3rd McClelland Lecture - Knut Andersen - Text

83 Pages·2015·55.02 MB·English

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