Citation:Pritchard, M and Allen, D (2017) International SEEDS Conference: Engineering Design for Society.International SEEDS Conference: Engineering Design for Society, Third. pp. 612-627.

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A more sustainable solution to geosynthetic products for short-termreinforcingapplicationsMartinPritchard1,DaveAllen2

1Reader,SchooloftheBuiltEnvironment&Engineering,LeedsBeckettUniversity,Leeds,LS28AG,UnitedKingdom.2OperationsManager,MetLase,AdvancedManufacturingPark,BrunelWay,Catcliffe,Sheffield,S605WG,UnitedKingdom.

Keywords:Geosynthetics,Limited-lifegeotextiles,Renewableresources,Soilreinforcement,Vegetablefibresgeotextiles.ABSTRACT

It is now very difficult to find a construction site that does not utilise any geosyntheticproducts. Materials used in the manufacture of geosynthetics are primarily syntheticpolymers–generallyderived from theby-productsof theoil industry.Asa resultof thefinite nature of these raw materials and their associated pollution streams, there isgrowingpressuretouserenewableresourcesforsustainableproduction.Also,themajorityof geosynthetic applications are only required to perform for a short period of time,therebyleavinganalienresidualinthegroundformanyyearstocome.Natural(vegetable)fibresprovideamoresustainablealternativetopolymericbasedmaterials,particularlyforshort-termapplications–termedlimited-lifegeotextiles(LLGs).Thispaperpresentsanoverviewofanextensive study thathasbeenundertakenon thedevelopment of reinforcing LLGs manufactured from renewable and biodegradablevegetable fibres for short-term applications. Initially, structural form is considered. It isshown that LLGs can have tensile strength of up to 100 kN.m-1, which is directlycomparabletoamid-rangegeosyntheticproduct.Theshear interactionpropertiesoftheLLGs was then compared to a number of different commercially available geotextilestructures – manufactured from both natural and synthetic materials. The resultsdemonstrate that coefficient of interaction values of around unity can be achievedwiththese LLGs. This is about 20–25%more shear resistance than their synthetic equivalent.Thedifferencestemmingprimarilyfromthecoarsenessofthevegetablefibresthemselvesbutalso fromthenovel structural form. In termsof longevity,durability testshavebeenundertaken on the LLGs in various ground conditions. The data obtained indicate thatdegradationratesaresensitivetofibretype,togetherwiththeamountofwaterpresentinthesoil.Coirfibreperformedthebestinworstdeteriorationenvironmenttested.Asimplebasalembankmentanalysis isthenpresentedtodemonstrateapotentialendapplicationfortheshort-termreinforcingLLGs.Inthisanalysis, it isshownthattherateatwhichtheunderlyingembankmentsoilgainsineffectivestress,duetothedissipationofexcessporewaterpressure,couldbedesignedtocorrespondtothedeclineintensilestrengthfromthedegradingLLG.

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1. INTRODUCTION

The construction industryover the last twodecadeshasexperienceda globalboom.Thishasplacedalargedemandonnaturalresources.Itisnowverydifficulttofindaconstructionsitethatdoesnotutiliseanygeotextileproducts;overthelastdecadethegeotextilemarkethas beenoneof themost thriving sectors in the technical textile industry. By the endof2017itispredictedthatjustoverfivebillionsquaremetresofgeotextileswouldhavebeenproduced,withanassociatedmarketvalueofaround£7billionpounds(GBP).Geotextilesare used, on a vast array of construction sites, to performone of five primary functions,namely: drainage, filtration, protection (erosion control), reinforcement and separation.Dependingontheapplication,thesefunctionscanperforminisolationorsimultaneously.The environmental effects of geotextiles manufactured from synthetic materials aretwofold;firstly,forshort-termapplication,analienresidueisleftinthegroundthatwillnotbiodegrade once the geotextile has served its purpose; secondly, andmore indirectly, bypollutingtheenvironmentthroughtheprocessofobtainingtherawmaterials, i.e.burningandflaringofoilandgas.Withtheneedtoembracemoresustainabledevelopmenttomeetthe triple bottom line on economic, environment and social security. It has nowbecomeimperative to use more environmental friendly resources to manufacture constructionmaterials.At present, limited-life geotextiles (LLGs) are constrained to woven and nonwoven gridstructures,and theirmainuse is forerosioncontrol.Theyaremanufactured fromasmallrange of fibres, primarily jute and coir, as illustrated in Table 1. Jute is easy to cultivate,widelyavailableonacommercialscale,cheap,biodegradableandcanholdfivetimesitsownweightofwater.Allthesefactors(especiallythe lasttwo)makeit ideallysuitedforthe initialestablishmentofvegetation,whichinturnprovidesanaturalerosionpreventionfacility.Bythetime vegetation has become well established the jute has started to rot/break down anddisappear(6to12months),withoutpollutingtheland.Coirhasalsobeenusedasgeotextiles,but not to the same extent as jute, for erosion control applications. However, in somecircumstances,suchasriverbankprotection,coirhasbeenfoundtobemoresuitablethanjuteduetoitbeingmoreresistanttorottingduetoitshighlignincontent.

Table1:Propertiesofcommercialerosioncontrolvegetablefibregeotextiles

Unit GeoJute GeoCoir Geocoir

Type Woven Woven NonwovenThickness mm 3 5 12Yarncount,warp No. 78 130 –Yarncount,weft No. 42 70 –Mass/unitarea g.m-2 460 900 820Openarea % 60 39 –Widewidthtensile,drywarpxweft kN.m-1 4.4x2.6 27.8x9.3 0.23x0.23Widewidthtensile,wet–warpxweft kN.m-1 1.8x0.9 21.4x6.8 –Elongationatfailure,dry–warpxweft % 10x10 68x32 19x19Elongationatfailure,wet–warpxweft % 11x11 82x49 –Durability years 1–2 5+ 3

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Apertinentfactorforageotextile,especiallyforreinforcement, isthat itmustpossessahightensilestrength.Thebestwayofobtainingthisisforthefibrestohaveahighratioofmolecularorientation.Thishighstrengthratioisachievednaturallybyvegetablefibres,butforsyntheticpolymersthemoleculeshavetobeartificiallyorientatedbyaprocessknownasstretchingordrawing.Ithasbeenshownthatflax,abacaandsisalfibrescanhavestrengthsintherangeof0.4 to 0.6 N.tex-1, which is directly comparable to that of polyester of around 0.4 N.tex-1(Leflaive, 1988). Also, vegetable fibres are much coarser than their synthetic equivalent,inherently offering more shear resistance – another important characteristic for reinforcinggeotextiles.Hence,natureprovidesidealfibrestobeusedinshort-term/temporaryreinforcingapplications–alsotermedvegetablefibregeotextiles(VFGs).2. SOILREINFORCEMENT

2.1TheconceptSoilisrelativelystrongincompressionbutweakintension(Fig.1a).Theconverseistrueforageotextile(Fig.1b).Therefore, iftheyareused in intimateassociationwitheachotheracompositematerialcanbeformed,whichisgoodinbothcompressionandtension(Fig.1c).If this concept is thenapplied toanunreinforced soilmass, it canbe shown thatwhenanormalloadisapplied,thesoiltriestodeformlaterally(Fig.1d)asthesoilparticlescannottakeanytensileload.However,whenageotextileis installedatvertical incrementswithinthesoilmass,andthereissufficientshearresistancealongthesoil/geotextileinterface,thetensile load will be taken by the geotextiles. Effectively, this provides an in-built lateralconfiningstresswhichpreventsdeformation (Fig.1e).This reinforcingsoil conceptcanbeextendedtoslopesandembankmentstabilisation.

Figure1:Theconceptofreinforcedsoil

Soilisstrongincompressionbutweakintension

(a) Soilparticles (b) Geotextile (c) Compositematerial

Strongintension

Goodincompressionandtension

Shearresistance

(d) Unreinforcedelementofsoildeformslaterally

(e)Reinforcedelementofsoil

Geotextile

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2.2Short-termapplicationShort-term reinforcing applications are frequently used to provide temporary support toengineered structures until excess pore water pressure has dissipated and the soil hasconsolidated. Typically, the geotextile only has to function during construction and for ashortperiodafterwards.Anexampleofsuchanapplicationisbasalreinforcement.Whenanembankmentisconstructedoversoftcompressibleground,theloadfromtheembankmentfill increasestheporewaterpressureintheunderlyingsoil,especiallyatthecentreoftheembankment. This corresponds to adecrease in the shear strengthof theunderlying soiland can result in the embankment failure, e.g. splitting, circular rotation and excessivesettlement(Fig.2a).However,whenabasalgeotextileisusedattheinterfacebetweentheembankment fill and underlying soft soil, the restraining lateral load provided by thegeotextile prevents the embankment from splitting or introduces a moment to resistrotation(Fig.2b).Settlementcanstillbeextensiveintheunderlyingsoil;thegeotextilewillhoweverensure itwillbemoreuniform.This typeof settlementcanbecompensated forduring construction. The stability of the embankment will improve in time as the excesspore water pressure in the underlying soil dissipates. Effective stress will then prevailresulting in the stabilising force from the basal reinforcement being surplus torequirements.Typically, thistimescalecouldbeanywherefromafewmonthsuptoafewyearsbutwouldultimatelydependon such factorsas coefficientof vertical consolidation(cv)oftheunderlyingsoilandlengthofthedrainagepath.Toincreasetherateofdissipationfindrainsaretypically installed. ItisproposedthisrateinsoilstrengthgaincanbedesigntocorrespondtothedeteriorationrateofthebasalLLGwithanappropriatefactor-of-safety(FOS)beingmaintained(Fig.2c).

Figure2:Basalgeotextile

Basalgeotextile

Rotationalfailure

Excessivesettlement

Splittingfailure

(a) Failures (b) Reinforcedembankment

(c) Soilstrength/porewaterpressurerelationshipforanembankmentimmediatelyafterconstruction

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2.3DurabilityThedeteriorationof anymaterial is the influence the environmenthas on thepropertieswiththepassageoftime;thisisparticularlytruefornaturalmaterials,suchasLLGs.Thereare numerous factors, which could affect the ageing process such as chemical (acid andalkaline) and biological (microorganisms) deterioration. Themain question is how long aparticular material can withstand the given degradation process, whilst maintaining therequisitepropertiesthroughoutitsdesignlife.Theanswerisultimatelyrelatedtothehostileenvironmentitisplacedinandthechemicalcompositionofthegeotextilematerial.2.4ShearinteractionTheshearingresistanceatthesoil/geotextileinterfaceisextremelyimportanttoenablethesoiltotransmitthetensileforcesfromthesoiltothegeotextilesuchthatthesoil/geotextilecompositeiseffective.Thefrictionalresistanceprovidedbythefabricstructureisassociatedwiththesurfaceroughness featuresof thegeotextile, i.e. soil slidingandthecapabilityofthesoiltoembedthefabric.Thelatterisrelatedtothegeotextile’saperturesrelativetotheparticle size of the soil, which influences both bond and bearing resistance as shown inFigure3.Bond resistance isdevelopedwhensoilparticlesembedwithin thegeotextile toretainsoilparticlesintheapertures,suchthatadjacentsoilaboveandbelowthegeotextilesurface are sheared against these retained particles. In comparison, bearing resistanceemanatesfromrestrictivemovementofsoilparticlesduetoridgesinthegeotextilesurface,orattheendoftheapertures,inthedirectionofshear.The coefficient of interaction (µ) is used to determine the efficiency of geotextiles indevelopingshearingresistance.Thevalue isdefinedbytheratioof the frictioncoefficientbetweensoilandgeotextile(tand)tothatofthefrictioncoefficientforsoilalong(tanf),asgiven in BS 6906: Part 8 (1991). Values for the coefficient of interaction typically rangebetween0.6and1areoftenquoted(RichardsandScott,1985).Thewidespreadrangeisaresultofsuchvariantsasthedifferentsoilstrengths,geotextiletypesandthetestmethodemployedtoderivethevalue.

Figure3:Shearformsofresistancealongthesurfaceofageotextile 3. RESEARCHREVIEWANDMETHODOLOGY

Processesfortheselection,specification,productionandutilisationofsyntheticgeotextilesarewellestablishedforsoilstrengtheningapplications.To-date,theuseofvegetablefibresfor soil reinforcement has not been investigated in depth because of preconceived ideasconcerning the durability, strength, extensibility and manufacturing capability of thesenatural materials. Their use has therefore been confined to erosion control applications.There are howevermany ground engineering situationswhere reinforcing geotextiles areonly required to function for a limited time period; whereas suitable synthetics have

Geotextile

Aperture

Sliding Bond Bearing

6

working lives longer than needed. Hence, are over designed but more importantly themanufactureanduseofsyntheticmaterialscausemanyformsofenvironmentalpollution.FromthewealthofknowledgeavailablerelatingtotheuseofsyntheticgeotextileswithinsoilitisalreadyknownwhatshouldbeexpectedfromreinforcingLLGs;however,theyhaveyettobemade,triedandtested.Themainaimsforthisworkweretodevelopthetechnologyappropriateforproductionof‘designer’LLGsforreinforcingapplications;tocharacterisetheirbehaviour;and,determinetheextenttowhichtheycanbeusedtostrengthensoil.Theprincipalfactorsaffectingthesuitability of these LLGs can be identified asmanufacturing feasibility, tensile properties,soil/geotextileinteractionanddurability.Tobeusablethesematerialsmustsatisfy/fulfilalloftheforegoingcriteriatosomedegree.Therefore,theoverallapproachofthisstudywasnot to ‘design’ and ‘test’ for a specific reinforcing application, but to determinewhetheracceptablebalancesofpropertiesandperformance canbeachieved.Apotentialuseof aLLG for a short-term reinforcing application is then concluded, in this paper, by thedevelopment of a computational finite difference model. The aim of this model is toillustrate how effective stress conditions will govern in time as the excess pore waterpressure dissipates. Hence, providing the timeframe the underlying embankment soilwillbecomeself-supportingwithouttheneedofabasalreinforcinggeotextile.4. RESEARCHMETHOD

ToenableadirectcomparisonwiththenovelLLGsdevelopedaspartofthisresearchwork,acommerciallyavailablewovencoirgeotextile(usedforerosioncontrolapplications)andawarpknittedpolyester grid (used for reinforcingapplications)werealso testedunder theexactsameconditions.4.1 StructuralformThe first aimof this researchworkwas todevelop anovel ‘designer’ geotextile structuremade from vegetable fibres which would be suitable for reinforcing applications. This isbecauseatthepresenttimetherangeofLLGs isvery limited, inthattheirmainuse is forerosioncontrol.Theprincipalcriterionsoughtofthegeotextileforerosioncontrolistohavesufficienttensilestrengthtoallowittobelaidonsiteandtoprovide,foralimitedtimeonly,someprotectiontothegrounde.g.retainsoilparticles,protectgrassseeds,holdwater,etc.LLGstructures forerosioncontrolapplicationshavebeenmainlyplainweave juteor coir.Nonwoven structures made from jute, coir and flax have also been used for mulchingapplications. Both of these types of structures have their limitations (woven structuresexhibit high elongation and nonwoven structures have low strength together with highelongation)andareunabletoformgeotextileswiththepropertiesandflexibilitypossessedby knitted structures, particularly directionally structured fabrics (DSF). These fabricsincorporate high strength straight inlaid yarns to provide high uniaxial strength in themachine(warp)direction(RankilorandRaz,1994a–d).Duetothecoarsenessandlackofpliabilityofvegetablefibreyarns,itpreventsthemfrombeing used on warp knitting machines. Weft knitting machines are however capable ofhandlingamuchwiderrangeofyarnstypes.Thelimitationofthesemachines,nevertheless,

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isthattheycannotincorporatehighstrengthinlayyarnsinthemachinedirectionduetothedesign of the mechanism which links the carriages on both beds. The initial aim of theresearch project was therefore to redesign this mechanism rather than constructing acompletelynewmachine,asthiswouldkeepthecostslowandenabletheLLGtobemassproducedveryquickly.Thus,theLLGstructurewasdesignedtobemanufacturedwiththefollowingcharacteristicstoreinforcesoil:1) Thehighestpossible strength inonedirection, togetherwith lowelongation combined

withtheeaseofhandlingandlayingonsite.2) Soilparticleinterlockandslidingresistancewiththegeotextiletosuchanextentthatthe

soil/geotextileinterfaceexhibitsthesameshearingresistanceasthesurroundingsoili.e.thesoil/geotextilecoefficientofinteractionisintheorderofunity.

3) Adegreeofprotectiontothehighstrengthinlaidyarnstoreduceinstallationdamage.4) Sufficient durability (with respect to degradation when buried in soil) to provide

reinforcementovertherequisitedesignlifeoftheconstruction.

4.2TensilestrengthAtensiletestingapparatuswasusedtodeterminethetensilepropertiesofthegeotextiles.Output of load and extension was data logged to an accuracy of 10 data points everysecond. Theoverall dimensions (includingapertures andabutments)of the samplesweredeterminedmanually – the thicknesswas determined from the ‘Shirley ThicknessGauge’using a circular plate 80mm in diameter under a 1000 g weight. The tensile tests wereconductedatthestandardtestingtemperatureandrelativehumidityof20±2°Cand65±2%respectivelyBS1051(1972).Thesampleswerealsoconditioned,atthistemperatureandhumidity,foratleast24hoursbeforetestingsothateachsamplewasinacomparablestatefortesting.Agauge lengthof200x50mmwasusedas inBSENISO13934-1,1999(stripmethod). As also recommended by standard five samples were tested in the strength(machineorwarp)directionataconstantstrainrateof100±10mmperminute.4.3CoefficientofinteractionThegeotextilesweretestedina300x300mm(plandimensions)partiallyfixeddirectshearbox.Theleadingsideofthelowershearboxhadthegeotextileclampedtoit.Thegeotextilewas then laid over the lower half of the shear box containing the Leighton Buzzard sand(LBS),whichwas flushwith the topof the lower shearbox.Ahydraulic ramwasused toapplyaverticalloadandaloadtransducermeasuredtheappliedpressure,enablingadirectmeasurementof thevertical stress to0.5kN.m-². Itwasalsopossible tokeep thenormalstressconstant,whilst thesamplewasdilating,by the loadtransducer.A100kNcapacityproving ringwasused tomeasure the shear forceand thisenabled the shear force toberecorded directly to 0.08 N (equivalent to a shear stress of 0.9 kN.m-²). The relativehorizontal displacement of the twohalves of the shear box, the change in sample heightduringshearingandtheverticaldisplacementof thetopfourcornersof theupperhalfoftheshearboxweremonitoredbylineardialgaugesreadingdirectlyto0.01mm.Thetestswere conducted at a strain rate of 0.3 mm per minute, up to 40 mm horizontaldisplacement.Theupperandlowerhalvesoftheshearboxwereeachcompactedinthreelayersofequalthickness using a vibrating hammer and tamping plate to a predetermined thickness toproduce nominal unit weight of 96% of the maximum nominal dry unit weight for the

8

compactedLBS.Thisfigurewaschosentorepresentthedensitylikelytobeachievedonsite,whilstmaintaininganaccuracyof±0.01Mg.m-³ fromthemeandrydensity in subsequentshearboxtests,asrecommendedbyBS6906:Part8(1991).Nominal effective normal stresses of 41, 68, 95 and 123 kN.m-2, to represent the likelyrangeofsoilpressureswhichwouldapplytofieldsituations,wereappliedtothesamples.Theserepresentedtheactualweightabovetheshearplane, i.e. theappliedpressureplustheweightof the soil in theupper-halfof the shearbox, theupper-halfof the shearboxitselfandthetopplaten.The LBSwas shearedwith no geotextile in the shear box (referred to as ‘plain’ sand) toestablish a datum. This enabled a direct comparison to be made when each of thegeotextileswereshearedwiththeLBS,henceallowingtheircorrespondingvalueofµtobecomputed.4.4DurabilitytestsThedurabilityofanygeotextileproductburiedinthegroundisofparamount importance,especially so for biodegradable vegetable fibre products. Physical and chemicaldeterioration conditions are commonly simulated in the laboratory under separateconditions.Itishoweverrecognised(Horrocks,1996)that,incombination,thesetwoeffectscan have amore detrimental impact on the product’s lifespan. To replicate this, a noveltestingrigwasdesignedandmanufactured(Fig.4).Thisenabledthegeotextilestructurestobeencapsulatedinthetestsoilswhilstsubjectingthemtobothatensileandconfiningloadinacontrolledenvironment.Aftersetperiods,thedeterioratingsamplesweretakenoutofthesoilandtestedtodeterminethepercentagelossintensilestrength.

Figure4:Durabilitytestrig

The test rig consisted of the durability boxes positioned on a loading frame; all ofwhichwerehousedinacontrolledroomwhichhadatemperatureandrelativehumidityof20–22°Cand60–65%respectively,inaccordancetoBSEN12224(2000).Eachboxwasdesignedtocontain three strips of geotextile surrounded by 35 mm of soil. The geotextile stripsencapsulatedinthesoilwere50mmwideby200mmlong,whichisinaccordancewithBSENISO13934-1,1999(stripmethod).

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Eachgeotextile stripwas loadedbyaspringmechanismviaabracket fixed to the rackingsystem to 5% of the geotextile’s maximum strength. The spring was adjusted at regularintervalstocompensateforanycreepinthegeotextile,hencemaintainaconstantload.Toensure soil/geotextile contact throughout the test period an effective normal stress of 2kN.m-2wasappliedasasurchargeontheboxlid.Tosimulatenaturalweatheringconditions(i.e.wetting/dryingcycles)thatoccursin-situduringinstallationandovertheworkinglifeofgeotextileproducts,anirrigationsystemwasusedtosaturatethesoilcontainedwithinthedurabilityboxesevery14days.5. RESULTS&DISCUSSION

5.1StructuralformModificationswerecarriedoutusingamechanicallyoperatedDubiedDC-25-gaugeV-bedweft-knittingmachinetoproduceanovelbasestructure(PatentNo.GB2339803).Themainmodificationtothemachineinvolvedremovingthebowlinkingthefrontandrearcarriagesasthisprohibitedverticalinlay.Thepurposeofthebowwastomaintainthesynchronisationof the front and rear carriages as they traverse the needle beds. The front and backcarriagesnowbeingconnectedbytwoendlesschainsconnectedviaaseriesofdoubleandsinglesprocketspositionedatbothendsofthemachine.

InreferencetothemanufacturingdesigncharacteristicsnotedinSection4.1,variationsofthis base structurewere thendeveloped toprovide the specific properties required fromgeotextilestostrengthensoil,essentially:1) The geotextile was designed to have the highest possible number of straight high

strength inlayyarns inonedirection,withthebasefabricstructure,madefromthinnermoreflexibleweaker/cheaperyarns,holdingtheinlayyarnsinplace.

2) Coarseyarns togetherwithabutmentsandapertures in thegeotextilewerecreated toproducehighshearresistanceinthemachinedirection.

3) Asacrificialbasestructure, formedfromacheapermoredegradableyarn,wasusedtoencapsulate thehigh strengthyarns.Byprovidingprotection to thehigh strength inlayyarnsthenecessitytointroducea largereductionfactor intothedesign,toaccountforinstallationdamagefromcertaintypesoffill/plant,isminimised.

4) Toachievedifferentdurability rateshighstrength inlayyarnscouldbewhollyorpartlychangedforamoredurableyarninaggressivegroundconditions.

Figure 5 illustrates the base structure that was developed to address the abovemanufacturing design characteristics. Essentially the structure is a directionally structure1x1 knitted rib, with alternate wales of face loops on each side. The inlay yarns areencapsulated within the knitted structure by the cross meshing between the face andreverse wale loops. This ensures the structure remains flat when cut and has a goodresistancetotear.Variations inboththe inlayandknittingyarnswerepossible.Toreducethenumberofvariables,threecombinationsofthebaseknittedstructurewereusedinthistesting programme, namely: (1) sisal inlay/knitted flax; (2) sisal inlay/knitted jute and (3)jute inlay/knitted flax. Table 2 summarises the fabric characteristics of these novel LLGstogetherwiththetwocommerciallyavailablegeotextiles(4&5).

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Figure5:Sisalinlay/knittedflax1x1ribbasestructure

Table2:Fabriccharacteristicsofthegeotextiles

No. Material

Inlayyarnsperm

No.ofcoursespercm

No.ofWalespercm

Stitchdensitycm2

%inlayyarn

%knittingyarn

1 Sisalinlay/knittedflax 110 8 4 32 52 482 Sisalinlay/knittedJute 110 8 4 32 53 473 Juteinlay/knittedflax 110 8 4 32 40 604 Wovencoir 90 0.9* 0.8# N/A 59 415 Warpknittedpolyestergrid 80 6 0.8 N/A 98 2

*No.ofwarpyarnspercm#No.ofweftyarnspercm5.2TensilestrengthFigure6 shows theultimate tensile strengthof the fivegeotextiles tested in themachine(warp)direction,withcorrespondingvaluesshowninTables3.Themainparametersunderconsiderationaretheloadthegeotextilescantakeforeverygivenmetre(kN.m-1)andthepercentagestrainresultingfromtheload.

Figure6:Tensilestrengthpropertiesoffivegeotextiles

0

20

40

60

80

100

120

0 5 10 15 20 25Strain (%)

Stre

ngth

(kN

/m)

(1) Sisal inlay/knitted flax (2) Sisal inlay/knitted jute (3) Jute inlay/knitted flax (4) Woven coir (5) Warp knitted polyester grid

4

2

3

51

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Table3:Physicalpropertiesofthefivegeotextiles

No. MaterialTensilestrengthkN.m-1

Strain%

ElasticmoduluskN.m-1

Massg.m-2

Thicknessmm

1 Sisalinlay/knittedflax 97.6 5.0 19.35 1380 52 Sisalinlay/knittedJute 97.4 5.3 18.52 1310 53 Juteinlay/knittedflax 43.8 5.4 8.18 1180 54 Wovencoir 16.6 13.2 1.26 900 45 Warpknittedpolyestergrid 106.2 22.0 4.83 430 1.5

Itcanbeseenthatthetensilestrengthofsisal inlaygeotextiles(1&2)arecomparabletothewarp knitted polyester geotextile (5). The jute inlay geotextile (3) has approximately50%ofthestrengthofsisal.Wovencoir(4)istheweakestofthegeotextilematerialstestedwithonly20%ofthetensilestrengthofsisal(1).Thisisduetothefactthatcoirisweakerasaresultofitschemicalcompositionandthisfabricstructurehaslessinlayyarnstocarrytheload.Onexaminationof theplot, sisal and jute fail in the samemanner.A ‘saw toothed’failure mode was created as individual inlay yarns break and the load is passed to theremainingyarns. Thewarpknittedpolyesterandwovencoir geotextileshave significantlyhigher ultimate strain capacities, which would ultimately result in more unfavourabledeformationinthereinforcedstructure.5.3CoefficientofinteractionThecoefficientofinteractionisdependentonboththegeotextileandsoiltype.Ideallythecoefficientshouldbeasclosetooneaspossible.FromtheLBSshearinteractionresults(Fig.7andTable4),coirhadthehighestcoefficientat0.99andthesyntheticgeotextilehadthelowestat0.80.Thestructuresmanufacturedfromsisal,juteandflaxwereallfoundtohavecoefficientsintherangeof0.91–0.97.

Figure7:InterfaceLBS–geotextilefrictionalvalues

A graph showing failure shear stress against effective normal stress

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140

Effective normal stress (kN.m-2)

Peak

she

ar s

tres

s (k

N.m

-2)

Sisal inlay/knitted flax Sisal inlay/knitted juteJute inlay/knitted flax Woven coirWarp knitted polyester

Sisal inlay/knitted flax y = 0.9273xSisal inlay/knitted jute y = 0.8383x

Jute inlay/knitted flax y = 0.9202xWoven coir y = 0.9442x

Warp knitted polyester y = 0.6992x

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Table4:Shearstrengthcharacteristics

No. MaterialPeakangleofbond

friction(o)Ф'p

Coefficientofinteraction

α LBS–LBS(nogeotextile) 44.0 1.001 Sisalinlay/knittedflax–LBS 42.8 0.972 Sisalinlay/knittedjute–LBS 40.0 0.913 Juteinlay/knittedflax–LBS 42.6 0.974 Wovencoir–LBS 43.4 0.995 Warpknittedpolyestergrid–LBS 35.0 0.80

5.4DurabilitytestsFigure8ashowstheresultsfromthetestscarriedoutatthesoilsnaturalmoisturecontentof0.1%(termeddry).Theresultsindicatethatnoreductionintensilestrengthoccurswithtime.Thiswasanticipatedasnorealmoistureispresenttosustainamicrobialcommunity.

(a) Drydurabilityboxes

(b) Wetdurabilityboxes

Figure8:Geotextilestrengths–durabilityboxes

020406080

100120

0 3 6 9 12 15 18 21 24

Test stage (months)

Stre

ngth

(kN

.m-1

)

Sisal inlay/knitted flax Sisal inlay/knitted JuteJute inlay/knitted flax Woven coirSynthetic grid

020

4060

80100

120

0 1 2 3 4 5 6

Test stage (months)

Stre

ngth

(kN

.m-1

)

020406080

100120

0 3 6 9 12 15 18 21 24

Time (months)

Stre

ngth

(kN

.m-1

)

Sisal inlay/knitted flax Sisal inlay/knitted JuteJute inlay/knitted flax Woven coirSynthetic grid

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Figure8bshowsthatoverthetestperiodthereweresignificantinitialreductionsintensilestrength of both the sisal and jute inlay geotextiles. After 1½ months the jute inlaygeotextile had only retained about 11% of its initial strength. The sisal inlay geotextileperforming slightly better, retaining 28%. The behaviour of the geotextiles at 3 and 4½monthsdidnothoweverfitalogicaldeteriorationtrend,butat6monthsthegeotextilehadnotreallylostanyfurtherstrength.Theoverallreductionsofaround70%and90%forsisalandjuteinlaygeotextileswereobtainedattheendofthetestingperiodrespectively.Afteran initial reduction in strength, thewoven coir geotextile retained on average 82%of itsinitialstrengththroughoutthedurationof thetestperiod.This is relatedto itshigh lignincontent,whichisdifficulttobreakdownandcanpersistforyearsinsoil(GrayandWilliams,1971).Thesyntheticgridalsoappearstohavelostonaverage9%ofitsinitialstrengthoverthetestingperiod.6. EMBANKMENTANALYSIS

Anumericalexample ispresentedbelowto illustratehowthesoftunderlyingsoilgains instrengthovertime;hence,demonstratingthatabasalgeotextilewouldonlyberequiredfora limited timeperiod. The finite difference software package thatwas used to develop anumerical solutionwas FLAC,which stands for Fast Lagrangian Analysis of Continua. Thissoftware package was developed by the Itasca Consulting Group, Inc. In this model, thefoundation soil was 10 m deep and groundwater was at ground level. The analysis wassimplifiedbytakingintoaccounthalfsymmetryandusinganappliedsurchargetosimulateembankmentloading(Fig.9).

Figure9:Modelparameters

11

10

9

8

7

6

5

4

3

2

11 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Embankment loading

10 m (half width of embankment)

= vertical displacement (yd)

= pore pressure (pp)

= effective stress (esyy)

= vertical displacement (yd)

= pore pressure (pp)

= effective stress (esyy)

10 m

1,1 2,1 3,1

1,2 2,2 3,2

1,3 2,3 3,3

4,1

4,2

5,1

1,5

1,4 2,4

Zones Grid points

Soil properties Dry density, ρ = 1.6 Mg.m-3 Drained poisson’s ratio, u = 0.2 Bulk modulus, k =16x103 kN.m-2 Soil constant, mm, M=1.0 Slope of normal consolidation line, λ = 1.0615 Slope of elastic swelling line, k = 0.0784 Pre-consolidation pressure, mpc = 160kN.m-2 Reference pressure, mp1 = 1 kN.m-2 Specific volume at reference pressure, mv_1, vλ=9.3

Grid points

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Surcharges relating to embankment heights of 3, 6 and 9mwere considered, and thesewere modelled as being applied instantaneously. The underlying soil was modelled as aCam-claymaterial,usingthepropertiesshownonFigure9.Mechanicalboundaryconditionscorresponded to fixed ‘x’ and ‘y’ displacements along the base of the grid to simulate aridgedbaseandrollerboundariesalongbothverticalboundariessothatdisplacementsareunrestricted.Asthe lowerboundarywasconsidered impermeable,drainageoccurredonlyatthesoilsurface.Duringcomputationalruntheporewaterpressure,effectivestressanddisplacements inthesoilweremonitoredatthethree locationsshownonFigure9, i.e.atcentre, toe and outside the embankment, with associated grid points 1,8; 11,8 and 16,8respectively.Figure10containsacombinedplotofporewaterpressureandeffectivestressforthethreelocations considered for the simulated embankment at 3 m high (similar plots wereobtained for theotherheights).Themonitoringofporewaterpressureshowedthatovertime the excess pore water pressure generated by the simulated embankment loaddissipated.Asthisoccurred,theeffectivestress increased illustratingthefactthatthesoilgainedstrengthasconsolidationanddrainagetookplace.Thepointatwhichtheeffectivestress line crosses the corresponding pore pressure line, indicates that the soil at thislocation has gained sufficient strength to become stable. Hence, sufficiently strong tosupport the embankment without the need of a basal reinforcing geotextile. As furtherdrainage/consolidationoccurs,anequivalentimprovementinstabilityisachieved.Thetimetakenforthistooccur,i.e.wheneffectivestressequalsporewaterpressure,issummarisedinTable5forthedifferentsimulatedembankmentheights.

Figure10:Porewaterpressureandeffectivestressplotsfora3mhighembankment

FLAC (Version 5.00)

LEGEND

13-Oct-09 14:37 step 11042Flow Time 1.6002E+07 HISTORY PLOT Y-axis : 7 Pore pressure ( 1, 8) 8 Pore pressure ( 11, 8) 9 Pore pressure ( 16, 8)Rev 10 Effective SYY ( 1, 8)Rev 11 Effective SYY ( 11, 8)Rev 12 Effective SYY ( 16, 8) X-axis : 13 Groundwater flow time

2 4 6 8 10 12 14

(10 ) 06

2.000

2.500

3.000

3.500

4.000

4.500

5.000

5.500

(10 ) 04

JOB TITLE : .

Civil Engineering Leeds Met

Thecrossingoftheporepressure&effectivestresslinesindicatesstabilityatthispoint.

Time(s)

Effectivestress&porepressure(N

.m-2)

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Table5:Stabilityresultsforvariousembankmentheights Centre Toe

Height(m)

Time(days)

Effectivestress=porepressure

(kN.m-2)

Timedays

Effectivestress=porepressure

(kN.m-2)3 20 44 14 316 27 65 102 369 30 88 104 480

For the 3 m high embankment, stability is achieved in 20 days at the centre of theembankment, and 14 days at the toe. As embankment height is increased the time forstability also increases. In addition, the time to reach stability at the toe becomes thecontrolling factor for embankments over 3 m. This is due to the pore water migratingthroughtheunderlyingsoiltoextremitiesoftheembankment.Thus,thesoilatthetoeofthe9mtookthegreatestamountoftimetostabilise–beingjustoverthreemonth(i.e.104days).Inreality,someformofsafetyfactorwouldalsobeaccommodatedwithinthedesign.Thiswillresultinthestabilityoftheunderlyingsoftsoiloccurringjustafterthecrossingoftheeffectivestressandporepressurelines(asillustrateddiagrammatically inFigure2).AsnotedinSection2.2,theinstallationoffindrainsintheunderlyingsoftsoilwouldincreasethe rate of pore pressure dissipate. The underlying soil would then take less time tostabilise,permittingthedesignlifeoftheLLGtoberelativelyshort.7. CONCLUSION&RECOMMENDATIONS

As part of this research project a novel directionally structured weft knitted vertical inlaygeotextile structure was manufactured from various vegetable fibres for short-time soilreinforcement applications. This structure was principally design to have the highestpossible strength in one direction, together with providing good soil/geotextile shearinteraction.Intermsof:

• Tensilestrength,thenovelsisalinlayLLGsweredirectlycomparabletoamid-rangegeosynthetic product tested under identical conditions; with strength values ofaround 100 kN.m-1. Also, the novel LLGs were up to six times stronger than thecommercially available woven coir geotextile, currently used for erosion controlapplications.

• Shearresistance,alltheLLGstestedoutperformedthesyntheticgeotextile;offeringbetween20and25%moreshearresistance.

• Durability,coirfibreretainedthehighestdegreeofstrength(i.e.justover80%ofitsinitial strength) when subjected to the worst deterioration environment underconsideration,i.e.cyclesofwetting/drying.

From the simplistic finite difference embankment analysis, it has been shown that thetimescaleforeffectivestressconditionstogovernrangedbetween20–30daysatthecentreoftheembankmentto14–104daysatthetoe,dependingontheheightoftheembankmentunder consideration. Potentially, this gain in strength timescale could be design tocorrespondtothedeclineintensilestrengthofthereinforcingLLG.

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It isrecommendedthatahybridofthenovelLLGs ismanufacturedandtested,containingbothsisalandcoirinlayyarns.Thesisalyarnsprovidinghighinitialstrength,whilstthecoiryarnsprovidinglongevity.Also,aninstrumentedsitetrialtophysicallytesttheperformanceoftheseLLGsinanaturalenvironmentwouldbeextremelybeneficialtoactuallydeterminetheirsuitabilityandperformancein-situ.Althoughvegetablefibreshavealwaysbeenavailablenoonevisualisedtheirpotentialasaformof geotextile until synthetic fibres enabled diverse use and applications for geotextiles toemerge.Thekeytodevelopinggeotextilesfromvegetablefibresistheconceptofdesigningbyfunction,i.e.toidentifythefunctionsandcharacteristicsrequiredtoovercomeagivenproblemand then manufacture the product accordingly. Provided the function can be satisfiedtechnically and economically then these can compete with synthetic materials and in somesituations,theywillhavesuperiorperformancetotheirartificialcounterpartsaswellasbeingfarmoresustainable.REFERENCES

BS 6906:8 (1991)Determination of sand-geotextile frictional behaviour by direct shear.BritishStandardsInstitution,MiltonKeynes.

BS 1051 (1972):Glossary of Terms Relating to the Conditioning and Testing of Textiles.BritishStandardsInstitution,MiltonKeynes.

BSENISO13934–1,(1999)Determinationofmaximumforceandelongationatmaximumforceusingthestripmethod.BritishStandardsInstitution,MiltonKeynes.

BSEN12224(2000)Geotextilesandgeotextile-relatedproducts.Determinationoftheresistancetoweathering.BritishStandardsInstitution,MiltonKeynes.

BSENISO13934–1(1999)Determinationofmaximumforceandelongationatmaximumforceusingthestripmethod.BritishStandardsInstitution,MiltonKeynes.

GB2339803(2002)Directionallystructuredfibregeotextiles.Patent,October.

Gray, T.R.G. & Williams, S.T. (1971) Soil microorganisms. University reviews in botany.Oliver&Boyd,Edinburgh.

Horrocks,A.R.(1996)TheEffectsofStressonGeosyntheticDurability.ProceedingsoftheFirst European Geosynthetics Conference Geosynthetics Applications Design andConstruction,Maastricht,Netherlands,30thSept.to2ndOct.,pp.629–636.

Ingold,T.S.(1994)Geotextilesandgeomembranesmanual.Elsevierscience,Oxford.

Leflaive,E. (1988)Theuseofnatural fibres ingeotextileengineer.Proceedingsof the1stIndian geotextiles conference on reinforced soil and geotextiles, NewDelhi, India,IndianGeotextilesSociety,Bombay,pp.81–84.

Rankilor, P. (1994)UTFGeosyntheticsmanual:A technicalmanual for thedesignofUFTgeosynthetics into civil and marine engineering projects. UCO technical fabrics,Belgium.

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Rankilor, P.R. and Raz, S. (1994a) Geotextiles and knitted structures – III. Man-madeTextilesinIndia,Nov.pp.493–497.

Rankilor,P.R.andRaz,S.(1994b)Geotextilesandknittedstructures–I.Man-madeTextilesinIndia,Sept.,pp.409–412.

Rankilor, P.R. and Raz, S. (1994c) Geotextiles and knitted structures – IV. Man-madeTextilesinIndia,Dec.,pp.531–534.

Rankilor, P.R. and Raz, S. (1994d) Geotextiles and Knitted Structures – V. Man-madeTextilesinIndia,Jan.

Richards,E.A.andScott,J.D.(1986)Stress-strainPropertiesofGeotextiles.ThirdInternationalConferenceonGeotextiles,Vienna,Austria,Vol.III,pp.873–878.