Quantifying the Use of Brush Mats in Reducing Forwarder Peak Loads and ... · PDF...

Originalscietificpaper–Izvorni znanstveni rad

Croat. j. for. eng. 33(2012)2 249

Quantifying the Use of Brush Mats

in Reducing Forwarder Peak Loads and Surface Contact Pressures

Eric R. Labelle, Dirk Jaeger

Abstract – Nacrtak

Forest biomass from timber harvesting residues is often used during mechanized forest operations to improve trafficability of strip roads (machine operating trails). In particular, during cut-to-length operations brush mats from harvesting residues are created on operating trails to reduce rutting. However, forest biomass is becoming increasingly important as a source of renewable energy. To maintain its full calorific value as a biofuel, brush (tree limbs, tops, and foliage) needs to be free of any mineral soil, which is considered a contaminant in this context. In cut-to-length operations, this eliminates any use of brush as a mat to improve trafficability on machine operating trails since it gets in direct contact with mineral soil. Using brush exclusively for biofuel will leave operating trails uncovered and can result in severe damage to forest soils. To manage the two competing uses of brush, it would be helpful to determine minimum brush amounts needed for efficient soil protection as it would potentially allow utilizing remaining brush as biofuel. This study assessed brush mats for their ability to distribute applied loads. As load distributing capacity of a brush mat increases, so does the resulting soil protecting effect. A total of 15 test scenarios were performed with a forwarder to analyze differences in peak loads recorded underneath brush mats of 5, 10, 15, 20, 25, and 30 kg m–2

(green mass) each subjected to 12 traffic cycles of a forwarder including unloaded and loaded movements. Highest loads were recorded within the first few forwarding cycles located on the 5 kg m–2 brush mat and then decreased on average by 23.5% as brush amount increased up to 30 kg m–2. When no brush was used (0 kg m–2) and the forwarder was in direct contact with the steel surface of the load test platform, we noticed that 97% of all peak surface contact pressures recorded exceeded the 150 kPa pressure threshold, compared to only 41% when the forwarder was driven over the 30 kg m–2 brush mat.

Keywords: biomass, brush mats, forest machinery, surface contact pressure, soil protection

1. Introduction – UvodUntilthefrequentuseofheavymachineryinforest

industrystartinginthe1960s,anthropogenicdistur-bancesonforestsoilsduringtimberharvestingopera-tionswerequitelow,bothinfrequencyandmagni-tude,andweremostoften limited to thedamagescausedbyhorsetraffic.Currently,tobeproductive,efficient,andsafe,mechanizedforestoperationsde-pendonheavyequipment toharvest,process,andtransporttrees.Soildisturbancesarepredominantlyassociatedwithin-standtimberextractionprocesseswhenmineralsoilisexposed,compacted,and/ordis-

placedwhilemachinestransporttimberfromthefell-ingsitetoaroadsidelanding.TheCanadianforestindustry applies twomainmechanizedharvestingmethods(cut-to-length,CTL;andfulltree)toharvestandextractwoodefficientlyandsafely.Thegrossmassofmachineryrangesfrom15to40metrictonsandexertsstaticsurfacecontactpressuresof70–180kPa(Kozlowski1999).Thismachineryoperatesdirectlyontheforestfloor,thushavingthepotentialtocausese-veresoildisturbance(Nugentetal.2003).Themostfrequentanddepletingdisturbanceissoilcompaction,whichisdefinedasanincreaseinsoildensity(Craig

E. R. Labelle and D. Jaeger Quantifying the Use of Brush Mats in Reducing Forwarder Peak Loads ... (249–274)

250 Croat. j. for. eng. 33(2012)2

2004).Byincreasingasoil’smechanicalresistancetopenetration,thedensificationprocesscanhaveadirectimpactonplantgrowththroughareductionofairex-changeandinfiltrationrate(ForristallandGessel1955;FroehlichandMcNabb1984;Corns1988).Themecha-nizedCTLharvestingmethod,whichdominatesinAtlanticCanada,usuallyrequiresaharvestertofellandprocesstreesandaforwardertotransportthelogsfromthemachineoperatingtrailstoroadside.WhenapplyingtheCTLmethod,harvestingequipmenttrav-elsontrailstypicallycoveredbyalayerofforestbio-massinformofharvestresidues(limbs,tops,andfoli-ageoftrees)resultingfromtheprocessingoffelledtrees(Fig.1).Thislayerofdebrisactsasaso-calledbrushmat,whichhelpstoimprovetractionandpro-longtrafficability.Thebrushmatdistributesmachineloadsoveragreaterarea,therebyloweringpeakloadsexertedonforestsoilsand,assuch,mitigatessoildis-turbances and related negative impacts on plantgrowth(Wronskietal.1990;BettingerandKellogg1993;RichardsonandMakkonen1994).However,thedesiretoreducecarbonemissionsfromfossilfuels(oilandnaturalgas)incombinationwiththeirhighandvolatilepricehasincreasedthefocusofforeststake-holdersinusingharvestresiduesasasourceofbioen-ergy.Apre-requisiteforanyviablebioenergyoperation

isthatbiomassremainsfreeofcontaminatingmineralsoil,thusmaintainingitsfullcalorificvalue(Eliasson2005).Toavoidsuchcontamination,operatorshavechangedthecommonpracticeofdelimbingtreesinfrontoftheharvestertherebycreatingabrushmatonthe trail todelimbing treeson thesideof the trail,whichleavesthetrailsbareandunprotected(Eliasson

2005).Withtheabsenceofbrush,amachine’ssurfacecontactpressureisdirectlyandfullyexertedtotheground,oftenresultinginincreasesofsoildensityandotherdisturbances.Conversely,brushusedonma-chineoperatingtrailsforsoilprotectioncannotbere-usedforbioenergygenerationduetocontaminants.Inordertooptimizethetwocompetingusesofbrush,knowledgeofminimumquantitiesandqualitiesofbrushforeffectivesoilprotectiononmachineoperat-ing trails is needed. With this knowledge, brushamountsnecessaryforsoilprotectioncouldbeallo-catedandtheremainingbrush,ifany,utilizedasbio-fuelwithoutcompromisingforestsoilintegrityalongmachineoperatingtrails.Thestudyattemptstopro-videnecessaryinformationinthisrespect.

1.1 Research objectives – Ciljevi istraživanjaPreventionofhighsurfacecontactpressuresex-

ertedbyforestmachineryisdiscussedwiththefollow-ingobjectives:1)Quantify(dynamic)forwarderloadstransmitted

belowbrushmatsof5,10,15,20,25,and30kgm–2 (greenmass)comparedtoloadswithoutapplyinganybrushmat(0kgm–2scenario).2)Determineminimumamountofbrushrequired

tolimitthedynamicsurfacecontactpressureactingonthesoiltoacertainthreshold.3)Quantify the impactof repetitive loadingsof

brushmatsontheirabilitytodistributemachineloadsaffectingthesoilbelowthemats.Inthisarticle,allbrushamountspresented,includ-

ingthoseusedasreferencefrompreviousstudies,areexpressedingreenmass.Wealsodefinedaloadingastheimpactassociatedwithatirepassingovertheloadtestarea.

1.2 Impacts of forest machinery on soil physical properties – Utjecaj šumskih vozila na fizikalne značajke tlaMostmachineryusedduringforestoperationsare

operatedinoff-roadconditionsdirectlyontheforestfloor.InCTLoperations,bothharvesterandforward-erregularlysharethesameoff-roadmachineoperat-ingtrails(normally3.5mwideandspacedby15–20mfromadjacent trailcentrelines),whichcoverap-proximately17–23%ofthetotalareatobelogged(Mc-NeelandBallard1992).Thisarea,directlyexposedtomachinetraffic,isoftensubjecttoseveresoilpropertyalterations(densityincreaseand/orsoildisplacementresultinginreducedporespaceandloweredhydrau-licconductivity)dueprimarilytothenormalloadex-ertedbyforestmachinery(150to400kN;Kozlowski

Fig. 1 Forwarder operated over a brush covered machine operating trailSlika 1. Forvarder na traktorskoj vlaci pokrivenoj zastorom granjevine

Quantifying the Use of Brush Mats in Reducing Forwarder Peak Loads ... (249–274) E. R. Labelle and D. Jaeger

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1999).Asasoilisbeingcompacted,soilparticlesaremovedclosertogether,whichreducesairvoids,inpar-ticularmacroporesessentialforplantgrowth(AdamsandFroehlich1984;TaizandZeiger1998;BradyandWeil1999).Thisreorganizationofsoilparticlesusu-allyleadstoincreaseddensity,whichdirectlyincreas-esthesoil’sresistancetopenetrationandlowersinfil-trationrates(Vepraskas1988).Soilcompactionbyaforwarderorskidderintherangeof15–25%comparedtopre-impactlevels,followingasingletrafficcycleisnotuncommonandhasbeenreportedbyVoorheesetal.(1978);BockandVanRees(2002);andLabelleandJaeger(2011).Thehighestincreaseofsoildensityusu-allyoccurswithinthefirstfewtrafficcycles(Froehlich1979;BradfordandPeterson2000;Hanetal. 2006;Syunevetal.2009).Continuationoftrafficfurtherin-creasessoildensitybutatlowerrates(LockabyandVidrine1984;Froehlichetal.1986;BraisandCamiré1998;Jamshidietal.2008;Hanetal.2009;LabelleandJaeger2011).Theeffectofmachineinducedsoildis-turbancemayalsopersistformanyyears.Residualforestsoilcompactionhasbeenidentifiedforperiodsranging from15 (Froehlichet al. 1986) to 55years(Power1974)onsitesintemperateclimatezoneswith-outwinterfrost.FiveyearsaftermechanizedCTLop-erationsonsoilsexposedtofrequentfreeze/thawcy-clesinNewBrunswick,Canada,LabelleandJaeger(2011)reportednosignofsoilbulkdensitynaturalrehabilitation.AtechniquecommonlyusedinCTLoperationsto

avoidruttingonmachineoperatingtrailsandtokeepthemoperationalduringhightrafficfrequenciesistofollowthecorduroyroadconstructionprincipleandreinforcethebearingcapacityofthetrailsbyhavingtheharvestercreateabrushlayeroflimbs,branches,andtreetops(McDonaldandSeixas1997).

1.3 Effects of brush on reducing soil compaction Djelovanje zastora granjevine na smanjenje zbijanja tlaTheCTLmethodwasdeveloped in theNordic

countriesintheearly1970s(Vidrineetal.1999).Theinnovationofthismethodwastheimmediateprocess-ingoffelledtreeson-site,directlyinthestand.Sinceitsbeginning,treeshavebeendelimbed,buckedtoap-propriatelength,andtoppedonthemachineoperat-ingtrail.Thison-traildelimbingprocessstartedduetoconveniencefortheoperatortoprocesstreesinfrontof theharvesterrather thanonthesideof the trailwhereboomoperatingspacemightberestrictedbyresidualtrees.Operatingforestmachineryonabrushmatprovedtobebeneficialinloweringrutdepthbyincreasingfloatability,whichalsohelpedtoextendthe

windowofoperationsintoseasonswithincreasedsoilmoisturecontent(TuftsandBrinker1993;McMahonandEvanson1994;EliassonandWästerlund2007).Sincetheearly1980sbrushhasbeenstudiedforits

potentialtoreducesoildisturbances.First,JakobsenandMoore(1981)investigatedtheimpactofbrushonsoil resistance topenetration (soil impedance) andhavebeenfollowedbyMcMahonandEvanson(1994);McDonaldandSeixas(1997);Hanetal.(2006);andHanetal.(2009).Thesestudiesfoundthatbrushmatsusedonmachineoperatingtrailshadabeneficialim-pactonmitigatingtheincreaseofsoilresistancetopenetration,inparticularwhenthematsconsistedofathickbrushlayerofapproximately20kgm–2.Ad-ditionally,brushmatsplacedontopofsoilswithhighmoisturecontenthelpedtoreducetheincreaseofpen-etrationresistancebyforwardertraffictoalargerex-tentcomparedtobrushmatsappliedondryersoils(McDonaldandSeixas1997;Hanetal.2006).Theaforementionedstudiesareimportantcontri-

butions inunderstanding thesignificanceofbrushmatsonmaintainingsoilintegritybutspecificeffectsdependbothonsiteandmachinecharacteristics.Duetothehighvariabilityofsoilconditionsatharvestingsites(soiltexture,soildepth,soilmoisturecontent,andorganicmattercontent),combinedwithvaryingchar-acteristicsofbrush,itappearsdifficulttopredictthegeneralreinforcingeffectthatbrushmighthaveonsoilbearingcapacity.Ideally,itwouldbepreferabletotesttheeffectsofvariousbrushamountsonanarrayofsoiltypes,soilmoisturecontents,andmachineconfigura-tions,butcurrentbudget,equipment,andlogisticre-strictionsdidnotallowforthesekindsoftests.How-ever,determiningtheeffectofbrushmatsofvaryingquantityandqualityonalterationofmachine loaddistributionbelowbrushmatsuncoupledofsoilprop-ertyvariationisoneapproachtoenhanceknowledgeaboutbrushmatcapabilitiesindependentofsitechar-acteristics.Shiftingtheuseofbrushawayfromsoilprotection,inparticularwithitsnewcompetinguseasasourceofbiofuel,couldprovecostlyconsideringtheassociatedrisksofreducedplantgrowthandtheslownaturalrecoveryrateofcompactedsoils.

1.4 Functions of a brush mat – Svrha zastora granjevineDuringtheprocessingphaseofCTLforestopera-

tions,branchesplaceddirectlyonthegroundinfrontofforestmachinescanprovideseveralkeyfunctionstoprotectforestsoils.Havingbranchesintertwinedwithinabrushmatincreasesfrictionandcreatesare-inforcedsurface,whichcanexpandthecontactareabetweenmachinerunninggearandtheforestsoil.At


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thebroadestlevel,thisincreasedcontactareashouldreduceaverageloadsperunitareabelowtheentirebrushmat.Thisreductionofaverageloadsisattrib-utedtoreducedpeakloadsdirectlylocatedunder-neathtiresoftheequipmentandloadtransfersinex-tendedareasadjacenttotheloadingimpactinbothlateralandlongitudinaldirections.

2. Methodology – Metode rada

2.1 Testing device – Naprava za istraživanjeTomeasureandrecorddynamicloadsexertedby

forestmachines,aprototypeloadtestplatformwasdesignedandconstructed.Thisplatformallowedfor

full-scaletestsofloadedforwardersdrivingovervar-iousbrushmatstoanalyzeloaddistributionbelowthemats.Theentireloadtestplatformwascomposedofthreeseparatesections:ramps,in-andout-feed,andtheloadtestareaitself(Fig.2A).Theprincipalpartofthestructurewastheloadtestareameasuring4.1mby2.5mforatotalareaof10.3m2andequippedwith24highcapacity(450kN)compressionloadcells,eachabletomeasureindependentloadsona30.5cmby30.5cmresolution.Thisresolutioncorrespondstothesizeofloadingplates,amatrixof104steelplatesform-inganengineeredsurfacebelowwhichloadcellswerepositioned.Thissectionwasconstructedandmountedtoaconcretebase.In-andout-feedsectionswerebuiltatthesameheight(19cm)astheloadtestareatoper-

Fig. 2A Three main sections of load test platform including in- and out-feed sections and load test area and 2B Forwarder traffic over the brush covered load test areaSlika 2A. Tri glavna dijela ispitne platforme (ulazni dio, mjerni dio, izlazni dio). 2B. Kretanje forvardera po mjernom dijelu platforme pokrivenom zastorom granjevine


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mittestingoftheforwarderinalevelposition(zeropercentgradient),thusavoidingpotentialwheelslipandachangeinmachinecentreofgravity,whilealsominimizingvariationsinbrushmatcompressibility.Furthermore,eachin-andout-feedsectionwasofsuf-ficientlengthtoallowthefullwheel-baseofthefor-wardertocompletelypasstheloadtestarea,akeydesignfeatureenablingthedistinctionbetweendis-creteloadingevents.Dependingontherequiredloadresolution,load

cellscouldbeplacedindifferentarrangements,inso-calledtestlayouts,withintheplatform.Twodifferenttestlayouts(clusteredandtransect)wereusedduringtesting.Tospecificallyquantifytheimpactofthefor-warder,the24loadcellswerefirstpositionedinaclus-teredtestlayout(fourclustersofsixloadcellseacharranged in two adjacent rows of three load cellswide),directlylocatedinforwardertracks(Fig.3).This

testlayoutofferedthehighestresolutiontocapturemachinefootprintswithatotalactivearea(91.5cmby61.0cm)percluster.Inadditiontoobtainingahighloadresolutionunderneathmachinetires,itwasalsoofinteresttounderstandhowthebrushcoulddistrib-uteapplied loadings laterally.Therefore, loadcellswerealsoinstalledinatransectlayoutontwoadjacentrowsthroughoutthefullwidthoftheloadtestarea(12loadcellswide;Fig.3).

2.2 Sampling procedure – Način uzorkovanja

2.2.1 Control parameters – Nadzor parametaraToestablishcontrolparameters,theTimbcofor-

warder(specificsinsection2.3)wasfirstdrivenun-loadedoverthebareloadtestplatform(withoutbrushcoverusingclusteredtestlayout).Theresultingdy-namicloadsfromeachtireweremeasuredbytheloadcells,enabledbya25inputchannelSystem5000scan-

Fig. 3 Top view schematic of test layouts used during testing. Black circles represent active load cells. Virtual active zones of the clustered test layout (four groups of six load cells) are identified by diagonal lines, while virtual active zones of the transect test layout (two full-width adjacent rows) are shown by horizontal linesSlika 3. Tlocrt skice korištenoga ispitnoga rasporeda. Crni krugovi prikazuju dinamometre. Mjerna područja grupimičnoga ispitnoga rasporeda (četiri grupe po šest dinamometara) prikazana su kosim crtama, a poprečni je mjerni raspored (cijelom širinom, jedan do drugoga dva reda dinamometara) prikazan ravnim crtama.


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ner,andstoredinaStrainSmartdataacquisitionsys-tem (VishayMicro-Measurements 2011). For dataanalysis,loads(kN)recordedbyeachloadcellwerealsoconvertedtosurfacecontactpressure(kPa)byrelatingthemeasuredloadtothesurfaceareaofaso-calledvirtualactivezone(930.3cm2,consistingoffourquartersoffouradjoiningloadingplates;(Fig.3).Thecontroltestwasreplicatedoncewiththeforwarderdrivenovertheplatforminthesamepositiontoverifytheaccuracyandprecisionoftheloadrecordingsys-tem.Thesameprocedurewasthenrepeatedwiththeforwarderloadedwith6,680kgofdrylogs.Inordertoavoidweightfluctuationsoftheloadedlogsduringthefourweeklongtestingperiod,drylogswithrela-tivelystablemoisturecontentwerechosen.Asaresult,weonlyhadapartialloadcomparedtothelogbunk’sfullcapacityof20metrictons.

2.2.2 Brush mat construction and forwarder traffic Izrada zastora granjevine i kretanje forvarderaAftercontrolparameterswereassessed,actualtest-

ingwithbrushofvaryingquantityandqualitywasperformed.Freshsoftwoodbrush(balsamfirAbies balsamea (L.)Mill. and black spruce Picea mariana (Mill.) B.S.P.) collected fromon-goingmechanizedCTL clear felling operations in Fredericton, NewBrunswick,Canadawasstoredinsidetheunheatedstoragehalltoreduceairdrying,whileprotectingitfromprecipitation.Priortoanybrushamounttest,branchesusedtocreatethebrushmatswerecharacter-ized individually by specie, diameter, and length.Asidefromspeciesidentification,brancheswereas-signedtooneoffourdiameterclasses(x≤10mm,10mm<x<30mm,30mm≤x≤60mm,andx>60mm)andtooneoffivelengthclasses(y≤1m,1m<y <2m,2m≤y≤3m,3m<y<4m,andy≥4m).Followingclassification,brancheswereweighed

withadigitalscaleandplacedperpendiculartothedirectionoftravelontheplatformtosimulatebranchpositioningofin-wooddelimbingbyaharvesteruntilthetargetbrushamountwasreached(McMahonandEvanson1994;Woodetal.2003;Hanetal.2006;Akayetal.2007).Ineachtest,brushsamples(approx.150g)werecollectedrandomlyfromthebrush(branchesandneedles)todeterminebranchmoisturecontentbyovendryingthematerialat105°Cuntilconstantmasswasreached.Priortoanyforwardertraffic,thethick-nessofeachbrushmatwasassessed.Sincethetopboundaryofthematwasdifficulttodeterminebe-causeofitsirregularshapedsurface,astandardmea-suringmethodwasdevelopedbyplacingaknownandconstantloadonameasuringboardontopofthemat.Thethicknessofthematwasthendeterminedbymeasuringtheverticaldistancebetweenthebottomof

themeasuringboardandthesurfaceofthesteelloadtestarea.Brushthicknesswasrecordedatsixloca-tions,fourpositionedintrackareasandtwoinout-side-trackareasbeforetrafficaswellasaftertwo,six,and12forwardingcycles(acycleiscomprisedoftwoforwarderpassesovertheloadtestplatform)toquan-tifybrushmatcompressibilityafterrepetitiveload-ings.Onceabrushmatwascompletedanditspre-im-

pactthicknessrecorded,allloadcellswerezeroedandforwardertrafficcommenced.Duetospacelimitationsatthetestingsite,theunloadedforwarderwasdrivenbackwards(ataspeedof1.5kmh–1)intothehalloverrampsandin-feedsectionontotheloadtestareaandfurtherontotheout-feedsectionuntilthefrontbogieaxlepassedtheloadtestarea(Fig.2B).Afterashortinterval,theforwarderwasdrivenfromtheout-feedsectionat thesamespeed ina forwardmovement,again,ontotheloadtestareaandoverthein-feedsec-tionwithrampsoutsidethehall.Duringthesema-chinepasses,all24loadcellswereactivatedtorecorddynamicloads.Afterwards,theforwarderwasloadedusingthesameloadasincontroltests(6,680kg)anddrivenovertheplatforminthesamepatternasex-plained previously. These two unloaded and twoloadedpassesoverthebrushmatconstitutedtwofor-wardingcycles.Similartoin-standforestoperations,where thecombinationofoneunloadedforwarderpassgoingintothecutblocktogatherwoodandoneloadedpassastheforwarderreturnstothelandingconstitutesoneforwardingcycle,wealsoconsideredoneunloadedandoneloadedpassovertheplatformasonecompletecycle.However,foreasinessofopera-tionwealwayspairedtheinwardandoutwardmove-mentsoftheunloadedandloadedforwarder.Atotalof12forwardingcyclesweretestedtodeterminethecapacityofthebrushmattoattenuatesurfacecontactpressureoverrepetitiveloadings.Whenalltrafficfrequencieswerecompletedona

specificbrushmat(combinationof12unloadedand12loadedpasses),theforwarderwasdrivenoutsidethehall,theplatformwasclearedofthecompressedbrush,andanewbrushmatwascreatedfromfreshbrushforthenexttest.Replacingbrushbetweentestswasessentialsincethepropertiesofbranches(com-pressibility,yieldpoint, tensile strength, etc.)werelikelyalteredbytheeffectofmachineloadings.Eachtestingscenariowithoneofthesixbrushamounts(5,10,15,20,25,and30kgm–2)overtheclusteredtestlayoutwasreplicatedtwice(threetestsintotal)toin-creasestatisticalpowerofanalyses.Followingthesetests,loadcellswerere-positionedintothetransecttestlayoutcoveringthefullwidthoftheloadtestareato


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assesstheabilityofbrushmatstodistributeloadslat-erally.Withthistransectlayout,threebrushamounts(10,20,and30kgm–2)weretesteddirectlyoverthesteelloadtestarea.Replicateswereomittedforthisanalysissincethesetestsweremainlyconductedtoverifytheoperationandaccuracyoftheloadcellsintheirnewpositionspriortotestingbrushmatsonthesandcovered loadtestarea.After testingthe threebrushmatswiththetransectlayout,brushmatsof10,20,and30kgm–2(eachbrushamountreplicatedtwice)weretestedontopofa20cmthicklayerofsandplacedontheloadtestareatotesttheloaddistributingcapa-bilitiesofbrushmatswhenappliedonaflexiblesur-face.Priortoanytests,thesandlayerlocatedontheloadtestareawascompactedusingaplatecompactorforthreeminutes.Followingthecompactionphase,weplacedabrushmatontopofthesandlayer.Beforeanytraffickingcommenced,allloadcellswerezeroedtoaccountfortheaddedmassofeitherthebrushmat(inclusteredlayoutoversteelloadtestarea)ormassofsandandbrushmatforalltestsperformedoverthesandcoveredloadtestarea.Onceloadcellswereze-roed,theforwarderwasdrivenoverthematinthesame sequence as for testswithout sand layerde-scribedbefore.Onceallforwardertrafficcycleshadbeencompletedforarespectivescenario,brushwasremovedfromtheloadtestareaanddiscarded.Sandwasthenloosenedwithashovelandre-compactedwiththeplatecompactorbeforetestingthenextbrushmatamount.Lastly,duringtestingofbrushmatsce-nariosoversand,wecollectedsandsamplestoobtaingravimetricmoisturecontentbyovendryingthema-terialat105°Cuntilconstantmasswasreached.Theapproachofdividingapeakloadfromasingle

loadcellbythesurfaceareaofaloadingplatetoderivesurfacecontactpressure(kPaorkNm–2)mentionedpreviouslywasstraightforward.However,thepres-enceofbrushandthesandlayerovertheloadtestareamadeitmoredifficulttomaintainconstantpositioningoftheforwarderinrelationtotheplatformduringtraf-fic.Todifferentiaterecordedloadsaffectedbytheloaddistributingcapabilityofvaryingbrushamountsfromthoseloadssimplyaffectedbymachinepositioning,wheretheloadwasdistributedbetweenadjacentloadcells,wesummedthepeakloadandthesecondhigh-estload(obtainedfromanadjacentloadcell).

2.3 Forwarder specifications – Značajke forvarderaA2000TimbcoTF820-Dforwarderwithataremass

of23,500kgandaloadcapacityof20,000kgwasusedforall tests.Thiseight-wheel (28L-26 tiresize) for-warderhadtwoindependentbogieaxles.TwoOlofs-

forssteelflexibletrackswithwideningplates,weigh-ing1,100kgeach,wereinstalledontherearbogieaxleduringalltestscenarios.Crossmembers(10cmwide)ofthesesteelflexibletrackswerespacedby25cm.Thefrontbogieaxleremainedwithoutsteelflexibletracksduringtestingforbettermanoeuvrabilityovertheloadtestareaoftheplatformandassociatedin-/out-feedsections.AccordingtothePASCALsoftware(FPIn-novations’groundpressurecalculator),nominalsur-facecontactpressureunderneaththefrontrubbertiredbogieaxlewas65kPaloadedand35kPabelowtherear loaded bogie axle equipped with tracks(Makkonen2007).Thesenominalsurfacecontactpres-suresarebasedona6,680kgload.Tireinflationpres-sureinalleighttiresremainedconstantat157kPathroughouttheexperiment.Unlikethemachinemove-ments during regular in-stand forest operations.wheretheboomoftheTimbcoisarticulatedinanup-rightpositioninfrontofthemachine,theboomwasstraightandfullyextendedinfrontoftheforwarderduringall tests toaccommodate the limitedheightclearanceinsidethetestingfacility.

2.4 Statistical analyses – Statistička analizaStatistical analyseswereperformedwith SPSS®

(SPSS2007)andMinitab®(MinitabInc.2010)statisticalsoftware.Dependantvariableswereanytypeofloadreadings(peakloads,sumofpeakandsecondhighestloads,thirdhighestloads)obtaineddirectlyfromtheloadcellsorpeaksurfacecontactpressuresonceloadswererelatedtothesurfaceareaofaloadingplate.Todetermine the impact of an independent variable(brushamount,logbunkloadstatus,forwardertrafficfrequency,etc.)onthechosendependantvariable,aseriesofone-wayanalysisofvariance(ANOVA)testswereperformedandaprobabilitylevelof0.05waschosenduringallstatisticaltests.

3. Results – RezultatiThissectionisstructuredaccordingtotestingsce-

nariosandwillfirstfocusonresultsobtainedwhentheforwarderwasdrivendirectlyoverthesteelloadtestarea,withoutbrushmats.Theseinitialanalyseswilladdress:1)peakloads,2)therelationshipbetweenpeakandsecondhighestloads,3)sumofpeakandsecondhighestloads,and4)theeffectofsandonstresspropagation.Afterwards,generalbrushmatcharac-teristicswillbepresented, followedbyanalysesofpeakloads,sumofpeakandsecondhighestloads,anddynamicsurfacecontactpressures recordedbelowvaryingbrushamountswhenexposedtodifferentfor-wardertrafficfrequencies.


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Fig. 4 Peak load frequency distributions for all three test scenarios (no brush). Numbers shown inside dotted lines indicate the percentage of loads within the respective 10 kN load class and the curves represents a normal distributionSlika 4. Raspodjele učestalosti vršnih opterećenja za sve tri inačice istraživanja, bez zastora granjevine. Brojevi unutar isprekidanih crta ozna-čuju udjele opterećenja unutar pojedinih razreda (raspona 10 kN), a krivulje predstavljaju normalne distribucije

3.1 Effects of machine related variables on load distribution (no brush) – Djelovanje varijabli vozila na raspodjelu opterećenja (bez zastora granjevine)

3.1.1 Peak loads – Vršna opterećenjaInthefollowingsection,allpeakloadspresented

wererecordedwhentheforwarderwasdrivendirectlyovertheloadtestarea,withoutanybrushmat.Toassess

recordedloads,wefirstanalyzedpeakloadfrequencydistributionperscenario(clusteredlayoutoversteel,transectlayoutoversteel,andtransectlayoutoversand)andpresentedtheseresultsinFig.4.Inadditiontothefrequencydistribution,wealsodeterminedthepercent-ageofpeakloadswithinfour10kNloadclasses(num-bersbetweendashedlines)andincludednormaldistri-butioncurvesasreference.


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Peakloadsvariedbetween7kN(1observation)and32kN(1observation)fortheclusteredtestlayout(Fig.4A).Mostofthepeakloads(91%)werelocatedbetween10and30kNandonly9%werebelow10kN.Peakloadsrecordedfromthetransecttestlayoutonsteelvariedfrom7kN(1observation)to35kN(2ob-servations).Withthistestlayout,5%ofpeakloads(10observations)exceeded30kNwhereas<1%wereex-ceedingthisloadduringtheclusteredtestlayout.Thepercentageofpeakloadslocatedbetween10and20kN(90%)wasalmost identical to theoneobtainedfromtheclusteredtestlayout.Resultsweredifferentwiththetransectlayoutover

sandscenario.Withtheadditionof thesandlayer,peakloadsvariedfrom5kN(2observations)to28kN(3observations)and27%(35observations)ofpeakloadswerebelow10kN.Thehigherpercentageofpeakloadslocatedbelow10kNincombinationwithnoloadsexceeding30kNclearlydemonstratetheef-fectoftheaddedsandlayerindistributingappliedloads.

3.1.2 Relationship between peak and second highest loads – Odnos između vršnoga i drugoga najvećega opterećenjaAstheareaofcontactunderneathsingletiresor

tracksoftheforwarderwasalwaysexceedingthesur-faceareaofasingleloadingplateandsincemachinelocationinrelationtotheloadtestareahadthepoten-tialtovarybetweenloadingevents,itwasnecessarytoconsidertheloadsofadjacentloadingplatesandoftherelatedloadcellsduringtheanalyses.Simplyduetomachinepositioning,thewheelloadexertedbytheforwardercouldeitherberecordedbyasingleloadcellwitharelativelylowloadonanadjacentloadcellinthecasewheremostoftheloadwaslocateddirect-lyaboveasingleloadcelloritcouldbesharedmoreequallybetweentwoormoreadjacentloadcells.Thisvariationmadeitdifficultwhenanalyzingpeakloadsofsinglecellstodifferentiatebetweentheeffectofma-chinepositioningandtheeffectofbrushmatsontherecordedloads.Therefore,wefirstanalyzedtherela-tiveloaddistributionbetweenadjacentloadcellstoidentifypotentialsimilarpatternbetweentestingsce-narioswithoutbrush.Thiswasdonebyfirstidentify-ingthehighestrecordedloadduringapassofatireortrackedtireandthenidentifyingthesecondhighestloadofanadjacentloadcell.Inasecondstep,thesec-ondhighestpeakloadwasdividedbythepeakloadtoderivetherelativemagnitudeofthesecondhighestloadinpercentofthepeakload.Fig.5showsthefrequencydistributionoftherela-

tivesecondhighestrecordedloadsforallthreetest

layouts.Ifhighfrequenciesoftherelativesecondhigh-estloadshadbeenlocatedinthelowerorupperlimitsof the abscissa, itwould indicate relatively loworequallyhighsecondhighestloadscomparedtopeakloads,respectively.Thiscouldalsoindicatelowvaria-tionofmachinepositioningbetweendifferenttests.Itwouldhavethenbeenappropriatetoconcentratetheanalysesmainlyonpeakloadssinceeithertheloadexerted to the second loadcell is relatively loworsimilartothepeakload.Resultsfromtheclusteredlayoutshowedalower

percentageofsecondhighestloadstopeakloadsinthe<20%classandahigherpercentageintheupper>80%classcomparedtothetransectlayoutoversteelscenario (Figs.5AandB).This indicates that loadsweresharedmoreequallyduringscenario1thantheywereinscenario2.Whentheforwarderwasoperateddirectlyoverthesandcoveredloadtestarea,only15%ofloadswerelocatedbelow40%andthenumberofobservationslocatedbetween80–100%increasedupto31%(Fig.5C)againshowingthatthesandlayerwasabletodistributeloadsmoreeffectivelybetweentwoadjacentloadingplates.However,sincethemajorityofthesecondhighestloads(approx.>70%)waswith-inthe20to80%rangeofthepeakloads,usingonlypeak loads or conversely secondhighest loads foranalyzingtheeffectsofvaryingbrushmatswouldnothavebeensufficient.For thisreason, the followinganalyseswillmostlyfocusonthesumofpeakandsecondhighestloadssincesinglepeaksmaybebiasedbymachinepositioning,whichcouldhidetheeffectdifferentbrushmatshaveonloaddistribution.

3.1.3 Sum of peak and second highest loads – Zbroj vršnoga i drugoga najvećega opterećenjaLoadsexertedbyaforwarderaredirectlyrelated

tothemassactingontherunninggear.Consequently,quantifying loads fromunloadedand loadedfrontandrearsingletiresandseparatingthemperaxlewasessentialindeterminingaccurateloads.Combiningloadsfromfrontandreartirescouldpotentiallyover-shadowtrendsthatmightbepresent.Whenindividu-alizingloadsbylogbunkloadstatus(unloadedandloaded),wenoticedastatisticaldifference(p=0.001)ofthemeansumofpeakandsecondhighestloadsbetweenunloadedandloadedreartires(Fig.6).Meansumofpeakandsecondhighestloadsobtainedfromsingletiresoftherearunloadedaxlewere16.2,16.1,and12.5kNandincreasedto28.9,29.1,and20.6kNwhenloadedforFigs.6A,B,andC,respectively.Thesesumofpeakandsecondhighestloadsweretherefore78.4,80.2,and64.6%higherforloadedthanunloadedscenariosforFigs.6A,B,andC,respectively.However,asthemajorityoftheloadonthelogbunkisdistrib-


258 Croat. j. for. eng. 33(2012)2

Fig.5 Percent of second highest load to peak load frequency distributions. Numbers shown inside dotted lines indicate the percentage of loads within the respective percentage classSlika 5. Raspodjela učestalosti udjela drugoga najvećega opterećenja u odnosu na vršno opterećenje. Brojevi unutar isprekidanih crta ozna-čuju udjele opterećenja unutar pojedinih postotnih razreda

utedtotherearbogieaxle,tiresfromthefrontaxleshowednostatisticaldifference(p=0.305)betweenthemeansumofpeakandsecondhighestloadsrecordedfromunloadedandloadedtestscenarios.Meansumof peak and secondhighest loads from front tiresshowedonlyadifferenceof0.3to1.2kN,(0.5to3.3%)betweentheunloadedandloadedtestscenarios.

Apriori,wewereexpectinghighestloadstobere-cordedunderneaththerearloadedtires.However,themodest load of 6,680 kg on the log bunkwas notenoughtoexertloadsequaltothoserecordedunder-neaththefronttires,whicharelocatedinproximitytotheheavyboom,hydraulicrotarysystem,andtheop-eratorcabin.Nevertheless,ouranalysesmainly fo-


Croat. j. for. eng. 33(2012)2 259

cusedontheloadsexertedbythereartiresbecausemostforwarderswithrearbogieaxlesareequippedwithsteelflexibletracks.Thedifferenceinloadsofthereartiresbetweenloadingregimes(unloadedvs.load-ed)waslesspronouncedforthetestsperformedoverthesandcoveredloadtestareaandreasonsforthiswillbeexplainedinthediscussion.Afterfurtheranal-ysis,wealsodeterminedthatloadsexertedbysinglerearaxletiresweresimilarbetweenthetwomachinesides,thusnotrequiringmachinesidestobeseparatedinupcominganalyses.

3.2 Effect of the sand layer on stress propagation Djelovanje sloja pijeska na rasprostiranje naprezanjaThusfar,meanloadsrecordedamongstdifferent

testlayoutshavebeenratherdifficulttocompareduetotheaddedsandlayer,whichinitselfreducedloadsbyplacingfurtherdistancebetweentheactualloadandtheloadcells.Sinceloadcellswerezeroedpriortoanytest,theverticalstressordeadloadcausedbythesanddidnotimpacttheloadreadingsrecordedbytheloadcells.However,becauseofshearingresistancewithinthesandlayer,appliedloadstothesandmasswerespreadlaterallywithincreasingdepthfromtheareaofapplication.Inshort,asdepthincreases,theareaoverwhichnewstressesoccurwillincreasebutthemagnitudeofthestresseswilldecrease(McCarthy2002).BasedonBoussinesq’s(1885)equationforstress

propagation,Steinbrenner(1936)developedinfluencecoefficients(Id)forcalculatingtheverticalstressdistri-butioninsoilscausedbyarectangularloadedareausingtheloadperunitarea(q)andtheverticalstressincrease(Δσz)resultingatacertaindepth(z)fromaloadedareaoflength(L)andwidth(B)asobtainedbyequation1.

Δσz = q · Id (1)

where:

, ,

Basedonequation1,wecalculatedthatonaload-edareaof0.195m2(L=0.273m,B=0.714m),the20cmlayerofsand(z=0.20m)addedontopofthesteelloadtestarealoweredappliedpressuresby34%comparedtopressuresrecordeddirectlybelowthetire(FangandDaniels2006).Theloadedareadescribedaboverelatestothecontactareabelowa28L-26Firestonetireas

equippedon theTimbcoforwarderandassumesa7.6cmverticalpenetrationwithinthesandlayer(Fire-stone2010).Forexample,awheelloadof30kNap-plied to a 28L-26 tire (same contact area as statedabove),wouldequalanominalgroundpressureof153.9kPaorkNm–2directlybelowthetire.Atadepthof20cm(verticaldistancefromthetire’scontactareatothelocationwherethestressiscalculated)thepres-surelocatedunderneaththetirewouldbereducedby34%toequal101.6kPaorkNm–2.ItisnoteworthytomentionthatBoussinesq’s(1885)stresspropagationtheorem,basedonelastictheory,isonlyanapproxi-mationsinceitwasderivedforhomogeneous,isotro-picmaterialsofsemi-infiniteextent.Steinbrenner’s(1936)influencecoefficientalsoassumesuniformloaddistribution below the loaded area (Liu andEvett1992).Themineralsandusedasacoveringlayerontopoftheloadtestareawasrelativelyhomogeneous(80%ofparticleswerewithin0.3mm≤x≤2.0mmandaveragemoisturecontentwas6.7%)butwaslocatedontopofarigidsurface,thusnothavingthesamepropertiesinalldirections.Unlikethebasicassump-tionofequation1,loaddistributionbelowaforward-ertireisnotuniformandishighlyinfluencedbytheoperatingsurface.Theseconditionslikelyaffectedthepercentreductiondescribedabove.

3.3 General brush mat characteristics – Glavne značajke zastora granjevineA total of 5,511branches for a combinedgreen

massof5,550kgwerecharacterizedbyspeciesthatcreatedthebrushmatsrequiredforalltestsscenarios.Besidesdifferingbetweenspecies,wealsocategorizedbranchesindiameterandlengthclasses.Weattemptedtocomposebrushmatsusingsimilardegreesofbranchsizeandtreetops.Thehighestfrequencyofbranchdiameter, regardless of brush amount,waswithinclasstwo(1to3cmdiameter;Fig.7).Over80%ofbranchesusedforallbrushmatswerewithindiameterclassoneandtwoandamaximumof4%wereinthediameterclassfour(>6cm).Whenaveragingallbrushmatreplicatesofthesameamount,sevenoutof12matsshowedahigherfrequencydistributioninbranchlengthclasstwo(1to2m),whiletheotherfivebrushmatshadahigherdistributioninlengthclassone.Thepercentageofbranchesgreaterthan3minlengthusedduringtestingwasquitelowandonlyvariedbetween0and4%oftotalbranchesused.Branchandneedlemoisturecontentwasalsomon-

itoredforeachbrushmatandvariedbetween45.3and55.2%ofthegreenmass.Ingeneral,brushmatsuseddirectlyoverthesteelloadtestareahadanaveragemoisture content of 52.1% compared to 48.8% for


260 Croat. j. for. eng. 33(2012)2

Fig. 6 Mean sum of peak and second highest loads presented by single tire and log bunk status directly over load test area (no brush). A different letter indicates a significant difference based on a one-way ANOVA test at the 0.05 probability levelSlika 6. Sredina zbroja vršnoga i drugoga najvećega opterećenja prikazanoga za pojedine kotače i (ne)utovarenost vozila neposredno na ispit-nom dijelu platforme bez zastora granjevine. Različita slova označuju značajnost razlike na osnovi jednostruke analize varijance za razinu vjero-jatnosti od 5 %

brushmatsusedoverthesandcoveredloadtestarea.Explanationsforthisslightvariationwillbepresentedinthediscussion.Oncethetargetamountofbrushmatwasreached

(e.g.20kgm–2)andbrancheswereplacedontheload

testarea,brushthicknessmeasurementswererecord-edpreimpactandalsofollowingtwo,six,and12for-wardingcyclesinbothtrack-andoutside-trackloca-tions.Thethicknessofallbrushmatspreimpactwassimilar for both track and outside-track areas and


Croat. j. for. eng. 33(2012)2 261

rangedfrom9cmforthe5kgm–2brushmatto70cmforthe30kgm–2brushmatinthetrackareasandfrom9cmforthe5kgm–2brushmatto74cmforthe30kg

m–2brushmatforoutside-trackareas(Fig.8).Logi-cally,brushmatthicknessincreasedwithincreasingbrushamount.Thelargestdecreaseinthicknesswas

Fig. 7 Branch diameter and length class frequency distributions. Each figure represents the total frequency for all replicates. Numbers shown inside dotted lines indicate the percentage of branches within the respective branch diameter or length class (rear loaded axle only). Branches were assigned to one out of four diameter classes 1 (x ≤ 10 mm), 2 (10 mm < x < 30 mm), 3 (30 mm ≤ x ≤ 60 mm), and 4 (x > 60 mm) and to one out of five length classes 1 (y ≤ 1 m), 2 (1 m < y < 2 m), 3 (2 m ≤ y ≤ 3 m), 4 (3 m < y < 4 m), and 5 (y ≥ 4 m)Slika 7. Raspodjele učestalosti razreda promjera i duljina grana. Svaki grafikon prikazuje ukupnu učestalost svih ponavljanja. Brojevi unutar isprekidanih crta označuju udjele unutar pojedinoga razreda promjera i duljina grana (samo opterećena stražnja osovina). Grane su pridružene u jedan od četiriju razreda promjera 1 (x ≤ 10 mm), 2 (10 mm < x < 30 mm), 3 (30 mm ≤ x ≤ 60 mm) i 4 (x > 60 mm) te jedan od pet du-ljinskih razreda 1 (y ≤ 1 m), 2 (1 m < y < 2 m), 3 (2 m ≤ y ≤ 3 m), 4 (3 m < y < 4 m) i 5 (y ≥ 4 m)


262 Croat. j. for. eng. 33(2012)2

measuredbetweenpreimpactandfollowingthefirsttwoforwardingcycles.Beyondthispoint,afurtherincreaseinloadingfrequencypertestseemedtoonlyslightlyreducebrushmatthickness.Regardlessofini-tialbrushmatamount,themeanbrushmatthicknessintracklocationswasreducedto<10cmfollowing12forwardingcycles.

3.4 Effects of brush on machine load distribution Djelovanje zastora granjevine na raspodjelu opterećenja vozila

3.4.1 Peak loads – Vršna opterećenjaFollowinganassessmentofmachinerelatedvari-

ablesonloadsexertedbytheforwarder,wefocused

Fig. 8 Mean brush mat thickness measured over repetitive loadings at two locations (track and outside track) on the load test area for each brush amountSlika 8. Srednja debljina zastora granjevine mjerena nakon višekratnih opterećenja na dva mjesta (u tragu i izvan traga) ispitne platforme za svaku gustoću zastora granjevine


Croat. j. for. eng. 33(2012)2 263

onanalyzingtheeffectofbrushquantityonmachineloaddistribution.Asafirstanalysis,peakloadfre-quencydistributionswereassessedperbrushamountandscenario(Fig.9).SimilartoFig.4,percentagesofpeakloadswithin10kNloadclassesareshownbe-tweendashedlinesandnormaldistributioncurvesareidentifiedforreference.Intheclusteredtestlayout,

frequencydistributionsincreasedinthelowerpeakloadclassesasbrushincreasedfrom5to30kgm–2,and45peakloadswerebelow10kN.Conversely,frequen-cydistributionsofloadsgreaterthan30kNdecreasedwithincreasingbrushamount.Specifically,fivepeakloadsexceeded30kNwiththe5kgm–2brushmatandnopeakloadwasbeyond30kNforthe20,25,and

Fig. 9 Peak load frequency distributions for varying brush amounts. Numbers shown inside dotted lines indicate the percentage of loads within the respective 10 kN load class and the curves represent a normal distributionSlika 9. Raspodjele učestalosti vršnih opterećenja za različite gustoće zastora granjevine. Brojevi unutar isprekidanih crta označuju udjele opterećenja unutar pojedinih razreda (raspona 10 kN), a krivulje predstavljaju normalne distribucije


264 Croat. j. for. eng. 33(2012)2

30kgm–2brushmats(Fig9A).Duringthetransectlayoutoversteel,thepercentagesofpeakloadsex-ceeding20kNwere36,21,and3%for10,20,and30kgm–2,respectively(Fig.9B).Generally,peakloaddistributionforrespectivebrushamountswassimilarbetweenscenarios1and2.Oncetheforwarderwasdrivenoverbrushmatsplacedonthesandcoveredloadtestarea,aconsiderablereductioninpeakloadswasobserved.Infact,only1%ofallpeakloadsex-ceeded20kNwiththe10and20kgm–2brushmatsandnonewerebeyond19kNwiththe30kgm–2mat.Atthemaximumbrushamounttested,closetoaquar-terofthepeakloadswerebelow10kN.

3.4.2 Mean peak loads and mean sum of peak and second highest loads – Srednja vršna opterećenja i sredina zbroja vršnoga i drugoga najvećega opterećenjaAsmentionedpreviously,duetothesizeofsingle

loadingplates,machinepositioningcouldinfluencepeakloadsbyeitherhavingthemajorityofthewheelloadrecordedbyasingle loadcellorsharedmoreequallybetweentwoorthreeadjacentloadcells.How-ever,foramorecomprehensiveanalysis,thisstudyalsoobservedmeanpeakloadsbecauseoftheirdirectimpactonsoilphysicalproperties.Meanpeakloadsdecreasedwiththepresenceofbrushcomparedtotestsdonedirectlyoverthesteelloadtestarea(Table1).Onaverage,theuseofa30kgm–2brushmatlow-eredmeanpeakloadsby32%comparedtowhentheequipmentwasdrivendirectlyoverthesteelloadtestarea(averagingbothtestlayoutsoversteelsurface).Asimilartrendofreducedloadwithanincreaseinbrushamountwasrevealedwhenanalyzingboththeabso-lutemaximumpeakloadsandthemeanofallloadsexceedingthemeanloadandtwostandarddeviations(Table1).Wealsofoundthatmeansumofpeakandsecond

highest loadsdecreasedwith an increase in brushamountfrom5to30kgm–2(clusteredandtransecttestlayoutsoversteelsurface).Thepercentreductionofmeanpeakandsecondhighestloadsrecordedforthesteelloadtestareavariedfrom2.5to5.7%undera5kgm–2brushmatandfrom22.1to24.9%underneatha30kgm–2brushmatinrelationtothenobrushsce-nario(Table1).Absolutemaximumsumofpeakandsecondhighestloadsrecordedunderneaththesteelload test area coveredwith a 5 kgm–2 brushmatrangedfrom35.0to37.6kNandfrom28.8to34.3kNwhenitwascoveredwiththe30kgm–2brushamount.Evenwhentheforwarderwasdrivenovera10kgm–2 brushmatplacedonthesandcoveredloadtestarea,absolutemaximumsumofpeakandsecondhighestloadsrangedbetween26.2and28.5kNandvaried

between25.6and28.1kNwiththe30kgm–2brushmat.Whencombiningallreplicates,loadsfromsingle

tires (meansumofpeakandsecondhighest loadsfromeachgroupof three load cells) from the rearloadedaxlerecordedduringtheclusteredlayouttestsdecreasedfrom28.9kNduringthenobrushscenarioto22.1kNfor the30kgm–2brushmat,equalinga23.5%reductioninmeanpeakload(Fig.10A).Modify-ingloadcellpositionfromaclusteredtoatransecttestlayoutprovidedsimilarresultsasmeansumofpeakandsecondhighestloadsdecreasedfrom29.1kNwiththenobrushscenario to22.0kNfor the30kgm–2 brushmat,whichtranslatedtoa24.6%loadreduction(Fig.10B).Basedonaone-wayANOVA,therewasastatistical difference ofmean load between 0 and10kgm–2brushmatsindicatingabeneficialeffectofhavingaminimumof10kgm–2ofbrushtostatisti-cally lowermachine loads (Figs.10A–B).A furtherincreaseofbrushalsoloweredmeanloadsuptothemaximumbrushamountstudiedof30kgm–2,withstatisticaldifferencesbetween eachbrush amount.Duringtestingwithbothofthesetestlayouts(Figs.10A–B),themeanpercentageofthethirdhighestloadtothesumofthreeloadcellswide(scenario1),andthesumoffourloadcellswide(scenario2),aso-calledhalfcluster(rightordinateofFig.10)increasedwithadditionalbrush.Thisresultsupportstheassumptionof enhanced lateral load distributing capability ofbrushmatsasbrushamountsincrease.Fromourdata,wecalculatedthattheadditionofa

sandlayer(20cmthick)ontopofthesteelloadtestarea,onaverage,loweredmeanloadsrecordedbelowthesandby25%incomparisontotestsdonedirectlyoverthesteelloadtestarea(Fig.10).Singletiresfromtherearaxleexertedlowermeansumofpeakandsec-ondhighestloadswhentheywereindirectcontactwiththesandthanwhentheloadtestareawascov-eredwith10and20kgm–2brushmatsplacedontopofthesand(Fig.10C).Thiswassurprisingsincewewereexpectingbrushplacedontopofthesandlayertofurtherdecreasemeanloads.However,uponfur-therinvestigation,wedeterminedthatthepercentageofthethirdhighestaverageloadtothesumofthehalfclusterwasmuchhigherwhenthemachinewasindirectcontactwiththesand(17%)thanwhenbrushwasadded(11,11,and13%forthe10,20,and30kgm–2 brushmats,respectively).Ahigherpercentagehereindicatesthatahigherloadisbeingdistributedtothethirdloadcell.Therefore,combiningallthreehighestloadsperloading(indicatedwithdashedlinesinFig.10C)showedtheexpecteddecreaseofloadfrom25.3kNforthenobrushscenarioto21.8kNfor30kgm–2 brushamount.


Croat. j. for. eng. 33(2012)2 265

Tabl

e 1

Desc

riptiv

e st

atist

ics o

f pea

k loa

ds a

nd s

um o

f pea

k and

sec

ond

highe

st lo

ads

for a

ll tes

ts (r

ear l

oade

d ax

le o

nly)

Tabl

ica

1. O

pisn

a sta

tistik

a vr

šnih

opt

ereć

enja

i zb

roja

vršn

ih i

drug

oga

najve

ćega

opt

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enja

za sv

a isp

itivan

ja (s

amo

straž

nja

opte

reće

na o

sovin

a)


266 Croat. j. for. eng. 33(2012)2

3.4.3 Peak surface contact pressures –Vršni površinski dodirni tlakoviTogainfurther insightonthecapacityofbrush

matstomitigatetheeffectsofmachinetraffic,were-

latedgroundpressures(obtainedbydividingapeakloadbythesurfaceareaofasingleloadingplate)toafixedvalue,referredtoasadynamicsurfacecontactpressure threshold (DSCPT), therebymakingcom-

Fig. 10 Mean sum of peak and second highest loads per brush amount. A different letter indicates a significant difference based on a one-way ANOVA test at the 0.05 probability level. Mean sum of peak, second, and third highest loads are indicated with dashed lines. Right or-dinate identified with rhombus corresponds to the percentage of the mean third highest load to half cluster sum (rear loaded axle only)Slika 10. Sredina zbroja vršnoga i drugoga najvećega opterećenja za različite gustoće zastora granjevine. Različita slova označuju značajnost razlike na osnovi jednostruke analize varijance za razinu vjerojatnosti od 5 %. Sredina zbroja vršnoga, drugoga i trećega najvećega opterećenja označena je isprekidanom crtom. Desna ordinata predstavljena podacima u obliku romba odnosi se na udjel sredine trećih najvećih optereće-nja u polovici zbroja klastera (samo stražnja opterećena osovina)


Croat. j. for. eng. 33(2012)2 267

parisonsamongstbrushmats,testlayouts,andoperat-ing surfacesmore equitable. Static surface contactpressure thresholds for maintaining soil integrityavailablefromtheliteraturevaryfrom35to80kPadependingonsoiltypeandsoilstrength(OlsenandWästerlund1989;Owendeetal.2002).However,loads

exertedbytheforwarderduringtestingattheloadtestplatformincludeddynamicloadsandwerethereforeconsiderablyhigherthantheirstaticcounterpart.Tocompensateforadditionalforces(vibratory,rolling,andtransient)associatedwithdynamictesting,weas-sumedaDSCPTof150kPaforasingleloadingplate.

Fig. 11 Peak ground pressures per brush amount in relation to a 150 kPa dynamic surface contact pressure threshold as identified by vertical dashed line. Percentages indicate the number of surface contact pressures that exceed the threshold. (Rear loaded axle only)Slika 11. Vršni površinski tlak za različite gustoće zastora granjevine u odnosu na granični prag dinamičkoga površinskoga dodirnoga tlaka od 150 kPa (prikazan uspravnim isprekidanim crtama). Postotni udjeli pokazuju broj površinskih dodirnih tlakova koji nadilaze granični prag (samo stražnja opterećena osovina)


268 Croat. j. for. eng. 33(2012)2

surfacecontactpressurescenariostothesamerefer-encevalue.Forwarderpeaksurfacecontactpressureswerereducedandgenerallymoreconcentrated(lowervariation) as brush amounts increased from 0 to30kgm–2(Fig.11).Whenfocusingontestsperformed

WederivedtheDSCPTof150kPausedforoneloadingplatebyapplyingamultiplicationfactorofthreetoaconservativethresholdof50kPa.Theintentionwasnottodetermineanexactthresholdatwhichsoilprop-ertiesarenegativelyaffected,butrathercompareall

Fig. 12 Mean sum of peak and second highest loads per brush amount and loaded forwarder passes. A different letter indicates a significant difference based on a one-way ANOVA test at the 0.05 probability level per loaded forwarder pass and brush amount (rear loaded axle only)Slika 12. Sredina zbroja vršnoga i drugoga najvećega opterećenja za različite gustoće zastora granjevine i prolaske opterećenoga forvardera. Različita slova označuju značajnost razlike po prolascima forvardera i gustoći zastora granjevine na osnovi jednostruke analize varijance za ra-zinu vjerojatnosti od 5 % (samo stražnja opterećena osovina)


Croat. j. for. eng. 33(2012)2 269

overthesteelloadtestarea(nosand)withloadcellsplacedintheclusteredtestlayout,thepercentageofrecordedsurfacecontactpressuresexceedingthe150kPaDSCPTdecreasedfrom97to43%for0and30kgm–2brushamounts,respectively.Asimilartrendwasapparentwiththetransecttestlayoutoversteelwhere96%ofpressuresexceededtheDSCPTthresholdwhentheforwarderwasoperateddirectlyonsteelandonly51%whenitwasoperatedovertheheaviest30kgm–2 brushmat(Fig.11B).Peak surface contact pressures exceeding the

150kPaDSCPTwiththetransecttestlayoutoversand,decreasedfrom54to21%forthe10and30kgm–2,respectively (Fig. 11C). However, even with the30kgm–2brushmats,thepercentageofpeakpressuresexceeding the threshold (21%)wasslightlygreaterthanwhentheforwarderwasoperateddirectlyoversandwithoutanybrush(19%).

3.5 Effects of repetitive loadings on load distri-bution below brush mats – Djelovanje višekratnoga opterećivanja na raspodjelu opterećenja ispod zastora granjevineAllresultspresentedsofarhavecombinedload-

ingsrecordedfromallforwardingtrafficfrequenciespertest.However,machinetrafficabilitythroughoutacutblockisafunctionofwoodlocationandvolumemakingtrafficfrequencyhighlyvariablerangingfromasinglecycleto12cyclesormorenearamainlandingwheretimberisbeingaccumulated.Todeterminetheabilityofabrushmattodistributeloadsoverrepeti-tiveloadings,weaveragedloadsrecordedduring1–2,3–6,and7–12loadedforwarderpassesandidentifiedthemastwo,six,and12passesinFig.12.Inthemajor-ityofcases,meanloadsincreasedslightlywithanin-creaseoftrafficfrequencyandbasedonaone-wayANOVAtestwerestatisticallysignificantbetweentwoand12loadedpassesforbrushmatsof15,20,25,and30kgm–2(Fig.12A).However,nostatisticaldifferencewasfoundin loadsrecordedbetweenthedifferentpassesforthelighter5and10kgm–2brushamounts.Wealsonotedthatthedifferencesbetweenloaddis-tributing/reducingeffectsofbrushmatsofvaryingquantitydecreasedastrafficfrequencyincreasedfromtwoto12loadedpasses.Thisindicatesthatbrushmatsofhigheramounts(e.g.30kgm–2)weremostbeneficialinreducingmeanpeakloadsatlowertrafficfrequen-cies.Nevertheless, evenafter12 loadedpasses the30kgm–2brushmatsshowedahighercapabilitytodistributemachineloadsthanthelighter10kgm–2

brushmatsafterjusttwopassesthroughastatisticallysignificantreductionofmeanloads(p=0.000,p=0.009,andp=0.000,forFigs.12A,B,andC,respectively).

4. Discussion – Rasprava

4.1 Brush mat properties – Značajke zastora granjevineDespiteseveralstudiesrelatingtheeffectofbrush

matstochangesofsoilproperties,thereislimitedlit-erature available on the mechanical properties ofbranchesandnoneavailableonthepropertiesofbrushmats formitigating soildisturbances.For effectiveloaddistribution,branchesusedtocomposebrushmatsshouldhavehighplasticityandbendingstrengthunderloading.Unlikestemwood,branchesarecomposedofreac-

tionwoodthatisclassifiedascompressionortensionwood.Compressionwoodformswithinthelowerpor-tionofleaningbranches,whereastensionwoodiscre-atedintheupperportion(Hsuetal.2005).Themodu-lusofelasticityandtensilestrengthofcompressionwoodarelowerthanthoseofnormalwood(Tsoumis1968).However,compressionparalleltothegrainandbendingstrengtharehigher(Hakkila1989inGurauetal. 2008).Gurauet al. (2008) found that scotspine(Pinus SylvestrisL.)branches(randomlycollectedfroma delimbing operation) had average compressionstrengthof31.8MPa,whichwas56%lessthanthatofstemwood.Branchwoodusuallydemonstratehigherplasticitythanstemwoodbybendingundercompres-sionratherthanfailinginshearingrupturetypicallyobservedforstemwood(Gurauetal.2008).Overall,branchwood seems to have adequatemechanicalpropertiesthatwouldallowbrushmatstobeeffectiveatlateralandlongitudinalloaddistribution.Gener-ally,abrushmatcomposedofvaryingbranchsizesandlengthswillnotreactinthesamemannertore-petitiveloadingscomparedtoasinglebranch.Branch-esaspartofamatcreateareinforcedgridthroughincreasedinternalfrictionandinterlockingeffect,thusincreasingtheoverallbendingandtensilestrengthsrelative to individual branches. Branch species,throughvaryingdensity,strength,andmoisturecon-tentcanpotentiallyinfluencehowaspecificmatwillreacttoloadingforces.Inthisproject,branchesfrombalsamfirandblack

sprucewereusedtocreatebrushmatsbecausethesearethemostcommonspeciesharvestedusingaCTLmethodinNewBrunswick,Canada.Inthisprovince,softwoodscomprise63%ofallCrown land forests(representing51%ofallNewBrunswickforests)andfromthisamount,80%areclassifiedasbalsamfirandspruce(NaturalResourcesCanada,2008).Thisisthemain reason why brush mats were composed ofbranches frombalsamfir andblack spruce in thisstudy.Furthermore,blackspruceisoftenlocatedon


270 Croat. j. for. eng. 33(2012)2

siteswithwetsoilssuchaslowlyingareaswithashal-lowdepthtowatertablewheretheneedofplacingabrushmatonthemachineoperatingtrailisincreasedduetothegreaterriskofsoildisturbance.Thecomposi-tionofbrushmats (species,diameterand lengthofbranches,andquantity)likelyimpactstheirabilitytoreducemachinesurfacecontactpressures.Astudyfo-cusingonsoftwoodandhardwoodbrushmatloaddis-tributionwillbepresentedinaseparatejournalarticle.Branchesusedtocreatebrushmatsfortestsonthe

sandcoveredloadtestareahadanaveragemoisturecontentof48.8%(percentgreenmass),whichwas3.3%lowerthantheaveragebrushusedfortestsoverthesteelloadtestarea.Lowerbranchmoisturecontentwouldimplythatmorematerialisrequiredtoreachatargetbrushamount,therebyincreasingtheaveragematthickness,thanwithbranchesofhighermoisturecontent.This trendwasobserved inour results ashigherbrushthicknessanditwasrecordedformostbrushmatsconstructedoverthesandcoveredloadtestarea(lowermoisturecontent)thantheonescre-ateddirectlyoversteelloadtestarea(highermoisturecontent).Thisdifferenceinmoisturecontentismainlyrelatedtothelengthoftimerequiredfortesting.Brushamountstestedoverthesandcoveredloadtestareawereperformedafterallbrushmatstesteddirectlyoverthesteelloadtestareahadbeencompleted.Thisstudyshowedthataveragebrushthickness

recordedbeforemachineimpact(allscenarioscom-bined)was19and40cmfor10and20kgm–2brushamounts,respectively(Fig.8),whichissimilartotheresultsobtainedbyMcMahonandEvanson (1994)where20and40cmthickradiatapine(Pinus radiataD.Don)brushmatscorrespondedto9.2and18.6kgm–2,respectively.Brushdiametercanalsocontributetotheefficiencyofbrushmatsindistributingmachineloads.AccordingtoJakobsenandMoore(1981),brushmatswithbranchesof20cmdiameterormoretendtoim-pedetheperformanceofthesteelflexibletrackandcanresultinincreasedsoildisturbance.Eventhoughweusedfourdiameterclasses,oneofwhichwas>6cm,nobranchesortreetopsusedinthisstudyexceeded10cmindiameter,sincethisistheoperationalthresh-oldformerchantablevolumeofmostconiferoustreespeciesinEasternCanada.Infact,morethan60%ofallbranchesusedwereintherangeof1to6cmdiam-eter.Basedonasamplesizeof133branchescollectedduringacommercialthinningofaradiatapineplanta-tion,McMahonandEvanson(1994)recordedanaver-agebranchdiameterof3.3cm(measured10cmawayfromtheendofthestem)inmaterialusedtocreatetheirbrushmats.Hanetal.(2006)usedDouglas-fir(Pseudotsuga menziesiivar.glauca)andwesternhem-

lock(Tsuga heterophylla)branches<7.6cmindiametertocreatebrushmatsof7.5kgm–2.Ideally,webelievethathavingacombinationofbranchdiametersandlengthswouldincreasetheoverallstabilityandeffi-ciencyofthemattodistributetheappliedloadsbe-causeofincreasedfrictionbetweenbranches.

4.2 Effects of brush on machine load distribution Djelovanje zastora granjevine na raspodjelu opterećenja vozilaPreviousstudiesfocusedondeterminingtheeffect

ofmachinetrafficoverbrushonsoilphysicalcondi-tionsmainlythroughtheassessmentofsoilmechanicalresistanceandsoildensitychangesbetweenpre-andpost-impactmeasurements.Even though thisstudyconcentratedonloadsrecordedunderneathdifferentbrushamounts,ourresultsofferedsimilartrendstowhatwasreportedbyHanetal.(2006),wherea15kgm–2 brushmatstatisticallyloweredpenetrationresistanceona soilofmediummoistureconditionata10cmdepth.Poltorak(2011)alsoreportedbenefitsofusinga20kgm–2brushmattostatisticallylowersoildensityincrease(compaction),soildisplacement,andsoilpen-etrationresistancecausedbymechanizedforestopera-tionscomparedtomachinetrafficdirectlyoverbaresoil. InasimilarstudyperformedbyWronskietal.(1990), itwasdetermined that forevery increaseof10kgm–2beyondaninitial10kgm–2brushdensity,soilbearingcapacitywasincreasedby25%.Inbothscenariosofthepresentstudy,wherebrush

wasindirectcontactwithsteelloadingplates,anin-creaseinbrushamountloweredthepeakloadsandalsothesumofpeakandsecondhighestloads.How-ever,thesamecouldnotbeconcludedforthesandcov-eredscenariosincelowermeansumofpeakandsecondhighestloadswereobservedwhentheforwarderwasdrivendirectlyoverthesandcomparedtowhenthesandwascoveredwith10and20kgm–2ofbrush.Weassumethatthereasonforthisistheincreasedsurfacecontactareabetweenbothtrackandtiretothesandincombinationwiththeabilityofsandtodistributeloadsdiagonallywithinthe20cmthicksandlayer.However,expandingthezoneofanalysesfromtwotothreeloadcellswideprovidedsimilarresults(i.e.lowerloadsasbrushincreasedfrom0to30kgm–2)astowhentheforwarderwasdrivenoversteelratherthansandcov-eredloadtestarea.Theamountofbrushrequiredtoprotectforestsoilsislargelydependentonsitecharac-teristics(soilmoisture,soiltexture,organiccontent,standtype,etc.)andisthereforedifficulttopredict(Hanetal.,2006). JakobsenandMoore (1981)sug-gestedthatthecriticalamountofbrushrequiredtoprotectforestsoilsis18kgm–2.Basedontheresults


Croat. j. for. eng. 33(2012)2 271

obtainedfromtheloadtestplatform,wewouldrecom-mendleavingaminimumbrushlayerof15to20kgm–2 onsensitivesites to lowermachinesurfacecontactpressure.Operatingheavyequipmentonhighlysus-ceptiblesoils(siltyclay,clayathighwatercontents)couldrequirethemaximumbrushamounttestedof30kgm–2ormoredependingonthenumberofpassesrequiredfortimberextraction.Itisinterestingtoexplorehowmuchbrushisneed-

edtocreatetherecommendedbrushmatof20kgm–2 onsensitivesitesandhowmuchbiomasswouldstillberemainingforbiofueluse.Assumingaveragesoft-woodstandconditionsinNewBrunswickwithamer-chantablevolumeof130m3ha–1 the totalavailablebrushfromclear-cutfellingoperationswouldamountto50greentonsperhectare.Approximately35greentonsperhectareor70%ofthetotalbrushavailablewouldbeneededtocreateabrushmatof20kgm–2 coveringtheentirenetworkofoperatingtrails(aver-agetrailwidthof3.5mandspacingof20mbetweenadjacenttrailcentrelines).Ifsiteconditionsallowforbiomassremovalandrelatedexportofnutrients,thiswouldleave15greentonsperhectareofforestbio-massavailableforbiofueluse.

4.3 Effects of repetitive loadings on load distri-bution below brush mats – Djelovanje višekratnoga opterećivanja na raspodjelu opterećenja ispod zastora granjevineInforestoperations,thefrequencyofoff-roadma-

chinetrafficisafunctionofharvestedwoodvolume,itslocationthroughoutacutblock,andthelocationsandcapacitiesofthelandings,andcanvarybetweenasin-glecycleto12cyclesormorenearamainlandingwherewoodisbeingaccumulated(JakobsenandMoore1981;McDonaldandSeixas1997;Hanetal.2009).Forthisreasonitwasofinteresttoquantifythereceivedloadsbelowvaryingbrushmatsduringrepetitiveloadings.Theabilityofabrushmattolowerloadstransferredbelowthematwasreducedwhentrafficfrequencyin-creasedfromtwoto12loadedforwarderpasses.Otherstudiesinforestedsettingsalsoshowedsimilarresultswhereinitialbrushmatsprovidedadequateprotectionbutdeterioratedwithincreasingmachinepasses,whichinturnmadethemlesseffectiveatminimizingsoildis-turbancefrommachinetraffic(Hanetal.2006;Akayetal.2007).Inparticular,Wronskietal.(1990)reportedthatbrushmatslessthan10kgm–2hadanegligibleef-fectonminimizingrutdepthfollowingthefirsttwomachinepasses.Asabrushmatwasbeingcompactedbyrepetitiveloadingsoftheforwarder,somebrancheswereshearedoffbythemachinerunninggearwhichdecreasedtheoverallstrengthofthemat.Thedifference

betweenmeanpeakloadsrecordedaftertwoand12loadedpassesincreasedwithincreasingbrushamount.Brushmatsofhigheramounts(20–30kgm–2)weremoreefficientinreducingmeanpeakloadsatlowertrafficfrequencies but their load distributing capacity re-mainedbeneficialevenafter12loadedpasses.According toafieldstudybyHanetal. (2006),

brushmatsof7.5and15kgm–2wereonlyeffectiveinminimizing the compactive energy of a forwarder(eight-wheelValmet890.1equippedwithsteelflexibletracksweighing31,434kgloaded)forthefirsttwotothreeloadedpassesafterwhichthebrushmatsdeterio-ratedandnolongerprovidedadequatesupport.Con-sideringthesimilaritybetweentheValmetusedinthestudybyHanetal.(2006)andtheTimbcousedinourstudy,wewerenotexpectingsuchadifferencebetweenthe abilityof brushmats to lowermachine impact.However,duringtheHanetal.(2006)study,over70%ofthemachineoperatingtrailwascoveredwithbrushgreater than7.6 cm indiameter and themaximumbrushamounttestedwasonly15kgm–2.Asacom-parison,only4%oftotalbranchesusedinthisstudywereclassifiedasgreaterthan6cmindiameterandourmaximumbrushamounttestedwas30kgm–2.

5. Conclusion – ZaključakInthisstudyweintendedtoquantifythecapabil-

ityofbrushmatstolaterallydistributeinducedloadsfromforwardertraffictoreducepeakloadsreceivedbelowthemats,whichhaveahighpotentialtosevere-lydisturbforestsoils.Thiswasdonebyanalyzingtherecordedloadsbelowmatsofvaryingbrushquantitybyusingaloadtestplatform.Softwoodbrushmats>10kgm–2wereproventobebeneficialinstatisticallyreducingpeakloadsofaneight-wheelforwardercom-paredtoanobrushscenario.However,basedonresultsfromthedynamicsurfacecontactpressureanalyses,wewouldrecommendleaving15–20kgm–2ofsoftwoodbrushonsensitivesitesanduptothemaximumamountavailableforsegmentsoftrailslocatedoverhighlysus-ceptiblesoils.Wealsoconcludedthatincreasingtrafficfrequency from two to 12 loaded forwarder passescausedbrushmatstolosesomeoftheirabilitytodis-tributetheappliedloadsbutstillremainedbeneficialatthehighesttrafficfrequencytested.Besidestheuseofbrushformatsonmachineoper-

atingtrailstolowermachineimpacts,itsalternativeuseasbiofuelwillfurtherincrease.However,leavingsuf-ficientamountsofbrushonmachineoperatingtrailsisessentialforproactivelymitigatingsoildisturbancesduringmechanizedforestoperationsandneedstore-mainanintegralpartofbestmanagementpractices.


272 Croat. j. for. eng. 33(2012)2

Acknowledgements – ZahvalaThisworkwasfinanciallysupportedbytheNatu-

ralSciencesandEngineeringResearchCouncilofCan-ada,theNewBrunswickInnovationFoundation,FPIn-novations,theUniversityofNewBrunswickResearchFund,andtheNewBrunswickDepartmentofTrans-portation.ForwarderandoperatorweregenerouslyprovidedbyDeblyForestServicesLimited.TheauthorsaregratefulformanuscriptrevisionsprovidedbyDr.RobertJ.RogersfromtheMechanicalEngineeringDe-partmentattheUniversityofNewBrunswickandMr.MarkPartingtonfromFPInnovations.Assistancewithstatisticalanalysesandprogramming taskswasob-tainedfromDr.WilliamKnightfromtheMathematicsDepartmentattheUniversityofNewBrunswick.Wearealsoappreciativeofthelabandfieldassistanceob-tainedfromMr.MarcelLabelle,Mrs.HélèneRioux,Mr.BenjaminJ.Poltorak,andMr.ScottFairbairn.

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Sažetak

Kvantificiranje uporabe zastora granjevine pri snižavanju vršnih opterećenja i površinskih dodirnih tlakova forvardera

Šumska biomasa iz ostatka pri sječi i izradbi drva često se upotrebljava tijekom mehaniziranoga pridobivanja drva za poboljšanje prohodnosti traktorskih vlaka. Takav postupak poboljšanja prohodnosti traktorskih vlaka, ali i smanjivanja dubine kolotraga, osobito je značajan pri pridobivanju drva sustavom harvester – forvarder. Međutim, šumska


274 Croat. j. for. eng. 33(2012)2

biomasa sve više postaje važan izvor obnovljive energije. Za očuvanje njezine ogrjevne vrijednosti kao biogoriva potrebno je da granjevina (grane, ovršine, ali i lišće) bude čista od mineralnih čestica tla, koje granjevinu onečišćuju pri njezinoj uporabi radi poboljšanja nosivosti tla. Zbog toga se u potpunosti otklanja uporaba zastora granjevine za poboljšanje prohodnosti trakorskih vlaka jer izravno dodiruje tlo. Uporaba granjevine isključivo kao biogoriva ostavlja traktorske vlake nepokrivene (nezaštićene), što dovodi do značajnih oštećenja šumskoga tla. Za te dvije suprotstavljene namjene granjevine trebalo bi utvrditi najmanju količinu granjevine potrebne za djelotvornu zaštitu tla, čime bi se omogućila uporaba preostaloga dijela granjevine kao biogoriva.

Ovo istraživanje utvrđuje sposobnost zastora granjevine da preraspodijeli opterećenja na tlo smanjujući vršna optere ćenja pri izvoženju drva forvarderom. Kako se navedena sposobnost zastora granjevine povećava, tako raste i učinak za štite tla. Ciljevi su istraživanja sljedeći: i) kvantificiranje dinamičkih opterećenja forvardera ispod zastora granjevine u usporedbi s opterećenjima bez uporabe zastora granjevine, ii) određivanje najmanje količine zastora granjevine potrebne za ograničavanje djelovanja dinamičkih površinskih dodirnih tlakova na tlo kao određeni prag, iii) kvantificiranje utjecaja višekratnoga opterećivanja zastora granjevine na njezinu sposobnost raspodjele opterećenja na tlo ispod zastora.

Za utvrđivanje razlika među naprezanjima tla zbog vertikalnih opterećenja nije se mogao upotrijebiti uobičajeni sustav vaga zbog dimenzija i mase vozila, ali i značajki grana korištenih za izradu zastora granjevine. Stoga je napravljena čelična testna naprava (ispitna platforma) po kojoj je omogućeno kretanje teških forvardera, gdje se utvrđivanje opterećenja tla vozilom mjeri pomoću ugrađenih dinamometara velikoga mjernoga raspona. Raspoređeni dinamometri neovisno mjere opterećenje vozilom s rezolucijom 30 cm × 30 cm. Ukupno je provedeno 15 inačica ispitivanja s osmokotačnim forvarderom Timbco TF820-D za analizu razlika u vršnim opterećenjima, izmjerenim ispod površine zastora granjevine gustoće 5, 10, 15, 20, 25 i 30 kg m–2 (u svježem stanju) koja je smještena na površinu ispitne naprave. Po svakom zastoru granjevine obavljeno je 12 prolazaka forvardera, ukljućujući neopterećeno i opterećeno kretanje.

Ključni rezultati ovoga istraživanja pokazuju da: 1) najmanja količina zastora granjevine od 10 kg m–2 potrebna je za statistički niža srednja vršna opterećenja izmjerena ispod zastora granjevine u usporedbi s opterećenjima bez uporabe zastora granjevine (kretanje forvardera po čeličnoj ispitnoj platformi), 2) gotovo pola od svih vršnih opterećenja nadilazi 20 kN kada se forvarder kretao po ispitnoj čeličnoj platformi, a samo 4 % pri kretanju vozila kada je platforma bila prekrivena zastorom granjevine gustoće 30 kg m–2, 3) kretanje opterećenoga forvardera (s teretom mase 6689 kg) po zastoru granjevine gustoće 30 kg m–2 smanjilo je vršna opterećenja za 30 % u odnosu na kretanje forvardera po čeličnoj ispitnoj platformi, 4) pri kretanju forvardera po ispitnoj platformi bez zastora granjevine (0 kg m–2) utvrđeno je da 97 % svih vršnih dodirnih tlakova nadilazi granični prag od 150 kPa, 5) najveće je smanjenje vršnih opterećenja uočeno tijekom prva dva prolaska forvardera nakon kojih zastor granjevine i dalje smanjuje vršna opterećenja, ali znatno manje.

Ključne riječi: biomasa, zastor granjevine, šumska vozila, površinski dodirni tlak, zaštita tla

Received(Primljeno):June15,2012Accepted(Prihvaćeno):August5,2012

Authors’address– Adresa autorâ:

EricR.Labelle,PhD.Candidatee-mail:e.r.labelle@unb.caFacultyofForestryandEnvironmentalManagementUniversityofNewBrunswickPOBox4400Fredericton,NewBrunswick,E3B5A3CANADA

Assoc.Prof.DirkJaeger,PhD.e-mail:[email protected]:dirk.jaeger@fobawi.uni-freiburg.deFacultyofForestryandEnvironmentalManagementUniversityofNewBrunswickCANADA

andInstituteofForestUtilizationandWorkScienceUniversityofFreiburgGERMANY

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