chapter 12 JM - U.S. DEPARTMENT OF THE INTERIOR volume engineering/N_Ch12_Roadway...materials such as gravel, coarse rocky soil, crushed aggregate, cobblestone, con-crete block, or

115LOW-VOLUME ROADS BMPS :

Chapter 12

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LOW-VOLUME ROAD surfaces and structuralsections are typically built from nativematerials that must support light vehicles and

may have to support heavy commercial truck traffic.In addition, low-volume roads should have a surfacethat, when wet, will not rut and will provide adequatetraction for vehicles. The surface of native soil roadsis also an exposed area that can produce significantamounts of sediment, especially if rutted (Photo12.1).

Roadway MaterialsIt is usually desirable and,

in many cases, necessary toadd subgrade structural sup-port or to improve the road-bed native soil surface withmaterials such as gravel,coarse rocky soil, crushedaggregate, cobblestone, con-crete block, or some type ofbituminous seal coat or as-phalt pavement, as shown inFigure 12.1. Surfacing im-proves the structural supportand reduces road surface ero-sion. The selection of surfac-ing type depends upon thetraffic volume, local soils,

“Select quality roadway materials that are durable,well-graded, and perform well on the road.

Maintain quality control.”

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available materials, ease of maintenance, and, ulti-mately, cost.

A range of options exists for improving the struc-tural capacity of the roadway in areas of soft soils orpoor subgrades. These commonly include:

• Adding material of higher strength and qualityover the soft soil, such as a layer of gravel or

Photo 12.1 Photo 12.1 Photo 12.1 Photo 12.1 Photo 12.1 A rutting road caused either by soft subgrade soil or inad-equate road drainage (or both).

LOW-VOLUME ROADS BMPS:116

a. Native Soil

b. Aggregate

c. Aggregate and Base

d. Cobblestone

e. Concrete Block

f. Asphalt Surfacing

g. Typical Aggregate Surfaced Road Template

— Native (In-Place) Soil

— Crushed Surface Aggregate or Gravel— Native Soil

— Native Soil

— Crushed Surface Aggregate or Gravel

— Aggregate Base

— Concrete Blocks

— Native Soil

— Sand

— Native Soil

— Sand

— Cobblestones

— Aggregate Base

— Asphalt Pavement

— Aggregate Sub-Base (Optional)

— Native Soil

Fill Slope Road Surface Ditch

FFFFFigurigurigurigurigure 12.1 e 12.1 e 12.1 e 12.1 e 12.1 Commonly used low-volume road surfacing types and structural sections.


crushed aggregate;

• Improving the soft soil inplace (in-situ) by mixing itwith stabilization additivessuch as lime, cement, as-phalt, or chemicals;

• Bridging over the soft soilwith materials such asgeotextiles or wood pieces(corduroy);

• Removing the soft or poorsoil and replacing it with ahigh quality soil or rockymaterial;

• Limiting the use of the roadduring periods of wetweather when clay soils aresoft;

• Compacting the native soilto increase its density andstrength; and

• Keeping moisture out of thesoil with effective roadwaydrainage or encapsulatingthe soil to keep water out.

Various soil stabilization ma-terials such as oils, lime, cements,resins, lignin, chlorides, enzymes,and chemicals may be used to im-prove the material properties ofthe in-place soil. They may be verycost-effective in areas where ag-gregate or other materials are dif-ficult to locate or are expensive.The best soil stabilization mate-rial to use depends on cost, soiltype, performance and local ex-perience. Test sections are oftenneeded to determine the most de-sirable and cost-effective product.However, many soil stabilizers still

need some type of wearing sur-face. A stabilized road surface im-proves traction and offers erosionprotection as well as structuralsupport.

Gravel, pit run rock, selectmaterial, or crushed aggregate arethe most common improved sur-facing materials used on low-vol-ume roads (Photo 12.2). Aggre-gate is sometimes used only as“fill” material in ruts. However, itis more desirable to place it as afull structural section, as shownin Figure 12.2. The roadway sur-facing aggregate must performtwo basic functions. It must havehigh enough quality and be thickenough to provide structural sup-port to the traffic and prevent rut-ting, and it must be well gradedand mixed with sufficient fines,preferably with some plasticity, toprevent raveling andwashboarding.

Necessary aggregate thick-ness typically ranges from 10 to

30 cm, depending on soil strength,traffic, and climate. Specific ag-gregate thickness design proce-dures are found in the SelectedReferences. Over very weak soils(CBR less than 3), aggregatethickness can be reduced with theuse of geotextile or geogridsubgrade reinforcement. Also,geotextile layers are useful oversoft soils to separate the aggre-gate from the soil, keep it uncon-taminated, and extend the usefullife of the aggregate.

Figure 12.3 presents some ofthe physical properties andtradeoffs of various soil-aggregatemixtures, first with no fines (nomaterial passing the #200 sieve,or .074 mm size), second with anideal percentage of fines (6-15%),and finally with excessive fines(over 15 to 30%). Figure 12.4shows the typical gradation rangesof aggregates used in road con-struction, how the materials, rang-ing from coarse to fine, best per-form for a road, and the approxi-

Photo 12.2Photo 12.2Photo 12.2Photo 12.2Photo 12.2 Stabilize the roadway surface with crushed rock (orother surfacing) on steep grades, in areas of soft soil, or in erosivesoils.


For surfacing aggregate usecrushed rock, gravel or 3 cmminus rock with fines.

If crushed rock or gravel isnot available, use coarse soil,wood chips or soil stabilizers.

0-30 cm min.

10-30 cm

(5-10 cm size or smaller)

POOR

MEDIOCRE– ADEQUATE

BEST

a. Minimal aggregatefilled into ruts whenthey develop.

b. Ruts filled plusaddition of 10-15 cm-thick layer of aggregate.

c. Full structural sectionplaced upon a reshapedcompacted subgrade.

Aggregate Surface orAsphalt Surfacing

Aggregate Base Course or CleanFractured Rock

FFFFFigurigurigurigurigure 12.2 e 12.2 e 12.2 e 12.2 e 12.2 Aggregate options to prevent rutting.


FFFFFigurigurigurigurigure 12.3e 12.3e 12.3e 12.3e 12.3 Physical states of soil-aggregate mixtures. (Adapted from Yoder andWitczak, 1975)

Aggregate withno Fines

• Grain-to-grain contact

• Variable density

• High Permeability

• Non-Frost Susceptible

• High stability whenconfined, low if uncon-fined

• Not affected by water

• Difficult to compact

• Ravels easily

Aggregate withSufficient Fines forMaximum Density

• Grain-to-grain contactwith increased resistanceagainst deformation

• Increased to maximumdensity

• Low permeability

• Frost susceptible

• Relatively high stabilityin confined or uncon-fined conditions

• Not greatly affected byadverse water conditions

• Moderately easy tocompact

• Good road performance

Aggregate with HighAmount of Fines

(>30 percent)

• Grain-to-grain contactdestroyed, aggregate is"floating" in soil

• Decreased density

• Low permeability

• Frost susceptible

• Low stability and lowstrength

• Greatly affected by water

• Easy to compact

• Dusts easily


PRACTICES TO AVOID

NOTE: Gradation Ranges Shown Are Approximate.

The best roadbed surfacing materials have some plasticity and are well graded. They havegradations parallel to the curves shown above, and are closest to the “Ideal” dashed curve in themiddle of the gradation ranges shown.

• Construction operations orheavy traffic during wet orrainy periods on roads withclay rich or fine-grained soilsurfaces that form ruts.

• Allowing ruts and potholes toform over 5 to 10 cm deep in

the roadway surface.

• Road surface stabilizationusing coarse rock larger thanabout 7.5 cm. Coarse rock isdifficult to drive upon or keepstabilized on the road surface,and it damages tires.

• Using surfacing materialsthat are fine grain soils,soft rock that will degradeto fine sediment, or clean,poorly graded coarse rockthat will erode, ravel, orwashboard.

FFFFFigurigurigurigurigure 12.4 e 12.4 e 12.4 e 12.4 e 12.4 Gradation ranges of roadway surfacing materials and their performance characteristics.(Adapted from R. Charles, 1997 and the Association of Asphalt Paving Technologists)

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mate limitations to the desirablegradation ranges. Note that thedesirable percentage of fines in anaggregate can be sensitive to theclimate or road environment. Insemi arid to desert regions, a rela-tively high percentage of fines,such as 15 to 20%, with moder-

RECOMMENDED PRACTICES

ate plasticity, is desirable. In a highrainfall “wet” environment, suchas tropical, coastal mountain, orjungle areas, a low percentage,such as 5 to 10% fines, is desir-able to prevent rutting and main-tain a stable road surface.

Ideally, aggregate surfacingmaterial is (1) hard, durable, andcrushed or screened to a minus 5cm size; (2) well graded toachieve maximum density; (3)contains 5-15% clayey binder toprevent raveling; and (4) has aPlasticity Index of 2 to 10. The

• Stabilize the roadwaysurface on roads that formruts or ravel excessively.Common surface stabiliza-tion techniques includeusing 10-15 cm of crushedaggregate; local pit run orgrid roll rocky material(Photo 12.4); cobblestonesurfacing; wood chips orfine logging slash; or soilsmixed and stabilized withcement, asphalt, lime,lignin, chlorides, chemi-cals, or enzymes.

• For heavy traffic on softsubgrade soils, use asingle, thick structuralsection consisting of atleast 20-30 cm of surfac-ing aggregate. Alterna-tively, use a structuralsection consisting of a 10-30 cm thick layer of baseaggregate or coarsefractured rock, cappedwith a 10-15 cm thicklayer of surfacing aggre-gate (Figure 12.2-BEST).Note that soft clay-richtropical soils and heavytire loads may require athicker structural section.The structural depthneeded is a function of thetraffic volume, loads andsoil type, and should

ideally be determinedthrough local experienceor testing, such as usingthe CBR (CaliforniaBearing Ratio) test.

• Maintain a 3-5% roadcross-slope with insloping,outsloping, or a crown torapidly move water off theroad surface (see Figure7.1).

• Grade or maintain theroadway surface beforesignificant potholes,washboarding, or rutsform (see Figure 4.5).

• Compact the embankmentmaterial, road surfacematerial or aggregateduring construction andmaintenance to achieve adense, smooth road sur-face and thus reduce theamount of water that cansoak into the road (Photo12.5).

• “Spot” stabilize local wetareas and soft areas with10-15 cm of coarse rockymaterial. Add more rockas needed (Figure 12.2).

• Stabilize the road surfacein sensitive areas nearstreams and at drainage

crossings to minimize roadsurface erosion.

• Control excessive roaddust with water, oils,wood chips, or use ofother dust palliatives.

• Blend coarse aggregateand fine clay-rich soil(when available) to pro-duce a desirable compositeroadway material that iscoarse yet well-gradedwith 5-15 % fines forbinder (see Figures 12.3and 12.4).

• Use project constructionquality control, throughvisual observation andmaterials sampling andtesting, to achieve speci-fied densities and quality,well-graded road materials(Photo 12.6).

• On higher standard, hightraffic volume roads(collectors, principals, orarterials) use appropriate,cost effective surfacingmaterials such as oils,cobblestone, paving blocks(Photo 12.7), bituminoussurface treatments (chipseals) (Photo 12.8), andasphalt concrete pave-ments.


Photo 12.3Photo 12.3Photo 12.3Photo 12.3Photo 12.3 A road in need ofmaintenance and surfacing.Add roadway surface stabiliza-tion or do maintenance withgrading and shaping of thesurface to remove ruts andpotholes before significant roaddamage occurs, to achievegood road surface drainage,and to define the roadbed.

Photo 12.4Photo 12.4Photo 12.4Photo 12.4Photo 12.4 A grid roller can beused to produce a desirablesurfacing material when thecoarse rock is relatively soft.Level and compact the roadwaysurface aggregate to achieve adense, smooth, well-drainedriding surface.

Photo 12.5Photo 12.5Photo 12.5Photo 12.5Photo 12.5 Compaction of soiland aggregate is typically theleast expensive way to improvethe strength and performanceof the material. Compaction isuseful and cost-effective bothfor the stability of fill embank-ments and for the road surface.


Photo 12.6Photo 12.6Photo 12.6Photo 12.6Photo 12.6 Here, a “nucleargauge” is being used to checkthe density of aggregate. Useproject construction qualitycontrol, gradation and densitytesting, etc., as needed toachieve the desirable materialsproperties for the project.

Photo 12.7Photo 12.7Photo 12.7Photo 12.7Photo 12.7 Concrete blocks(Adoquin) or cobblestone offeran intermediate alternative toaggregate and pavement roadsurfacing. These materials arelabor intensive to construct andmaintain, but are very cost-effective in many areas.

Photo 12.8Photo 12.8Photo 12.8Photo 12.8Photo 12.8 A chip seal roadsurface being compacted. Avariety of surfacing materialscan be used, depending onavailability, cost, and perfor-mance.


surfacing applied to the road mustbe maintainable in order to pre-vent rutting and erosion. Signifi-cant deterioration of the road canoccur if ruts, raveling,washboarding, or surface erosionare not controlled (Photo 12.3).Road damage can be greatly re-duced by restricting road use dur-ing wet conditions if road man-agement allows for this option.

Compaction is usually themost cost-effective method to im-prove the quality, includingstrength and water resistance, ofsubgrade soils and to improve theperformance of aggregate surfac-ing. It increases the density andreduces the void spaces in thematerial, making it less susceptibleto moisture. Thus, compaction isuseful to protect the investmentin road aggregate, maximize itsstrength, minimize loss of fines,and prevent raveling. Road per-formance has been excellent insome semi-arid regions with theuse of blended local materials,very high compaction standards,and a waterproof membrane suchas a bituminous seal coat.

Compaction can best beachieved with a minimum of ef-fort if the soil or aggregate is wellgraded and if it is moist. Ideally,it should be close to the “optimummoisture content” as determinedby tests such as the “Proctor”Moisture-Density Tests. Expan-sive soils should be compacted onthe wet side of optimum. Handtamping can be effective, but onlywhen done in thin lifts (2-8 cm)and ideally at a moisture contenta few percent above optimum.

The best compaction equip-ment for granular soils and aggre-gate is a vibratory roller. A tamp-ing, or sheepsfoot roller is mosteffective on clay soils. A smoothdrum, steel wheel roller is ideal forcompaction of the roadway sur-face. Vibratory plates or rammers,such as “wackers”, are ideal inconfined spaces. No one piece ofequipment is ideal for all soils, butthe best all-purpose equipment forearthwork in most mixed soils isa pneumatic tire roller that pro-duces good compaction in a widerange of soil types, from aggre-gates to cohesive silty soils.

Materials SourcesThe use of local materials

sources, such as borrow pits andquarries, can produce major costsavings for a project compared tothe cost of hauling materials fromdistant, often commercial,sources. However, the quarry orborrow pit material quality mustbe adequate. Sources may benearby rock outcrops or granulardeposits adjacent to the road orwithin the roadway. Road widen-ing or lowering road grade in frac-tured, rocky areas may producegood construction materials in anarea already impacted by con-struction. Rock excavation andproduction may be by hand (Photo12.9), or with the use of varioustypes of equipment, such asscreens and crushers. Relativelylow-cost, on-site materials can re-sult in the application of consid-erably more roadway surfacingand more slope protection withrock since the materials are readilyavailable and inexpensive. How-ever, poor quality materials willrequire more road maintenance

and may have poor performance.

Borrow pits and quarries canhave major adverse impacts, in-cluding sediment from a large de-nuded area, a change in land use,impacts on wildlife, safety prob-lems, and visual impacts. Thusquarry site planning, location, anddevelopment should usually bedone in conjunction with Environ-mental Analysis to determine thesuitability of the site and con-straints. A Pit Development Planshould be required for any quarryor pit development to define andcontrol the use of the site and thematerials being extracted. A pitdevelopment plan typically definesthe location of the materials de-posit, the working equipment,stockpile and extraction areas(Photo 12.10), access roads,property boundaries, watersources, and final shape of the pitand back slopes. Materials sourceextraction can cause long-termland use changes, so good siteanalysis is needed.

In-channel gravel deposits orstream terrace deposits are oftenused as materials sources. Ideally,deposits in or near streams or riv-ers should not be used. Gravel ex-traction in active stream channelscan cause significant damage tothe stream, both on-site anddownstream (or upstream) of thesite. However, it may be reason-able to remove some materialsfrom the channel with adequatestudy of the fluvial system andcare in the operation. Some gravelbar or terrace deposits may beappropriate for a materials source,particularly if taken from abovethe active river channel. Equip-


Photo 12.9 Photo 12.9 Photo 12.9 Photo 12.9 Photo 12.9 Develop quarriesand borrow sites (materialssources) close to the projectarea whenever possible. Eitherhand labor or equipment may beappropriate, depending on thesite conditions and productionrates.

Photo 12.10 Photo 12.10 Photo 12.10 Photo 12.10 Photo 12.10 Quarries andborrow sites (materialssources) can provide an excel-lent, relatively inexpensivesource of project materials. Asite may require simple excava-tion, screening, or crushing toproduce the desired materials.Control use of the area with aPit Development Plan.

ment should not work in the wa-ter.

Site reclamation is typicallyneeded after materials extraction,and reclamation should be an in-tegral part of site developmentand included in the materials cost.Reclamation work should be de-fined in a Pit Reclamation Plan.Reclamation can include conserv-ing and reapplying topsoil, reshap-ing the pit, revegetation, drainage,erosion control, and safety mea-sures. Often, interim site use, clo-

sure, and future reuse must alsobe addressed. A site may be usedfor many years but be closed be-tween projects, so interim recla-mation may be needed. Roadsideborrow areas are commonly usedas close, inexpensive sources ofmaterial (Photo 12.11). These ar-eas ideally should be located outof sight of the road, and they tooneed reclamation work after use.

The quality of the local mate-rial may be variable or marginal,and the use of local material of-

ten requires extra processing orquality control. Low quality ma-terial may be produced at a costmuch lower than commerciallyavailable material, but may notperform well. Zones of good andbad material may have to be sepa-rated. The use of local materials,however, can be very desirableand cost-effective when availableand suitable.


RECOMMENDED PRACTICES• Develop local borrow pits,

quarries and pit-runmaterial sources whereverpractical in a project area.Ensure that EnvironmentalAnalysis has been done forthe establishment of newmaterials sources.

• Use a Pit DevelopmentPlan to define and controlthe use of local materials.A Pit Development Planshould include the locationof the site, extent ofdevelopment, excavation,stockpiling and workingareas, shape of the pit,volume of useable mate-rial, site limitations, a planview, cross-sections of thearea, and so on. A planshould also address interimor temporary closures andfuture operations.

• Develop a Pit Reclama-tion Plan in conjunctionwith pit planning to return

PRACTICES TO AVOID

• In-stream channel gravelextraction operations andworking with equipment inthe stream.

• Developing materialssources without planning

and implementing reclama-tion measures.

• Using low quality, ques-tionable, or unprovenmaterials without adequateinvestigation and testing.

the area to other long-termproductive uses. A PitReclamation Plan shouldinclude information suchas topsoil conservationand reapplication, finalslopes and shaping, drain-age needs, safety mea-sures, revegetation, anderosion control measures(Photo 12.12).

• Reshape, revegetate andcontrol erosion in roadsideborrow areas to minimizetheir visual and environ-mental impacts (Figure12.5). Locate materialssources either within theroadway or out of view ofthe road.

• Maintain project qualitycontrol with materialstesting to guarantee theproduction of suitablequality material fromquarry and borrow pitsources.


Good Practices for Quarry Development

Poor Practices for Quarry Development

• Screen pit area from road• Leave gentle slopes• Reshape and smooth the area• Leave pockets of vegetation• Seed and mulch the area• Use drainage control measures• Replace Topsoil

DO!

Ideal Location and Sequence of Excavation

41

2

3

DO NOT!• Expose large, open area• Leave area barren• Leave steep or vertical slopes

Road

Locate borrow areas out of sight of the road.(NOTE: Safe backslope excavation height depends on soil type.Keep backslopes low, sloped or terraced for safety purposes.)

FFFFFigurigurigurigurigure 12.5 e 12.5 e 12.5 e 12.5 e 12.5 Good and bad roadside quarry development practices. (Adapted from Visual QualityBest Management Practices for Forest Management in Minnesota, 1996)


Photo 12.12 Photo 12.12 Photo 12.12 Photo 12.12 Photo 12.12 A reclaimed and revegetated borrow site. Reshape, drain,plant vegetation, and rehabilitate borrow pits and quarries once theusable materials are removed and use of the area is completed.

Photo 12.11 Photo 12.11 Photo 12.11 Photo 12.11 Photo 12.11 This roadside borrow area lacks drainage and erosioncontrol. Roadside quarry development can be inexpensive and useful, butthe areas should be hidden if possible, and the areas should be reclaimedonce the project is completed.

chapter 12 JM - U.S. DEPARTMENT OF THE INTERIOR volume engineering/N_Ch12_Roadway...materials such as gravel, coarse rocky soil, crushed aggregate, cobblestone, con-crete block, or

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