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StudyonthedeformationcharacteristicsanddynamicmechanismofseismicfracturesinWenchuanearthquakeshatteredslopesJinKeYUAN,RunQiuHUANGandXiangjunPEIStateKeyLaboratoryofGeohazardPreventionandGeoenvironmentProtection,ChengduUniversityofTechnology,Chengdu610059,SichuanE-mail:yuanjingke@163.
comAbstractNumerousshatteredslopestriggeredbywenchuanearthquake,theshatteringareawerelargeandtheseismicfractureswerewidedistribution,mostlythreatenedthepost-hazardreconstruction.
Accordingtothecharacteristicsofseismicfracturesinshatteredslopes,theresearchresultsshow:(1)Theseismicfractureswasdividedintosixtypes:surfacetype,transfixingtype,striketype,tendencytype,closuretype,extensiontypefromthroughsurveythemorphologicalcharacteristicsanddeformationindications;(2)OnthewholetheSeismicfractureswasN30~60Etrending,whichaccordswiththewholestrikeofactivefaultinwenchuanearthquake;(3)mainlydistributedinseismogenicfaultsareaorridge、andsteepwithslowborderarea;(4)wellextension,highopendegree,upperwideandlowernarrowofprofilemorphology,fillingslessorno,mainlymixedstonefragments,withaerialphenomena.
Basedonthetheoryoffracturemechanics,themechanismofseismicfractureswasbelongtotensilecrack,thedeformationandfailureprocessofstructuralplanesunderdynamicactionwasdividedintorupture,expansionandarrestabilitythreestages.
Keywords:wenchuanearthquake,shatteredrockmass,seismicfracture,geohazards1.
IntroductionTensofthousandsofcollapse,landslidesandsecondarygeologicaldisasterstriggeredbyWenchuanEarthquake.
Earthquakeinducedalargenumberofslopegeologicaldisasters,alsocausedapotentialdisaster—Slopeshattereddeformablebodywhichnamelyshatteredrockmass.
Theshatteredrockmassasageologicalphenomenon,itwasmainlyaffectedbytheearthquakeforceunderthecertaingeologicalconditions(Liangetal.
2003;Fengetal.
2009;Liangetal.
2009).
Thedynamiceffectofseismicwaveinrockmassdestroyedtherockmassstructure,itswholestructurevaried(Shao2005;Zuoetal.
2008).
Themosttypicalfeatureofslopeshatter-destroyedisthattheseismicfracturesdevelopedThedeformation,instabilityofshatteredrockmasshascontrolledbyseismicfractures,itisapotentialdisaster.
Theshatteredrockmasshasnomacrodestructionfeaturessuchaslargedeformation,largedisplacementasotherdestructionforms,itwasaninherentrockmassdamage,alsoitwasthemaincauseofsecondarydisastersaftertheearthquaketakenplacefrequently.
Studytheseseismicfractureshasimportantsignificancetorecognizethedeforma-tionandfailure,formationmechanismsofshatteredrockmasscorrectly.
2.
SeismicfractureclassificationsandfailurecharacteristicsTherockmassstructurewerecrackedopenbyearthquakeforce,sotheseismicfractureformed.
Theclassificationofseismicfractureshouldcorrespondstoitsformationmechanisms,nowtheclassificationresearchtofracturesmainlybasedonthevisual,intuitiveshapeofsurvey(Xie1988).
Theclassificationanalysistofracturesmainlybasedonitsdistribution,developmentcharacteristics,deformati-onfeatures,andmakeitpossibletopredictitsdeformationanddestruction.
Basedonthisconsideration,theseismicfractureswasdividedintosixtypes:surfacetype,transfixingtype,striketype,tendencytype,closuretyp,extensiontypefromthemorphologicalcharacteristicsanddeformationfeatures.
2.
1SurfacetypeseismicfracturefeaturesandfailurecharacteristicsSurfacetypeseismicfracturewasrefertotherockmasssurfaceshatteredlongerlengthfromitscharacteristics,butitswidthandvisibledepthisgenerallysmall.
Smallscaleofsuchfracturesweredevelopedinallkindsofslopestructure,andthereisnoobviousdirectionaldistribution,itdistributedintherockmasssurface,andhadpoordeepinterconnectivity(Fig1).
(Ⅰ)(Ⅱ)Fig.
1SurfaceseismicfractureInaccordancewiththeextensioncharacteristicsofsurfacetypeseismicfracturecanbedividedintobrokenlinetype,Lineartype.
Fromitscombinationfeaturescanbedividedintoen-echelontype,crossovertype.
[1]brokenlinetype:Thespreadingdirectioninplaneofseismicfractureswasturning、bentpointofthepolylineorarc-shaped,andthencontinuetoextend(Fig.
1Ⅰ).
[2]Lineartype:Theoverallspreadingdirectioninplaneofseismicfractureswasextendedstraight,littlechangeindirection,nosignificantturningorbendingintheprocessofextending(Fig.
1Ⅱ).
[3]En-echelontype:itformedbyaseriesofseismicfractureswhosedirectionsubstantiallythesame,en-echelondistributionpresentedontheplane,seismicfractureswerebrokenlineorlineartype(Fig2).
Fig.
2Distributionschematicdiagramofen-echelontypeseismicfracture[4]crossovertype:itformedbythetwoorthreegroupsofseismicfractureswhoseextensiondirectionweredifferent,crosseachother,thedistributionshapeontheplanewas"X"or"Y"shaped(Fig3).
(Ⅰ)"X"shapedseismicfracture(Ⅱ)"Y"shapedseismicfractureFig.
3CrossovertypeseismicfractureSurfaceseismicfracturewereexistintherockmasssurface,theycan'tcontrolthedeformation、instabilityofrockmass.
However,thistypeofseismicfracturesdistributedgenerally,higherdensitydevelopedinlocalposition,therebyitdestroyedtheoverallstructureofrockmass.
Anditeasytobecomeagoodwaytorainwaterinfiltrationwhenexperiencerainfallconditions.
2.
2TransfixingtypeseismicfracturefeaturesandfailurecharacteristicsComparativelyspeakingtoseismicfracturesurface,thetransfixingseismicfracturenotonlylongextendinglength,butalsolargeopeningdegree,visibledepthwasupto10m.
Thefracturehadgoodinterconnectivity,linearextensioninplane,jointinterfacejagged,throughoutinsidetherockmassbytheformofpinch-out.
Forsurvey,themajorityofseismicfracturesbelongtothistype(Fig.
4).
Fig.
4SeismicfracturethroughoutinsiderockmassThedevelopmentscaleofransfixingseismicfractureswerelargeandhadgoodinterconnectivity,itcausedsignificantdamagetorockmassstructureandaffectthedeformationandinstabilityofrockmass.
Throughtheinvestigationoffracturesonslope,suchfracturesdevelopedinbetterpositionasslopetop,steepslowborder,freefacetoformedthetrailingedgeorsideedgeboundaryofcuttingrockmass,itbecomethemaincontrollingfactorsofshatteredrockmassstability.
Seismicfracturescuttingrockmasstoformedthedangerousrockbodyonslope(Fig.
5).
Fig.
5DangerousrockformedbyseismicfracturecuttingSeismicfractureswasfillinglessorno.
Theycouldfurtherextendedwhenencounteredtherainfallorvibrateinterference.
Whenthefracturescontinuedtorunthroughthelockedsegmentinshatteredrockmass,thelockedsegmentcouldsuddenlysnippedoncethetensilestressofrockmasswasgreaterthanthetensilestrengtundertheactionofgravity,earthquakeforceevenwind.
Thendangerousrockseparatedfromthematernalrocktodropandformedrollingrock.
Throughthecollapsesinvestigation,theperforateseismicfracturesbecamethemaincontrollingfactorofcollapsedisastersafterearthquake.
Suchseismicfracturesweredevelopedinalargescal、stronginterconnectivity,theyoftencausedgreatcollapsehazard.
2.
3StriketypeseismicfracturefeaturesandfailurecharacteristicsAccordingtotheextensiondirectionofseismicfractures,thestriketypeseismicfracturewasrefertoitslengthalongthestrikedirectionwaslargerthanalongthedipdirection,fracturedevelopedmainlyalongitsstrike.
Theextensiondirectionofstriketypeseismicfracturewasidenticalorsimilartothedirectionoftectonicforce(suchasthestrikeoffault).
Theseseismicfracturesextendedalongitsstrike,theyhadstrongdirectional(Fig6).
Sincethistypeofseismicfracturesdevelopedwasmainlyduetotheeffectoftectonicforce,sotheirdevelopmentalscaleweregenerallylarger.
Fig.
6SeismicfracturedevelopednearthefaultStriketypeseismicfracturesexposedonslopebylesserextentofsubjectedtotheinfluenceofterrainandsurfacefeatures,theirextensionlengthalongthestrikedirectionwaslarge.
Seismicfracturesweremainlydistributedinthevicinityoffault.
Itwasduetothestrongearthquakeforceexistedatthefault,higherdegreeofrockmassshattered,fracturescouldeasilydevelopedalongthestrikedirectionoffault.
Andseismicfractureswhichexistedatotherlocations,owningtotheirextendprocesswaslimitedbytheinfluenceofterrainandsurfacefeatures,sothecharacteristicsalongthestrikeextensionwasnotobvious.
ThestrikeofthistypeseismicfracturewasN30~60Ebyfieldinvestigation,seismicfractureswhichdevelopedalongtheirstrikedirectionwerelocatedinthevicinityoffault.
Thelengthalongstrikeofstriketypeseismicfracturewaslonger,itcouldconstitutedtheinstabilityrangeboundaryofshatteredrockmassandcontrolledawiderangeofrockmassunstable.
Therockmassstructurearoundthistypeofseismicfractureswaspoor,higherdegreeofshatter-damaged.
Oncetheshatteredrockmassproduceddeformationandinstability,majordestructiveformed.
2.
4TendencytypeseismicfracturefeaturesandfailurecharacteristicsThetendencytypeseismicfracturewasrefertoitsextensiondirectionmainlyalongitsdipdirection.
Forthedevelopmentalmorphology,thelengthalongdipdirectionwasgreaterthanalongitsstrikedirection,itsdevelopmentwasextendedintothedeepofrockmass.
Thistypeofseismicfracturesdevelopedinvariouslocationsonslope,especiallydevelopedatridge、steepwithslowborderarea.
Mostseismicfracturesbelongedtothistendencytypethroughinvestigatedonslope.
Althoughitslengthexposedongroundwasshort,owningtoitwasextendedintothedeepalongitsdipdirection,itsvisibledepthwasupto10minthedeepofrockmass.
Therearecertaindistributioncharacteristicsinsuchseismicfractures,theLaddertype、"Y"-shapetypeandTrace-backtypehadpresentedinthemorphologyofprofilecombination(Fig.
7).
Laddertypeseismicfractureswascharacterizedbyaseriesoffractureswhichinclined、extendedtothesamedirection,parallelornearlyparalleltoeachother.
Duetothevisibledepthlimitedinsuchseismicfractures,sothistypefracturesmayalsointersectedinthedeeperpositionofrockmass.
Thecharacteristicsof"Y"-shapetypeseismicfractureswasthataseriesofdifferenttendenciesseismicfracturescrossedintheextendprocesstothedeep,"Y"-shapepresentedintheprofile.
Trace-backtypeseismicfracturewasrefertothefracturesoccurredoneormorebranches,thesecondaryseismicfractureswhichcontinuousorintermittentappearedinitsextendprocessintothedeep.
Fig.
7CombinationschematicdiagramoftendencytypeseismicfracturesTendencytypeseismicfracturescuttingrockmass,constitutedtheinstabilityboundariesofshatteredrockmass,formeddangerousrockhangedontheslope.
Owningtothelengthofseismicfractureextendingtodeep,goodcontinuitywithaerialphenomenon,itoftenbecomeagoodchannelforrainwater、groundwaterinfiltration,couldeasilycausetheinstabilityofshatteredrockmasstocollapse.
Thistypeofseismicfractureswaswidedistributiononslope,sotheformationofdamageisrelativelylarge.
2.
5ClosuretypeseismicfracturefeaturesandfailurecharacteristicsFortheextendingtrendenceinthedistributionplaneofseismicfractures,thejointinterfaceonthistypeoffracturesuneveninextensionprocess,atbothendsofthefractureclosedornearlyclosed,buttheintermediateportionisopen,smallopeningwidth(Fig.
8).
Fig.
8ClosuretypeseismicfractureClosuretypeseismicfracturesweremainlydistributedinthebetterpartsofrockmassstructure.
Thebettertheoverallstructureofrockmass,thelessdamageextentbyseismicforcestotherockmassstructure.
Seismicfracturesoccurredbyrockmasstensioncrackintheextendingprocesswasrestricted,itdiscontinuousdistributedbytheformofbeaded-shapeinslopesurface.
Thisfracturewassmalldegreeopening,thejointinterfaceonbothsidesinrockmassgraduallyfillingatlaterundertheactionofgravity、fracturedeposits.
Sincethistypeofseismicfracturesintermittentdistributed,poorcontinuity,smalldegreeopening,thejointinterfacegraduallyhealinglater,devastatinglimited,butitdiscontinuousdistributedintheslopesurface,withacertaindegreeofductility,itdestroiedtheoverallstructureofslope.
Seismicfracturefillinglessorno,overheadrockmass,itcouldconstituteagoodchannelforrainwater、groundwaterinfiltration.
Onceitconnectwithotherfractureswhichwaslargeopening、goodcontinuity、stronginterconnectivity,thesefracturescouldcausecrackingscaling,acceleratingtherockmassdeformationinstability,sodestructive.
2.
6ExtendedtypeseismicfracturefeaturesandfailurecharacteristicsExtendedtypeseismicfracturereferstoitsextensioninthesection,oneendwasgraduallyopen,andextendedtotheslopesurfacewithopeninggraduallylarge,oneendgraduallypinchoutintothedeep,andhadthewedge-shapedgapofwideupper-narrowlower.
Thiskindsoffractureswaslargeopeningdegree,fillinglessorno,thejointinterfacewasroughandjagged(Fig.
9).
Fig.
9ExtendedtypeseismicfractureThroughslopeseismicfracturesinvestigation,thefracturewhichwaswideupper-narrowlowershapedinsection,largedegreeopening,overheadrockmass.
Itmostlybelongtotheextensionseismicfracture.
Thisseismicfractureformatedbythehighdegreeofshatterdamagedcausedbyseismicforcestotherockmassstructure,severelydamaged,thedevelopmentoffracturesextendedalongthedirectionofseismicforce.
Usingthetheoryoffracturemechanics,fracturesbegintoexpandwhenitstipstressintensityfactorisgreaterthanrockmassintensityfactorbyseismicforce,thiscanberoughlydividedintothreestages:initialshatteredstage,fractureformed,developmentstage,fracturepropagation、extendingtosurfacestage(Linetal.
1986).
Extendedtypeseismicfractureswerewidelydevelopedontheslopesaccordingtothesurvey,especiallydevelopedatcrest、ridgeorsteepwithslowborderarea,acertainsizedistributed.
Thisseismicfracturesnotonlydestroiedtherockmassstructure,butalsolargedegreeopening,overheadrockmass.
Aftertheearthquake,frequentaftershocks、rainfallandwindforceswereconstantlychangedthestateofshatteredrockmassbalance,thesemadetheseismicfracturescontinuestoexpand、extend.
Thesefracturesleadtherockmassinstabilityandcollapse.
3.
SeismicfracturedevelopmentcharacteristicsInvestigatedthebasiccharacteristicsofseismicfractures,theirspread、developmentcharacteristics、deformationsignshadsomecorrespondingcharacteristics.
3.
1SpatialdistributioncharacteristicsSeismicfractureswasN30~60Etrending(Fig.
10),itisbasicallythesamewiththetriggeringseismicfaultsinWenchuanearthquake,theextensiondirectionoffractureshasobviouslyeffectedbythetriggeringseismicfault.
Somelargerdegreeofshatteredopeningfracturesspreadbylinearshaped,goodextension,generallynotsubjectedtotheinfluenceoftheterrainandsurfacefeaturesinitsextendingdirection.
Someseismicfractureswerecorrespondingwiththefaultwhichinfluencedtheirdevelopment.
Americanscholars(Leonard1929;Holzeretal.
1987)putforwardedtheviewoftectonicorigininseismicfracturesbyanalysistheearthquakeforce,theythinkedthetectonicisthedominantfactorinformationanddevelopmentofseismicfractures.
Fig.
10StatisticalgraphoftypicalseismicfracturesThedistributionofseismicfracturesweremainlyduetotheeffectoftriggeringseismicfaults,theyweremainlydistributedinthevicinityofcausativefaultorridgespositionandsteepslowborderinslope,thesepositionswerealsothesiteofstrongseismicforce.
3.
2ExtendedlengthcharacteristicsTheextensionlengthofseismicfracturereferstoitscontinuouslengthexposedontheslopesurface.
Thefracturesextendedlengthgenerallylonger,morethan6m.
Someshatteredseriousfractureswasupto10minlength,portionextendedtothetopofslope.
Collapsepit、tensionalslotwereexposedintheprocessofextending(Fig.
11).
Largenumberofsecondaryseismicfracturesweredistributedaroundthemainseismicfractures.
Thesecondaryseismicfracturesdiscontinuousdistributedandpoorcontinuity.
Fig.
11Tensilegroovedeveloped3.
3OpeningdegreecharacteristicsTheopeningdegreeofseismicfracturemainlyreferstothewidthofcrackinrockandsoilmassbyshockedtensionintheearthquakeforce.
Throughthesurvey,theopeningdegreeofseismicfracturewastheformofwideupper-narrowlowershaped,pinchoutintothedeep(Fig.
12).
CaiandZhang(2001)indicatedthattheseenseismicfracturewaswedgeshapedfracturesonthewideupper-narrowlowerform.
Forsomebetteroverallstructureofrockmass,theopeningdegreewassmall,generallyonlyafewcentimeterstoadozencentimeters;Somerockmasswithpoorstructureandstructuralplanesignificantlydeveloped,theirfracturesopenedagreaterdegree.
Andtheseismicfractureslocatedaroundthetriggeringseismicfaultshadthewedge-shapedgapofwideupper-narrowlower,theopeningdegreeintopwasclosedto1m,itisillustratedtherockmassaroundthefaultswasseriousshattered.
Fig.
12SchematicsectionofseismicfractureTheshatteredwidthofSurveiedfractureswere20cmormore,inclinationanglegreaterthan60,extendedtodeepbyjagged-shaped,butitextendingtothedeep,theshatteredwidthbecamesmallergradually.
3.
4FillingcharacteristicsTheseseismicfractureswerelargeopeningdegreegenerally,fillinglessorno,overheadrockmass.
Thejointinterfacewasrelativelynewandbedrockexposed,straightorjagged,extendinglonger.
Wideupper-narrowlowershapedintheformoffractures,shatteredstonefragmentswerefilledinit.
Bothsidesofrockandsoilcollapsedwascausedbysomeseismicfractureswhichhadlargedegreeopening(Fig.
13).
Fig.
13Collapseinducedbythesloperockmasswereshattered4.
SeismicfracturemechanicsmechanismanalysisSeismicfracturewasmainlycausedbytheearthquakeforce.
Refractionandreflectionofseismicwavesoccurredinthestructuralplanewhentheseismicwavesaffectedit(Fig.
14),sotherockmassinstructuralplaneshatteredcrackduetothetensioncrackwhichcausedbytheaccumulatedstress(Zhangetal.
1993).
Theslopedamagecausedbyseismicforcewascloselyrelatedtotheeffectofseismicwaves,seismicwaveisdividedintoP-wave(longitudinalwave)andS-wave(shearwave),bothjointactionatthestructuralplaneofrockmass.
P-waveandS-wavewithfrequencyup-downandhorizontalvibration,thisgeneratedn-pull、anti-pullrepeatedstresstorockmassinthestructuralplane,androckmassinthestructuralplanecrackedopenalongthestructureplaneunderthedynamicactionofpowerfulearthquakeforce(Zhangetal.
2005;Fengetal.
2009;Newmark1965).
Theshearstrengthofstructureplanequicklylowerowningtoitsufferedrepeatedshearstress,rockmasswascuttedalongthestructureplanebytheactionofstrongshearstress,rockmassopenedanddeformed.
Myer(1990)studiedthepropagationcharacteristicsofseismicwavesthroughtherockmassjointsurfaceandtheforcebetweentherockmassinterfacecontrolledbytheCoulombcriterion.
Liang(2003)holdedthatthestructureplaneformedshatteredfacebytheearthquakeforce,therockmassabovethesurfacewouldoutoftheparentrockandcontinuetomovement,andtherockmassbelowthesurfacewouldgenerateanewfreesurfaceduetotheinfluenceofuppershatteredface.
Fig.
14SchematicdiagramofstresswavepropagationeffectedbythestructuralplaneSeismicfractureformedbytheprocessofrupture、perforation、extendinrockmass,internalfracturewasfurtherdevelopedintolargefractureintheslopeatlast.
Therupturedofrockmasswasmainlyduetoseismicwaves,Fig15showsasimplifiedseismicwavesschedulefigure,Fig16isafigureofsimplifiedequivalentstressintensityfactorcurves.
Usethefracturemechanicstheorytoanalysistheprocessofrupture、propagationandcrackarrestinrockmass(Li2010).
Fig.
15CurvediagramofFig.
16Curvediagramofseismicwavecoursestressintensityfactor4.
1FracturecriterionCombinedwithfracturemechanics,therupturecriterionofrockmassisthemaximumstressintensityfactorcriterion,lawofK,therockmasswillproducetensilefailureifthestressintensityisgreaterthanthefracturetoughness.
Attimet=0,seismicwavesisnotyetineffect,thestructureplaneonlyinthestaticstressfield,KIt2,theeffectcourseofseismicwaveswillenduntilt=t3.
Theeffectofseismicwaveswillstopaftert>t3,extendedfractureisinthestaticstressfield.
Ifthestressintensityfactorisstillgreaterthanthefracturetoughnessofrockmass,thefracturewillcontinuetoextendinthestaticstressfield,itstoptheextensionuntilt=t4.
4.
3CrackarrestevidenceAstheseismicintensitydecreases,theenergyreduce,thefracturestoprupturewhenthestressintensityfactorislessthanthefracturetoughness.
Forthestructuralplaneeffectedbystrongearthquakeforce,itmayhavehigherstressintensityfactoraftertheearthquakestop.
Thefractureinthestaticstressfieldmaycontinuetoextend、untilthecrackarrestconditionsismet.
5.
ConclusionsInsummary,throughdetailedinvestigationandanalysistothedeformationcharacteristics、deformationandfailuremechanisms、deformationkinematiccharacteristicsofslopeseismicfracture,theresultsshowthat:Basedonthemorphologicalcharacteristicsanddeformationdeformationfeaturesofslopeseismicfracture,theseismicfractureswasdividedintosixtypes:surfacetype、transfixingtype、striketype、tendencytype、closuretype、extensiontype.
Surfaceseismicfractureinaccordancewithitsextensioncharacteristicscouldbedividedintobrokenlinetype、Lineartype.
Fromitscombinationfeaturescanbedividedintoen-echelontype、crossovertype.
SlopeseismicfractureswasN30~60Etrending,theyweremainlydistributedinthevicinityofcausativefaultorridgespositionandsteepslowborderinslope.
Seismicfracturewaswellextension、highopendegree、obviousconnectivityandlargescale.
Upperwide-lowernarrowofprofilemorphology,pinchoutintothedeep,fillingslessorno.
Basedontherockfracturemechanicsmechanism,combinedwiththestressintensityfactortoanalysistheshatterdamagemechanismofthestructureplane.
Themechanicstypeofslopefracturesmainlybelongtothetensioncrack,rockstructureplanewouldrupturetogeneratecrackunderthedynamicaction,crackgraduallyextendtobecomethepenetratingtensionalseismicfracture.
AcknowledgementsFinancialsupportfromkeyprojectoftheNaturalScienceFoundationofChina(NSFC,NO.
40972195)andtheIndependentresearchoftheStatekeyLaboratoryofGeologicalHazardPrevention&GeologicalEnvironmentProtection(HGY-2012-08)isgratefullyacknowledged.
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