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CHAPTER 11

G e o t e x t i l e s

Other techniques are available for improv-ing the condition of a soil besides mechanicalblending and chemical stabilization. Thesetechniques incorporate geotextiles in variouspavement applications.

The term geotextile refers to any permeabletextile used with foundation, soil, rock, earth,or any other geotechnical engineering-relatedmaterial as an integral part of a human-madeproject, structure, or system. Geotextiles arecommonly referred to as geofabrics, engineer-ing fabrics, or just fabrics.

APPLICATIONSGeotextiles serve four primary functions:

Reinforcement.Separation.Drainage.Filtration.

In many situations, using these fabrics canreplace soil, which saves time, materials, andequipment costs. In theater-of-operationshorizontal construction, the primary concernis with separating and reinforcing low load-bearing soils to reduce construction time.

ReinforcementThe design engineer attempts to reduce the

thickness of a pavement structure wheneverpossible. Tests show that for low load-bearing soils (generally 5), the use ofgeofabrics can often decrease the amount ofsubbase and base course materials required.The fabric lends its tensile strength to the soil

to increase the overall design strength. Fig-ure 11-1, shows an example of this concept.

Swamps, peat bogs, and beach sands canalso be quickly stabilized by the use ofgeofabrics. Tank trails have been success-fully built across peat bogs using commercialgeofabric.

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SeparationConstruction across soft soils creates a

dilemma for the engineer. The constructionproceeds at a slow pace because much time isspent recovering equipment mired in muckand hauling large quantities of fill to provideadequate bearing strength. Traditionally,the following options may be considered:

Bypass the area.Remove and replace the soil.Build directly on the soft soil.Stabilize mechanically or with an ad-mixture.

Bypass. This course of action is often ne-gated by the tactical situation or other physi-cal boundaries.

Remove and Replace. Commonly referredto as “mucking,” this option is sometimes avery difficult and time-consuming procedure.It can only be used if the area has good, stablesoil underneath the poor soil. Furthermore, asuitable fill material must be found nearby.

Build On Directly. Base course construc-tion material is often placed directly on theweak soil; however, the base course layer isusually very thick and the solution is tem-porary. A “pumping” action causes fines tointrude into the base course, which causes thebase course to sink into the weak soil (see Fig-ure 11-2). As a result, the base course itselfbecomes weak. The remedy is to dump morematerial on the site.

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Stabilize. As discussed in Chapter 10,stabilizing can be done mechanically or withan admixture could be used, but it may bevery time consuming and costly.

For the last three options, the poor soileventually intrudes into the base course, suchas in a swampy area, or simply moves underthe loads. By using geofabrics, the poor soilscan be separated and confined to prevent in-trusion or loss of soil (see Figure 11-3).

DrainageGeotextiles placed in situations where

water is transmitted in the plane of theirstructure provide a drainage junction. Ex-amples are geotextiles used as a substitute forgranular material in trench drains, blanketdrains, and drainage columns next to struc-tures. This woven fabric offers poor drainagecharacteristics; thick nonwoven fabrics haveconsiderably more void space in their struc-ture available for water transmission. A gooddrainage geotextile allows free water flow(but not soil loss) in the plane of the fabric.

FiltrationIn filter applications, the geotextile is

placed in contact with soil to be drained andallows water and any particles suspended inthe water to flow out of the soil while prevent-ing unsuspended soil particles from beingcarried away by the seepage. Filter fabricsare routinely used under riprap in coastal,river, and stream bank protection systems toprevent bank erosion. Another example ofusing a geotextile as a filter is a geotextile-lined drainage ditch along the edge of a roadpavement.

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UNPAVED AGGREGATEROAD DESIGN

The widespread acceptance of geotextilesfor use in engineering design has led to aproliferation of geotextile manufacturers anda multitude of geofabrics, each with differentengineering characteristics. The designguidelines and methodology that follow willassist in selecting the right geofabric to meetconstruction requirements.

Site ReconnaissanceAs with any construction project, a site

reconnaissance provides the designer insightinto the requirements and the problems thatmight be encountered during construction.

Subgrade Soil Type and StrengthIdentify the subgrade soil and determine

its strength as outlined in Chapter 9. If pos-sible, determine the soil’s shear strength (C)in psi. If you are unable to determine C, usethe nomograph in Figure 11-4 to convert theCBR value or Cone Index to C.

Subgrade Soil Permissible LoadThe amount of load that can be applied

without causing the subgrade soil to fail isreferred to as the permissible stress (S).

Permissible subgrade stress without ageotextile:

S = (2.8) C

Permissible subgrade stress with ageotextile:

S = (5.0) C

Wheel Load, Contact Pressure,and Contact Area

Estimate wheel load, contact pressure, andcontact area dimensions (see Table 11-1, page11-4). For the purpose of geotextile design,both single and dual wheels are represented

as single-wheel loads (L) equal to one-half theaxle load. The wheel load exerted by a singlewheel is applied at a surface contact pressure(P) equal to the tire inflation pressure. Dualwheel loads apply a P equal to 75 percent of

the tire inflation pressure. Tandem axlesexert 20 percent more than their actualweight to the subgrade soil due to overlappingstress from the adjacent axle in the tandemset.

Estimate the area being loaded (B2):

B = = length of one side of the squarecontact area

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Aggregate Base Thickness Using the calculated values of X andAssuming that wheel loads will be applied X geotextile, use Table 11-2 to find

over a square area, use the Boussinesq theory the corresponding value of M and Mof load distribution to determine the ag- geotextile.gregate section thickness required to support Then solve for the aggregate basethe design load. The Boussinesq theory coef-ficients are found in Table 11-2. thickness H and H geotextile.

First, solve for X: Without a geotextile:SWithout a geotextile: X = —

(4)P With a geotextile:With a geotextile:

S geotextile H geotextile =X geotextile = (4) P

H=

B

B (inches)(2) M

(2) M geotextile

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The difference between H and H geotextileis the aggregate savings due to the geotextile.

Aggregate Quality. Adjust the aggregatesection thickness for aggregate quality. Thedesign method is based on the assumptionthat a good quality of aggregate (with a min-imum CBR value of 80) is used. If a lowerquality is used, the aggregate section thick-ness must be adjusted.

Table 11-3, page 11-6, contains typical com-pacted strength properties of commonstructural materials. These values are ap-proximations; use more specific data if it isavailable. Extract the appropriate thicknessequivalent factor from Table 11-3, page 11-6,then divide H by that factor to determine theadjusted aggregate section thickness.

Service Life. Adjust the aggregate basethickness for the service life. The designmethod assumes that the pavement will besubjected to one thousand 18,000-poundequivalent vehicle passes. If you anticipatemore than 1,000 equivalent passes, you willneed to increase the design thickness by 30percent and monitor the performance of theroad.

A second method of determining minimumrequired cover above a subgrade for wheeledvehicles with and without a geotextile re-quires fewer input parameters. Again, useFigure 11-4, page 11-3, to correct CBR or coneindex values to a C value. Determine the per-missible stress (S) on the subgrade soil bymultiplying C times 2.8 without a geotextileand 5.0 with a geotextile. Select the heaviestvehicle using the road and the design vehiclefor each wheel load configuration: single,dual, or tandem. Using the appropriategraph (see Figures 11-5, 11-6, or 11-7, pages11-7 and 11-8) enter the graph at S. Roundthe design-vehicle wheel loads to the nexthigher wheel-load weight curve (for example,a dual wheel load of 10,500 pounds is roundedto 12,000 pounds (see Figure 11-6, page 11-7)).Determine the intersection between the ap-propriate wheel-load curve and S (with andwithout a geotextile) then read the minimumrequired thickness on the left axis. Use thegreatest thickness values as the design thick-ness with and without a geotextile. Corn parethe cost of the material saved with the cost ofthe geotextile to determine if using thegeotextile is cost effective.

SELECTING A GEOTEXTILEUp to this point in the geotextile design

process, we have been concerned with generaldesign properties for designing unpaved ag-gregate roads. Now you must decide which

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geotextile fabric best meets your project re-quirements.

There are two major types of geotextilefabric: woven and nonwoven. Woven fabricshave filaments woven into a regular, usuallyrectangular, pattern with openings that arefairly evenly spaced and sized. Nonwovenfabrics have filaments connected in a methodother than weaving, typically needle punch-ing or head bonding at intersection points ofthe filaments. The pattern and the spacingand size of the openings are irregular in non-woven fabrics. Woven fabrics are usuallystronger than nonwoven fabrics of the samefabric weight. Woven geotextiles typicallyreach peak strength at between 5 and 25 per-cent strain. Nonwoven fabrics have a highelongation of 50 percent or more at maximumstrength.

Table 11-4, page 11-9, provides informationon important criteria and principle propertiesto consider when selecting or specifying ageotextile for a particular application. Thetype of equipment used to construct the roador airfield pavement structure on top of thegeotextile must be considered. Equipmentground pressure (in psi) is an important fac-tor in determining the geotextile fabricthickness. A thicker fabric is necessary tostand up to high equipment ground pressure(see Table 11-5, page 11-10).

Once the required degree of geotextile sur-vivability is determined, minimum specifi-cation requirements can be established basedon ASTM standards (see Table 11-6, page11-10). After determining the set of testingstandards the geotextile will be required to

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withstand to meet the use and constructionrequirements, either specify a geotextile forordering or evaluate on-hand stock.

Roadway ConstructionThere is no singular way to construct with

geofabrics. However, there are several ap-plications and general guidelines that can beused.

Prepare the Site. Clear, grub, and excavatethe site to design grade, filling in ruts and sur-face irregularities deeper than 3 inches (seeFigure 11-8, page 11-11 ). Lightly compact thesubgrade if the soil is CBR 1. The light com-paction aids in locating unsuitable materialsthat may damage the fabric. Remove thesematerials when it is practical to do so.

When constructing over extremely softsoils, such as peat bogs, the surface materials,such as the root mat, may be advantageousand should be disturbed as little as possible.Use sand or sawdust to cover protruding

roots, stumps, or stalks to cushion the geotex-tile and reduce the potential for fabricpuncture. Nonwoven geotextiles, with theirhigh elongation properties, are preferredwhen the soil surface is uneven.

Lay the Fabric. The fabric should be rolledout by hand, ahead of the backfilling anddirectly on the soil subgrade. The fabric iscommonly, but not always, laid in the direc-tion of the roadway. Where the subgradecross section has large areas and leveling isnot practical, the fabric may be cut and laidtransverse to the roadway. Large wrinklesshould be avoided. In the case of wide roads,multiple widths of fabric are laid to overlap.The lap length normally depends on the sub-grade strength. Table 11-7, page 11-12, pro-vides general guidelines for lap lengths.

Lay the Base. If angular rock is to form thebase, it is a common procedure to first place aprotective layer of 6 to 8 inches of finermaterial. The base material is then dumpeddirectly onto the previously spread load,

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pushed out over the fabric, and spread from 11-13, shows one method that can be adaptedthe center using a bulldozer. It is critical thatthe vehicles not drive directly on the fabricnor puncture it. Small tracked bulldozerswith a maximum ground pressure of 2 psi arecommonly used. The blade is kept high toavoid driving rock down into the fabric.Finally, compaction and grading can be car-ried out with standard compactionequipment. If the installation has sidedrains, these are constructed after the pave-ment.

Earth Retaining WallsAs with road construction, there is no

specific or preferred method for using geotex-tiles for retaining walls. Figure 11-9, page

to the specific needs of the engineer. Thebackfill material can be coarse-grained, fine-grained, or alternating layers of coarse- andfine-g-rained materials.

Construction on SandConstruction on sand, such as a beach or a

desert, presents a severe trafficability prob-lem. The construction of an expedient roadthrough this soil can be expedited by using aplastic geocell material called “sand grid”.This material is in the Army’s inventorystockage (National Stock Number (NSN)5680-01-198-7955). The sand grid is ahoneycomb-shaped geotextile measuring 20feet long, 8 feet wide, and 8 inches deep when

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fully expanded (see Figure 11-10, page11-14).

Sand grids are very useful when developinga beachhead for logistics-over-the-shore(LOTS) operations. Construction of sand-gridroadways proceeds rapidly. A squad-sizedelement augmented with a scoop loader, lightbulldozers, and compaction equipment are allthat is required to construct a sand-grid road.

Procechures. Use the following constructionprocedures for a sand-grid road:

Lay out the road. Establish a center-line that follows the course of theproposed road.Perform the earthwork necessary tolevel the roadway.

Distribute folded sand grids along theroadway.Expand the sand-grid sections andsecure them in place. Use shovels tofill the end cells and some of the sidecells with sand.Use a scoop loader to fill the grids.They should be overfilled to allow fordensification when compacted.Compact the sand, using compactionequipment.Use a scoop loader to back drag ex-cess sand to the road shoulder area ifasphalt or gravel surfacing is used.If available, apply an asphalt surfacetreatment over the filled sand grids toenhance their service life.

Sand grids perform well under wheeled-vehicle traffic. Tracked-vehicle traffic is verydestructive to the sand-grid road. Asphaltsurface treatments reduce sand-grid damagewhen a limited number of tracked vehiclesmust use the road.

Maintenance. Sand-grid roads are easilymaintained. Entire damaged sections can be re-moved or the damaged portion can be cut andremoved and a new grid fitted in its place.

Sand grids have many uses in the theater ofoperations other than just roads. They canalso be used to construct bunkers, revet-ments, retaining walls, and a host of otherexpedient structures.

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