV series machines that can beequipped with either a smooth or padfoot drum. Equipped with high-traction, hydrostatic drivethat synchronizes the rear wheels with the compaction drum and a low center of gravity,Cases SV compactors are suitable for work on steep grades. The drums vibration oscillationcan be set to either low amplitude and high frequency for granular materials or highamplitude and low frequency for cohesive soils.

Sakai America offers a wide range of high-force vibratory soil compactors designed for rapidcompaction of all types of soil, rockfill, recycled base materials, and soil cement at the lowestpossible cost. Featuring high-centrifugal-force outputs; dual amplitude/dual frequency; choiceof drum configurations; drum and axle drives for traction; heavy-duty, center hitch design;and three braking choices, Sakai has four compactors in its SV510 line ranging in weightfrom 24,140 to 29,875 pounds. Sakai also offers a full complement of high-frequencyvibratory asphalt rollers for superior density and smoothness.

Reflecting its overall diversified product lines, Ingersoll-Rand produces a line of SD serieslarge soil compactors with diverse applications. This product line includes the 16,350-poundSD-77 (smooth or two different padded foot drums); the SD-116, which operates well onsteep grades due to its Ultra-Grade traction system and axles and drive motors designed forclimbing; the high-powered SD-160 with its six-cylinder turbocharged engine, which canhandle thicker lifts with fewer passes; and the 205-hp SD-200 for heavy-duty applications.Ingersoll-Rand also provides an auxiliary line of small soil compactors ranging from the 2.6-ton SD-2D TF to the SD-70 D/F. All the companys models come with two-piece, clamp-on

padfoot shell kits for conversion to cohesive soil compaction.

Pneumatic TiresPneumatic-tired rollers utilize heavy wheels aligned to produce a rolled track the width of thecompaction vehicle. When not a tow-behind single-axle model, a pneumatic tire compactorusually is configured on a two-axle self-propelled ride-on vehicle. Dead loads for pneumatictire compactors range from 12 to 40 tons. The tires provide soil compaction through akneading action and are best suited for both coarse and fine grained soils, with the exceptionof very soft clays and variable soils. Pneumatic tire compactors are extensively used inroadway construction to compact base, sub-base, and wearing course materials.

In addition to sheepsfoot, padded foot, and smooth drum rollers, Caterpillaralso

manufactures a line of pneumatic tire compactors. From the 23,100-pound PF-300B to thehigh-productivity PS-360C (which can operate as heavy as 40,785 pounds when its drumsare filled with wet sand), these pneumatic compactors are suitable for use on base, sub-base, or wear course materials by providing a kneading action and a high weight-per-wheelratio.

Daniel P. Duffy, P.E., is an environmental engineer employed by URS Corp. in Akron, OH.


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Let's face it. Soil compaction is an old-hat subject. After all, it has been practicedby man for thousands of years. And the fundamental need has remained yearafter year, millennium after millennium, and it still exists in the millennium justbeginning. Today, compaction is being routinely used in virtually everyconstruction project. After all, soil compaction increases load-bearing capability,prevents soil settling and frost damage, provides stability, and reduces waterseepage, swelling, and contraction. What's more, building codes and inspectorsdemand it.

Now, as then, there are just two principal types of compaction force: static andvibratory. To quote from Multiquip's Soil Compaction Handbook:

"Static force is simply the deadweight of the machine, applying downward forceon the soil surface, compressing the soil particles. The only way to change theeffective compaction force is by adding or subtracting the weight of the machine.Static compaction is confined to the upper soil layers and is limited to anyappreciable depth. Kneading and pressure are two examples of staticcompaction.

"Vibratory force uses a mechanism, usually engine-driven, to create a downwardforce in addition to the machine's static weight. The compactors deliver a rapidsequence of blows (impacts) to the surface, thereby affecting the top layers aswell as deeper layers. Vibration moves through the [soil], setting particles inmotion and moving them closer together for the highest density possible."

While these are indeed the fundamentals of soil compaction, there are a varietyof conditions and applications that dictate the design of the many different soil


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compaction products on the market. Therefore, it is useful to review theseconditions and applications to gain an understanding of the design rationale ofthe products and what conditions dictate the use of each.

Soil Classification

Both the American Society for Testing and Materials and the AmericanAssociation of State Highway and Transportation Officials classify soil as eithergranular or cohesive on the basis of a sieve analysis. Granular soil consistsmainly of sands and gravels, whereas cohesive soil consists mainly of silts andclays. J im Layton of the Wacker Corporation in Menomonee Falls, WI, explainsthe structural differences.

"In granular soil, the particles are held in position due to the frictional force thatexists at the contact surfaces. In the dry state, granular soil particles can beeasily separated and identified. In a moist state, a granular material such as sand

may be formed to desired shapes but will crumble easily as soon as it isdisturbed.

"Granular soils are best compacted by vibration. This is because the vibrationaction reduces the frictional forces at the contact surfaces, thus allowing theparticles to fall freely of their own weight. At the same time, as soil particles areset in vibration, they become momentarily separated from each other, allowingthem to twist and turn until they can assume a position that limits theirmovements. This settling action and repositioning of particles is compaction. Allthe air voids that were previously present in the soil mass are now replaced bysolidly packed soil.

"In cohesive soil, the molecular attraction between soil particles is the force thatholds the soil in place. As these particles are very small in size, high in number,and densely arranged, the cohesive force within the soil is very high. Cohesivesoils are very hard in the dry state. When moist, they are plastic and can bemolded or rolled into almost any shape.

"Cohesive soils are best compacted by impact force. Cohesive soils do not settleunder vibration due to the natural binding forces between the tiny soil particles.These soils tend to lump, forming continuous laminations with air pockets inbetween. Therefore, cohesive soils such as silt and clay are more effectivelycompacted using impact force because it produces a shearing effect thatsqueezes the air pockets and excess water to the surface and moves theparticles closer together."

Of course, these are generalizations because there are many types of cohesivesoils and granular soils. For example, the Unified Soil Classification Systembreaks down soil types into 15 types and indicates the quality of each asconstruction material. This and other classification systems take into account


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such factors as particle sizes, grain-size distribution, and the effect of moistureon the soil. Because of the wide variations among soils that might beencountered on a specific job site, soil testing is wise (and usually mandated).

Soil Testing

Prior to the start of excavation, samples of the soil on the site should be taken toa soil test lab for a Proctor Test to determine its density value. The Proctor Testwill measure the density that can be attained for that soil and express it as astandard. It will also determine the effect of moisture on soil density. This is not ahigh-tech test. A standard weight is dropped 25 times on each soil sample fromthe job site. Each soil sample is weighed, oven-dried for 12 hours, and thenreweighed. The procedure is repeated, adding different amounts of water to thesoil with each repetition.

At a certain moisture, the soil reaches a maximum density when a specific

amount of compaction energy is applied. The maximum density reached underthese conditions is called 100% Proctor density, and this value is used as a basisfor comparing the degree of compaction of the same type of soil on the job site.The compaction specification for the site may be expressed as a percentage ofthe maximum density (e.g., 85% Proctor).

This Standard Proctor Test, developed in the early 1930s by R.R. Proctor, a fieldengineer for the City of Los Angeles, has become universally accepted for mostconstruction projects. For heavier structures such as nuclear power plants, aModified Proctor Test was developed. The principles and procedures are thesame, but it uses a heavier weight and a longer drop.

After compaction, the site must be tested to determine whether it meets thedensity specification determined in the laboratory tests. There are several testsused, says Steve Spence, compaction product manager for Multiquip Inc. ofCarson, CA. "The two most widely used are the sand cone test and the nucleardensity test.

"In the sand cone test, a 6- by 6-inch hole is dug in the compacted soil to betested. The soil is removed and weighed, then dried and weighed again todetermine its moisture content. The dry weight of the soil removed is divided bythe volume of the sand needed to refill the hole. This gives us the density of thecompacted soil in pounds per cubic foot. This density is [divided] by themaximum Proctor density obtained earlier (o determine whether the compactionmeets the specified Proctor percentage).

"Nuclear density meters use a radioactive isotope source, Cesium 137, at the soilsurface or from a probe placed into the soil. The isotope source gives offphotons, usually Gamma rays, which radiate back to the meter's detectors on thebottom of the unit. Dense soil absorbs more radiation than loose soi, and the


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readings reflect overall density. A relative Proctor density is obtained aftercomparing maximum density with the compaction results from the test."

The cost of the sand cone test is quite low, but the process takes time, soequipment operation must be halted while the test results are derived.

Conversely, the nuclear density test is much faster, but its cost is relatively high.Moreover, if the results of these field tests do not meet the compactionspecification, the site will have to be recompacted and the field test repeated,thereby cutting into the productivity of the construction project.

At least two manufacturers have taken steps to resolve this dilemma. Both MBWInc. of Slinger, WI, and Compaction America of Kewanee, IL, have developedsoil compaction instrumentation that enables field crews to measure compactionin real time in lieu of laboratory testing.

"Our Soil Compaction Meter is independently tested and correlates with 95%

Standard Proctor," says MBW's Brad Derosa. "To use it, a contractor places adisposable piezoelectric sensor at the bottom of his excavation before filling. Ascompaction begins, the sensor transmits voltage based on the pressure waveamplitude of the compaction process. Once the voltage signals a predeterminedsoil density, the system's hand-held meter flashes a stop light. Not only does thisenable the contractor to measure compaction in real time without under- orovercompacting, but our Soil Compaction Supervisor unit permits a fast, easytransfer of compaction data to a computer, thereby providing evidence that thecompaction specification was met."

Compaction America's Terrameter works somewhat differently, according to

Manager of Marketing Services Steve Wilson. "It's mounted on the instrumentpanel of our Bomag roller compactors within easy reach of the operator," hesays. "As the roller passes across the ground compacting the soil, theTerrameter monitors interaction between the acceleration of the roller's vibratingdrum and the dynamic stiffness of the soil. Thus, the measuring systemcontinually produces, stores, and displays a measurement of compaction qualitycalled an Omega value. The higher the Omega value, the better the compaction.During each pass, the Terrameter calculates the average Omega value andcompares it with previous passes.

"A green indicator light indicates that the Bomag roller is compacting effectively.If the Omega value that meets the specification is achieved before the green lightgoes out, the operator may stop compacting. Alternatively, if the average Omegavalue increase between two passes is minimal, the green light will go out,signifying that maximum economic compaction has been attained. Thus, throughthe indication of Omega values, the Terrameter provides assurance of uniformcompaction quality without the delay of laboratory testing. This leads tosignificantly increased compaction quality too. Because conventional testmethods are applied only at sample points, they only provide partial compaction


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data. Conversely, the Terrameter assesses the entire area, thereby reducing therisk of under- and overcompacting throughout the compacted area."

Types of Compaction Equipment

Soil compaction equipment is available in a variety of different forms. Theequipment can be self-propelled or it can be mounted on and use the hydraulicsystems of earthmoving equipment. The self-propelled equipment falls into thefollowing four major categories (although there are many different models andvariations within each category):

1. forward vibratory plates2. reversible vibratory plates3. rollers4. rammers

The primary factor that determines the selection of the optimum equipment for agiven application is the type of soil to be compacted, although such complicatingfactors as confinement, trench depth and width, and cost can be importantdifferentiators as well. Table 1 provides a rule-of-thumb guide to the effect of soiltype on equipment selection.

Table1. Effect of Soil Type on Equipment selection

VibratingSheepsfootRammer

StaticSheepsfootSmoothRoller

VibratingPlateVibratingRoller

Lift Thickness Impact Pressure Vibration

Gravel 12 in. Poor No Good

Sand 10 in. Poor No Excellent

Silt 6 in. Good Good Poor

Clay 6 in. Excellent Very Good No

Forward Vibratory Plates

As indicated in Table 1, granular soils (sands and gravels) are best compacted

by vibratory energy. Very small particles, such as sands, will respond best to veryhigh frequencies, in the range of 10,000-15,000 vibrations per minute (vpm),whereas larger gravels will respond best to lower frequencies in the range of2,000-4,000 vpm. Therefore, it is best to match the frequency of the vibrationcompactor to the most prevalent particles present in the soil to be compacted.

Forward vibratory plates are low-amplitude, high-frequency devices, Spencesays. Gasoline or diesel engines drive an eccentric weight at a high speed to


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develop compaction force and vibrations that compact granular soils. (Theengine and the handle are vibration-isolated from the vibrating plate.) Thefrequency range is usually from 2,500 vpm to 6,000 vpm to accommodate arange of granular soils.

The exciter design and the total static weight both play an important role in theefficiency and performance of the vibratory plate, Layton adds. "Exciter unitsoperate on the principle of turning an unbalanced eccentric weight at high speedto produce centrifugal force. It is this centrifugal force, which varies with thesquare power of the exciter speed, that causes the machine to vibrate, moveforward, and compact the soil.

"The static weight of a small vibratory plate - 150- to 300-pound weight class - isusually negligible compared to the centrifugal force that is generated in theexciter. Here, the vibratory force is the dominant force that acts on soil particlesduring the compaction process. The heavier the plate, however, the more

compaction force it generates. Therefore, for vibratory plates above 300 pounds,the static weight and the vibratory action have a combined effect on soil particles.The total effect is to vibrate and squeeze soil particles together to achievecompaction."

Reversib le Vibratory Plates

Inherent to the design of the forward vibratory plate compactor is the fact that itcan only move in one direction, a situation that limits its maneuverability,particularly in confined areas. Conversely, a reversible vibratory plate compactorcan move in both directions because it has an exciter system with two eccentric

weights that revolve in opposite directions. These weights are arranged such thatthe plate will move in the opposite direction every time the relative position of oneeccentric is changed 180 with respect to the other.

This is accomplished in different ways depending on the manufacturer's design.In the Mikasa MVH hydraulic system shown in Figure 1, forward and reverse arechanged by switching pressurized oil between the servo pistons located on theeccentric case. The servo positions change the position of the eccentric weights.In the Weber system, one of the weights is keyed solid with constant pitch whilethe other weight is allowed to move 180 in pitch. The reversibility comes fromsimply varying the pitch of the movable weight. The Wacker mechanism uses asleeve gear in the exciter. The lever the operator holds controls a hydraulicallyactuated piston that connects to this sleeve gear. As the piston moves in and out,the sleeve gear rotates, changing the relationship between the two eccentricweights thereby determining the direction of travel.


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Whatever the mechanism, as Ron McCannell, vice president of operations forWeber Machines USA points out, the fact that there are two weights moving inthe exciter case creates twice as much force as a comparably powered forwardvibratory plate compactor. What's more, changing the direction of a reversingplate occurs instantaneously at full shaft speed, without the necessity of stoppingthe machine. In fact, the eccentrics can be changed in infinite increments fromfull forward to full reverse, thereby achieving maximum maneuverability. Theplate can even be held in place with no forward or reverse motion so that the fullcentrifugal force can be applied for spot compaction.


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"Reversibles do cost more than forwardplates," McCannell concedes. "A 200-pound reversing plate lists at $3,700 ascompared to $2,500 for a forward-platemachine - all other things being equal.

That's a very small cost differenceconsidering the far greater productivityinherent to the reversible. A contractorcan easily justify the added cost based onfirst-year labor savings alone."

In material provided by spokespersonKathy Reissig, Stone ConstructionEquipment points out that there's goodreason for the added cost of a reversibleplate compactor. "Reversible plates

pound the ground harder than forwardplates with as much as three-plus timesthe impact force - from 5,200 pounds to12,880 pounds. Also, reversible platesare very versatile. They can be equippedwith a remote control capability that letsan operator compact a trench withoutever actually getting into one. While someforward plates are designed for trench

work, they still require someone to walk behind them to keep them undercontrol."

Steve Stone questions the maneuverability of other reversibles in trenches orother confined areas. "Most designs only allow a remote-controlled reversibleplate to steer forward, 90 to the right or left, or go reverse," he points out. "Onlyrecently have we developed stepless steering,' a configuration that features onelarge eccentric assembly with a weight arrangement that allows a 180 freedomforward and a 130 freedom of rear motion. Therefore, wherever you point thejoystick on the remote, that's where the machine will go."

Stepless steering would seem to be icing on a cake that has already earned theenthusiasm of contractors and manufacturers alike. Stone sums it up succinctly,saying, "Dollar for dollar, a reversible is possibly the best compaction buy youcan find."

Rollers

There are four general types of rollers: static, vibratory, sheepsfoot, andpneumatic tire. However, the use of static rollers for soil compaction has beensteadily declining since the introduction of vibratory rollers because static rollers


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must be very heavy to handle even moderate soil lifts. By the same token, theuse of pneumatic tire rollers is primarily limited to surface compaction witheffective compaction depths of no more than 6 in.

A vibratory roller has exciter weights in at least one of its drums to generate

vibratory action in addition to the effect of its static weight. The vibratory impulsesbreak up the frictional force between the soil particles. Since this allows deeperlayers of soil to vibrate and settle, vibratory rollers can accommodate larger liftsand provide quicker and more effective compaction than static rollers.

As Wacker's Contractor's Update points out, however, walk-behind vibratoryrollers are not as cost-effective as reversible vibratory plates. To quote that reportdirectly, "Most reversible plates cost less than walk-behind rollers but have amuch larger cubic-yard capacity. The larger contact area of the baseplatetransmits more vibration to the surface producing (deeper soil lifts and) moreeffective compaction. This greater compaction capability gives the contractor a

more productive, less costly means of compaction (than is possible with rollers)."See Figure 2.

The sheepsfoot roller is one of the most recognizable compaction devices and is

used throughout the world. These rollers have drums with many protruding studs,each similar to a sheepsfoot, that provide a kneading action. It works on a widerange of materials but is most effective for compaction of plastic soils like clay orsilt. When used on more granular materials, sheepsfoot rollers tend to shoverather than compact such soils.

According to information supplied by Steve Wilson, "The sheepsfoot compactsfrom the bottom of each lift to the top. High contact pressures cause the feet to


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penetrate through the loose material and actually compact the soil directlybeneath the foot tip. Towed sheepsfoot rollers can only work at speeds from 4 to6 miles per hour, which prevents any benefit being received from the forces ofimpact or vibration. A high number of coverage passes are required withsheepsfoot rollers because of the small contact area compacted by each foot.

"Self-propelled sheepsfoot rollers equipped with fill spreading bladesarecapable of higher productivity than towed sheepsfoot rollers. They are moreexpensive to own and operate than towed sheepsfoot rollers, however.

"The tamping foot roller incorporates the advantages of the sheepsfoot and steelwheel into a high-speed compaction tool. Like the sheepsfoot roller, it compactsfrom the bottom to the top of the lift for uniform density. And like the steel wheel,it also compacts from the top of the lift. The tamping foot roller is capable of highrolling speeds without throwing material because of the design profile of thetamping foot. [Unlike the sheepsfoot roller], it leaves a relatively smooth, sealed

surface so that haul units are able to maintain good speeds when traveling overthe fill. In some cases, the added productivity from this advantage can offset thecost of compaction."


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Rammers

Rammers (see Figure 3) are hand-operated impact devices that deliver high-impact force through a rectangular shoe plate roughly 1 ft.2 in size. Capable ofdelivering up to 4,500 lb. of impact per blow and up to 800 blows per minute,rammers are an excellent choice for cohesive and semicohesive soils. Thecompaction force is generated by a small gasoline or diesel engine powering alarge piston set with two sets of springs. The hand-operated rammer, which canweigh in excess of 200 lb., is inclined at a forward angle to allow forward travelas the machine jumps. As a result, rammers can travel at more than 50 ft./min.and therefore compact more than 3,000 ft.2/hr. (Stone markets a rammer with aforward travel speed of up to 90 ft./min., boasting a productivity of 4,950 ft.2/hr.)

Rammer specialists such as Stone and Multiquip have as many as 10 differentmodels in their lines to accommodate different field needs. Multiquip's MikasaMTR-35HS, for example, weighs just 90 lb., making it easier to use in the fieldbut at the cost of delivering an impact of just 1,212 lb. per blow, less than half


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that of any other rammer in its line. Shoe size is another variable. Whereas 10 to11 in. x 13 in. is the popular shoe size of all manufacturers, that same MTR-35HS has a shoe size with a width of less than 6 in. Why? Lower weight was justone of the reasons, Steve Spence explains. A standard-width chain trenchercreates a 6-in. trench, so a 5.9-in. shoe is

ideal for this application.

Engine type is another differentiator,Spence says. Contractors who operate alltheir equipment with diesel fuel orcontractors who do work on job siteswhere gasoline-powered equipment isrestricted [such as a petrochemical plantor an oil refinery] have little choice. Theymust use diesel-powered rammers. Andtwo-cycle gasoline engines on rammers

are changing now. Today, modern two-cycle rammer engines are oil-injected,with a separate tank for the gasoline anda separate tank for the oil. That way, onlygasoline runs through the carburetor; theoil is mixed with the fuel in the enginecylinder. Finally, EPA air-qualityrestrictions threaten to ban conventionaltwo-cycle engines. The solution? A four-cycle engine running at a little lowerRPM. Not only does it generate lower emissions, there is also less harsh noiseand a 27% increase in fuel efficiency. A four-cycle engine costs about 2% moreand delivers about 15% less impact, so there are tradeoffs for a contractor toconsider."

Carrier-Mounted Equipment

While self-propelled compactors are widely used in the construction industry,they are by no means the only entry in the field. Compactors as attachments toearthmoving equipment represent an alternative that many contractors are using,and suppliers of these attachments have attractive product lines. For example,Allied Construction Products of Cleveland, OH, markets a line of vibratorycompactor/drivers that attach to and operate off the hydraulic systems of skid-steers, backhoes, loaders, and excavators and are in the attachment catalogs ofmost original equipment manufacturers.

Allied, which claims to have pioneered the concept of hydraulically operatedvibratory compactor drivers, currently markets five models of these Ho-Pacmachines, ranging from a model designed for attachment to skid-steers up to apowerful model designed to attach to excavators of at least 45,000 lb.


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"Attached to the carrier's boom, the Ho-Pacs use an eccentric, rotating weightthat creates vibration and impulse energy," says Manager of Sales and MarketingSteve Sabo. "Specially designed rubber mounts direct the energy to the Ho-Pac'scompaction plate, not the carrier's boom. The compactor operates off thecarrier's hydraulic system and reaches out to work anywhere the carrier's boom

can reach. Static downpressure and high-impulse vibration forces produced bythe compactor are ideal for compacting granular soils. The vibrations generatestress waves that bring the soil's air to the surface. As a result, the soil particlesare rearranged, compressed, and compacted."

This carrier-mounted compactor configuration can generate powerful compactionforces. Allied's Model 9801, for example, operates off a 45,000-lb.-classexcavator and can generate 20,000 lb. of impulse force at 2,000 cycles perminute. Depending on job conditions, the company says, it can compact in 4- to6-ft. lifts to densities in excess of 95% Proctor with a production of 160 yd.3 ormore per hour. And, Sabo points out, each Ho-Pac model can function as a

driver as well as a contractor, thereby adding versatility to a contractor's fleet.

Pack Wheel of Madison, TN, also markets a carrier-mounted compactor. Calledthe Pack Wheel, this static wheel compactor is designed for use on virtually allexcavating equipment, owner J im Thilmony says. "It uses the power of themachine to achieve compaction levels equal to or greater than standardconstruction specifications. Each Pack Wheel uses eight compaction feet oneach 32- or 18-inch-diameter wheel. The openness of the Pack Wheel allowspenetration through the backfill, packing from the bottom upward. The spacingbetween the wheels, combined with the slotted rims, enables it to penetrate from18 to 24 inches into the fill. It rolls back and forth, mixing and packing as it goes,

rather than riding over the top as a conventional hydraulic packer does. What'smore, the open-wheel design allows almost continual backfilling. Therefore,fewer lifts are necessary, and that further reduces backfill and packing time onthe job."

MBW also manufactures a carrier-mounted machine: its EXA vibratory rollerattachment for backhoes and excavators up to 60,000 lb. The boom-mountedEXA essentially combines the features of a static wheel and a vibratory plate,combining the static rolling process with an intense vibration. "The EXA's smallfootprint is important," says MBW's Brad Derosa. "Because of it, the pounds persquare inch that the EXA supplies is approximately three to four times theintensity of the same carrier using a boom-mounted vibratory plate. The vibratoryforce is also 50% to 100% greater on a per-square-inch basis, which enhancesthe placement of granular materials, particularly larger particles like crushed rockor pit-run gravel."


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Self-Propelled or Machine-Mounted Compactors?

There is considerable controversy as towhether carrier-mounted compaction iscost-effective in the long run. Sabo

believes that machine-mountedcompactors are more productive thanself-propelled models. After all, hecontends, the range and the static weightof the carrier are inherent advantages. Ofcourse, Sabo represents a company thatmakes only machine-mountedcompactors, but both Derosa and

Thilmony agree with his contention, and their companies make self-poweredmodels as well as machine-mounted ones. Derosa asserts that a machine-mounted compactor is "five to six times more productive than a walk-behind,

even one equipped with a remote control." Thilmony laconically adds that"contractors usually have some unit standing around that they can mount acompactor on."

Both Spencer and McCannell believe this line of reasoning is misleading.Spencer dismisses it out of hand, saying, "An excavator is an awfully expensivecompactor." McCannell elaborates on this point: "If a contractor owns one ofthose big machines and he's using it to tamp dirt, it's just a waste of money. Mostmachines can only deliver 11,000 to 12,000 pounds of force for compaction, andit will cost them about $10,000 for the attachment plus the cost of hooking it upeach time. I can sell him a self-propelled vibratory plate compactor listing at

$15,000 that will deliver that much power, and he won't have to tie up a $160,000machine. It's the money per yard that a contractor makes moving dirt that keepshim in business, not a little compacting."

Durability

Everyone seems to agree on one basic point: Compactors are one item that youshouldn't buy on the cheap. McCannell states the issue most forcefully. "This isequipment designed to beat itself to death while it pounds the ground. Becauseof the constant vibration and the working environment created as a soilcompactor operates on a daily basis, logically it becomes necessary to acceptthe fact that eventually there will be a limit to the productive life of thesemachines. Therefore, it just makes sense that when a machine is beingdesigned, consideration should be given to the fact that at some point in time, itwill need to be repaired and even rebuilt. With this thought in mind, Weber hasbeen building forward and reversible compactors that are completely rebuildableat a reasonable cost. For example, all Weber models have a sealed vibratorhousing with oil-bath lubrication for the bearings. And all bearings, seals, andbelts are available over the counter locally. Also, the vibrators are detachable


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from the base plate to facilitate repair or rebuilding. I could go on and on. As anindustry, we owe it to our customers to extend the life of our products as long aspossible."

Wilson of Compaction America agrees that manufacturers should plan for

rebuilding, adding that "except for the drum surface, you can recondition a wheel-type compactor indefinitely. The engine can be rebuilt, and all the hydrauliccomponents, the drive components, the differential, and the transmission can bereplaced." And Wacker publishes specific maintenance and trouble-shootingfeatures to extend compactor life. The company points out with considerablepride that rammers and plates it built 40 years ago are still being used bycontractors today.

That sounds like the age-old solutions to the age-old needs weren't all that bad.Innovations since have just made things better - in terms of durability, in terms ofperformance, and in terms of value.

Charles D. Bader is with Dateline II Communications in Los Angeles, CA.

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