Soil is the foundation material for allroads, whether in the form of native undis­turbed subgrade materials, transported ma­terial, or native embankment material.Vehicle loads are transmitted to theroadbed and if adequate support does notexist, severe rutting or washboarding willresult.

In the fme-grained materials of NewHampshire's roadbeds, the stability is mois­ture dependent. In areas where there ispoor drainage or subsurface water condi­tions it is almost impossible to keep theroadway from rutting. Even though thegravel is added to these locations, theroadway will produce ruts as soon as thenext rainfall occurs. Placing a geotextile, orfabric as it is sometimes called, with 4 inchesof gravel on these locations will eliminatethe rutting and provide all weather servicefor the roadway. ) \

Selecting a Geotextile

First, let's look at the two distinct typesof fabrics available for roadway applications.The first type is the woven geotextile. Thistype resembles burlap in appearance in thatthe fibers are actually machine woven.Woven fabrics are very high in tensilestrength properties, however, the propertiesof elongation and permeability are lowerthan non-woven fabrics. Woven fabrics arenot recommended for separation applica­tions on low volume gravel surfaced roads .

The second type, and the type which isrecommended for this particular applica­tion, are the non-woven fabrics . Thesefabrics are made from fibers placed in arandom arrangement and bonded togetherby various processes such as needlepunched, spunbonded, melt bond, etc.These fabrics resemble the lining in thetrunks of automobiles. Non-woven fabricsdo not have the tensile strengths of thewovens but they have tremendous elonga­tion and permeability properties whichenable them to give the excellent perform-ance on the problem spots on low volume 1roads. The minimum requirements for a

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Installation Of Ge­otextiles On GravelRoads

Selection and installation of theproper geotextile and aggregatesfor eliminating perennial boggyspots and trouble areas

page 6

fabric to be used on a low volume gravelsurfaced road are as follows:

Minimum Elongation (at break)50% ASTM D 1682

Minimum Grab Tensile90 lbs. ASTM D 1682

ASTMD 1682Puncture Strength

30

Equivalent Opening (US std. sieve)60 min. Corps of Engrs .170 max. CW-D2215-77

Minimum Weight4 oz/sq.yd, ASTM D 1910

Note: If excessive vehicle weights or high vol­umejhigh velocity vehicle traffic is expectedthe minimum values should be increased.

.)

Site Preparation

Ideally, the roadbed should be gradedsmooth and a crown established in theroadbed. If the roadway has deep ruts whenplacing the fabric , it will take additionalgravel to fill the ruts adding to the cost ofthe operation. Secondly, and most impor­tant, a loaded dump truck with its bedraised could tip over on its side if the rearwheels should fall off into the rut.

It is not mandatory to fully compact thesection of roadway to be repaired prior toplacing the fabric and gravel. In most cases,this would be very difficult to achieve chieflybecause this area usually is highly saturatedwith water. When the roadbed material isdry enough to hold up a motor grader andloose enough to fill in the ruts; use themotor grader to shape the roadbed, filling inthe ruts and establishing a crown.

In the case of an emergency or unusualcondition such as a spring in the roadbed, itwill not be possible to shape the roadbed.These conditions will be discussed later.

Placing The Fabric

Once the roadbed has been shaped thefabric can be rolled out. Since mostmanufacturers produce the textiles in rollwidths of 12.5 feet to 15 feet, it generallywill take 2 rolls side by side to completely

continued 011 p. 8

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Geotextiles...continued from p. 7

cover the roadbed. The first roll is lined upon one edge of the roadbed, starting about20 feet away from the problem areas, androlled out for about 20 feet . The second rollis lined up on the other edge of the roadwayand overlaps the first roll near the center­line of the road. All overlaps should be aminimum of 12 inches.

Continue to roll the first roll outkeeping about 20 feet ahead of the secondroll. This will allow for periodic shifting ofthe rolls to keep the alignment correct. Ifthere are windy conditions present whenunrolling the fabric, the ends and sides willneed to be weighted down to keep the fabricfrom blowing. This can be accomplished byputting shovel fulls of gravel or soil from theborrow ditch of the ends and sides to keepthe fabric down. Depending on the velocityof the wind, a spacing of eight to ten feet isgenerally sufficient to keep the fabricweighted down. It is recommended to placethe first roll on the down wind side of theroad . The second roll will overlap the firstroll in the direction the wind is blowing,keeping the wind from picking up the edgeof the first roll at the centerline.

When the two rolls are rolled out andadditional rolls are needed to be placed,fold the ends of the first two rolls back overon top of themselves abo ut two feet . Startthe additional rolls in the same manner asthe first rolls with the ends next to the foldsof the first rolls. As soon as the additionalrolls are rolled out a short distance pull thefolded fabric over the ends of the additionalrolls. Continue lapping the fabric in thesame direction each time new rolls areplaced. This is necessary as the grovel mustbe spread in the same direction the fabric islapped. If the fabric has folds in it, flattenthe folds in the same direction the fabric is .lapped and weight the folds down as flat aspossible.

Selecting The Gravel

The type of gravel to be placed on thefabric will chiefly depend on the localsources available. The gravel should rangein size from a maximum of 3/4 to 11/2inches in diameter and have at least 10%fines or dust. This dust should not containany significant amount of clay. The gravelshould not contain any significant amount ofclay. The gravel should have angular sidesas with crushed stone as opposed to a roundriver gravel. Crusher run gravel is probablythe best type of gravel to use. The idealmaterial would conform to the followingspecifications:

Gradation Requirements (Percent Passing)

Sieve Size (mm)

11/2 in. (37.5) 100

3/4 in. (19.0) 90-100

No.4 (4.75) 30-85

No. 200 (0.075) 10-25

Plasticity Index 6-15

Liquid Limit Not more than 35

Placing The Gravel

As stated previously, the gravel must bespread in the same direction the fabric islapped. Spreading the gravel into the lapcan cause the fabric to separate resulting inunprotected spots in the roadbed. Once theroadbed has been shaped and the fabricrolled out over the problem area the graveltrucks can dump the gravel.

The two most common operations willbe using end dump or belly dump trucks .The end dump trucks can either dump thegravel while driving forward or they mayback dump. An experienced driver canchain the tailgate and spread the gravel veryclose to the required thickness of fourinches . This will save considerable time inthe blading operation. The belly dumptrucks will dump in windrows and the motorgrader will pull the material from thewindrows and blade the gravel across thefabric .

The truck drivers should be cautionedagainst sudden hard stops or takeoffs whilethe trucks are on the fabric . Sudden slidingor spinning of the truck tires can tear thefabric . A smooth continuous rollingmovement is advised.

Blading The Gravel

A motor grader will be needed tospread the gravel evenly across the fabric .The moldboard on the grader should betilted forward to give a dragging action .The grader operator should not try tospread the gravel in thin lifts (less than 21/2 inches) . Trying to blade a thin lift willcause the gravel to lock toget her and willdrag the gravel across the fabric. This willresult in tearing of the fabric .

The end result should be a four inch

page 8

thickness of gravel across the fabric . Avoidturning the front wheels of the grader whileit is not moving. This will tear the fabric.

Unusual Conditions

In the event of having to place fabricand gravel in boggy or extremely wet andsoft locations the following techniques arerecommended:

• Roll the fabric across the problem area,starting about 30 feet from the soft area

• End dump trucks can be used to placethe gravel if the trucks back dump thegravel, keeping at least four inches ofgravel between the tires and the fabric

• End dump trucks can also be used todump the gravel in a pile on one end ofthe fabric . A small dozer or front endloader can then push the gravel outahead of the machine as long as fourinches of gravel is between the tracks ortires of the equipment and the fabric .

• Don't use a motor grader to spread theinitial layer of gravel. The front wheelsof the grader are about ten feet in frontof the blade and will be on the barefabric . This will cause the fabric to bepushed down into the mud making largeruts .

Summary

1. Shape the roadway and establish acrown.

2. Roll the fabric across the problem area.Weight the ends , sides, and folds asnecessary.

3. Spread the gravel in the appropriatemanner and in the direction of the laps.

4. In soft conditions, keep four inches ofgravel between the machinery and thefabric.

The above Information was taken from a re­search report published by Oklahoma StateUnivers ity. John Hopkins, Director of theOkiahoma T2 Center, has spearheaded in­vestigations Into the use of geotextiles withgravel roads through a grant from theFederal Highway Administration. For moreInformation on the SUbject please contactthe New Hampshire Technology TransferCenter at 1-800-423-0060. •