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D.LEYNAUD&

J.MIENERT

文献出处：

MarineGeology213

（1）:

457-480,2018（如觉得年份太老,可改为近2年，毕竟很多毕业生都这样做）

英文2916单词，18408字符（字符就是印刷符），中文4673汉字。

（如果字数多了，可自行删减，大多数学校都是要求选取外文的一部分内容进行翻译的。

）

SlopeStabilityAssessmentoftheTranadjupetSlideArea

OffshoreTheMid-NorwegianMargin

Abstract：

Large-scalesubmarineslidesoccurredduringtheHoloceneonthecontinentalslopeoffshoremid-Norway,northandsouthoftheV0ringPlateau.TheTraenadjupetslideeventthataffectedanareaof14100km2islocatednorthoftheV0ringPlateau.Itoccurredabout4,300yearsB.P.,4000yearsafterthegiantStoreggaslidethataffectedanareaofabout112,500km2.Aslopestabilityevaluationwasperformedinordertoexplainwhytheslidingtookplaceonaverygentleslope（1degree）.ThiswasdonefirstwiththedeterministicapproachusingtheLimitEquilibriumandtheFiniteElementmethods,forstatic,pseudo-staticanddynamiccases.Thentheprobabilisticapproachwasappliedusingthelimitequilibriummethodwiththe1stand2ndorderreliabilitymethods（FORMandSORM）andtheMonteCarlosimulationtoincludetheparameteruncertainties（soilsparameters,seismicloading）.TheFiniteElementmodellingindicatesthattheslidetriggeringimpactedpreferablytheupper40metersofthesedimentcolumn.ThetriggercouldhavebeencausedbyonelargeearthquakeofmagnitudelargerthanMS5.8（retrogressivefailures）butcyclicloadingduetoseveralearthquakescouldalsoexplaintheslide,affectingtheshearingresistanceintheNYKcontouritedriftunit（weaklayer）byexcessporepressuregeneration.

Keywords:

SlopeStability,Submarineslide,LimitEquilibrium,FiniteElement,FORM&

SORM

1Introduction

Submarinelandslidesarecommonlyobservedonpassiveandactivecontinentalmargins,particularlyonthecontinentalslopewherethesteeperpartofthemarginincreasestheeffectofgravityonthedownslopeforcesactingonacertainvolumeofsediment.Amongthemostobvioustriggersofsubmarineslopefailuresonecanfindcyclicloadingfromearthquakesorwaves,gashydratedecompositionandexcessporepressure,over-steepening,andundercuttingofslopes.Thedifficultyonefacesinassessingslopestabilityisthefactthatsubmarineslopefailuresmayoccurevenonverygentleslopeswherethedownslopeforcesareminor.Thefailuremechanismisfarfromverywellunderstoodbyusingonlygeotechnicalin-situmeasurements.Asmanyparametersareinvolvedinthismechanism,theprobabilisticapproachisusedtoobservetheeffectsofuncertaintyonthelikelihoodoffailure.Itwillbeusedtoimproveourknowledgeaboutthesedimentthicknessvulnerabletofailureandthefailureprobabilityduringaspecifictimeperiod.

2Traenadjupetslidearea:

GeologicalandGeotechnicalsettings

TheTraenadjupetslidefieldislocatedtothenorthoftheV0ringPlateau（Figure1）whiletheStoreggaSlideliestothesouthofit.BothslidesoccurredduringtheHolocene,theStoreggaslideduringamultiphaseeventat8300yrsBP（Haflidasonetal.,2001）andtheTraenadjupetslideatapproximately4000yrsBP（Labergetal.2002）（Figure2）.ThemeancontinentalslopeangleoutsidetheTraenadjupetslideareaisapproximately1degree（Lindberg,2000）.Theaveragegradientwithintheslidescarareais1.25degreesandatthesidewall25degrees（Labergetal.,2002）.Theslideheadwallislocatedatawaterdepthof300meters.ThegeotechnicalparametersofthelastglacialinterglacialsedimentsdepositedjustnorthandsouthoftheV0ringPlateauareassumedtobesimilar.Wehaveusedthegeotechnicaldatafromborehole6606/3-GB1（850mwaterdepth）ofthesouthernV0ringPlateau.Foursoilunitsweredefineddowntoamaximumdepthof106meters（Tables1and2）.Theidentifiedsoilunitsaredescribedasverysoftclay（unit1）,mediumtostiffsiltysandyclay（unit2）,stifftoverystiffclay（unit3）andverystifftohardclay（unit4）.

Figure1:

LocationofthetraenadjupetslideoffchoreNorway（fiomVorrenetal.,1998）andborehole6606/3-GB1.

SketchoftheTraonadjupetslide

Figure2:

SketchofprofilealongtheTrwnadjupetSlide（fromLabergetal,2002）.

Table1:

Summaryofsoilconditionsandthebasicrecommendedsoilparametersforborehole6606/3-GB1NYKslope.

Table2:

PhysicalandgeotechnicalpropertiesofthelateweichselianglacigenicsedimentsandtheNykcontouritedriftsediments（fromLabergetal,2002）

3MethodologyandBasicconcepts

Thetotalstressactingonsedimentsofasubmarineslopeisrelatedtotheweightofthewater（abovetheseafloorandintheporesspaceofthesediment）andtheweightofthe（solids）sedimentwithinthisvolume.Thus,therealstressactingonthesedimentmatrixisreducedbytheeffectofwaterpressure（TerzaghiandPeck,1967）andiscalledtheeffectivestress.Theeffectiveunitweightofthesoilisthenconsideredastherealunitweightlesstheweightofwaterandiscalledthesubmergedunitweightofthesoil.ThemodelusedinthisstudydefiningthemechanicalbehaviourofthesedimentstopredictthefailurepotentialistheMohr-Coulombmodel.Inthiscommonlyusedmodel,theshearingresistancesperunitofareaisrelatedtothenormalstressactingonthesoilataspecificdepth,usinganempiricalequation.

4DeterministicSlopeStabilityEvaluationMethods

4.1LIMITEQUILIBRIUMMETHOD

Thelimitequilibriummethodevaluatestheforces（orstresses）resultingalonganassumedfailuresurface.Thismeansthatthefailureoccurswhentheshearstrengthisfullymobilized（staticequilibrium）.Forconcavefailuresurfaces,wehavetousethemethodofsliceswhichdividesthesoilvolumeabovetheslipsurfaceintoverticalslicesandconsiderstheequilibriumofeachslice.Theforcesareestimatedatthebaseofeachsliceandaresummedoverthelengthofthefailuresurfacetogetanestimateofthestability.

TheFiniteElementmethod（FEM）isbasedontheconceptofmodellinganobjectwithsimpleblocksorsmallelements.Oncethestructureisdefinedwithelementsandnodes,onecandescribethephysicalbehaviourofeachelement.Thentheelementsareconnectedtoapproximatethewholesoilbehaviour.Also,onecanestimatethestrainandstressatselectedelements.Elasto-plasticanalysesofgeotechnicalproblemsusingthefiniteelement（FE）methodhavebeenwidelyacceptedasamoreaccurateprocedure.

5DescriptionoftheProbabilisticapproach

Whilethedeterministicapproachusesonlyaconstantvalue（meanvalue）foreachparameterrequiredtodescribethesoilbehaviour,theprobabilisticapproachconsiderthespatialvariabilityoftheseparametersanddefinethemusingaprobabilisticdensityfunction.

5.1M0NTECARLOSIMULATION

Onewaytoestimatetheexpectedvalueandthestandarddeviationoftheperformancefunctionistheuseofsimulationmethods,oftenreferredasMonteCarlosimulation.Theperformancefunctiondefinesthelimitstatebetweenthesafeandthefailuredomains.IntheMonteCarlosimulation,valuesoftherandomvariablesaregeneratedfollowingtheirprobabilitydistribution,andtheperformancefunctioniscalculatedforeachgeneratedset.Thisprocessisrepeatednumeroustimes,typicallythousands,andtheexpectedvalue,standarddeviationandprobabilitydistributionoftheperformancefunctionareestimatedfromthecalculatedvalues.

5.2FIRSTANDSECOND-ORDERRELIABILITYMETHODS

Thefirst-andsecond-orderreliabilitymethods（FORMandSORMrespectively）areemployedtoapproximatetheprobabilitybylinearizationoftheboundaryofthefailuredomain.ThemaintaskistodefinethesafetyfactorsummingthedifferentforcesappliedonthewedgesandthendefinealimitstatefunctionorperformancefunctiongX,suchthatgX0whentheslopeisstableandgX0whentheslopehasfailed.Xrepresentsavectorofrandomvariablesincludingsoilproperties,loadeffects,geometryparametersandmodellinguncertainty.ThesubroutinesdevelopedbyGollwitzeretal.（1988）wereusedfortheFORMapproximation.

6.2DYNAMICAPPROACHWITHFINITEELEMENTS

FortheFiniteElementmethod,arepresentativeaccelerogram（accelerationvs.time）isnormalizedtothePeakGroundAccelerationvalueexpectedintheareaforaspecificreturnperiod（assumingasimplelinearbehaviour）inordertocreateaneventprovidingtheexpectedseismicaccelerations.TheearthquakerecordusedtomodeltheseismicloadinginthestudyareaistheFriuliTarcentoearthquakewithadurationof33.18seconds（sampling:

0.02sec）.

ThePeakGroundAcceleration（PGA）representsthemaximumvalueoftheaccelerationexperiencedbyasmallparticleofthesoilduringtheearthquakemotion.Thehorizontalcomponentofthisparameterisusedinthepseudo-staticapproachtohaveanestimateoftheacceleration-inducedshearstressdevelopedinthesoil.AcommonwayistoconsiderthePGAwithaprobabilityofnoexceedanceduringacertainperiodoftime.IntheEurocode-8regulations,andforconventionalbuildings,aPGAvaluewith90%probabilityofnoexceedanceduring50yearsisrequired,whichcorrespondstoa475-yearreturnperiod.ThemaximumPGAvaluesconsideredfor475and10000yearreturnperiodsforNorwayareshowninTable3（NORSAR,1998）:

Returnperiod（years）

PGA（g）

475

0.10g

10000

0.35g

Table3:

PeakGroundaccelerationestimatedfor475and10000yearreturnperiod

7Softwares

7.1SLOPEAV（LimitEquilibrium）/QUAKEAV（FiniteElement）

SLOPE/W（GEO-SLOPE,2001）isagraphicalsoftwareproductthatuseslim