萨尔瓦多地震诱发的拉斯科琳娜滑坡稳定性分析外文翻译EARTHQUAKEINDUCED LANDSLIDE STABILITY ANALYSISOWord格式.docx

萨尔瓦多地震诱发的拉斯科琳娜滑坡稳定性分析外文翻译EARTHQUAKEINDUCED LANDSLIDE STABILITY ANALYSISOWord格式.docx

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Abstract:

TheJanuary13,2001earthquake（M7.6）offthecoastofElSalvadortriggeredwidespreaddamaginglandslidesinmanypartsofthecountry.TheLasColinaslandslide,locatedinBalsamoRidgewestofSanSalvador,causedthegreatestlossoflifeinasinglelocationfromtheearthquake.Thepreearthquakeandpost-earthquakeseismicstabilityanalysisoftheLasColinasslopeisstudiedusingthelimitequilibriumandfiniteelementmethodsinthispaper.Thesafetyfactorofthepre-earthquakeslopewaslikelyover1.50basedonthelowgroundwaterlevel,whichwasnotedbylocalresidentspriortotheJanuary13earthquake.Theslopefailedasthehorizontalearthquakecoefficient,kc,reachedbetween0.18gand0.31gdependingonthefailureanalysismethods（i.e.circleandtrialsurfacemethods）andthelowsoilstrengthassumed.Thesafetyfactorofthepost-earthquakeslopefallsinarangefrom1.851atlowwatersaturationto1.337whenfullysaturated.Ascomparedwiththepre-earthquakeslope,thestabilityisabout8%higherforthepost-earthquakeslope.

Keywords:

Earthquake,landslide,seismic,stability,limitequilibrium,finiteelement.

1、INTRODUCTION

ElSalvadorlocatesonthePacificcoastoftheisthmusandborderedbyGuatemalatothewestandHondurastothenorthandtheeast.Thecountryhasexperienced,onaverage,onedestructiveearthquakeeverydecadeduringthelasthundredyears.TheearthquakeofJanuary13,2001thatstruckElSalvadorwasthefirstmajorseismic

disasterofthethirdmillenniumandthefifthdestructiveearthquaketoaffectthissmallCentralAmericarepublicin50years（BommerandBenito,2002）.

TheJanuary13earthquaketriggeredtheLasColinaslandslideoffthesteepnorthernflankofBalsamoRidge.Thelandslideoriginatedatanelevationofabout1,040mto1,070mandtravelednorthwardabout700mto800mintotheLasColinasneighborhoodofSantaTecla,westofSanSalvador.Theverticaldropfromtheridgetotheneighborhoodisabout160m.Thevolumeofthelandslidematerialisestimatedatabout250,000m3（JibsonandCrone,2001）.

Accordingtoapriorinvestigation（JibsonandCrone,2001）,thelandslidematerialexposedintheheadwallscarpappearedsomewhatmoistbutnotsaturated,butoncethismaterialmobilizeditbehavedasasemi-liquidmasswithasoupyconsistency;

thisallowedthemasstotravelanunusuallylongdistancefromthebaseoftheslope.Thetransformationofthelandslidematerialfrombehavingassolidtofluidintheabsenceoflargequantitiesofwater（suchasinaheavystorm）andthelongrunoutdistanceindicateanabnormalmaterialbehaviorthatrequiresdetailedinvestigationandanalysis.

Landslidehazardsandstabilitystudyiscarriedoutbyseveralorganizationsintheearthquakerelatedareas.Thepre-earthquakeandpostearthquakeseismicstabilityanalysisandsensitivityanalysisoftheLasColinaslandslidearestudiedusingboththelimitequilibriumandthefiniteelementmethodsinthispaper.Safetyanalysisand

suggestionsaregiveninthefollowingsectionsofthepaper.

2.GEOLOGYANDVOLCANICSOIL

ThelandslidescarpoftheLasColinasslopeexposestheupper25m-30mofdepositsinvolvedinthelandslide.ThestratigraphyofthesedepositsistypicalofthevolcanicdepositsforBalsamoRidgeandmuchofthesurroundingCordilleraBalsamo.BasedonthereconnaissanceworkbyUSGS,thelocationsofabundantearthquakeinducedlandslidesintheCordilleraBalsamoseemtobeinfluencedbythepresenceofrelativelythickdepositsofTB（TierraBlancatephra）,whicheruptedfromavolcanicsourcenearLagodeIlopangoabout10,000yearsago,andtheoverlyingfreshandweatheredtephra.

TierraBlancaisadalaciticpumiceashcomposedofacidicandepiclasticdeposits,whichcoversmostoftheupperpartofSanSalvador,andispoorlyconsolidatedandoriginatedfrommultiplevolcaniceruptionsthatcanreachupto50mthick.Studydemonstratedthatoneofthemostimportantfactorsintermsofseismicslopestabilityistherelativelyweakcementationthatisbrokenwhenthesoilissubjectedtoevensmallstrain（Bommeretal.,2001）.Itismostprobablythatthesoilsmayexperienceadrasticreductionofshearstrengthduringearthquakeandhenceassoonastheslopefailurebeginsthematerialundergoesalmostadebrisflow.

Thestronggroundshaking,alongwiththepresenceofthick,loosetopoorlyconsolidated,youngvolcanicdeposits,thesteeptopographyonthenorthernflankofBalsamoRidge,andpossiblythepresenceofarelativelyimpermeableancientsoilatthetopoftheBalsamoFormationwerecontributingfactorstotheLasColinaslandslide.Thecross-sectionoftheLasColinasslopeisshowninFigure1.Therelativelythinlayerofpaleo-soilmighthaveplayedanimportantroleininitiatingthemovement.Therearefourmaingeologicalstrataintheslopesection:

1）pyroclasts;

2）brownashes;

3）1.0m~1.5mpaleo-soils;

and4）consolidatedtuffsandpyroclasticflows.Figure2isaprofileatthetopoftheLasColinaslandslidewherelandslidemitigationisunderconstruction.Thelightcolorformationnearthebottomoftheprofileispaleo-soil.

Figure1.Cross-sectionoftheLasColinasslopeshowingthegeologicalstratum

Figure2.AprofileatthetopoftheLasColinaslandslide

Oneofthemostcriticalstepsinslopestabilityanalysisistodeterminetheshearstrengthparameters（candf）alongtheslidingsurface.Laboratoryandfieldsoiltesting（e.g.Standardpenetrationtest,downholeseismicvelocity,unconsolidatedtriaxialtest,etc.）wereperformedbyC.LottiandAssociati（2001）.ThephysicalandmechanicalpropertiesofthesoilsatthesitearesummarizedinTable1.

Theavailablestronggroundmotionrecordings（UCA2001）duringtheearthquakeareshowninFigure3.Thelargestpeakgroundvelocity（PGV）wasobservedattheSantaTeclaStation（Te）withavalueofapproximately57cm/sandapeakgroundacceleration（PGA）of485cm/s2（around0.5g）.Thisgroundmotionwassufficientlylargetotriggerlargelandslideinthearea.

Table1.SoilpropertiesoftheLasColinasslope（C.Lotti&

Associati,2001）

Figure3.AccelerationhistoryfromTEstation

3.SLOPESTABILITYANALYSIS

Thepseudo-staticlimitequilibriumanalysisandthefiniteelementmethodwereemployedinslopestabilityanalysis.TheprimarymethodofevaluationwasbasedontheModifiedBishopmethodsforcircularandtrialsurfacefailures.Forcomparison,JanbucircleandrandomsurfaceandSarmamethodswerealsousedtoanalyzethesafetyfactorofthelandslide.Thepotentialslidingsurface,circleorpolygonal,canbepre-specifiedorrandomlygenerated.Theprograms,PCSTABL5（Achilleos,1988）developedatPurdueUniversityandReinforcedSlopeStability（RSS）byGeocompCorporation（1996）,wereusedinthestaticandpseudo-staticlimitequilibriumanalyses.ABAQUS/Standardwasusedinthefiniteelementanalysis.

3.1Staticanalysis

3.1.1Pre-earthquake

Thefailureplanewaslikelyrestrictedbytheconsolidatedpyroclasticflows,whichhavemuchhigherstrengthpropertiesthantheotherformationsabove.Themostprobableslidingplanewouldbealong（orthrough）thethirdstratum,paleo-soilsthatisrelativelythinandhastheloweststrength.BothBishopandJanbucirclemethodsemployautomaticsearchforafailuresurface.However,thetrialsurfacemethodrequiresapre-determinedslidingsurface.Inthestudy,aslipsurfacefollowingtheweakpaleo-soillayerwastreatedasapotentialslidingplane.Theeffectofgroundwaterontheslopestabilitywasassessedatdifferentwatersaturationconditions.ThelocationsofgroundwatertableareshowninFigure4.

Threestrengthgroups（i.e.low,medium,andhigh）wereconsideredforthesensitivityanalysisofslopestability（Table2）thatincludesrangesofcandffoundfromthefieldandlabtesting.Table3summarizestheanalysisresultsbasedondifferentmethodsandfailuremodeswiththeassumedwatertablemoreorlessfollowingthepaleo-soillayer.

Table3.Thesafetyfactorofpre-earthquakestabilityanalysiswithwatertablefollowingthepaleo-soillayer.

Table2.Strengthparametersgroupsusedinstabilityanalysis.

Theresultsofpre-earthquakestaticstabilityanalysisindicatethattheslopewiththeloweststrengthpropertiesandunderthesaturationconditionnotedinthefieldwouldhavebeenstableifnotbecauseoftheearthquake.Eveniftheslopewasfullysaturated,itwouldstillbestablewithsafetyfactorlargerthan1.Themostprobablecircularfailuresurfaceofafullysaturatedslopeisacirclecuttingthroughthetopthreestrata,followingtherelativelyweakthirdstratum,andthenemergingoutneartheboundarybetweenthefirstandsecondstrata.ThesamecircularfailuresurfacewasanalyzedusingtheSarmamethod,anditgivesasafetyfactorof1.52.

Themostcriticalsurface,however,isdeterminedbasedonthetrialsurfacethatfollowstheactualslipsurfaceobservedinthefield（Figure4）.Theminimumsafetyfactorforafullysaturatedslopewiththeloweststrengthpropertiesis1.235basedontheJanbutrialsurfacemethod.TheBishopTrialsurfacemethodyieldedasimilarResult.

3.1.2Post–earthquake

Duringearthquake,thepotentiallyunstablesoilmassdetachesfromthestrata,andresultsinamorestableslope.Theresultsofpost-earthquakeslopeanalysis（i.e.thecurrentslopeatthesite）arelistedinTable4.Therangesofsafetyfactorarehigherthanthatofthepre-earthquakeslopeunde