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 ANALYSISO.docx
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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EARTHQUAKEINDUCEDLANDSLIDESTABILITYANALYSISO
1英语原文及翻译:
EARTHQUAKE-INDUCEDLANDSLIDESTABILITYANALYSISOFTHELASCOLINASLANDSLIDEINELSALVADOR
H.Y.LUO,W.ZHOU,S.L.HUANG,G.CHEN
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