Erratum Frontiers in Offshore Geotechnics III Editor: Vaughan Meyer ISBN: 978-1-138-02848-7 After the publication of the volume, it was noticed that amendment of a few figures in the Third ISSMGE McClelland Lecture ‘Cyclic soil parameters for offshore foundation design’ by K.H. Andersen on pages 5-82 was required. Please find the correct version of the paper here. The Publishers ISFOG CH001.tex 9/7/2015 16:32 Page1 TheThird ISSMGE McClelland Lecture ISFOG CH001.tex 9/7/2015 16:32 Page2 ISFOG CH001.tex 9/7/2015 16:32 Page3 FrontiersinOffshoreGeotechnicsIII–Meyer(Ed.) ©2015Taylor&FrancisGroup,London,ISBN:978-1-138-02848-7 Introduction to Knut H.Andersen, TheThird ISSMGE McClelland Lecturer KnutH.Andersenwasbornin1945inOslo,Norway, Knuthashadtheopportunitytoworkcloselywith wherehehasalsolivedandworked,apartfrom4years andco-authorpublicationswithallthefiveNGIdirec- studyinTrondheimandoneyearmilitaryservicein tors.InhisfirstyearsatNGI,Knuthadtheprivilege theNorwegianCorpsofEngineers. toassistDr.Bjerruminhisworkonstabilityandset- HereceivedhisMScfromtheNorwegianUniver- tlement of embankments and structures on soft clay sityofScienceandTechnology(NTNU),thennamed andstabilityofexcavations,whichwasbasisforpart NTH,inTrondheimin1968andselectedSoilMechan- of Bjerrum’s State-of-the-Art report to the Interna- icsashisMajor,influencedbytheinspiringlectures tionalConferenceonSoilMechanicsandFoundation ofProf.NilmarJanbu.HewenttoNGIforhismaster Engineering, ICSMFE, in Moscow in 1973. Knut thesisin1968withCarlJ.F.Clausenassupervisor.The developedanin-situtechniquetomeasurelateralpres- subjectwasinterpretationofsettlementrecordsoftwo suresinclaysbyhydraulicfracturingduringthesefirst heavy buildings on soft clay (Oslo Jernbanetollsted) years,andheanalyzedstressesanddisplacementsin which had settled 50 and 70cm, respectively, over rockfilldamsbythefiniteelementmethod.Hewas, a 50 year period.The thesis work involved valuable along with Carl J. F. Clausen, active in introducing practicalexperiencewithsiteinvestigationandlabo- andutilizingfiniteelementprogramsingeotechnical ratory testing in addition to the theoretical analyses. engineering at NGI.This included development and Dr.LauritsBjerrum,NGI’sfirstdirector,followedthe implementationofmaterialmodels. work very closely and gave encouraging input and ThediscoveryoftheEkofiskfieldintheNorthSea comments. in the late 1960s caught Dr. Bjerrum’s enthusiastic Dr.BjerrumofferedKnutapositionafterhisthe- attention and lead to significant new and interesting sis, and Knut started as a regular employee at NGI challenges for NGI. Knut’s work was then directed on 1 January 1970, after a year military service in towards offshore foundation engineering, which has 1969.The continued co-operation with Dr. Bjerrum been his focus since early 1972. This has involved untilhisprematuredeathin1973shapedKnutasan designofoffshoregravitystructures,jacketstructures, engineer and has been decisive for his professional jack-ups,seabedstructures,seabedslopestability,and career.InlinewithNGI’sphilosophy,hehasworked suctionanchorsinclay,silt,sandandcarbonatesoil. onbothconsultingandresearchprojects,takingadvan- The work has included practical foundation design, tageofthebeneficialinteractionbetweenconsulting concept development, development of design meth- andresearch. ods, determination of foundation design parameters, 3 ISFOG CH001.tex 9/7/2015 16:32 Page4 and planning and interpretation of laboratory tests, haveincludedcyclicbehaviorofsand;interpretation modeltestsandprototypemonitoring. ofprototypeperformanceobservations;performance Thefirstinvolvementwasthefoundationdesignof andinterpretationof1gfieldandmodeltestsonmono- the Ekofisk oil storage tank, which is the first GBS podandtripodgravitystructuresandsuctionanchors; installedintheNorthSeaon30thJune1973.Laterthe planningandinterpretationofcentrifugetestsongrav- involvementhasincluded,todifferentdegrees,most ityplatformsandsuctionanchors;foundationdesign ofthegravityplatformsinstalledintheNorthSeaand proceduresforgravityplatforms,skirtedfoundations, anumberofothersworldwide,coveringdifferentsoil suctionanchorsandjack-ups;conductorsettingdepth conditions,rangingfromverysoftclaystoverydense for drilling of oil wells; slope stability under cyclic sands,silts,hardclaysandcarbonatesoils.Thedeepest loading from earthquakes and vibrations; interpreta- oneistheTrollPlatformin330mwater. tion of T-bar and ball penetrometer tests; and soil Knut participated in the foundation design of the samplingdisturbanceandmeansofcorrection. first North Sea tension leg platform with skirted Several of the projects have been performed in anchors at the Snorre field and in the development co-operation with geotechnical companies and uni- ofskirtedanchors(suctionanchors)foranchoringof versities in countries outside Norway and sponsored floaters and skirted foundations for jackets. He was by international oil companies, certifying agencies, project manager for the large 1-g field testing pro- research councils, and construction companies. He gram of the SnorreTLP skirted anchors at Lysaker has also participated in the EU funded projects on inNorwayandhasbeeninvolvedininstallationand foundationdesignofcaissonbreakwaters. holdingcapacitydesignforsuctionanchorsinvarious Knuthasbeenkeynotespeakerandchairmanand soilconditionsworldwide. givenpresentationsatanumberofinternationalcon- He has also participated in deepwater geohazard ferences.Hegavethe21stBjerrumLecturein2007 and submarine slope stability evaluations offshore and the Distinguished Lecture at the Memorial Uni- Norway,offshoreWestAfricaandintheGulfofMex- versity of Newfoundland, Canada in 1997. He was ico.Hehasestablishedsoilparametersforearthquake examiner for several PhD and MSc theses at AUC, analysesofstructuresandslopesforvariousonshore Denmark in the period 1992–96. He is author or andoffshorelocationswithdifferentsoilconditions, co-author of more than 100 contributions to profes- andhasparticipatedinevaluationandremediationof sionaljournals,booksandconferences. accidentalleakagefromoilwells. Knut has been member of several international His offshore knowledge and experience has been technicalcommittees,includingISSMGE’sTechnical usedfordesignofharboursandseafloodprotection CommitteeTC209 on Offshore Geotechnics (2010), barriers, like the Oosterschelde storm surge barrier ISSMGE’s Technical Committee TC1 on Offshore in the Netherlands. He also analyzed and evaluated andNearshoreGeotechnicalEngineering(2005),ISO thepiersofthewesternpartoftheStorebæltBridge TC67/SC7WG8ArcticStructuresPart2:TP3:Foun- betweenFunenandZealand,Denmark,forcyclicload- dation Design (2007), and API RG7 Geotechnical ingfromwavesandfromicesheetdriftingfromthe ResourceGroup,Risersandflowlines(2006–2008). BalticSea. He was chairman of the board of NGI Inc. in Duringthelast10to15yearsKnuthasbeenactive Houstonfromitsstartin2002until2008. inthedevelopmentofoffshorewindturbinefounda- Knuthaswidemanagementexperienceasproject tions.Thisincludesconceptdevelopment(monopods, managerforconsultingprojectsandmajorjointindus- monopiles,skirtedmultipods,piles,etc.),foundation try sponsored research programs, and management design analyses (installation, stability, soil stiffness positionsatNGI.Hewastechnicaldirectorfrom1996 forstructuraldynamicanalyses,cyclicdisplacements, to 2012, when he also coordinated NGI’s research settlements,permanentdisplacements,etc.),planning activities.Hereachedhisretirementagein2012and andspecificationoflaboratoryandfieldtestingpro- nowservesastechnicaladvisoratNGI. gramsofthefoundationsoil,interpretationoflabora- Overthelasttwentyyears,Ihavehadtheprivilege tory and field test results, and determination of soil and honor to work with Knut on many challenging parametersforfoundationdesignforseveralfields. deepwaterprojects.Itisthereforewithgreatprideand Knut has participated in a number of joint indus- pleasurethatI,onbehalfoftheInternationalSociety trysponsoredresearchprojects(JIP)andbeenproject ofSoilMechanicsandGeotechnicalEngineeringand managerfor12ofthem.Thefirstonewasthestudyon its Technical Committee 209 on Offshore Geotech- cyclicbehaviorofclay,whichwasinitiatedin1974in nics, hereby present him with the Third ISSMGE co-operationwith12industrycompanieswhosawthe McClellandLectureaward. need for more knowledge about cyclic soil behavior inconnectionwiththefoundationdesignofthefirst PhilippeJeanjean,Ph.D.,P.E.,M.ASCE NorthSeagravityplatforms. Chairman,ISSMGETC209,OffshoreGeotechnics TheresultsfromthisJIPhaveformedthebasisfor June10th,2015 NGI’s modelling of cyclic soil behavior. Later JIPs 4 ISFOG CH001.tex 9/7/2015 16:32 Page5 FrontiersinOffshoreGeotechnicsIII–Meyer(Ed.) ©2015Taylor&FrancisGroup,London,ISBN:978-1-138-02848-7 Cyclic soil parameters for offshore foundation design K.H.Andersen NorwegianGeotechnicalInstitute,Oslo,Norway ABSTRACT: Foundationdesignofstructuressubjectedtocyclicloading,includingstability,cyclicandper- manentdisplacements,soilstiffnessforuseindynamicanalyses,andsoilreactions,requiresthateffectofcyclic loadingisaccountedforinthesoilparameters.Aprimarygoalofthispaperistoprovidecorrelationsofthese parameterswithindexparametersforuseinpracticaldesign.Thecontourdiagramframeworkforinterpretation andpresentationofcyclicsoilbehaviorissummarized,aswellasporepressureandcyclicstrainaccumulation procedurestoestablishequivalentnumberofmaximumloads,N ,thatgivesthesamedegradationastheirreg- eq ularcyclicloadhistory.Guidanceisgivenonhowthesoilparameterscanbeappliedindesign,andcalculated andmeasuredprototypeobservationsandmodeltestresultsareusedtovalidatethesoilparameterframework. Focusisgiventofoundationdesignofstructures,butaproceduretoanalyzethestabilityofslopessubjectedto cyclicloadingisalsoincluded. 1 INTRODUCTION index parameters that can be used to establish the parameters of this framework for various soil types. I feel honored being asked to present this 3rd Correlationswithindexparametersarealsogivenfor McClelland Lecture, and I would like to thank the initial shear modulus, static shear strength, friction ISSMGETC209committeeforinvitingme. angleandconsolidationcharacteristics(compressibil- I did not have the privilege to meet Bramlette ityandpermeability),sincetheseparametersarealso McClelland.TheclosestIgotwasinconnectionwith neededincyclicfoundationdesign. thedesignoftheEkofiskoilstoragetank,whichwas Thecorrelationscanhelpestimateparametersfor installedintheNorthSeain1973.Thegeotechnical feasibility studies before site specific data are avail- siteinvestigationwasdonebyMcClellandEngineers, ableandguidespecificationandinterpretationofsite and NGI did the geotechnical design verification specificlaboratoryprograms.Thisguidancewillhelp on behalf of the Norwegian government through its reducetherequirednumberofsitespecifictestsand agent, DNV. I was then a young engineer and was providequalitycontrolofthetestresults.Thecorrela- greatlyinspiredbyMcClelland’sachievementsinoff- tionsnaturallycontainsomescatterandcoveralimited shoregeotechnicalengineeringthatIlearntaboutin numberofsoiltypes.Forfinaldesign,theparameters connection with the Ekofisk project and from the from correlations should be verified by site specific literature. teststoavoidunwantederrororconservatism. One of the special issues with offshore structures Some of the correlations have been presented in isthattheyhavetowithstandseverecyclicwaveload- previous publications, but they have been updated ing.Inlateryearswindpowerstructureshavealsobeen and revised by including more data and additional placedoffshore,andthesestructureswillbesubjected variables.Thecorrelationsareprimarilyvalidfornon- to significant cyclic loading from the wind in addi- calcareoussoils,butsomecorrelationsforDSStype tion to the wave loading. Cyclic loading effects can ofshearingarealsogivenforcalcareoussoils. also be important for structures along the coast and The paper first gives examples of cases where onland.Cyclicloadingwillinfluencethestrengthand cyclicloadingisimportant,andpresentsthefounda- deformationcharacteristicsofthesoil,andfoundation tion design aspects and the parameters required for design of these structures requires that the effect of thefoundationdesign.Itcontinuesbyexplainingwhat cyclicloadingisaccountedforinthesoilparameters happenstothesoilwhensubjectedtocyclicloading thatareapplied. andpresentsacontourdiagramframeworktocharac- The aim of this paper is to present a framework terize the cyclic soil behavior.The contour diagram for cyclic soil behavior that can be used for practi- framework has been applied in offshore foundation calfoundationdesignofstructuressubjectedtocyclic design for many years (e.g. Andersen et al. 1988, loading, and to provide data and correlations with Andersen&Lauritzsen1988,Andersen&Høeg1991). 5 ISFOG CH001.tex 9/7/2015 16:32 Page6 Itisthereforenotanewconcept,butitissummarized 7.2 Cyclicshearstrength herein as background for the correlations.Advice is 7.3 Shearstrainsasfunctionsofaverageand also given about aspects that are important to con- cyclicshearstressesforaconstant siderwhenestablishingcycliccontourdiagrams.The numberofcycles contour diagrams can be used directly in design, as 7.4 Permanentshearstrain shown later, or they can be used to develop mathe- 7.5 Shearstrainsasfunctionsofcyclicshear maticalcyclicsoilmodels.Sincethecontourdiagrams stressandnumberofcyclesforconstant representnon-manipulateddata,mathematicalmodels averageshearstress shouldbeverifiedbycheckingthattheycanreproduce 7.6 Porepressure thecontourdiagramsandthevariousstressconditions 7.7 Damping thatthesediagramscover. 7.8 Importantparameters Thecyclicloadhistoryisnormallynon-symmetrical 7.9 Testingstrategy and irregular, and may need to be transformed into 8 Cyclicshearstrengthanddeformationproperties a simplified, more regular form. It is shown how foradesignstorm thiscanbedoneandhowtheregularhistorycanbe 8.1 Designstormcompositionandcycle applied in a pore pressure or a cyclic strain accu- counting mulationproceduretoestablishanequivalentnumber 8.2 Equivalentnumberofcycles,Neq of maximum cyclic loads, N , that gives the same 9 Samplepreparationandlaboratorytesting eq cyclicdegradationastheirregularcyclicloadhistory. 9.1 Samplepreparation Theaccumulationprocedureshavealsobeenapplied 9.2 Effectofconsolidationtime formanyyears(e.g.Andersen1976,Andersenetal. 9.3 Loadperiod 1992&1994)buttheirusetoestablishN aswellas 10 Staticstrengthcorrelations eq stressstrainrelationsandcyclicshearstrengthhasnot 10.1 StaticDSSstrengthofNCsandandsilt beenfullyexplainedinpreviouspublications. 10.2 Staticshearstrengthanisotropy,sand Following individual cycles through a cyclic load andsilt history with hundreds or thousands of cycles is not 10.3 StaticDSSstrengthofNCclay considered feasible in practical design, and the con- 10.4 EffectofOCRonstaticstrength tourdiagramframeworkisdefinedintermsofcyclic 10.5 Slopeoffailureenvelopeineffectivestress amplitudes, meaning that the stress-strain relation pathplots within individual cycles is not defined.A procedure 11 Monotonicstress-straincharacteristics is proposed, however, that can be used to define the 11.1 Normallyconsolidatedsandandsilt loaddisplacementrelationshipwithinacycleatagiven 11.2 Normallyconsolidatedclay timeintheloadhistory. 11.3 EffectofOCRonstress-strain Focus is given to the foundation design of struc- characteristics tures,butaproceduretoanalyzethestabilityofaslope 11.4 Initialshearmodulus,G max subjectedtocyclicloading,asfromanearthquake,is 12 Cyclicstrengthcorrelations proposedtowardstheendofthepaper. 12.1 CyclicDSSstrengthofNCsandandsilt Commentsandguidancearegivenonhowthesoil 12.2 CyclictriaxialstrengthofNCsandandsilt parameter framework can be applied in design, and 12.3 CyclicDSSshearstrengthofNCclay predicted and backcalculated prototype observations 12.4 CyclictriaxialstrengthofNCclay andmodeltestresultsarepresentedasvalidation. 12.5 Cyclicshearstrengthanisotropy Valuable work on cyclic soil behavior is done 12.6 Effectofpreshearing in many organizations internationally, but this paper 12.7 EffectofOCR concentratesonmodelsdevelopedandappliedatNGI. 12.8 Gravelandwellgradedsoil Thecontentofthepaperisorganizedinsectionsas 12.9 Carbonatesoils(non-cemented) follows: 13 Correlationsforcyclicstress-strain 2 Caseswithcyclicloading characteristics 3 Cyclicloadingcharacteristics 13.1 CyclicstressstrainasafunctionofNin 4 Foundationdesignrequirements DSStestsonNCsandandsilt 5 Soilparametersforcyclicfoundationdesign 13.2 ShearstrainsinDSStestsonNCsand 5.1 Cyclicsoildata andsilt 5.2 Monotonicdata 13.3 ShearstrainsasfunctionsofNin 5.3 Consolidationcharacteristics triaxialtestsonNCsandandsilt 6 Cyclicsoilbehavior 13.4 ShearstrainsintriaxialtestsonNC 6.1 Typicalstressconditions sandandsilt 6.2 Soilbehaviorundercyclicloading 13.5 Shearstrainsincyclictestsonclay 6.3 Examplesoflaboratorytestresults 13.6 Cyclicstress-strainanisotropy 6.4 Strengthlimitationduetodrainagewithina 13.7 EffectofOCR cycleandcavitation 14 Porepressure 7 Cycliccontourdiagramconcept 14.1 PorepressureinDSStestsonNCsand 7.1 Numberofcyclestofailure andsilt 6 ISFOG CH001.tex 9/7/2015 16:32 Page7 14.2 PorepressureintriaxialtestsonNC cyclicloadingcharacteristicscanthusvaryconsider- sandandsilt ably.Forinstance,waveloadingwilltypicallyhavea 14.3 Porepressureincyclictestsonclay periodof10to20sandthestormeventcanhaveadura- 15 Damping tionoftheorderof1dayandcontainsomethousand 16 Consolidationcharacteristics cycles.Thecyclicloadhistorywillbeirregularwitha 16.1 Constrainedmodulusformulation, cyclicamplitudevaryingfromonewavetothenext.In sandandsilt manycasestherecanalsobeanaverageloadcompo- 16.2 Correlationsformodulusformulation, nentthatcanvaryduringthestorm.Ontheotherhand, sandandsilt earthquakesmayhaveadurationofabout10to30s,a 16.3 Constrainedmodulusforclay loadperiodof∼1sandsometensofcycles,whereas 17 Slopestabilityundercyclicloading tidalforcesandstoragevariationscanhaveaperiod 17.1 Failuremechanismandstressconditions of12hrsandmore.Differentsourcesmayalsogener- 17.2 Laboratorytesting atecyclicloadingsimultaneously,likewindandwave 17.3 Laboratorytestresults for an offshore wind power structure. Resonance of 17.4 Timetofailure thestructurecanalsobeasourcethatgeneratesaddi- 17.5 Strengthrepairfromporepressure tional cyclic loading on the soil as a reaction to the dissipation primarysource.AnexampleistheGreatBeltBridge 17.6 Designprocedure wherebreakingicesheetssetthepillarsinmotion,thus 18 Calculationprocedures generatingcyclicloadingwithaperiodof∼1ssuper- 18.1 Capacity imposedontheprimary∼10scyclicloadingperiod 18.2 Cyclicdisplacements fromtheicesheets(Andersen2009). 18.3 Permanentdisplacements More examples and details of cases with cyclic 18.4 Equivalentsoilspringstiffnesses loadingcanbefoundinAndersen(2009),Jardineet anddamping al.(2012)andAndersenetal.(2013). 18.5 Foundationspringsforindividualcycles 19 Verificationbyprototypeobservations andmodeltests 20 Summaryandconcludingremarks 4 FOUNDATIONDESIGNREQUIREMENTS Acknowledgment References The major requirements to be addressed in cyclic foundationdesignareto: 2 CASESWITHCYCLICLOADING • ensuresufficientcapacity.Thecapacityundercyclic loading can differ significantly from the capac- Cyclicloadingeffectshavebeengivenmostattention ityundermonotonicdrainedorundrainedloading, inconnectionwithfoundationdesignofoffshorestruc- especially if the cyclic loading involves net load tures,traditionallyforoilandgasproduction,andmore reversal.Oneexampleshowingtheimportanceof recentlyalsoforoffshorewindpowerstructures.The loadreversalisthecapacityofdrivenpilesforjacket offshorestructurescanbefixedtotheseafloorbytheir structures where the capacity of the piles of the ownweight(gravityplatforms)orbypiles,orthefoun- windwardlegcanbelowerthanfortheleewardleg dationscanbemonopilesorskirtedfoundations.The becauseofgreaterloadreversalforthewindward offshorestructurescanalsobefloating,andthefoun- leg(Jardineetal.2012,Andersenetal.2013). dationdesignthenconsistsofdesigningtheanchors Forstructuresitisnormallythecapacitytocarry forthemooringsystem,whichcanbesuctionanchors, the cyclic loads that is critical, since the safety piles,draganchorsorgravityanchors. against a failure under static load is high in most Cyclic effects can also be important for founda- cases.Forslopestability,however,thesafetyagainst tiondesignofstructuresalongthecoastandonland, failureunderthestaticslopeweightcanbelow,and such as harbors, breakwaters, storm surge barriers, onemustalsoensurethatthetemporarilyreduced windpowerstructures,andforvibrationsfrominfras- staticcapacityduetocyclicloading(e.g.froman tructure and industry. Earthquakes will create cyclic earthquake)duringandsometimeafterthecyclic stressesinthesoil,andinfluencethestabilityofslopes eventissufficienttocarrytheweightoftheslope. andthebehaviorofbuildingsandotherstructures,both • demonstrate that cyclic displacements are tolera- onlandandoffshore. ble. Cyclic displacements can be a serviceability problem,andmayalsoinducestressesinstructural elementsinthesoilorconnectionstothestructure, 3 CYCLICLOADINGCHARACTERISTICS like oil wells, risers and pipeline connections.All displacementcomponents(vertical,horizontaland Cyclicloadscanhavedifferentoriginsandvarysignif- rotational)needtobedetermined,asforinstancea icantlyinamplitude,periodandduration.Theorigin rotationatseabedcanleadtosignificanthorizontal can be waves, wind, drifting ice sheets, icebergs, displacementsatdecklevelforatallstructure. earthquakes,tidalvariations,traffic,blasting,machine • provideequivalentsoilspringstiffnessesanddamp- vibrations, and emptying and filling of storage.The ingforuseinglobaldynamicsoil-structureanalyses 7 ISFOG CH001.tex 9/7/2015 16:32 Page8 andearthquakeanalyses.Thecyclicsoilspringstiff- Asmentionedearlier,allfoundationrequirements nessesareespeciallyimportantfortallandslender inSection4arenotrelevantforalltypesofstructures, structuresastheycaninfluencetheresonancefre- andthetypeandamountwillbecasespecific.Further quency.Iftheresonancefrequencyapproachesthe discussionaboutsoilparametersneededfordifferent cyclicloadfrequencythedynamicloadamplifica- casesandtheirdeterminationcanbefoundinAndersen tion will increase.The cyclic soil spring stiffness etal.(2013). is especially important for wind turbines, where the resonance frequency should be within a rela- tively narrow band determined by the operational 5.1 Cyclicsoildata rateofrotation.Soilspringstiffnessesintermsof Cyclicsoilbehaviorandparametersaredescribedin both cyclic and average components can also be detail and defined in the next section, but a brief important for multi-legged structures, as the soil summaryofrequiredcyclicparametersisgivenbelow. stiffnesswilldeterminethedistributionoftheloads Cyclicshearstrengthwillberequiredasafunction betweenthelegs.Soildampingisnotalwaysimpor- ofaverageshearstressandnumberofcycles. tant, but may be of relevance for tall and slender Thefollowingparameterswillbeneededasfunc- structures. tionsofcyclicandaverageshearstressesandnumber • assesswhetherincreasedlongtermpermanentdis- ofcycles: placementsduetocyclicloadingaretolerable.The predominantpermanentdisplacementwillnormally • Cyclic,averageandpermanentshearstrains beincreasedverticalsettlementsincaseofagrav- • Permanentporepressure itybasedstructureorstructureswithmoreorless • Volumetricstrain symmetrical loads, but permanent rotational dis- • Damping placementsmustalsobeconsiderediftheloadsare • Postcyclicstaticshearstrength notsymmetricaloriftherearelateralvariationsin The cyclic soil parameters are anisotropic and thesoilprofile.Forstructureswithlargehorizontal depend on stress path.Thus, compression, DSS and loads, like wind turbines, there can be significant extension tests are needed, unless the foundation permanenthorizontalandrotationaldisplacements. behaviorisgovernedbyonetypeofstresspath,asfor Thepermanentdisplacementswillaccumulateover horizontalslidingofagravityplatform.Thepostcyclic thelifetimeofthestructure,anditisnecessaryto shearstrengthisnormallyonlyneededinspecialcases, considermorethanasingledesignstorm.Thecal- likeslopestabilityunderearthquakeloading.Damping culationofpermanentdisplacementsmustinclude maybemostimportantfortallandslenderstructures componentsfrombothpermanentshearstrainsdur- andearthquakes.Volumetricstrainscanbecalculated ingcyclicloading,increasedcreepratedueeffective basedonpermanentporepressureandreconsolidation stressreductionfromcyclicloading,andvolumetric modulus,ordeterminedfromdrainedcyclictests,as strainsfromdissipationofcyclicallyinducedpore discussedinSection18.3. pressureandduetoshearinduceddilatancy. • assesshowcyclicloadingcanalterthebaseandside soilreactionstresses.Thecyclicloadingcancause 5.2 Monotonicdata redistribution along the base or the side, but also Monotonic data provide a useful reference for the a redistribution between the base and the side, as cyclicsoilparametersandareneededtoconstructthe observed in model tests of skirted foundations in cycliccontourdiagramspresentedinsubsequentsec- sand(Jostadetal.1997). tions.Inclays,itisconvenienttonormalizethecyclic Allrequirementsarenotrelevantforallstructures. parametersbytheundrainedmonotonicshearstrength. For anchors, for instance, sufficient capacity is the Forpiles,thecapacityformonotonicloadingisneeded mainrequirement,anddisplacementsarenotimpor- ifthecycliccapacityisevaluatedfrominteractiondia- tant.The requirements above are discussed in more gramsordegradationlawswherethecycliccapacity detailandillustratedbyexamplesinAndersen(2004) isnormalizedtothemonotoniccapacity(Jardineetal. andAndersenetal.(2013). 2012,Andersen et al. 2013). Monotonic parameters will also be needed if the cyclic load history or the drainage conditions are such that there is little pore pressuregenerationpriortothemaximumwave. 5 SOILPARAMETERSFORCYCLIC FOUNDATIONDESIGN 5.2.1 Undrainedparameters Undrainedmonotonicparametersthatmaybeneeded Thesoilparametersneededtoaddressthefoundation forbothclayandsandare: designrequirementsaregroupedintocyclicsoildata, • Undrainedshearstrength monotonicsoildataandconsolidationcharacteristics. • Undrainedstress-strainresponse Moreconventionalparameters,likeindexproperties, preconsolidationpressureandoverconsolidationratio Boththeshearstrengthandthestressstrainresponse arenotaddressedherein,eveniftheyarealsorequired areanisotropic,andcompression,DSSandextension inacyclicfoundationdesign. testsmaybeneeded. 8 ISFOG CH001.tex 9/7/2015 16:32 Page9 5.2.2 Effectivestressparameters The following effective stress parameters may be requiredforsandincaseswhereundrainedconditions maynotbeassumedtoapply: – Drained triaxial peak friction angle and slope of failurelineindrainedDSStests – Undrainedtriaxialeffectivestressfrictionangleand slope of effective stress failure line in undrained DSStests – Dilatancyangletodetermineshearinducedvolume changes – Interfacefrictionanglestoconsiderbaseslidingfor shallowfoundationsandshaftfrictionforpiles. Figure 6.1. Simplified stress conditions for typical ele- 5.2.3 Initialshearmodulus ments along a potential failure surface beneath a shallow foundation. Thestressstrainresponseisnon-linear,anditisimpor- tanttoknowtheinitialshearmodulusinordertomodel thestressstrainbehaviorproperly. Inthispaper,τ denotestheshearstressonthe45◦ Theinitialshearmodulusmaybeneedednotonly planeincompressionandextensionelementsandon nearthestructure.Thesmallstrainresponseinthefar- thehorizontalplaneinDSStests.Thecyclicloading fieldcanbeimportantbecausethestrainisintegrated isstress-controlled,sincethisisconsideredtobethe over a large volume and can give an important con- bestrepresentationforcycliceventsdefinedinterms tributiontothecyclicdisplacementsandsoilstiffness ofloads.ThisisfurtherdiscussedinSection8.2.4. evenifthemodulusishigh. The average shear stress, τ , can be expressed as a Theinitialshearmoduluscanbedeterminedfrom τ =τ +(cid:2)τ ,where: a 0 a bender elements, resonant column tests and in-situ shearwavetesting. – τ0 istheinitialshearstressinthesoilpriortothe installationofthestructure,τ =0.5·(1−K )·p(cid:3) intriaxialtests,andτ =0in0DSStests.p(cid:3) 0isthe0 0 0 5.3 Consolidationcharacteristics vertical effective overburden pressure, and K0 is thecoefficientofearthpressureatrest.Theinitial Theconsolidationcharacteristicsareneededtocalcu- shearstress,τ ,actsunderdrainedconditions,and 0 latethedissipationrateoftheporepressuregenerated thesoilisconsolidatedunderthisstress. by cyclic loading and the magnitude and rate of – (cid:2)τ ,istheadditionalshearstresswhichisinduced a permanent displacements due to this pore pressure bythesubmergedweightofthestructureandany dissipation. average environmental loads. (cid:2)τ will first act a Theconsolidationcharacteristicsaredefinedby: underundrainedconditions,butasthesoilconsol- idates,(cid:2)τ willalsoactunderdrainedconditions. – Virgin,unloadingandreloadingmoduli a Inthecaseofsand,drainagewilloccurrelatively – Coefficientofconsolidation. rapidly, and it is reasonable to assume that the Theconsolidationcharacteristicscanbedetermined soil consolidates under the weight of the plat- fromoedometertests,butforcleansandtheflowresis- formbeforethedesignstormarrives.Whetherthe tanceinfiltersandtubesmaybehighcomparedtothe part of (cid:2)τa due to environmental loads will act flowresistanceinthesandspecimen,andtriaxialtests undrainedordrainedwilldependondrainagedis- withlowflowresistancecanberequiredtomeasure tance,consolidationcharacteristicsofthesandand thecoefficientofconsolidation. thevariationoftheaverageloadduringthecyclic loadingevent.Asshownlater,thedrainagecondi- tions for (cid:2)τ can have significant impact on the a 6 CYCLICSOILBEHAVIOR cyclic shear strength. Consolidation occurs much slowerforclays,andwhenconservativeitmustbe 6.1 Typicalstressconditions assumed that the design storm occurs before any significantconsolidationhastakenplace. Thestressconditionsinthesoilaroundafoundation subjected to cyclic loading are complicated.A sim- Thecyclicshearstress,τ ,iscausedbythecyclic cy plifiedpictureoftheshearstressesalongapotential loads.Ingeneral,environmentalloadsandperiodvary failuresurfaceinthesoilbeneathashallowfoundation continuouslyfromonecycletothenext,andthecyclic is shown as an example in Figure 6.1.The elements shearstresswillalsovaryfromcycletocycle. follow various stress paths (compression, DSS and Todeterminethesoilpropertiesneededinthefoun- extension), and they will experience different com- dationdesignanalyses,thelaboratorytestsshouldfirst binationsofaverageshearstress,τa,andcyclicshear be consolidated to the in situ effective stresses, and stress,τcy. thensubjectedtoshearstressesthatsimulatethestress 9