沥青路面结构设计的低成本农村道路.docx
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沥青路面结构设计的低成本农村道路
沥青路面结构设计的低成本
structuralDesignofAsphaltPavementforLowCost
农村道路
RuralRoads
structuralDesignofAsphaltPavementforLowCostRuralRoads
Indevelopingcountries,ruralroadconstructionismostlycumberedbyshortageoffunds.Engineersconcernsmostinruralareasishowtobuildroadswhichnotonlycostlessbutalsomeetthetrafficdemands.EspeciallyinvastruralareasofChina,thereareagreatvarietyoftransportationpatterns,andthetrafficcompositionisverycomplex.Comparedwithothercountries,thetrafficcompositioninChinaruralareashaveitsownfeatures.Therefore,thereisnoexperienceabouttheruralroadsconstructionforreference.Inrecentyears,thecentralgovernmentofChinahasincreasedthestrengthforruralroadconstruction.Atthesametime,alotofresearchesaboutruralroadconstructionhavebeendonebyresearchersinChina,andsomeconclusionsaboutchinaruralroadshavebeenmade.Intheauthors'opinion,theselectionofthepavementstructurematerialisthekeymeasuretoreducetheconstructioncostofruralroadsaftertheroutehasbeendetermined.Comparedwithconcretepavement,asphaltpavementrelativelycostslessandisthefirstchoiceforruralroadsinChina.Andthen,accordingtotheresearchachievementsaboutruralroadsconstruction,theauthorshavedonesomepreliminaryresearchesonthestructuredesignforlow-costasphaltpavementsforruralroads.
1TrafficCompositionofRuralRoad
Ruralroadsincludecountyroads,townroadsandvillageroads.
Thetrafficonruralroadsisusuallymixed.Onacountyroad,trafficvolumeisbetween300to1500veh/dinaverage,andinacountywithadevelopedeconomy,itreaches1000to2000veh/d.Thetrafficvolumebetweencountyandtownis100to300veh/d,andthetrafficvolumebetweentownsisusuallylessthan100to300veh/d.Inamixedtrafficflow,trucksaccountfor40%to70%ofthetrafficvolume,whicharemainlylighttruckscarryinglessthan2.5tons(includingagriculturalvehiclessuchaselectro-tricycles,walkingtractorsetc.)andmedium-sizetrucksof2.5to5tons.Mostoftheselightormediumtrucksareoverloaded.Theproportionofheavytruckislessthan9%.Onsomeroadstocounties,theproportionofoverloadedtrucksis5%to32%,whileonsomecountyroadsconnectingtonationalorprovincialtrunkhighways,theproportionofoverloadedvehiclesusuallyamountsto20%to32%.
Thetrafficvolumeonruralroadsisnotheavy.However,consideringthepracticalsituationinChina,aswellastheexitenceofoverloadedvehicles,100kN,orBZZ-100wasadoptedasstandardaxleloadintheresearch.
Thepavementdeflectionortheflexural-tensilestressatthebottomofasphaltsurfaceistakenasthedesignparameter.Theaxleloadwascalculatedin
accordancewiththefollowingformula:
和
-theaxleweightofani-levelaxleinkNandtheactionfrequency;
-theaxleweightofstandardaxlein100kNandtheactionfrequency;
Ifthedistancebetweenaxlesislessthan3m,axleloadsarecalculatedas
adouble-axleormulti-axleloads,and
Iftheflexural-tensilestressatthebottomofsemi-rigidbaseistakenasthedesignparameter,theaxleloadiscalculatedinaccordancewiththefollowingformula:
Ifthedistancebetweenaxlesislessthan3m,
2TrafficVolumeonRuralRoads
Minibusesareadoptedasthestandardvehicleforthedesignofruralroads.Table1showsitsexternaldimensions.
Table1Externaldimensionsofthepassengercarm
LengthWidthHeightFrontoverhangDistancebetweenaxlesRearoverhang
6.01.82.00.83.81.4
ThetypicalvehicletypesonruralroadsarelistedinTable2.Andotherssuchasnon-power-drivenvehicles,animal-drawnvehicles,andbicyclescanbetakenintoaccountinthecalculationoftrafficvolumeonruralroads,inviewoftheirroadsideinterference.
Inaccordancewiththetrafficcompositionandvolumes,ruralroadsaredividedintofivegrades.ThetrafficvolumeofeachgradeisshowninTable3.
TrafficvolumespecifiedinTable3wasobtainedbytakingtheminibusasthestandardvehicletype,andconvertingdifferenttypesvehiclesaccordingtothevehicleconversioncoefficientsgiveninTable2.InTable3,
Nereferstothecumulativeequivalentaxleloadactionfrequency;
Nsreferstotheequivalentaxleloadactionfrequencyinthedesignedtrafficlaneinthebeginningoperationperiodofruralroads;
yreferstotheaverageannualgrowthrateoftrafficvolume;
ηreferstolanecoefficient,and1.0forasinglelaneand0.6一0.7foraduallane.
3StrengthofRoadbed
Themodulusofresilienceofroadbedvariesgreatly.Forconvenience,thestrengthofroadbedcanbedividedintofourclassesaccordingtoitsmoisturecontentandmodulusofresilience,asshowninTable4.
TheparametersinTable5aredeterminedbycombiningdesignprincipleswithpracticalexperience.ByapplyingelasticmultilayertheorytothepavementstructurespecifiedinTable5,theinfluenceofNeonthepavementthicknessofruralroadswasanalyzed,andtheresultshowthatforgivenh,h2,E0,theroadbasethicknessforneighboringtrafficclasseschangesinarangeof4-5cm.ThisresultindicatesthattheclassificationoftrafficvolumeonruralroadsshowninTable3isreasonableandfeasibleintermsofthedesignandconstructionofasphaltpavementstructures.
Byusingtheelasticmultilayertheory,theasphaltpavementstructureofordinaryruralroadinTable5isanalyzed.WhenNe,thecumulativeequivalentaxleloadactionfrequency,thethicknessofroadsurface(h=3cm),andthethicknessofsubbase(h2=20cm)remainthesame,theinfluenceofneighboringroadbedstrengthclassificationsonthethicknessofroadbaseis3cm一5cm.Thisconclusionindicatesthatthestrengthclassificationofroadbedisreasonableandapplicabletothedesignandconstructionofasphaltpavementstructure.
4DeterminationofThicknessesofAsphaltPavementStructure
Sensitivityanalysisofthedesignparametersofroadbedandpavementstructuresistofindouttherelationshipbetweenstructuralstrengthofasphaltpavementstructureandthedesignparametersofeachlayer,anddeterminethemostsensitivelayerinthepavementstructure.Theasphaltpavementstructureofruralroadsisgenerallycomposedofaroadsurface,aroadbase,andasubbase,asshowninTable6.Thepavementstructurewasanalyzedaccordingtoelasticmultiplayertheoryunderthedoublecircularuniformload,withanassumptionthatthereiscontinuouscontactbetweentheadjacentlayersoftheasphaltpavementstructure.ThebasicparametersusedinthecalculationandanalysisofasphaltpavementstructurearelistedinTable7.Byanalyzingtheeffectsofthechangeofalltheparametersofpavementstructureonthedistortionoftheroadsurface,roadbase,androadbed,thefollowingconclusionshavebeendrawn.
(1)Increasingthethicknessoftheroadsurfaceeffectivelydecreasestheroadsurfacedeflection,butraisesthecost.Thecomparativelyeconomicalandeffectivemethodistoincreasethethicknessofthesubbase,whichissuperiortoincreasingthethicknessofroadbase,whileincreasingthethicknessoftheroadsurfaceisthelastchoice.
(2)Asthethicknessofpavementstructureincreases,thechangeofroadsurfacedeflectionwilltrendtobegentle.Whenthethicknessofroadsurfacereachesacertainvalue,thevarianceintheroadsurfacedeflectionwillnotbeobvious,andthenitisineffectivetoenhancethebearingcapacityofasphaltpavementstructurebyincreasingthethicknessofroadsurface.Itisrecommendedthatthethicknessesoftheroadbaseandthesubbaseshouldbeequaltoorlarger
than18and20cm,respectively,indesignofasphaltpavementstructuresofruralroads.Fig.1showstheeffectsofthechangesinthethicknessofeachlayeronroadsurfacedeflection.
(3)Roadsurfacedeflectionisverysensitivetothechangeofmodulusoftheroadbed.Theincreaseinthemodulusofroadbaseorsubbaseisalsoeffectivetodecreasethedeflectionoftheroadsurface.Ontheotherhand,thedeflectionoftheroadsurfacedecreasesgraduallywhenthemodulusofthesurfaceincreases,beingtheleasteffectivefactor.Whenthemodulusoftheroadsurfaceincreasestoacertainvalue,decreaseinroadsurfacedeflectionisnotapparent.Fig.2showstheeffectofthemodulusofeachlayeronroadsurfacedeflection.Fromtheabovediscussion,weconcludethatthemostsensitivelayerforroadsurfacedeflectionissubbase,andthenextisroadbase.Todecreasetheroadsurfacedeflectionoflow-costruralroads,thestrengthandstabilityoftheroadbedshouldbeenhanced,andthematerialswithacertainthicknessandrelativelyhighdensityshouldbeusedtopavethesubbase.
Thetrafficvolumeortheaccumulativeequivalentaxleloadactiontimes(frequency)withinthedesignedlifeofroadisusedtodeterminethetypeandthicknessoftheasphaltpavementroadsurface,andtheresultsarelistedinTable8,whereveh/dmeansthenumberoftheequivalentthepassengercarsperday.
ForalowtrafficvolumeruralroadwithNe500000,gradedbrokenstones(orgravel)canbeusedasaflexiblebase.Theflexiblebasehasgoodstrengthandeffectivelypreventsreflectioncracksoftheasphaltpavementroadsurface,providedthegradedbrokenstones(orgravel)meetstherequirementsforhighdensity(degreeofcompaction,100%.Toensurethesufficientstrengthandstabilityoftheflexiblebase,itsthicknessisnotlessthan15cm,thethicknessoftheaggregatesubbaseisnotlessthan20cm,
Asemi-rigidbaseusuallyhasagoodbearingcapacityFortheruralroadswithNe)500000,orthosewithlowtrafficvolumesbutrelatively,theminimumthicknessofsemirigidbase