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a basic handbook

What is soil?

Soil is formed in place or deposited by various forcesof nature—such as glaciers, wind, lakes and rivers—residually or organically. Following are importantelements in soil compaction:

■ Soil type■ Soil moisture content■ Compaction effort required

Why compact?

There are five principle reasons to compact soil:

■ Increases load-bearing capacity■ Prevents soil settlement and frost damage■ Provides stability■ Reduces water seepage, swelling and contraction■ Reduces settling of soil

Types of compaction

There are four types of compaction effort on soil orasphalt:■ Vibration■ Impact■ Kneading■ Pressure

Soil Compaction

Soil compaction is defined as the method of mechanically increasingthe density of soil. In construction, this is a significant part of the buildingprocess. If performed improperly, settlement of the soil could occurand result in unnecessarymaintenance costs orstructure failure. Almost alltypes of building sites andconstruction projects utilizemechanical compactiontechniques.

These different types of effort are found in the twoprinciple types of compaction force: static andvibratory.

Static force is simply the deadweight of the machine,applying downward force on the soil surface, com-pressing the soil particles. The only way to changethe effective compaction force is by adding orsubtracting the weight of the machine. Static com-paction is confined to upper soil layers and is limitedto any appreciable depth. Kneading and pressure aretwo examples of static compaction.

Vibratory force uses a mechanism, usually engine-driven, to create a downward force in addition to themachine’s static weight. The vibrating mechanism isusually a rotating eccentric weight or piston/springcombination (in rammers). The compactors deliver arapid sequence of blows (impacts) to the surface,thereby affecting the top layers as well as deeperlayers. Vibration moves through the material, settingparticles in motion and moving them closer togetherfor the highest density possible. Based on thematerials being compacted, a certain amount of forcemust be used to overcome the cohesive nature ofparticular particles.

SOIL COMPACTION HANDBOOK 1

Loose Soil (poor load support)

Compacted Soil (improved load support)

SOIL DENSITY

Figure 1

Figure 2

These illustrations show the results of improper compaction and how proper compaction can ensure a longer structural life,eliminating future foundation problems.

Soil Types and Conditions

Every soil type behaves differently with respect tomaximum density and optimum moisture. There-fore, each soil type has its own unique requirementsand controls both in the field and for testing pur-poses. Soil types are commonly classified by grainsize, determined by passing the soil through a seriesof sieves to screen or separate the different grainsizes. [See Figure 3]

Soil classification is categorized into 15 groups, asystem set up by AASHTO (American Association ofState Highway and Transportation Officials). Soilsfound in nature are almost always a combination ofsoil types. A well-graded soil consists of a wide rangeof particle sizes with the smaller particles fillingvoids between larger particles. The result is a dense

structure that lends itself well to compaction. A soil’smakeup determines the best compaction method touse.

There are three basic soil groups:

■ Cohesive■ Granular■ Organic (this soil is not suitable for compaction

and will not be discussed here)

RESULTS OF POOR COMPACTION

SOIL COMPACTION HANDBOOK2

Cohesive soils

Cohesive soils have the smallest particles. Clay has aparticle size range of .00004” to .002”. Silt ranges from.0002” to .003”. Clay is used in embankment fills andretaining pond beds.

Characteristics

Cohesive soils are dense and tightly bound togetherby molecular attraction. They are plastic when wetand can be molded, but become very hard when dry.Proper water content, evenly distributed, is criticalfor proper compaction. Cohesive soils usuallyrequire a force such as impact or pressure. Silt has anoticeably lower cohesion than clay. However, silt isstill heavily reliant on water content. [See Figure 4]

Granular soils

Granular soils range in particle size from .003” to .08”(sand) and .08” to 1.0” (fine to medium gravel).Granular soils are known for their water-drainingproperties.

Characteristics

Sand and gravel obtain maximum density in either afully dry or saturated state. Testing curves are rela-tively flat so density can be obtained regardless ofwater content.

The tables on the following pages give a basicindication of soils used in particular constructionapplications. [See Figures 5, 6 & 7]

Figure 3

SIEVE TEST

What to look for

Granular soils, finesands and silts.

Cohesive soils, mixesand clays.

Water movement

When water and soilare shaken in palm ofhand, they mix. Whenshaking is stopped,they separate.

When water and soilare shaken in palm ofhand, they will not mix.

When moist...

Very little or noplasticity.

Plastic and sticky.Can be rolled.

When dry...

Little or no cohesivestrength when dry.Soil sample willcrumble easily.

Has high strengthwhen dry. Crumbleswith difficulty. Slowsaturation in water.

Figure 4GUIDE TO SOIL TYPES

Appearance/feel

Coarse grains can beseen. Feels grittywhen rubbed betweenfingers.

Grains cannot be seenby naked eye. Feelssmooth and greasywhen rubbed betweenfingers.


-- -- 1 1 -- -- 1 1 1 3

-- -- 2 2 -- -- 3 3 3 --

2 4 -- 4 4 1 4 4 9 5

1 1 -- 3 1 2 6 5 5 1

-- -- 3* 6 -- -- 2 2 2 4

-- -- 4* 7* -- -- 5 6 4 --

4 5 -- 8* 5** 3 7 6 10 6

3 2 -- 5 2 4 8 7 6 2

6 6 -- -- 6** 6 9 10 11 --

5 3 -- 9 3 5 10 9 7 7

8 8 -- -- 7** 7 11 11 12 --

9 9 -- -- -- 8 12 12 13 --

7 7 -- 10 8*** 9 13 13 8 --

10 10 -- -- -- 10 14 14 14 --

GW Well-graded gravels, gravel/sand mixtures, little or no fines

GP Poorly-graded gravels, gravel/sand mixtures, little or no fines

GM Silty gravels, poorly-gradedgravel/sand/silt mixtures

GC Clay-like gravels, poorly gradedgravel/sand/clay mixtures

SW Well-graded sands, gravellysands, little or no fines

SP Poorly-graded sands, gravellysands, little or no fines

SM Silty sands, poorly-graded sand/silt mixtures

SC Clay-like sands, poorly-gradedsand/clay mixtures

ML Inorganic silts and very finesands, rock flour, silty or clay-likefine sands with slight plasticity

CL Inorganic clays of low to mediumplasticity, gravelly clays, sandyclays, silty clays, lean clays

OL Organic silts and organic silt-claysof low plasticity

MN Organic silts, micaceous ordiatomaceous fine sandy or siltysoils, elastic silts

CH Inorganic clays of high plasticity,fat clays

OH Organic clays of medium highplasticity

GroupSymbol Soil Type H

omog

eneo

usEm

bank

men

t

Cor

e

Shel

l

Eros

ion

Res

ista

nce

Com

pact

edEa

rth L

inin

g

Seep

age

Impo

rtant

Seep

age

Not

Impo

rtant

Fros

t Hea

veN

ot P

ossi

ble

Fills

Fros

t Hea

vePo

ssib

le

Surfa

cing

(NAVFAC DM-7.2, MAY 1982)

* if gravelly** erosion critical

*** volume change critical-- not appropriate for this type of use

Figure 5

SA

ND

SLE

AN

FAT

CLA

YS

& S

ILTS

GR

AVE

LS

Rolled Earth Fill Dams Canal Sections Foundations Roadways

Relative Desirability for Various Uses(1=best; 14=least desirable)

RELATIVE DESIRABILITY OF SOILS AS COMPACTED FILL


Effect of moisture

The response of soil to moisture is very important, asthe soil must carry the load year-round. Rain, forexample, may transform soil into a plastic state oreven into a liquid. In this state, soil has very little orno load-bearing ability.

Moisture vs soil density

Moisture content of the soil is vital to proper com-paction. Moisture acts as a lubricant within soil,sliding the particles together. Too little moisturemeans inadequate compaction—the particles cannot

move past each other to achieve density. Too muchmoisture leaves water-filled voids and subsequentlyweakens the load-bearing ability. The highest densityfor most soils is at a certain water content for a givencompaction effort. The drier the soil, the moreresistant it is to compaction. In a water-saturatedstate the voids between particles are partially filledwith water, creating an apparent cohesion that bindsthem together. This cohesion increases as the particlesize decreases (as in clay-type soils). [See Figure 8]

Figure 6

IMPACT PRESSUREwith kneading

VIBRATION KNEADINGwith pressure

Vibrating SheepsfootRammer

Static SheepsfootGrid RollerScraper

VibratingPlate Compactor

Vibrating RollerVibrating Sheepsfoot

ScraperRubber-tired RollerLoaderGrid Roller

GRAVEL 12+ Poor No Good Very Good

SAND 10+/- Poor No Excellent Good

SILT 6+/- Good Good Poor Excellent

CLAY 6+/- Excellent Very Good No Good

LiftThickness

PermeabilityFoundation

SupportPavementSubgrade Expansive

CompactionDifficulty

GRAVEL Very High Excellent Excellent No Very Easy

SAND Medium Good Good No Easy

SILT Medium Low Poor Poor Some Some

CLAY None+ Moderate Poor Difficult Very Difficult

ORGANIC Low Very Poor Not Acceptable Some Very Difficult

Figure 7

FILL MATERIALS

MATERIALS


Soil density tests

To determine if proper soilcompaction is achieved forany specific constructionapplication, several methodswere developed. The mostprominent by far is soildensity.

Why test

Soil testing accomplishes thefollowing:

■ Measures density of soilfor comparing the degreeof compaction vs specs

■ Measures the effect ofmoisture on soil densityvs specs

■ Provides a moisturedensity curve identifyingoptimum moisture

MOISTURE VS SOIL DENSITY

A quick method of determining moisture density is knownas the “Hand Test.”

Pick up a handful of soil. Squeeze it in your hand. Openyour hand.

If the soil is powdery and will not retain the shape made byyour hand, it is too dry. If it shatters when dropped, it is toodry.

If the soil is moldable and breaks into only a couple ofpieces when dropped, it has the right amount of moisturefor proper compaction.

If the soil is plastic in your hand, leaves small traces ofmoisture on your fingers and stays in one piece whendropped, it has too much moisture for compaction.

HAND TEST

Figure 9

Types of tests

Tests to determine optimum moisture content are donein the laboratory. The most common is the Proctor Test,or Modified Proctor Test. A particular soil needs tohave an ideal (or optimum) amount of moisture toachieve maximum density. This is important not onlyfor durability, but will save money because less com-paction effort is needed to achieve the desired results.

Proctor Test (ASTM D1557-91)

The Proctor, or Modified Proctor Test, determines themaximum density of a soil needed for a specific jobsite. The test first determines the maximum densityachievable for the materials and uses this figure as areference. Secondly, it tests the effects of moisture onsoil density. The soil reference value is expressed as apercentage of density. These values are determinedbefore any compaction takes place to develop thecompaction specifications. Modified Proctor valuesare higher because they take into account higherdensities needed for certain types of constructionprojects. Test methods are similar for both tests. [SeeFigure 10]

Field tests

It is important to know and control the soil densityduring compaction. Following are common fieldtests to determine on the spot if compaction densitiesare being reached.

Figure 8


PROCTOR TEST

Figure 10

Figure 11

• Large sample• Accurate

• Void under plate• Sand bulking• Sand compacted• Soil pumping

• Many steps• Large area required• Slow• Halt equipment• Tempting to accept flukes

Cost

Disadvantages

Errors

Advantages

• Moderate • High• Low

• Large sample• Direct reading obtained• Open graded material

• Fast• Deep sample• Under pipe haunches

• Fast• Easy to redo• More tests (statistical reliability)

• Overdrive• Rocks in path• Plastic soil

• Small sample• No gravel• Sample not always

retained

• Surface not level• Soil pumping• Void under plate

• Slow• Balloon breakage• Awkward

• Miscalibrated• Rocks in path• Surface prep required• Backscatter

• No sample• Radiation• Moisture suspect• Encouragesamateurs

Sand Cone Balloon Densometer Shelby Tube Nuclear Gauge

FIELD DENSITY TESTING METHODS

• Low


Sand Cone Test (ASTM D1556-90)

A small hole (6” x 6” deep) is dug in the compactedmaterial to be tested. The soil is removed andweighed, then dried and weighed again to deter-mine its moisture content. A soil’s moisture is figuredas a percentage. The specific volume of the hole isdetermined by filling it with calibrated dry sandfrom a jar and cone device. The dry weight of thesoil removed is divided by the volume of sandneeded to fill the hole. This gives us the density ofthe compacted soil in lbs per cubic foot. This densityis compared to the maximum Proctor density ob-tained earlier, which gives us the relative density ofthe soil that was just compacted. [See Figure 12]

Nuclear Density (ASTM D2922-91)

Nuclear Density meters are a quick and fairly accu-rate way of determining density and moisturecontent. The meter uses a radioactive isotope source(Cesium 137) at the soil surface (backscatter) or froma probe placed into the soil (direct transmission). Theisotope source gives off photons (usually Gammarays) which radiate back to the meter ’s detectors onthe bottom of the unit. Dense soil absorbs moreradiation than loose soil and the readings reflectoverall density. Water content (ASTM D3017) can alsobe read, all within a few minutes. A relative ProctorDensity is obtained after comparing maximumdensity with the compaction results from the test.[See Figure 13]

NUCLEAR TEST

Figure 13

Soil Modulus (soil stiffness)

This field-test method is a very recent developmentthat replaces soil density testing. Soil stiffness is theratio of force-to-displacement. Testing is done by amachine that sends vibrations into the soil and thenmeasures the deflection of the soil from the vibra-

tions. This is a very fast, safe method of testing soilstiffness. Soil stiffness is the desired engineeringproperty, not just dry density and water content.This method is currently being researched and testedby the Federal Highway Administration.

SAND CONE TEST

Figure 12


Compaction Equipment

Applications

The desired level of compaction is best achieved bymatching the soil type with its proper compactionmethod. Other factors must be considered as well,such as compaction specs and job site conditions.

■ Cohesive soils—clay is cohesive; its particles sticktogether.* Therefore, a machine with a highimpact force is required to ram the soil and forcethe air out, arranging the particles. A rammer isthe best choice, or a pad-foot vibratory roller ifhigher production is needed. [See Figure 14]*The particles must be sheared to compact.

■ Granular soils—since granular soils are notcohesive and the particles require a shaking orvibratory action to move them, vibratory plates(forward travel) are the best choice.

P33/24 FMRRoller

Figure 14

COHESIVE SOILS

MT-75HSRammer

Reversible plates and smooth drum vibratory rollersare appropriate for production work. Granularsoil particles respond to different frequencies(vibrations) depending on particle size. Thesmaller the particle, the higher the frequencynecessary to move it. As you compact soils withlarger particles, move up to larger equipment toobtain lower frequencies and higher compactionforces. [See Figure 15]


MVH-200GAReversible Plate

T-26Tandem Vibratory Roller

MVC-90HVibratory Plate

GRANULAR SOILS

Figure 15

Soil can also be over-compacted if the compactor makestoo many passes (a pass is the machine going across alift in one direction). Over-compaction is like con-stantly hitting concrete with a sledgehammer. Crackswill eventually appear, reducing density. This is awaste of man-hours and adds unnecessary wear to themachine.

Compaction specifications

A word about meeting job site specifications. Gener-ally, compaction performance parameters are givenon a construction project in one of two ways:■ Method Specification—detailed instructions

specify machine type, lift depths, number ofpasses, machine speed and moisture content. A“recipe” is given as part of the job specs toaccomplish the compaction needed. This methodis outdated, as machine technology has faroutpaced common method specification require-ments.

■ End-Result Specification—engineers indicatefinal compaction requirements, thus giving thecontractor much more flexibility in determiningthe best, most economical method of meeting therequired specs. Fortunately, this is the trend,allowing the contractor to take advantage of thelatest technology available.

Normally, soils are mixtures of clay and granularmaterials, making the selection of compactionequipment more difficult. It is a good idea to choosethe machine appropriate for the larger percentage ofthe mixture. Equipment testing may be required tomatch the best machine to the job.Asphalt is considered granular due to its base ofmixed aggregate sizes (crushed stone, gravel, sandand fines) mixed with bitumen binder (asphaltcement). Consequently, asphalt must be compactedwith pressure (static) or vibration.

Compaction machine characteristics

Two factors are important in determining the type offorce a compaction machine produces: frequency andamplitude.Frequency is the speed at which an eccentric shaftrotates or the machine jumps. Each compactionmachine is designed to operate at an optimumfrequency to supply the maximum force. Frequencyis usually given in terms of vibrations per minute(vpm).

Amplitude (or nominal amplitude) is the maximummovement of a vibrating body from its axis in onedirection. Double amplitude is the maximum dis-tance a vibrating body moves in both directions fromits axis. The apparent amplitude varies for eachmachine under different job site conditions. Theapparent amplitude increases as the material be-comes more dense and compacted.

Lift height and machine performance

Lift height (depth of the soil layer) is an importantfactor that affects machine performance and com-paction cost. Vibratory and rammer-type equipmentcompact soil in the same direction: from top tobottom and bottom to top. As the machine hits thesoil, the impact travels to the hard surface below andthen returns upward. This sets all particles in motionand compaction takes place.As the soil becomes compacted, the impact has ashorter distance to travel. More force returns to themachine, making it lift off the ground higher in itsstroke cycle. If the lift is too deep, the machine willtake longer to compact the soil and a layer withinthe lift will not be compacted.[See Figure 16]

Proper Lift Improper Lift

Figure 16

LIFT HEIGHT


Equipment types

Rammers

Rammers deliver a high impact force (high amplitude)making them an excellent choice for cohesive and semi-cohesive soils. Frequency range is 500 to 750 blows perminute. Rammers get compaction force from a smallgasoline or diesel engine powering a large piston setwith two sets of springs. The rammer is inclined at aforward angle to allow forward travel as the machinejumps. Rammers cover three types of compaction:impact, vibration and kneading. [See Figure 17]

Vibratory plates

Vibratory plates are low amplitude and high frequency,designed to compact granular soils and asphalt.Gasoline or diesel engines drive one or two eccentricweights at a high speed to develop compaction force.The resulting vibrations cause forward motion. Theengine and handle are vibration-isolated from thevibrating plate. The heavier the plate, the more com-paction force it generates. Frequency range is usually2500 vpm to 6000 vpm. Plates used for asphalt have awater tank and sprinkler system to prevent asphaltfrom sticking to the bottom of the baseplate. Vibrationis the one principal compaction effect. [See Figure 17]

Reversible vibratory plates

In addition to some of the standard vibratory platefeatures, reversible plates have two eccentric weightsthat allow smooth transition for forward or reversetravel, plus increased compaction force as the resultof dual weights. Due to their weight and force,reversible plates are ideal for semi-cohesive soils.

A reversible is possibly the best compaction buy dollarfor dollar. Unlike standard plates, the reversible’sforward travel may be stopped and the machine will maintain its force for “spot” compaction.[See Figure 17]

Granular Soils Sand and Clay Cohesive Clay Asphalt

Rammers Not recommended Testing recommended Best application Not recommended

Vibratory Plates Best application Testing recommended Not recommended Best application

Reversible Plates Testing recommended Best application Best application Not recommended

Vibratory Rollers Not recommended Best application Testing recommended Best application

Rammax Rollers Testing recommended Best application Best application Not recommended

Figure 18EQUIPMENT APPLICATIONS


EQUIPMENT TYPES

MVH-402DSReversible Plate

MT-76DDiesel-Powered Rammer

MVC-77HVibratory Plate

Figure 17

Walk-behind

SmoothA popular design for many years, smooth-drummachines are ideal for both soil and asphalt. Dualsteel drums are mounted on a rigid frame andpowered by gasoline or diesel engines. Steering isdone by manually turning the machine handle.

ROLLER TYPES

Figure 19

Frequency is around 4000 vpm and amplitudes rangefrom .018 to .020. Vibration is provided by eccentricshafts placed in the drums or mounted on the frame.

Rollers

Rollers are available in several categories: walk-behind and ride-on, which are available as smooth drum,padded drum and rubber-tired models; and are further divided into static and vibratory sub-categories.[See Figure 18]

MR-8GVibratory Roller

P48Ride-on Roller

P33/34Trench Roller

T-26Ride-on Vibratory Roller

V30-3EVibratory Roller


Safety and General Guidelines

As with all construction equipment, there are manysafety practices that should be followed while usingcompaction equipment. While this handbook is notdesigned to cover all aspects of job site safety, wewish to mention some of the more obvious items inregard to compaction equipment. Ideally, equipmentoperators should familiarize themselves with all oftheir company’s safety regulations, as well as anyOSHA, state agency or local agency regulationspertaining to job safety. Basic personal protection,consisting of durable work gloves, eye protection,ear protection, approved hard hat and work clothes,should be standard issue on any job and availablefor immediate use.In the case of walk-behind compaction equipment,additional toe protection devices should be available,depending on applicable regulations. All personneloperating powered compaction equipment shouldread all operating and safety instructions for eachpiece of equipment. Additionally, training should beprovided so that the operator is aware of all aspectsof operation.No minors should be allowed to operate constructionequipment. No operator should run construction

equipment when under the influence of medication,illegal drugs or alcohol. Serious injury or death couldoccur as a result of improper use or neglect of safetypractices and attitudes. This applies to both the newworker as well as the seasoned professional.

Shoring

Trench work brings a new set of safety practices andregulations for the compaction equipment operator.This section does not intend to cover the regulationspertaining to trench safety (OSHA Part 1926, SubpartP). The operator should have knowledge of what isrequired before compacting in a trench or confinedarea. Be certain a “competent person” (as defined byOSHA in Part 1926.650 revised July 1, 1998) hasinspected the trench and follows the OSHA guide-lines for inspection during the duration of the job.Besides the obvious danger of a trench cave-in, theworker must also be protected from falling objects.Unshored (or shored) trenches can be compactedwith the use of remote control compaction equip-ment. This allows the operator to stay outside thetrench while operating the equipment.Safety first!

PaddedPadded rollers are also known as trench rollers dueto their effective use in trenches and excavations.These machines feature hydraulic or hydrostaticsteering and operation. Powered by diesel engines,trench rollers are built to withstand the rigors ofconfined compaction. Trench rollers are either skid-steer or equipped with articulated steering. Opera-tion can be by manual or remote control. Largeeccentric units provide high impact force and highamplitude (for rollers) that are appropriate forcohesive soils. The drum pads provide a kneadingaction on soil. Use these machines for high produc-tivity.Ride-onConfigured as static steel-wheel rollers, ride-ons areused primarily for asphalt surface sealing andfinishing work in the larger (8 to 15 ton) range. Smallride-on units are used for patch jobs with thin lifts.The trend is toward vibratory rollers. Tandemvibratory rollers are usually found with drum widths

of 30” up to 110”, with the most common being 48”.Suitable for soil, sub-base and asphalt compaction,tandem rollers use the dynamic force of eccentricvibrator assemblies for high production work.Single-drum machines feature a single vibratingdrum with pneumatic drive wheels. The drum isavailable as smooth for sub-base or rock fill, orpadded for soil compaction. Additionally, a ride-onversion of the pad foot trench roller is available forvery high productivity in confined areas, with eithermanual or remote control operation.

Rubber-tireThese rollers are equipped with 7 to 11 pneumatictires with the front and rear tires overlapping. Astatic roller by nature, compaction force is altered bythe addition or removal of weight added as ballast inthe form of water or sand. Weight ranges vary from10 to 35 tons. The compaction effort is pressure andkneading, primarily with asphalt finish rolling. Tirepressures on some machines can be decreased whilerolling to adjust ground contact pressure for differ-ent job conditions.


Glossary

AASHO American Association of State HighwayOfficials.

Adhesion a property of soil which causes the par-ticles to stick together.

Aggregate stone or gravel that was crushed andscreened to various sizes for use in concrete, asphaltor road surfaces.

Amplitude the total vertical distance the vibratingdrum or plate is displaced from a resting or neutralposition from the eccentric moment.

ASTM American Society for Testing Materials.

Backfill materials used in refilling a cut or otherexcavation, or the act of such refilling.

Ballast heavy material, such as water, sand ormetal which has no function in a machine except toincrease its weight.

Bank a mass of soil rising above an average level.Generally, any soil which is to be dug from itsnatural position.

Bank Gravel a natural mixture of cobbles, gravel,sand and fines.

Bank Yards soil or rock measured in its originalposition before digging.

Base the course or layer of materials in a roadwaysection on which the actual pavement is placed. Itmay be of different types of materials ranging fromselected soils to crushed stone or gravel.

Berm an artificial ridge of earth, generally side-slopes of a roadbed.

Binder fines which fill voids or hold gravel togetherwhen dry.

Borrow Pit an excavation from which fill material istaken.

BPR U.S. Bureau of Public Roads.

BUREC U.S. Bureau of Reclamation.

Capillary a phenomenon of soil which allows waterto be absorbed either upward or laterally.

Centrifugal Force the force generated from theunbalanced condition of eccentric shaft rotation at agiven speed.

Clay material composed and derived from thedecomposition of rock which consists of microscopicparticles.

Clean free of foreign material; in reference to sandor gravel, lack of a binder.

Cohesion a property of soil which holdstheparticles together by sticking. Also, the soil’sability to resist shear is determined by its degree ofcohesiveness.

Cohesive Material a soil having properties ofcohesion.

Compacted Yards measurement of soil or rock afterit is placed and compacted in a fill.

Compressibility a property of soil which permitsdeformation when subjected to a load.

Core a cylindrical piece of an underground forma-tion, cut and raised by a rotary drill with a hollowbit. The impervious center of an earthfill dam.

Crown the elevation of a road surface at its edges,to encourage drainage.

Datum any level surface taken as a plane of refer-ence from which to measure elevations.

Density the ratio of the weight of a substance to itsvolume.

Embankment a fill with a top higher than theadjoining natural surface.

Elasticity a characteristic of soil which allowsdeformation during a subjected load, but returnsalmost to its original configuration after removal ofthe force.

Fines clay or silt particles in soil.

Finish Grade the final grade required by specifica-tions.

Foot in tamping rollers, one of a number ofprojections from a cylindrical drum.

Frequency referring to rotational speed of theeccentric shaft—usually rated in “Vibrations PerMinute”—which is equal to the RPM of the shaft.

Frost Line the greatest depth to which ground isexpected to freeze in a given location.


Grade usually the surface elevation of the ground atpoints where it meets a structure. Also, surface slope.

Grain Size Curve a soil graph analysis showing thepercentage size variations by weight.

Granular Material a sandy type of soil withparticles that are coarser than cohesive material anddo not stick to each other.

Gravel a cohesionless aggregate of rock fragmentswith varying dimensions of 3.0 to .08 inches.

Gumbo material in the plastic state identified by asoapy or waxy appearance.

Humus organic material formed by the decomposi-tion of vegetation.

Impervious resistant to movement of water.

In Situ natural undisturbed soil in place.

Lift a layer of fill as spread or compacted.

Liquid Limit the water content at which soil passesfrom a plastic to a liquid state.

Loam a soft, easily worked soil containing sand, silt,clay and decayed vegetation.

Optimum Moisture Content that percent of moistureat which the greatest density of a soil can beobtained through compaction.

Pass a working trip or passage of an excavating,grading or compaction machine from point A topoint B. (One direction only.)

Permeability a characteristic of soil which allowswater to flow through it because of gravity.

Plastic the ability of a soil to be rolled into a finethread at a certain moisture content.

Plastic Limit the lowest moisture content at whicha soil can be rolled into a 1/8" diameter threadwithout breaking.

Proctor a method developed by R.R. Proctor fordetermining the density/moisture relationship insoils. It is almost universally used to determine themaximum density of any soil so that specificationsmay be properly prepared for field constructionrequirements.

Proctor, Modified a moisture density test of morerigid specification than Proctor. The basic differenceis the use of heavier weight dropped from a greaterdistance in laboratory tests.

Quicksand fine sand or silt that is prevented fromsettling firmly together by upward movement ofunderground water.

Sand a cohesiveless aggregate of round and angularfragments of rock with a particle size between 2.0and .05mm.

Shearing Resistance a soil’s ability to resist slidingagainst neighboring soil grains when force isapplied. Internal friction and cohesion determineshear resistance.

Shrinkage soil volume which is reduced whensubjected to moisture; usually occurs in fine grainsoils.

Silt soil material composed of particles between .005and .05mm in diameter.

Soil the loose surface material of the earth’s crust.

Stabilize to make soil firm and prevent it frommoving.

Sub-base the layer of material placed to furnishstrength to the base of a road.

Subgrade the surface produced by grading nativeearth, or cheap imported materials which serve as abase for more expensive paving.

Acknowledgements: Budinger & Associates, Geotechnical &Material Engineers, for their assistance with some material.


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