EFFECT OFCONCENTRATEDLOADS ONSHALLOWBURIEDPOLYVINYLCHLORIDEANDPOLYETHYLENETUBINGbyTIMOTHYJAMESMCGRATHBS, Northeastern University(1973)Submittedin partial fulfillmentof therequirements forthedegree ofMaster of Sciencein Civil Engineeringat theMassachusettsInsti tuteof TechnologySeptember, 1975Si gnature of Author r" . .....- ./..- 0 ... ~ .....Departtfnent of Civil Engineering, June27, 1975~ ~ .,.; ~ / ~ P >Certified by. 0 e -.-- 0 ThesisSupervisorAccepted by II .,- p .Chairman, Departmental Committee onGraduateStudentsof theDepartment of Civil EngineeringABSTRACTEFFECT OF CONCENTRATEDLOADS ONSHALLOWBURIEDPOLYVINYL CHLORIDE ANDPOLYETHYLENETUBINGbyTIMOTHY JAMESMCGRATHSubmittedtothe Department of Civil Engineering on June27, 1975inpartial ful-fillment oftherequirementsforthedegree of Master ofScienceinCivil Engineer-ing0Thisthesis presents astudy ofthebehavior ofPVCandcorrugatedPEplasticpipeunder simulatedwheel loads. Dataispresented onpipedeflections, soil-pipeinterfacepressures, and soil strains forpipeburiedwith6, 12 and18inches ofsoil cover andsurfaceloadedwith 10,000pounds on a 10inchdiameter plate.In5 of theteststhe PVCpipewasinstrumentedwithstraingages ontheinsideand outside of thepipe atthecrown, invert andspringlines. Thetest resultsarecomparedwithanelastictheoryfordeeplyburied pipetoobserveiftheelasticpredictions can beused as adesignbasisfortheshallow burial-concen-tratedloadproblem.ThesisSupervisor:Title:2HerbertH. EinsteinAssociateProfessor of Civil EngineeringACKNOWLEDGEMENTSThis proiect wasfundedinpart byTheNational CooperativeHighwayResearchProgram, Proiect 4-11whoseassistancewasgreatly appreciated. Thanks arealsoextendedtoall theemployees of SimpsonGumpertz andHegerInc. whohelpedinpreparation of thefinal manuscript, especially Mr. RichardE. Chambers, whosetime I frequentlymonopolizedinsearchof advice andsupport. Mr. JoeRixnerofHaley& AldrichInc. helpedmeout withmany ofmysoilsproblems, Dr e ErnestT. Sel ig oftheUniversityot'Buffaloprovided a greatdeal of histimeand equip-ment tohelpconduct thetestprogram, andProfessor Herbert H. Einstein of MITaidedindevelopingthe testprogram, andprovidedmany useful suggestionsinwritingthemanuscript. Mygreatest thanks aredue tomy wife, Ginny, whomanagedtosupply a great deal ofmoral support andgivebirthtoabeautiful babydaughterinthemiddleof it all.3CONTENTSPageTitlePageAbstract 2Acknowl edgements 3Table of Contents 4List of Tables 6List of Figures 7CHAPTER INTRODUCTION 9CHAPTER 2 THEORETICAL ANALYSIS122.1 Material Properties 122.2 BuriedPipeDesign 142.2.1 Soil StructureInteraction 142.2.2 FiniteElements 172.2.3 ElasticitySolutions 202.3 PreviousStudies 21CHAPTER 3 TESTINGPROGRAM273.1 Apparatus 273.2 Samples 303.3 TestProcedure 323.4 TestResul ts 353.4.1 Loads 3543.4.2 Defl ections 403.4.3 PVCStrains 433.4.4 Effect of Flat Bedding 503e 4.5 LoadPlateSettlements 503.4.6 Effect of TankSize onPipePerformance 50CHAPTER 4 DISCUSSION544. 1 CriteriaFor Establ ishing aMinimumDepth of Cover 544.2 AllowableMinimum Cover 584.3 DesignForTheConcentratedLoad-ShallowBurialSituation 63CHAPTER 5 CONCLUSIONS 675. 1 Minimum Cover 675.2 DesignMethod685.3 RecommendationsForFutureResearch69References 71APPENDIX A BURNS ANDRICHARDELASTICITYSOLUTION 73APPENDIX B SOILPROPERTIES 775LIST OFTABLESNumber Title Page2.1 Physical Properties ofPVC andPE133.1 Physical Properties of Samples UsedInTestProgram 333.2 Summary ofTest Conditions 364.1 Deflections andStresses at 10,000PoundLoad 604.2 Comparison of TestResul ts WithBurns and RichardTheory 656Number2. 12. 22. 32. 42. 52. 63. 13. 23. 33. 43. 53. 63. 73. 83. 93.103.113.123.13LIST OFFIGURESTitleStrengthVariation WithTemperature InPEDetermination of One-Dimensional ModulusRadial Pressures OnPipeforVariousLoad WidthsRadial Pressures OnPipeforVaryingDepth of CoverHoopBendingMoment InPipeforVariousLoadWidthsPiossonEffect In anElasticMaterialCross-SectionThroughTestTank and Load ApparatusInstrumentationLocations AroundTest PipeStressesInSandWithout PipeCrownSoil Stressesfor PEPipeTypical PressureDistribution AroundTest PipeEffect of BearingFailure OnPipeDeflectionsDeflectionProfile AlongPVCPipeDeflectionProfileAlongPEPipePVC- Vertical Deflection Vs. LoadPE- Vertical Deflection Vs. LoadCrownStrains- PVC- DenseSandCrown, Invert andSpringl ineStrainsVariationInCrownBendingStrains WithLoad Condition7181823232525283137393941424244444547483.14 VariationInSpringlineThrust WithLoadCondition493.15 Comparison of Flat and90DegreeBeddingInfluence OnVertical Deflections513.16 Comparison of Flat and90DegreeBeddingInfluenceOnBendingStrains524. 1 Extrapolation ofDeflectionsInLooseSoil to10,000PoundLoad594. 2 Stresses forUse InBurns andRichard Analysis 64B1 Sta ndard CompactionTestResul ts7882 One-Dimensional Modulus for ConcreteSand 798CHAPTER1INTRODUCTIONOverthepast twentyyears plasticpipehasseenincreasingusagein awidevarietyof applications. Amongtheseapplicationsisdrainageof roadways, bothintheformof stormsewers, andunderdrains. Plasticpipes arelight, flexible, and,sincetheyaremanufactured byextrusion processes, canbeproducedin anyreason-ablelengthdesired. Thesefactorshelpinreducingconstructioncostsbyreducingthemanpower andequipmentneededtoplaceindividual lengths, andbyreducingthenumber offieldioints. Plasticpipingsystemscan alsobedesirabledue totheirresistancetothecorrosiveeffects ofmanyfluids, which'NOuldshortentheIife of other pipematerials.Structurally, buriedpipesmust bedesignedfortwotypes of load afterinstallation,earthloads, which aresimplythe'Neight of soil bearing onthepipe; andliveloads,whichareprimarilywheel loadsimposedfrorri thesurface. Soil pressuresduetowheel loads aregenerallyinsignificantforsmaller pipes buried overten feet, buttheyincreaseveryrapidlywithdecreasingburial depthuntil they arepredominantatdepthslessthan about four feet. Duringroadconstruction, particularlyhigh-ways, theweight of constructionequipment rolling over pipeinstallationsbeforebackfillingiscompletedcancreate a moresevereloadingconditionthanthepipe9will ever besubiectedto again. Toreduce theseloads, ASTMspecification02321, for theinstallation ofthermoplasticsewerpipesdoesnot permitheavyequipment totravel overburiedpipesuntil 2.5 feet of cover is placed andcom-pacted overthecrown ofthepipe. If suchprovisionswereremoved orlessenedthecost ofroadconstructionmight bereducedbyallowingcontractorsmorefreedom; however, culvert technologyhasnot asyet providedamethodtodesignforwheel loads andshallow burial. Until sucha methodisdeveloped, theserestrictionsmustbeusedtoprevent damage todrainagesystems.Thepurposeofthisthesisistostudythestructural performance ofburiedplasticse'Ner anddrainpipeundersimulatedwheel loads, andtoestablishthefollowing:1. Anindication ofminimumcoverrequiredtoprotect plasticpipeinstallationsfromconstruction equipment loads. Thisdepthisgovernedbythemagnitude oftheload, therateatwhichtheloadattenuateswithincreasingdepthandthephysical propertiesofthepipeandsurroundingsoil.2. A rational designapproachtotreat theconcentrated loadproblem.Dueto the multitude oftypes andshapes of plasticpipe available, it isnecessarytorestrictthisthesistotwoparticular piping systems, polyvinyl chloride (PVC)andcorrugatedpolyethylene (PE). Both ofthesesystems arenow beingconsidered10bymanytransportationdepartmentsfor use indrainageof highways.11CHAPTER2THEORETICAL ANALYSIS2. 1 Material PropertiesStructural analysis of plasticpipemust ofnecessitybegin withabrief look at plas-tics andtheprocessesthatmakethemintopipes. Plastics areorganiccompoundswhoseproperties aredependent uponmolecular structure. PVCandPE areboththermoplastics, that istheir properties vary withtemperature. Figure2.1showsthis variationforpolyethylene. Theeffect onPVCis similar but less pronounced.Thermoplastics aremanufacturedintopipeformby 1) heatingaresin, 2) forcingitthroughan extruder, 3) performinganymolding operationsnecessary(corruga-tions, belled ends)and 4) coolingthefinishedproduct. Theproperties of thefinishedproduct are affectedby suchfactorsasthetypeandformulation of resin,themanufacturingprocess, andtheprocessingtemperature. Thestructural pro-perties of plastics vary alsowithtime, however,sincethisthesisis studyingshorttermloads, onlyshorttermproperties will bediscussed.CorrugatedPEunderdrain pipeis specifiedin ASTM standardF405-74 whichinturncalls formaterial conformancewithASTMD1248. Some of thephysical pro-perties ofPEfromD1248 areshowninTable 2. 1.12PVCsewer pipeisgoverned by several specifications(02729, D3033, D3034).D2729isfor athinwalled pipe, whileD3033 andD3034 specify pipewiththickerwalls and arealmost identical. All threespecificationsrefertothematerial stan-dard 01784fortheplasticspecification. Some of thephysical propertiesfor PVCfrom01784 areshowninTable 2.1.TABLE2.1 PHYSICALPROPERTIES OFPE ANDPVCPropertyTensileStrength(psi)Elongation, min. (%)Brittleness Temp., max. fC)Impact Strength, min. (lzod)ft-Ib/in. of notchModul us of Elasticitytension(psi)Diameter(in.)PE(D1248)1800 -3200100 -500-75to-6060,000-180,000*3 - 8PVC(D1784)6000 -700040- 80*0.65-1.5360,OOO-SOO,0004 - 15*TakenfromRef. 1, not specifiedbyASTMSinceboth of thesematerial specifications wereprimarilyintended forpressurepipes, whicharenormally stressedmostlyintension, nocompressiveor flexuralstrengths arespecified. PVCplasticstypicallyhavecompressivestrengths ofabout 9500psiandflexural strengths of about 12,000 psi. PE plasticshave acompressivestrength of about 3100 psiand afl exural strength of about SOOO psi.132.2 BuriedPipeDesignHistoricallythedesign of buriedtubes (i.e., pipes, culverts, etc.) hasbeendividedintotwofields, onefor IIrigidllconduitssuchasconcretepipe andtheotherfor lIf1exiblellconduitssuchascorrugatedmetal culverts. Kigidconduitsaredesignedtoresist earth andIiveloadsprimarilybyinternal forces. Thismeams that themainformofresistanceisthroughbendingmoments, resultinginrather stiff sections andthereforetheclassificationIIrigid. II Flexibleconduitshowever aredesignedtodeflect laterallyunderloadin ordertoutilizethepassiveresistance ofthesurrounding soil. Thistypeof designresistsloadsmorebymem-brane actionthan bybending andresul tsinmuchthinner sections. Themaiordifficultywiththesetraditional culvert designpracticesisthattheyweredevelopedempiricallyforspecifictypes of pipeanddonot providean adequatedesignmethodfor all buried pipes. That is, thereisanintermediaterange of stiffnessesfor whichneitherdesignmethodis adequate, asisdiscussedbelow.2.2.1 Soil-StructureInteractionThetraditional designmethodsmentionedclassifyinstallations asflexibleorrigid solelyfromthebendingstiffness ofthepipe wall, calledthe IIringstiffness":s=EID3where S =Ringstiffness(psi)14E = Modulus of elasticity of pipematerial (psi)I =Moment of inertia of pipewall per unit length(in. 4;in.)withrespecttoringbendingD=Meandiameter of pipe (i n .)Inrecent years, however, abetterunderstanding ofthesoil structureinteractionaroundpipeshasbrought intousea newparameter, theflexibilitycoefficient:MsF =-Swhere F = Flexibility coefficient (dimensionless)Ms = One-dimensional compressionmodulus of soil (psi) I(Seebelowfordiscussion ofthis parameter.)Theflexibil i tycoeffi cientis amoresui tableparameterforthedescription of buriedpipeinstallationsbecauseit showsthat performanceisgoverned bytherelativestiffness of soil andpipe, ratherthanthe absolutevalue of either one. That is,agiven pipeinstallationcouldbeconsidered either flexibleor rigiddependinguponthesoil inwhichitis buried.It has alsobeen shownbyLew(3) andSelig(4), that theperformance of a buriedpipecanbeinfluenced bytheringcompressibility ofthepipe. Thiscan bethecaseinmetal culvertswheresl ip can occurin bol ted ioints, andinplasticpipes,whichtypicallyhavelow elasticmoduli. Theringcompressibility of a pipeis15described bythecompressibil itycoefficient:where C = Compressibilitycoefficient (dimensionless)A= Cross-sectional area of pipewall per unit length(in.2/in.)Allgood(5) suggests that asystemfor whichF is greaterthan1000 beclassifiedas flexible (inthetraditional sense)and anysystemforwhichF islessthan orequal to 10 beclassified asrigid. Theareain betYleenistheaforementionedlIintermediate"range. Consideringthat atypical valuefortheringstiffness ofPVC or PEpipeis about 1.0 psi I thenby All good's cl assi fi cation anypipeburiedin asoil withMs lessthan1000 psi would beintheintermediaterange. Thisindicates that for plasticpipeit isdesirabletofindanew designmethod whichcantreattheentirerangeof pipestiffness.Both of the abovecoefficients usetheone-dimensional modulus(MJas thepara-meter todescribesoil stiffness. Thisreplacesthesoil parameter traditionallyusedinflexibleconduit design, themodulus of soil reaction(E'). Themodulus of soilreactionis used withtheIowaformulafordetermining flexibleconduit deflections,butis highly empirical. Krizek(6) presented anumber of attempts at correlatingE'withother soil properties but thesehavenever proved entirely satisfactory.The one-dimensional modulushowever canbedeterminedfromasimplelaboratory16test. Theprocedurecalls fordetermination of astress-strain curveforthetestsoil initsnatural state. Theone-dimensional modulusis thenselected as theslope of thesecant fromzerotothepoint onthecurvewhichrepresentsthecal-culated load. This procedureis showninFigure 2.2. It isimportant tonotethatthis procedureinvolvesthe assumption of linear stress-straincharacteristics inthesoil. Thisis adequateingeneral, however someinvestigators(6, 7) haveintroducedmethods of treatingthetruenon-linear behavior of soil.Methods that arepresently availablefortheanalysis anddesign of buried pipethatcantreat circumferential stiffness, aswell astheintermediateringstiffnessrange, fall intotwogeneral categories, closedformelasticitysol utions, andnumerical methodsintheform of finiteelemert analyses. Photoelastic andholographicinterferometric (8) methodshave alsobeenusedtoanalyzepipeproblems, butthesearenotintended asdesignmethods.2.2.2 FiniteElementsPerhapsthemost comprehensivemethod availablefortheanalysis of buried pipesisthecomputerized finiteelement approach. Finite elements intheirmost com-pleteformcanbeusedtoanalyze almost anysituation, such as concentrated loads,poor bedding, non-uniform soil ornon-linear soil behaviorc Some solutions haveeven beenmadetotreatlongitudinal effects alongthelength of the pipe.Katona et al. (7)arepresently developing athree-level computer analysis/design17/0000~ -~~ -"""IIIIl-""""""'Ill ~'"""'..................~-'ilIIIl'--~~ ."~ "-20 0 +20 +4fO ~ ~ -1-/00TEMPeRATURe (DC.)Yiel...o SrteE5S(psi)--- UL.rIMATE T!!!!NSIL-ES'rRENarrt (psi)Flauli'62./ 5'rlCeNarHVARIATIONWItHIFMPeIfArU/fc IN Pc(I=R'OJW ~ e F . 2)p-.... .......STRAIN(e)18programforculverts calledCANDE(Culvert Analysis andDesign). T\I\ co1-NO/.J.:;.=77:1:;;;07'c?':J/..I.ClaA% l{41LOOSlZ 501/DtZn6/Z 5011---18"Covar G'l CO\.4Zr _ __'Notrz: C'-'rvtZ6 -fOr lootsc2' 5011OI'?Z ,-e,,- /OQa'e /b.e.CUA