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SOIL REINFORCEMENT PRACTICE FOR FILLS OVER SOFT GROUND IN JAPAN

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31 GEOSYNTHETICS INTERNATIONAL S 1996, VOL. 3, NO. 1 Technical Paper byH.Ochiai,Y.Watari andY.Tsukamoto SOIL REINFORCEMENT PRACTICE FOR FILLS OVER SOFT GROUNDINJ APAN ABSTRACT: This paper summarizes thetheory and practiceof geosynthetic rein- forcementof fillsoverextremelysoftgroundinJapan. Emphasisisplacedonartificial- lyreclaimedsoftgroundcomposedof marinesoilswhicharecharacterizedbyhighwa- tercontentsandlowshearstrengths. Theuseof geosyntheticsfacilitatesthemovement of constructionequipmenta
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  31 GEOSYNTHETICS INTERNATIONAL  S  1996,VOL.3,NO.1 TechnicalPaperbyH.Ochiai,Y.WatariandY.Tsukamoto S OIL  R EINFORCEMENT P RACTICE FOR F ILLS O VER S OFT G ROUNDIN  J APAN ABSTRACT:  Thispaper summarizesthetheoryandpracticeof geosyntheticrein-forcementoffillsoverextremelysoftgroundinJapan.Emphasisisplacedonartificial-lyreclaimedsoftgroundcomposedofmarinesoilswhicharecharacterizedbyhighwa-tercontentsandlowshearstrengths.Theuseofgeosyntheticsfacilitatesthemovementofconstructionequipmentandtheplacementoffill.Thebearingcapacityofthegroundisimprovedbyusinghighstrengthgeosynthetics. Classical bearingcapacitytheory,cabletheorycombinedwithmodulusofsubgradereactiontheory,andplatetheoryarefoundtobeusefulindesigningembankmentsonsoftground.Theempiricalparametersusedinthesetheoriesareavailableformostprojects.Theuniquegeotechnicalcondi-tionsof thesoftgroundtypicallyinhibittheconstructionproceduresspecifiedinthedesign.Theexperiencegainedfrompastgeosyntheticreinforcedfill projects,oneof whichishighlightedinthepaper, facilitatestheimprovementof fill placementandspreadingtechniquesonsoftground KEYWORDS:  Soft ground, Reinforcement, Geosynthetic, Bearingcapacity, Casehistory,Fill,Embankment. AUTHORS:  H.Ochiai,Professor,DepartmentofCivilEngineering(Suiko),KyushuUniversity, 6-10-1 Hakozaki Higashi-ku, Fukuoka 812, Japan, Telephone:81/92-641-1101, Telefax: 81/92-641-5195, Y. Watari, President, MizunoInstituteof  Technology, MiuraBuilding, 2-22Nakajima-cho, Naka-ku, Hiroshima730, Japan, Telephone: 81/82-246-3311, Telefax: 81/82-246-3370, andY. Tsukamoto, ResearchAssociate, Department of Civil Engineering, Science University of Tokyo, 2641, Yamazaki, Noda City, Chiba 278, Japan, Telephone: 81/471-24-1501, Telefax:81/471-23-9766. PUBLICATION:  Geosynthetics International   ispublishedbytheIndustrial FabricsAssociationInternational,345CedarSt.,Suite800,St.Paul,Minnesota55101,USA, Telephone:1/612-222-2508,Telefax:1/612-222-8215. GeosyntheticsInternational   isregisteredunderISSN1072-6349. DATES:  Original manuscriptreceived2August1995, revisedversionreceived21September1995andaccepted23November1995.Discussionopenuntil1September1996. REFERENCE:  Ochiai,H.,Watari,Y.andTsukamoto,Y.,1996,“SoilReinforcementPracticeforFillsOverSoftGroundinJapan”, GeosyntheticsInternational  ,Vol.3,No.1,pp.31-48.  OCHIAI,WATARIANDTSUKAMOTO  D  SoilReinforcementforFillsOverSoftGround 32  GEOSYNTHETICS INTERNATIONAL  S  1996,VOL.3,NO.1 1 INTRODUCTION SoilreinforcingpracticeforembankmentsandareafillsoversoftgroundinJapanisdescribedinthispaper.ThemostcommonlyencounteredsoilsinJapanare:softclayeysoilsonalluvial plainswhichhavepoorbearingcapacityandlargecompressibility;and,looselydepositedsandysoils.Also,thereisagreatdemandforextendingtheap-plicationof soil reinforcementtofillsover soft, artificially reclaimedground. Thebenefitofusinghightensilestrengthgeosyntheticsforfillsoversoftgroundisrealizedthroughincreasedbearingcapacityandreduceddifferentialsettlement.Akeyobjectiveforusinggeosyntheticstoreinforcefillsistoloadthefoundationsoilasuniformlyaspossiblesoastoachieveuniformsettlement.Thisuseofageosyntheticreducesdiffer-ential settlement andimprovesstability of any superstructure. Thedeformationof foundationsoils, causedbyembankmentconstruction, canbegenerallycategorizedintofourclassesasfollows(Figure1):(a)Relativelystifffoundationsoils,whichdonotnormallypresentproblems.(b)Foundationsoilswithadequatebearingcapacity,butsusceptibletosettlement.Soilreinforcementisusedtominimizedifferential settlement.(c)Foundationsoilswithpoorbearingcapacity.Soilreinforcementisusedtoimprovebearingcapacity.(d)Verysoftfoundationsoilswithminimalbearingcapacity.Geotextilereinforcementmustbeanchoredtoprovidemembranesupportforconstructiontrafficandsubse-quentfilling. Thefollowingsectionsdescribehowsoilreinforcementmaybeusedtoimprovebear-ingcapacityandreducedifferentialsettlementsoffillsoversoftground,thusfacilitatingthemovementofconstructionequipmentandsubsequentembankmentconstruction. Figure1. Deformationpatternsoffoundationsoilsandembankmentafterembankmentconstruction: (a) relatively stiff foundation soils; (b) foundation soils susceptible tosettlement;(c)foundationsoilswithinsufficientbearingcapacity;(d)verysoftfoundationsoils. (a)(b)(c)(d)  OCHIAI,WATARIANDTSUKAMOTO  D  SoilReinforcementforFillsOverSoftGround 33 GEOSYNTHETICS INTERNATIONAL  S  1996,VOL.3,NO.1 2 AREAFILLING2.1 ClassificationandConcepts Landreclamationinvolvesthedepositionofdredgedmarinesoilswhichareinitiallyextremelysoftduetoahighwatercontent.Evenafterconsolidationunderself-weightthesedepositsareoftentooweaktosupportevenpedestriantraffic.A decadeago,re-claimedlandwasdrainedusingavarietyofmethodsuntilitwasstrongenoughtosup-portlowgroundpressureconstructionequipmentwhichcouldthenplaceandspreadfill.Morerecently,therehasbeenademandforearlieraccesstoreclaimedlandandthusithasbecomecommonpracticetousegeosyntheticstoenablefillingimmediatelyafterreclamation. Thefillingmethodrequiresacombinationofgeosyntheticplacementandfillspread-ingusingtheoptionsoutlinedinFigure2(Watari1984a).Sinceverysoftgroundoffersnegligiblebearingcapacitymostoftheweightofthefillissupportedbytensileforcesgeneratedinthegeosyntheticreinforcement. EachoptionshowninFigure2maybecharacterizedbythewayinwhichreactionforcesaregenerated(Watari 1984b).Intechniquessuchas“ropesheet”reinforcement,thegeosyntheticgenerallyrequiresanchoragetoadjacentstablegroundtosupportthetensileloadsgeneratedintherein-forcement.However,geotextileorgeonetreinforcementmayworkfairlywellwithoutanchorageif thegroundconditionsarebetter.Bamboosheetreinforcementsareabletomobilizereactionforceswithinthegroundduetotheirflexural rigidity.Fill spreadingmaybeperformedoverareaswherethegroundwatertableisbelowgrade,oroverareassubmergedinwater.Intheformercase,thinlayersoffillarespreadusinglight-weightconstructionequipment,andinthelattercase,fill spreadingboatsareusedtodumpthefillinthewater.Duetothebuoyancyeffect,thickerlayersoffillcanbedumpedinthewaterasopposedtospreadingfillinareaswherethegroundwatertableisatorbelowthesurfaceandthegroundisverysoft.Inall casesitisimportant Figure2. Classificationofearthfillingmethods. Surface soil stabilizationBamboo raft and rope sheetGeotextileRope sheetGeonetEarth filling methodBearing capacityimprovementFill spreadingLight weight construction equipmentSmall hydraulic fill pumpSoil spreading bargeSoil reinforcementCementLime  OCHIAI,WATARIANDTSUKAMOTO  D  SoilReinforcementforFillsOverSoftGround 34  GEOSYNTHETICS INTERNATIONAL  S  1996,VOL.3,NO.1 thatthefill bedepositeduniformlytoavoiddifferential fill penetrationintothesoftfoundationsoils. 2.2 DesignConcepts AsshowninFigure3a,settlementoccursbeneaththeembankmentandheaveoccursatitsedges.Althoughvariousdesignmethodsarebeingdevelopedtotakeintoaccountthesedeformations, insitumonitoringandtestingareusually employedtocontrolconstructionactivities.Mathematical expressionsforbearingcapacityandsettlementofreinforcedfillsmaybecategorizedasfollows: S  Formulasbasedonclassical bearingcapacitytheory(Fukuzumi andNishibayashi1967;YamanouchiandGotoh1979)thattakeintoaccountthetensileforcegeneratedinthegeotextileandsurchargeeffectsduetosettlementandheave. S  Formulasbasedoncabletheorycombinedwithmodulusofsubgradereactiontheory(Shimizu1978)topredictthedeformedshapeandtensileforcegeneratedinthegeo-textileduetosettlement. S  Formulasbasedonplatetheory(Yamanouchietal.1979)predictingsettlementbasedontheassumptionthatmeshopenings,suchasthoseingeonetsandgeogrids,becomepackedwithsoil andformaplate-likestructure. S  Formulasbasedonair-membraneinteractiontheory(WatariandHiguchi1985)thatassumethatgeotextilesbehaveasstressedmembranesanchoredattheiredges.Classicalbearingcapacitytheoryisthemostfrequentlyusedoftheabovementionedtheories(Figure3b).Theultimatebearingcapacityofthefoundationsoil isassumedtocomprisethefollowingfourcomponents:bearingcapacityduetotheshearstrength Figure3. Designconceptfor embankmentconstruction:(a)grounddeformationunderembankment;(b)modellingofgrounddeformation. Embankment Dead load due toembankment self-weightSoft foundation soils BT D f   r T  r  θ q d  (a)(b) θ
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