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.. , 1.ReportNo.2GovernmentAccessionNo. CFHR3-8-71-161-1 4.TitleandSubtitle itASurveyofEarthSlopeFailuresandRemedial MeasuresinTexas" 7.Authorl s) TimothyG.AbramsandStephenG.Wright 9.PerformingOrgonizotionNameandAddress CenterforHighwayResearch TheUniversityofTexasatAustin Austin,Texas78712 _......12.SponsoringAgencyNameandAddres. TexasHighwayDepartment 11thandBrazos Austin,Texas78701 15.SupplementoryNotes Researchperformedin HighwayAdministration ResearchStudyTitle: 16.Ab.tract cooperationwithDepartmentof ts tabi lityofEarthS lopes It TECHNICALREPORTSTANDARDTITLEPAGE 3.Recipient'sCotologNo. 5.ReportDote December1972 16.PerformingOrganizationCode S.PerformingOrgani zationReportNo. ResearchReport161-1 10.WorkUnitNo. 11.ContractorGrantNo. ResearchStudy3-8-71-161 13.Type01ReportandPeriodCovered Interim Sept.1970- June1972 14.SponsoringAgencyCode Transportation,Federal TheresultsofasurveyundertakentoidentifyregionsinTexaswheretherehas beenahighincidentrateofslopefailureandtoidentifysomeofthefactors responsiblefortheseslidesarepresented.Inconjunctionwiththisstudyareview ofpresentslopedesignproceduresandtheremedialmeasuresemployedbytheTexas HighwayDepartmentforrepairandmaintenanceofearthslopesiscontainedinthis report. Themajorslopefailuresofsignificancewerefoundtobeassociatedwith primarilyexcavated(cut)slopesinstiff-fissuredclaysandclayshales,although failuresinembankment(fill)slopesconstructedofhighplasticityclayswerealso encounteredinseveralareas.Anumberofremedialmeasureshavebeenemployedfor repairofthesefailures,includingregrading,stabilizationwithlimeandcement additives,andvariousformsofrestraintorbuttressingstructures.Whileexces-siveamountsofgroundandsurfacewaterwereusuallypresentatmostsiteswhere slopefailureshaveoccurred,drainageofwaterhasnotbeenextenSivelyemployed asaremedialorpreventativemeasure. 17.KeyWord.18.Di,tributionStatement Soilmechanics,slopestability 19.SecurityClouif. (of thi'teport)20.SecurityCiani!. (ofthi spagel21.No.ofPages22.Price UnclassifiedUnclassified 109 FormDOTF1700.7(S-69) '. ASURVEYOFEARTHSLOPEFAILURES ANDREMEDIALMEASURESINTEXAS by TimothyG.Abrams StephenG.Wright ResearchReportNumber161-1 StabilityofEarthSlopes ResearchProject3-8-71-161 conductedfor TheTexasHighwayDepartment incooperationwiththe U.S.DepartmentofTransportation FederalHighwayAdministration bythe CENTERFORHIGHWAYRESEARCH THEUNIVERSITYOFTEXASATAUSTIN December1972 Thecontentsofthisreportreflecttheviewsofthe authors,whoareresponsibleforthefactsandthe accuracyofthedatapresentedherein.Thecontents donotnecessarilyreflecttheofficialviewsor policiesoftheFederalHighwayAdministration.This reportdoesnotconstituteastandard,specification, orregulation. ii ., ,\ PREFACE ThisreportisthefirstreportonthefindingsofResearchProject 3-8-71-161,"StabilityofEarthSlopes."Includedhereinaretheresultsand summaryofasurveyofearthslopefailuresalongTexashighwaysandofthe remedialmethodsemployedforrepair.TheareasofTexaswhereslopeproblems appearsignificantareidentifiedandavailableinformationontheselection, designandperformanceofremedialmeasuresisreviewed. Theauthorswishtoacknowledgethevaluableassistance,service,and informationprovidedbythemanypersonneloftheDistrictOfficesofthe TexasHighwayDepartment.TheassistanceandadviceofMr.ChesterMcDowell oftheCenterforHighwayResearch,Messrs.JimBrownandBobGuinnofthe TexasHighwayDepartmentandMr.TonyBalloftheFederalHighwayAdministra-tionarealsoappreciated. December1972 iii TimothyG.Abrams StephenG.Wright ABSTRACT TheresultsofasurveyundertakentoidentifyregionsinTexaswhere therehasbeenahighincidentrateofslopefailuresandtoidentifysomeof thefactorsresponsiblefortheseslidesarepresented.Inconjunctionwith thisstudyareviewofpresentslopedesignproceduresandtheremedialmeasures employedbytheTexasHighwayDepartmentforrepairandmaintenanceofearth slopesiscontainedinthisreport. Themajorslopefailuresofsignificancewerefoundtobeassociated withprimarilyexcavated(cut)slopesinstiff-fissuredclaysandclayshales, althoughfailuresinembankment(fill)slopesconstructedofhighplasticity clayswerealsoencounteredinseveralareas.Anumberofremedialmeasures havebeenemployedforrepairofthesefailures,includingregrading,stabiliza-tionwithlimeandcementadditives,andvariousformsofrestraintorbuttres-singstructures.Whileexcessiveamountsofgroundandsurfacewaterwere usuallypresentatmostsiteswhereslopefailureshaveoccurred,drainageof waterhasnotbeenextensivelyemployedasaremedialorpreventativemeasure. KEYWORDS:soilmechanics,slopestability. iv ' .. SUMMARY TheresultsofthissurveyofearthslopefailuresinTexasindicate thatfailureshavedevelopedmostextensivelyincutslopesinmediumtohighly plastic,overconsolidated,stiff-fissuredclaysintheDallas,FortWorth, Waco,AustinandSanAntonioregionsofTexas.Intheseregions,failures tendedtobeshallow,semi-circularslideswhich,inmanyinstances,developed severalyearsaftertheconstructionoftheslopesinvolved.Moderateto excessiveamountsofgroundwaterandsurfacewaterwereobservedinallbut afewoftheseslidessuggestingthatswellingandpositiveporewaterpressures weretheprincipalcausesoffailure. Thereviewofthecurrentearthslopedesignproceduresandremedial measuresemployedbytheTexasHighwayDepartmentindicatesthatatthepresent timehighwayslopesaregenerallydesignedempiricallyandrepairedwhenthey fail.Oneormoreofthefollowingremedialmeasuresaretypicallyused: 1.Stabilizationoftheslidematerialbytheadditionoflimeor cement. 2.Substitutionofsandsandgravelsforslidematerial,generally inthevicinityofthetoeoftheslide. 3.Constructionofrestraintstructures,usuallypilingandcast-in-placedrilledshafts,withorwithoutretainingwallsorsimilar structuresattached. 4.Controlofgroundwaterwithinterceptortrenchesandsurface waterwithditches,curbing,crackfilling,andslopeplanting. 5.Constructionofflatterslopegrades. 6.Constructionofconcreterip-raponthefacesofunstableslopes. Whilealloftheseremedialmeasureshavebeeneffectivetoadegree,thereare manyinstanceswheretheeffectivenessofasingleoneisuncertainduetothe combineduseofseveralmeasuresforrepairofasingleslide.Specificat-tentionisgiveninthisreporttotherestraintstructurereferredtoasa "slidesuppressorwall"andasuggesteddesignprocedureispresentedfor evaluatingthismeasure. v , ' .. IMPLEMENTATIONSTATEMENT Theresultsofthisresearchindicatethattheinstabilityofearth slopesalonghighwaysinTexasrepresentsaproblemwhichmayinvolvesignifi-cantcostsformaintenance.Themajorslopeproblemareawhichshouldbe recognizedencompassesmanycut(excavated)slopesinstiffclaysandclay shalesintheSanAntonio,Austin,Waco,DallasandForthWorthregionsof Texas.Onesolutiontothestabilityproblemsintheseareasistheadoption offlatterslopesfordesign;however,untilthesecanbeeconomicallyjustified andbecauseanumberofexistingslopesareanticipatedtopresentfuture stabilityproblems,continuedmaintenanceofearthslopesinTexasshouldbe plannedfor. Theresultsofthisresearchshouldserveasausefulguidetothe highwayengineerforjudgingtheeffectivenessofalternateremedialmainte-nancemeasures,basedonpastexperienceinTexas.Oneofthesemeasures,the "slidesuppressorwall,"mayalsobeevaluatedquantitativelyusingthedesign chartandprocedurepresentedinthisreport.Achartforback-calculating shearstrengthparametervaluesfromobservedslopefailuresisemployedin thisprocedure.Thischartmaybeusedfordeterminationofshearstrength parametervaluesforuseinthedesignofotherremedialmeasuresaswell. vi .... TABLEOFCONTENTS PREFACE ABSTRACT SUMMARY IMPLEMENTATIONSTATEMENT LISTOFTABLES LIS TOFFIGURES NOMENCIATURE CHAPTERI.INTRODUCTION CHAPTERII.EARTHSLOPEFAILURES Introduction CharacteristicsofSlopeFailures GeologicConditions HydrologicConditions Summary CHAPTE RII 1.REMEDIALMEASURES Introduction StabilizationwithAdditives SoilSubstitution RestraintStructures ControlofWater SlopeAlteration. ConcreteRip-Rap CHAPTERIV.ESTIMATINGSOILSTRENGTHPARAMETERVALUES BackCalculationofcand ApplicationofCharts SummaryandConclusions vii iii iv v vi ix x xii 1 3 4 5 12 15 16 16 24 25 30 34 34 38 44 47 \-CHAPTERV.EARTHPRESSUREFORCESFORARESTRAINTSTRUCTURE ProcedureforCalculationofEarthPressureForces Non-DimensionalCoefficients InfluenceofSoilProfileCharacterizations ModificationofEarthPressureCoefficients PointofApplicationofEarthPressureForces ApplicationofCharts... ComparisonofEarthPressureCoefficientProcedure withConventionalProcedures. SummaryandConclusions CHAPTERVI.SUMMARYANDCONCLUSIONS APPENDICES AppendixI. AppendixII. AppendixIII. AppendixIV. AppendixV. REFERENCES AtterburgLimitTests ParametersUsedinAnalyses ValuesofN p PointofApplicationofEarthPressure SummaryofTrial-WedgeAnalyses. Forces viii 51 53 56 64 66 66 74 80 81 84 87 91 93 95 96 Table 3.1 5.1 5.2 LISTOFTABLES SummaryofRecentSlopeFailuresWhichHaveReceived ExtensiveRemedialTreatment EarthPressureForces(E)andNValuesforA==10 pc and2: 1Slope ComparisonsofEarthPressureForcesDeterminedfrom Equivalent-FluidProcedureandNProcedure p ix Page 17 54 76 '. Figure 2.1 2.2 2.3 2.4 2.5 LISTOFFIGURES TypicalfailurewhichoccursinTexashighwaycutand embankmentslopes TexasHighwayDepartmentDistricts Atypicalslopefailureinahighlyweatheredzoneofan overconsolidatedstiff-fissuredclayslope CrosssectionofaslidewhichoccurredonU.S.281 northofEvant CrosssectionofslopewhichfailedalongI.H.45near Centerville.. 2.6Varioussequencesofperviousandimperviousstrata Page 6 7 8 10 11 inclayslopes....13 3.1 4.1 4.2 4.3 5.1 5.2 5.3 5.4 5.5 TypicalcrosssectionofBoggyCreeknearU.S.183in southeastAustin. Criticalcentersfortoecircleswithoriginof coordinatesattoeofslope.. Valuesofc/ -yHandtancorrespondingtoafactorof safetyofunity.. (a)CaseforwhichchartsillustratedinFigs.4.1and 4.2maybeused.(b)Caseforwhichchartsarenot applicable.. Slopestabilizedwithslidesuppressorwall Variationofinters licesideforcesalongthelengthof theslope.(a)Locationofslices.(b)Variationof inters liceforces Failuresurfacefor-y= 10and2: 1slope c Totalstressshearstrengthcharacterizationsofthe soi 1strengthprofile.(a)CaseI.(b)CaseII ComparisonofCaseIandIIearthpressurecoefficients withvaluesofNevaluatedfora2:1slopeatL/3 p x 27 41 43 46 50 52 55 57 59 Figure 5.6 5.7 5.8 5.9 5.10 5.11 5.12 .. 5.13 5.14 5.15 5.16 5.17 5.18 -. 5.19 ComparisonofCaseIandIIearthpressurecoefficients withvaluesofNevaluatedfora3:1slopeatL/3 p Piezometriclinesusedineffectivestressanalysesfor (a)2:1and(b)3:1slopes... ComparisonofCaseIIIandIVearthpressurecoefficients withvaluesofNevaluatedfora2:1slopeatL/3 p ComparisonofCaseIIIandIVearthpressurecoefficients withvaluesofNevaluatedfora3:1slopeatL/3 p Typicalslopecrosssectionusedtoevaluatevaluesof NI.. p Comparisonofearthpressurecoefficientsdeterminedfrom totalstressandeffectivestressanalyseswithvaluesof N'foraslidesuppressorwallinstalledone-thirdofthe p distanceupa2:1slope.. Comparisonofearthpressurecoefficientsdeterminedfrom totalstressandeffectivestressanalyseswithvaluesof N'foraslidesuppressorwallinstalledone-thirdthe p distanceupa3:1slope.. N'valuesforslidesuppressorwallplacedone-fourth p ofthedistanceupa2:1or3:1slope.. N'valuesforslidesuppressorwallinstalledone-third p ofthedistanceupa2:1or3:1slope. N , valuesforslidesuppressorwallplacedone-halfthe p distanceupa2: 1or3:1slope SimplifiedcrosssectionoftheslopeonI.H.35under theMooreStreetBridge.. Assumedfluidpressuredistributionusedinthemodified equivalent-fluidprocedure Slopecrosssectionshowingtheassumptionsmadeinthe trialwedgeanalyses Comparisonofearthpressureforcesdeterminedfrom trial-wedgeanalyses(planefailuresurfaces)and Spencer'smethod-of-slicesprocedure(circularfailure surfaces).. . . . . . . . . . . . xi Page 60 61 62 63 65 67 68 69 70 71 73 75 77 79 Symbol ~c Co d E F H HI L LI Ncf Np Np I ru .. V X Y z a ~y Yc ~ ~ z ~ z~NOMENCLATURE Definition cohesion cohesionatthecrestoftheslope maximumdepthofslidemeasurednormaltoslopeface earthpressureforceinhorizontaldirection factorofsafety slopeheight modifiedslopeheight distancefromtoetocrestofslopemeasuredhorizontally modifieddistancefromtoeofslidetocrestofslope measuredhorizontally stabilitynumber earthpressurecoefficient modifiedearthpressurecoefficient ratioofporewaterpressuretooverburdenpressure earthpressureforceinverticaldirection X-coordinateofcriticaltoecircle Y-coordinateofcriticaltoecircle resultantearthpressureforce inclinationoftrial-wedgefailureplane inclinationofslope unitweightofsoil equivalentfluiddensity dimensionlessparameterrelatingy,H,,andc dimensionlessparameterrelatingy,H,cand~ zangleofinternalfriction rateofincreaseinshearstrengthwithdepth xii .-CHAPTERI INTRODUCTION Slopefailures,inadditiontocreatinghazards,delays,andinconven-iencesforthehighwayuser,increasethecostofhighwaymaintenance.The oftenhighcostofrepairingaslopesometimesresults r o ~ theindirectuse ofhighfactorsofsafetyfortheremedialmeasureinanefforttopreventthe recurrenceoffailure.Inotherinstances,wherelessconservativemeasures havebeenused,failurehasoccurredrepeatedly.Byidentifyingsomeofthe factorscontributingtoslopefailuresandevaluatingcurrentlyusedremedial measures,helpfulguidelinesmaybeestablishedforavoidingfutureslope failuresanddesigningeffective,economicalremedialmeasures. Presentedinthisreportisasurveyofearthslopefailureswhichwas conductedtodeterminegeologicregionswheretherehavebeenhighincident ratesofslopefailuresandtoidentifysomeofthefactorswhichmayhave contributedtothesefailures.Toaccomplishtheseobjectives,engineersand geologistsfromtheTexasHighwayDepartmentwereinterviewed,landslidesand repairedslopeswereinspected,recordedinformationonsoilandgeologic conditionswascollected,anddesigndrawingsandreportspertainingtoslope failureswereobtained.Fromdiscussionswithhighwaypersonnelassociated withslopefailuresandfromon-siteinspections,someofthecharacteristics ofthefailuresandtheinfluenceofthesecharacteristicsontheeconomyand successofvariousremedialmeasuresweredeterminedforvariousgeologic formations.Althoughstrengthdatawerenotobtained,AtterburgLimittests wereperformedonsoilsamplestakenfromseveralsites,andtheseresultsare summarizedinAppendixI. Inconjunctionwiththiscollectionofinformationonslopefailures, areviewwasmadeofcurrentearthslopedesignproceduresandremedialmeasures employedbytheTexasHighwayDepartment.Slopefailureshavecommonlybeen repairedbyregradingtheslope;however,insituationswherearegraded slopehascontinuedtofailorwheretheslopefailurehasendangereda 1 2 highwayorbridgestructure,moreextensivemeasureshavebeenused.Twenty-six recentslopefailureswheremeasuresmoreextensivethanjustregradingwere employedwereinvestigatedtodeterminethetypesofremedialmeasuresusedand theireffectiveness. Thedesignofaremedialmeasureisoftendependentontheshearstrength ofthesoil.Whileshearstrengthdatamaybeobtainedfromlaboratoryor in-situtests,theexpenseanduncertaintyassociatedwithobtainingrepresenta-tivesoilsamplesandperforminglaboratoryorin-situtestsmayinmanyin-stancesoffsetthebenefitsderivedfromsuchtests.Analternativeapproach istoback-calculatetheshearstrengthparametervaluesfromtheobserved failuresurface.Thisapproachisusedinthisreportandachartisdeveloped forestimatingshearstrengthparametervaluesfromanobservedfailuresurface. Byemployingvaluesofback-calculatedshearstrengths,aseriesof analyseswereperformedtoevaluateonetypeofremedialmeasureusedbythe TexasHighwayDepartment.Thedesignofthismeasure,aretainingstructure foundedondrilledpiers,generallyrequiressomeknowledgeorestimateof thelateralforcesthestructuremustwithstand.Byperforminganalysesusing thebackcalculatedshearstrengthparametervaluesandaproceduredescribed hereinthelateralforcesmaybedetermined.Theforcesobtainedinthis mannerarecomparedwiththevaluesdeterminedbyconventionalprocedures, whicharecommonlyusedforthispurpose.Thenewproceduredevelopedinthis reportisbelievedtobesomewhatmorerationalthantheexistingprocedures andaseriesofsimpledesignchartshasbeendevelopedtofacilitateitsuse. Theinformationpresentedinthisreportisdividedintofivemajor parts:(1)EarthSlopeFailures,(2)RemedialMeasures,(3)EstimatingSoil StrengthParameterValues,(4)EarthPressureForcesforRestraintStructures, and(5)SummaryandConclusions. CHAPTERII EARTHSLOPEFAILURES Introduction EarthslopefailuresinTexashavedevelopedprimarilyinmediumto highlyplastic,overconsolidated,stiff-fissuredclaysinwhichmoderateto excessiveamountsofgroundwaterandsurfacewaterwerepresent.Thesuccess-fuldesignofslopesinthesematerialsdependstoalargeextentonareliable predictionoftheshearstrengthofthesoilandthehydrologicconditions withintheslope.However,predictionsoftheappropriateshearstrength valuesforoverconsolidated,stiff-fissuredclaysiscomplicatedbymany factors(DuncanandDunlop,1969;SkemptonandHutchinson,1969).These include: (1)thereductioninshearstrengthduetoswelling, (2)thereductioninshearstrengthwhenthestrainsinaheavily overconsolidatedclayreachthefailurestrain, (3)theanisotropicshearstrengthvariationsresultingfrom geologicprocesses, (4)thereductioninshearstrengthduetofissures,fractures andslickensides,and (5)theerrorsassociatedwithsamplingthesoilandperforming laboratorytests. Inordertoestablishtheinfluenceofthesefactorsontheshearstrength andstabilityofaparticularslope,anumberofextensiveandsophisticated testproceduresandanalyticalmethodsare 'commonlyrequired. Theevaluationofthestabilityofslopesinthestiffclayformations commonlyencounteredinTexasisfurthercomplicatedbywhatappeartobe randomlydeveloped,seasonalgroundwaterconditions.Toadequatelydetermine thegroundwaterregime,extensiveboringswouldberequired,toinsurethat isolatedandrandomlydevelopedconcentrationsofwaterarediscovered. Furthermore,inmanyinstancesitmaybenecessarytomonitorsuchborings forextendedperiodsoftimebecauseoftheseasonalnatureofsomeofthe groundwatersourcesandtherelativelylowpermeabilityofmanyofthegeologic 3 .. 4 formationsinvolved.Inaddition,theexcavationofaslopemaychangethe seepagepatterninamannernotanticipated.Fortheabovereasons,the determinationoftheshearstrengthandhydrologicconditionsformanyhighway slopesinTexaswouldbeexpensiveanddifficult.Anoftenmoreeconomical alternate,andanapproachgenerallyusedbytheTexasHighwayDepartment,is tobasetheslopedesignonexperienceandtorepairslidesifandwhenthey occur. Theheightandinclinationofanumberofembankmentandcutslopes constructedbytheTexasHighwayDepartmentaredictatedbyright-of-way restrictions,highwaygeometry,maintenancerequirementsl,andexperience. Computerassistedslopestabilityanalyses(HoustonUrbanOfficeandDistrict 12)orstabilitycharts(District9)haveoccasionallybeenusedtoaidinthe designofaslope;however,right-of-wayrestrictionsandmaintenancerequire-mentsgenerallygovernthefinalslopedesigns.Typically,theheightsof theseslopesrangefrom20to40feet,withslopeinclinationsrangingfrom 2(horizontal):1(vertical)to3:1,withslopessteeperthan2:1generally rip-rappedwithconcreteslabs. Thetypeofsoilusedintheconstructionofembankmentsisgenerally notcontrolled.However,inDistrict12oftheTexasHighwayDepartment,soils withliquidlimitsgreaterthan65percentarenotusedforfillmaterials exceptinlimitedcases,andinDistrict2,iftheplasticityindexofthefill materialexceeds40percent,thelastfourorfivefeetoftheembankmentare limestabilized. CharacteristicsofSlopeFailures WhilethepresentdesignpracticeoftheTexasHighwayDepartment appearsinmanyinstancestobesatisfactory,therehavebeenanumberof slopefailures.Thesehavedevelopedassmallslumps,shallowsemi-circular slides,andlargerotationalslides,withsomeoftheslidesextendingseveral hundredfeetparalleltotheslopecrestandinvolvingseveralthousand cubicyardsofmaterial.Ingeneral,whentheseslidesdevelop,theheadof theslidemassdrops,leavinga4to12footscarp,whilethetoeoftheslide bulgesandflowsdownoroffthefaceoftheslope,asillustratedbythe 1Maintenancerequirementsmaygoverntheslopedesigninsomeinstances becausegrassmowerscannotoperateefficientlyonslopegradesmuch steeperthan3:1. 5 typicalcrosssectioninFig.2.1.Althoughtherehavebeenseveralinstances offailuresduringconstructionofbothcutandembankmentslopes,failures havegenerallyoccurredthreetoeightyearsafterconstruction.Theselong-termfailuresmayhaveresultedfromthegradualreductioninshearstrength duetoswellingandtothedevelopmentofpositiveporewaterpressures.Fur-thermore,instiff-fissuredclaystheexcavationofaslopemaycausesome fissurestoopen;theseopenfissures,inadditiontobeingsurfacesofweak-ness,providenaturalpathsforwatermigrationandtherebymayleadtothe softeningofthestiff-fissuredclaymassandareductioninshearstrength (TerzaghiandPeck,1967).Themajorityoftheslideshaveoccurredinfive TexasHighwayDepartmentDistricts:2(FortWorth),9(Waco),14(Austin), 15(SanAntonio),and18(Dallas)(seeFig.2.2).Someslopefailureshave occurredinotherDistrictsbutwithapparentlymuchlowerfrequencythanin thepreviouslymentionedDistricts.ThefiveDistrictscitedencompasssome ofthemostunfavorablegeologicandhydrologicconditionsforthestability ofearthslopesinTexas. GeologicConditions Slopesexcavatedingeologicformationsconsistingofmediumtohighly plastic,overconsolidated,stiff-fissuredclayshaveproventobethemost susceptibletofailure.TheoverconsolidatedclaysintheTaylorandNavarro geologicgroupshavecausedthegreatestproblemsinSanAntonio(District15). Thesegeologicgroups,andinadditiontheDelRioclayformation,havealso causedproblemsinAustin(District14).Theplasticityindexesoftheclays sampledfromslidesinthesegeologicunitshaverangedfrom40to60percent. SlopesintheTaylorandNavarroclaystypicallyhavefailedalongshallow semi-circularrupturesurfaceswhichappearedtodevelopinamorehighly weatheredzoneextendingapproximatelyfivetoeightfeetindepthbelowthe slope,asillustratedinFig.2.3.Forexample,therupturesurfaceofa slidenearHeep'sDairyonI.H.35southofAustinina20-foothigh,21/2:1 slopewasshallow,withthemaximumdepthoftheslide(measurednormaltothe slopeface)beingaboutsixfeet. InWaco(District9)theprincipalgeologicformationscausinginsta-bilityaretheSouthBosqueShale,Walnut,DelRio,andEagleFord;theplas-ticityindexesoftheseformationsrangefrom38to60(FontandWilliamson, 1970).Thestiff-fissuredclaysandmarlsinsomeoftheseformationsalternate Fig.2.1.TypicalfailurewhichoccursinTexashighway cutandembankmentslopes. 6 4 5 Childress:3 Lubbock 8 Abilene 6 Odessa 1 Fig.2.2. SanAngelo TexasHighwayDepartmentDistricts. oftheslopefailureshaveoccurred whicharecross-hatched. 7 Themajority inthedistricts Fig.2.3.Atypicalslopefailureinahighlyweathered zoneofanoverconsolidatedstiff-fissured clayslope. 8 9 withthinlayersofeithersandstoneorlimestone.Thesethinrocklayerstend tocontrolthegroundwaterconditionsandthemodeoffailureintheselayered slopes.Randomlyfracturedandjointedrockhastendedtocauseerratic seepagepatternswhilecontinuousintactrockhasledtotheformationof perchedwatertables.Inaddition,failuresurfacesinthesematerials tendtobenon-circular,asthesurfaceofslidingtendstodevelopnearthe baseofclaylayersadjacenttomoreresistantrocklayers.Atypicalcross sectionofsuchaslide,whichoccurrednorthofEvantonU.S.281,isillus-tratedinFig.2.4. IntheFortWorthandDallasDistricts,theMineralWells,EagleFord, Taylormarl,Denton,andKiamichiformationsaresomeoftheprimarygeologic formationsinwhichlandslideshaveoccurred.Manyoftheclaysinvolved inthesefailuresaremoderatelytohighlyplasticandsome,suchastheEagle Ford,arehighlyexpansive(TexasHighwayDepartment,1966).Plasticityindexes rangingfrom35to50havebeenmeasuredinanumberofslidesinDistrict2. Typically,inmanyoftheseformationsthinlayersoflimestoneorshell conglomeratesalternatewiththickerclayandmarlbeds.Perchedwatertables maydevelopabovethinrocklayersor,insomecases,watermayseeplaterally throughthelayerscausingseepstoemergeonthefaceoftheslope.Failures intheseformationsaregenerallysimilartothoseshowninFigures2.3and 2.4. Inadditiontoformationsofstiff-fissuredclayswithsomeinter-beddedlimestonelayers,unfavorablesequencesofclayandsandlayershave alsocontributedtolandslidesinseveralHighwayDepartmentDistricts.For example,attheI.H.45andStateHighway7interchangenearCenterville,a layerofWhechesclay,tentofifteenfeetthick,isunderlainbytheQueen CitysandandoverlainbySpartasand,awaterbearingfinegrainedsand,as illustratedinFig.2.5.TheplasticityindexoftheWhechesclayinthisarea rangesfrom28to35percent.Duringtheexcavationfortheconstructionof thissectionofI.H.45,thesaturatedWhechesclaywasexposedin4:1and flatterslopes,resultinginthedevelopmentofseveralshallowslides.These slideswereattributedtothelowshearstrengthoftheWhechesclayresulting fromseepageintheoverlyingSpartasand. Slopefailureshaveoccurredinslopeswhichwereoriginallyformed adjacenttolimestonecliffsbyfallingrockdebris(talus).Thesefailures havegenerallybeeninitiatedinsaturatednmterialswhenthetoeofthe _______ .......----....... - ---=:::::........ -...... ThinLimestoneLoyer Fig.2.4.Crosssectionofaslidewhichoccurredon U.S.281northofEvant. 10 ..t\ " .... .. , ., J... ... . -, .. .', .' #OueenCitySand . . .' . . " ! ... ..~... ..~" , ..... ... ExtentofE)(cavation WhenFailureOccurred ~ ." .. " .. . ' . 11 . ".....",' . ~ . ' . .. .... Fig.2,5.Crosssectionofslopewhichfailedalong I.H.45nearCenterville. I 12 naturalslopewasremovedbyexcavation.Severalsuchfailureshaveoccurred inWaco(District9),andinthesetheslidingsurfacesappearedtodevelop alongthecontactofthetalusandtheunderlyingmaterial. Embankmentsconstructedofmediumtohighlyplastic,overconso1idated clayshavealsobeensusceptibletofailures.Failuresweregenerallyobserved toinvolveonlytheembankmentmaterialasinthecaseoftheslideatthe intersectionofI.H.410andMarbachRoadinnorthwestSanAntonio.Inthis instance,therupturesurfacepassedthroughthecrestandtoeoftheslope andthetoeoftheslideflowedafewfeetbeyondthetoeoftheslope.Only oneembankmentfailureinvolvingthefoundationmaterialwasobserved.In thisinstancethefoundationfailedbeneatha46-foothighembankmentwhich wasbeingbuilteastofDallastoraiseI.H.30abovetheelevationofthe thentobebuiltLakeHubbard.Theembankmentwasdesignedwith3:1side slopes;however,inordernottointerrupttheflowoftrafficontheexisting I.H.30,thesouthslopewasbuilttemporarilyona2:1grade.Duringconstruc-tion,twofailuresinvolvingapproximatelyonethosandfeetofthesouth slopeofthenewembankmentdeveloped.Thefailuresurfacespassedthrough theweakfoundationsoilsandbeyondthetoeoftheslope. HydrologicConditions Wateroriginatingasgroundwaterorsurfacerunoffappearstobeone oftheprincipalfactorscontributingtotheinstabilityofclayslopesin Texas.Inmanyofthegeologicformationspreviouslydescribedthepresence ofrandomlydistributedlimestoneandsandstonestrata,varyingfromcontinuous andintacttohighlyjointed,mayresultinanumberofdifferentpatternsof seepage.Thus,thepredictionofwaterconditionsisconsiderablymorecomplex anduncertainintheseformationsthaninhomogeneoussoildeposits.In addition,thepresenceoffissures,astheycommonlyexistinstiffclay formationsinTexas,furthercomplicatestheseepagepatternsandinfluences therandomnessofgroundwaterconditions. Seepagewithinrelativelyperviouszonesinaslopesuchassandand gravelseams,porousandfracturedlimestones,sandstonesandshellconglomer-ates,asshowninFig.2.6a,mayoftenbeamajorfactorininitiatingslope failures.Waterfromasandyclayseamisbelievedtohaveinitiatedtwo slidesona30-foot,21/2:1slopealongI.H.35atMooreStreetinSan Antonio.Thefirstslideoccurredinasectionoftheslopewestofthe (a) .. ..... '" ..t7 ... '.".*......'., ....... "'. .. '" ...... "."'=!!=" ,,- '" ' ... ...... (b) (c) Fig.2.6.Varioussequencesofperviousandimpervious stratainclayslopes. 13 MooreStreetbridgeaboutthreeyearsaftertheslopehadbeenexcavated. Priortothefailure,seepshadformedonthefaceoftheslope,andafter 14 thefailure,theslidedebrisappearedtobesaturated.Boringsmadenearthe crestoftheslopedisclosedawaterbearingseamapproximatelyelevenfeet belowthecrestoftheslope.Severalyearsafterthisslidehadbeenrepaired asecondslidedevelopedintheconcreterip-rappedslopebeneaththeMoore Streetbridge.Inanefforttoarresttheslide,holesweredrillednearthe toeoftherip-rap.Initially,waterflowedfreelyfromtheseholes,and later,whentherip-rapwasremovedtocorrecttheslide,theslidematerial wasobservedtobesaturated.Bothoftheseslidesarebelievedtohavebeen initiatedbywaterseepingthroughsandyclayseamsinthepredominately overconsolidatedclay. Waterperchedinsand,gravel,andperviousrocklayersbyunderlying clays,asillustratedinFig.2.6b,mayalsocontributetoslopefailuresin theclaybyreducingitsshearstrength.This,forexample,wasthecasein thepreviouslydiscussedCentervilleslides.Whiletheseslidesoccurred duringconstruction,anumberofyearsmaypassbeforewaterfromanoverlying strata will initiatealandslide.Forinstance,waterseepingfromafractured limestonelayerintoa40-foot,2:1slopeonI.H.820nearGroveOakDrivein FortWorthappearstohavecontributedtothefailureofthissidehillcut, severalyearsaftertheslopehadbeenexcavated. Watermayalsobeperchedwithintheclaystrataitselfbyanunder-lyingrocklayeroflowerpermeability,asdepictedinFig.2.6c.Aslide occurringonu.S.281northofEvantwasbelievedtohavebeeninitiatedby waterperchedinsuchamanneraboveathinlimestonelayer.Theslopein whichtheslideoccurredhadbeencutin1959;thefirsttwelvefeetofthe slopewerecutona2:1gradeandtheremaining28feetona3:1grade,as previouslyshowninFig.2.4.Theslideprofileconsistedofalternating unitsofthinlimestonelayersandthickerclaylayers.Inthelate1960's,a slideoccurredalongathincontinuouslimestonelayerlyingabouttenfeet abovethetoeoftheslope.Thetoeoftheslidewassaturated,andborings disclosedhighconcentrationsofwaterabovethelimestonelayerwherethe failureoccurred. Ingeneral,slideshavedevelopedduringorafterperiodsofheavy rainfall,whichmayreplenishorincreasetheflowinanaquifersuchasa sandorgravelseam.ThenaturalhighlyplasticclayslopealongBoggyCreek 15 neartheU.S.183highwaybridgeinsoutheastAustinwassteepenedtoa2:1 gradetostraightenthecreek.Asectionofthis30-foot-highslopefailed shortlyafteraperiodofheavyrainfall,approximatelytwomonthsafterthe cuthadbeenmade.Inthefailedsectiontherewasaburiedchannelwherewater infiltratingfromabovetheslopeisbelievedtohaveinitiatedthefailure. Duringperiodsofdryweather,shrinkagecracksmaydeveloptoconsider-abledepthsinhighlyplasticclays,suchastheEagleFordorTaylorMarl formations,thusprovidinganaturalpathforsurfacewaterinfiltration. Surfacewaterwhichenteredshrinkagecracksisbelievedtohavecontributed toseveralembankmentfailuresonI.H.35EinDallas.Thematerialinthese slopesconsistedofahighlyplasticsiltyclaywithaplasticityindex rangingfrom30to50.Duringaperiodofseveralmonths,twoseparateslides developedinsaturatedmaterialatthislocationfollowingperiodsofheavy rainfall.Theseslidesinvolvedatotalofapproximately500feetofthe21/2:1 to3:1embankmentslopes.Athreetofour-footscarpdevelopedalongtheedge ofthehighwayshoulderandthetoeoftheseslidespassedthroughthelower thirdpointoftheslopeface.Shrinkagecrackswereobservedalongthe shoulderofthehighwayseveralmonthsaftertheslideshadoccurred,suggest-ingthatsurfacewaterwhichinfiltratedtheslopethroughsimilarcracksmay havecontributedtothefailures. Summary Themajorityoftheearthslopefailureshavedevelopedinmediumto highlyplastic,overconsolidated,stiff-fissuredclayslocatedintheDallas, FortWorth,Waco,Austin,andSanAntonioregionofTexas.Slopedesignis generallybasedonexperience,duetothedifficultiesandexpenseofadequately definingtheshearstrengthoftheclaysandthegroundwaterconditions withintheslope.Generally,shallowsemi-circularfailureshaveoccurred inembankmentandcutslopesandmayhavedevelopedasaresultofthegradual reductioninshearstrengthduetoswellingandthedevelopmentofpositive porewaterpressurescausedbyseepagefromperchedwatertables,aquifers,and fissures. Introduction CHAPTERIII REMEDIALMEASURES Anumberofdifferentremedialmeasureshavebeenutilizedbythe TexasHighwayDepartmenttocorrectearthslopefailures.Thesemeasures include: (1)stabilizationwithaddit ives, (2)soilsubstitution, (3 )restraintstructures, (4)controlofwater, (5 )slopealteration,and (6)concreterip-rap_ Whileslopesareoftenrepairedbypushingtheslidematerialbackontothe slope(regrading),thisapproachismoreofanefforttoretaintheoriginal slopegradethantocorrectoroffsetthoseconditionswhichinitiatedfailure. Althoughslopeshaveremainedstableafterbeingregraded,theimproved stabilityprobablyresultsfromachangeintheconditionswhichinitiated failure,suchastheequilibrationofporewaterpressuresorthedrainageof apocketofwater,ratherthanfromanyimprovementintheshearstrengthofthe soilachievedbypushingtheslidedebrisbackontotheslope.Therefore,for thesereasons,regradinghasnotbeenconsideredasaremedialmeasureinthis report.Inordertoevaluatethesixremedialmeasureslistedabove,twenty-sixrecentlycorrectedslopefailures,summarizedinTable3.1,werereviewed withattentionfocusedontheapplicationandeffectivenessofthesemeasures. StabilizationwithAdditives TheTexasHighwayDepartmenthasutilizedbothlimeandcementadditives forstabilizationandrepairofearthslopefailures.Ingeneraltheuseof limehasbeenrestrictedtohighlyplasticclayswhilecementhasbeen utilizedwithbothclaysandgranularmaterials.Theadditionoflimetothe soilhasbeenaccomplishedbydirectmixingwiththesoil(shallowtreatment) 16 Table3.1SummaryofRecentSlopeFailuresWhichHave ReceivedExtensiveRemedialTreatment SLOPET.R.D.SLOPESLOPE LOCATIONDISTRICTHEIGHrRATIO U.S.80Westof LakeArlington225'2:1 TarrantCountyC* U.S.180nearBrad220'11/2:1 PaloPintoCountyC S.H.174 JohnsonCounty215'2:1 C U.S.377south-eastTarrant240'3:1 CountyC U.S.67near Alvarado 225'2: 1 C U.S.180nearBrad PaloPintoCounty2L.O'2:1 C U.S.281near0-12'2:1 Evant912-40'3:1 HamiltonCountyCcut I.H.35and U.S.81918'2:1 McLennanCountyC U.S.59near GreenbriarStreet1218'2.6: 1 HarrisCountyF *c:Cut,excavated,ornaturalslope F:Fillorembankmentslope CONDITIONSATSITE GEOLOGICHYDROLOGIC DelRioclayDry HighlyExcesswater plasticclay SiltyclayExcesswater NaturalslopeFailedafter heavyrain Excesswater SidehillExcesswater fill WalnutExcesswater format ion TaylorKarlSeepsat baseof rip-rap HighlyExcesswater plasticclay AGE OFSLOPEREMEDIALMEASURES ATFAILURE Soilmixedwithlimeandcompacted ? of$130,000onslopeatacost FailedduringRetainingwallbuiltoutofI-beams constructionandtimberplanks;drainagetrench placed 15yearsRetainingwallbui ltoutofI-beams andtimberplanks ?Retainingwallbui ltoutofsteel I-beamsandsteelguardrails 8-9yearsFlattenedslopeto4:1andmixed limewithrecompactedsoil 5-6yearsConstructedl2-ft,.. deeptrench drain 1yearDrilledholesfilledwithhydrated lime;limestonebrokenupwith dynamite;seeptappedwithpipe 1yearFlattenedslopeto3: 1and constructedlowretainingwall Treatedtimberpilesusedto ? arrestslopem::::>vement.Toeof slidereplacedwithsand REMARKS Slopefailedoneyearafterbeing limestabilized.Theslopewill bebridgedover. Slideextendedlaterally1025ft. sucessfullystabi lizedatacost of$12,500 Successfullystabilizedatacost of $4,300 Waterfromutilityditchmayhave init iatedfailure Slopesuccess fullystabilized Slopesuccessfullystabilized i Fai luredevelopedalongclayand 1 imestonecC!llI'ltact;stab i lized Slopesuccessfullys tabil izec Costofplacementoftimberpi ling andsand $100,000 Slidesinvolved1400ft.ofslope (Continued) Table3.1(Continued) SLOPET.R.D.SLOPESLOPE CONDITIONSAT SITEAGE OFSLOPEREMEDIALMEASURESREMARKS LOCATIONDISTRICTHEIGHTRATIO GEOLOGICHYDROLOGIC AT FAILURE S.H.225andT.N.HighlyExcesswater6-7yearsSlidearrestedwithtimberpi lesSlidedeve lopedunderrip-rap &O.Overpass1225'2-3: 1plastic HarrisCountyCfill loH.35nearFourteen-foot-deepdra inagetrenchAfterdrainagetrenchwasbuilt, Reep'sDairyin 1420'21/2: 1TaylorMarlExcesswater placedaboveandparalleltocrest;theslopebegantobulge SouthAustin ? timberpilingusedonlaterslopeThismovementwascorrec tedwith C TravisCountytOOve me ntpiles U.S.183atBoggyFailed2monthsTworowsofcast-in-placedrilledFirstslideinthissectionof Creek1430'2:1TaylolMarlafterheavyshaftson6ft.centersslope TravisCountyCrain U.S.183under1'1.0staggeredrowsofpi lingonLimeslurryplacedindri lled BoggyCreekbridge 1430'2:1TaylorMarl10months 6andl2-footcenters;slopeholesandconcreterip-rapwas TravisCounty C steepenedto1.4: 1downs l0i>eusedinanunsucessfulattempt (secondslide)oftimberpiles. topreventf ai lure U.S.183atFailedafter4yearsLimemixedwithslidematerial;Twosmallslidesoccurred;one BoggyCreek1430'1.4: 1TaylorMarlheavyrainsloperegradedtoformergradeoneachsideofbridge TravisCountyC U.S.87northeastLimeplacedindrilledholesandSucessfullystabilizedatacost SanAntonio1530'11/2: 1TaylorMarlExcesswater _6yearsmixedwithslidematerial;concreteof- $12,000 BexarCountyCditchbuiltalongcrest loH.35andMooreTaylorMarl I Excesswater3-4yearsLimeplacedindrilledholesandSlidewhichoccurredwestof Street1530'21/2:1P .l.- 54mixedwithslidematerial;a14- MooreStreetbridgewasrepaired BexarCountyCft.drainagetrenchbui It atcrestatacostof_$24,000 I.H.35andMooreSlidesuppressorwallplaced1/3Slidecleve lopedunderrip-rap Street1523'21/2: 1TaylorMarlExcesswater7-8yearsthedistanceuptheslopeofMooreStreetbridge BexarCountyC loH.10andNewSlidesuppressorwallplaced1/3Slidedevelopedin400ft.ofthe BraunfelsAvenue1525'3:1TaylorMarlExcesswater3yearsthedistanceuptheslopesouthslopeofl.H.10;repaired BexarCountyCatacostof $33,500 .-(Continued) Table3.1(Continued) SLOPET .H.D.SLOPESLOPECONDITIONSATSITEAGE OFSLOPE REMEDIALMEASURESREMARKSLOCATIONDISTRICTHEIGIITRATIO GEOLOGICHYDROLOGIC ATFAILURE I.H.10andNewThirteen,20 -ft.longdrilledSlideinnorthslopeofI.H.10 BraunfelsAvenue15 23'3:1TaylorMarl..Excesswater~ 3yearsshaftson8-ft.centersplacedwassuccessfullystabilizedata BexarCountyC1/3thedistanceuptheslopecostof_$14,900. I.H.10&RolandHighlySlidesuppressorwallplacedone- Failureoccurredbeneathrip-rap Avenue1518' [1/2:1 plasticclayExcesswater~ 3yearsthirdofthedistanceuptheslope BexarCountyC I.H.37andSouthHighlySlidesuppressorwallplacedone- Failureoccurredbeneathrip-rap CrossBoulevard1517'2:1plasticclayExcesswater?thirdofthedistanceuptheslope BexarCountyC I.H.37andNew HighlySlidesuppressorwallplacedone- Failureoccurredbeneathrip-rap BraunfelsAvenue 1517'2:1plasticclayExcesswater?thirdofthedistanceuptheslope BexarCounty C I Loop4l0Wand MarbachRoad1522'11/2:1HoustonclayExcesswater1-2yearsSlidematerialmixedwithlimeSuccessfullystabilized BexarCountyF I.H.45andS.H.7 Spartasand SaturatedDuringSystemofinterceptorandlateralSlopewassuccessfullystabilized I nearCenterville1755'4:1 Wheechesclay clay ~ t i o ndra insatacostof_$27,000. LeonCountyC QueenCity sand SouthI.H.35E ISlopefailuresrepairedbymixing northofBachmanRd1835'2:1P .1.30-50Excess'Water!2 -3yearsSloperebuiltwithsamematerial limewiththeslideuebrishave DallasCountyF i occurredagaininthismaterial. I.H.30atLakeThreefailuresoccurredinthis Hubbard1846'2:1TaylorMarlDryDuringSlopeflat tenedto3:1embankmentinvolving1,000ft.of DallasCountyFconstructionslope 20 orbyplacingthelimeindrilledholes(deeptreatment),whilecementstabili-zationhasbeenrestrictedtodirectmixingtechniques(shallowtreatment). LimeStabilization(ShallowTreatment).Thestabilizationofslides bytheadditionandmixingoflimewiththesoilisusuallyrestrictedto relativelyshallowdepthsofuptoapproximately4feet.However,inone instancelimestabilizationbydirectmixingwascarriedouttodepthsas greatasninefeet.Theadditionandmixingoflimewiththesoilareeither donein-placeonthefaceoftheslopeortheslidedebrisisremovedfromthe slope,mixedwithlimeandthenplacedbackontheslope.Thestabilized materialisthenrecompactedwithtractorsorconventionalcompactionequipment; however,thecompactionmaynotbecontrolled. Forlargeslides,generallyinvolvingover400cubicyardsofmaterial, astageconstructionprocedurehassometimesbeenusedforstabilizationof theslidematerial.Intheseinstancestheslidedebrisisexcavatedby benchingtheslope,mixedwithlime,replacedontheslopeandrecompactedto thefinalslopegrade.Adjacentstripsarethenexcavatedandtheprocedure repeateduntiltheentireslideareahasbeenstabilized. Theleveloflimetreatmentusedisgenerallyintherangeof3to4 percentlimebyweightandnolaboratorystudiesaremadetoselectoptimum limecontent.Thisleveloftreatmentistypicalofthelimecontentsused formodificationofpavementsubbasematerials. Whilelimestabilizationappearsinseveralinstancestohavebeena successfulcorrectivemeasure,failureshaveoccurredinslopeswhichwere previouslystabilizedwithshallowlimetreatment.Thereasonforthe recurringslidesisuncertain,andgenerallyinsufficientinformationis availabletodecisivelyascertainthecoursesoftherecurringslides.However, severalfactorsprobablycontributetotheinadequacyoftheremedialmeasures: (1)poormixingofthelimewiththesoil, (2)poorcompactionofthelimestabilizedmaterial - nocompaction control, (3)insufficientdepthoftreatmentandfailuretostabilizethe zoneofmovement,particularlyfordeepslides,and (4)improperconsiderationofthecausesoffailure. Oneofthelargestshallowlimestabilizationprojectswascarriedoutby FortWorth(District2oftheTexasHighwayDepartment)tocorrectaslide alongU.S.80westofArlington.Thisslideoccurredasadeeprotational . . 21 failurealonga1,200footsectionofa35-foothigh,2:1cutslopeinthe DelRioformation,ahighlyplastic,stiff-fissuredclayshale.Theslide debriswasremovedinl2-footwidestripsrangingindepthfromfivetonine feet,mixedwithlime,andrecompactedtotheoriginalslopegrade;however, thelimestabilizedsurfacedidnotinterceptthefailuresurfaceoftheslide. Approximatelythreeyearsaftertherepairsweremadein1969,theslopefailed again,withthestabilizedstripsofsoilbeingdisplacedasmoreorless continuousblocksinthesaturatedslidematerial.Sixmonthspriortothis failure,severaldeep,widecrackswereobservedalongthefaceoftheslope, apparentlyattheinterfacebetweentheadjacentstabilizedstrips.These cracks,whichmayhavedevelopedasaresultofrenewedmovementalongthe initialfailuresurface,providednaturalpathsforsurfacewatertosaturate andweakentheslopemass.Therehadbeenanumberofotherslidesinthis sectionofu.S.80beforethismorerecentoneaudinordertopreventfuture problems,thisslidezonewillbebridgedoverandtheslopebeneaththebridge willbeflattened. Inthemajorityoftheslopefailureswhichhaveoccurredthepresence ofexcesssurfacewaterandthegroundwaterconditionsappeartohaveplayeda dominantroleintheinitiationoftheslides.Intheseinstancesstabiliza-tionandsealingtheslopefacewithlimemayimpedeinternaldrainageand resultinthedevelopmentofexcesshydrostaticwaterpressuresinthesoil withacorrespondingreductioninstrengthandstability. Oneofthelimitationsplacedontheuseoflimestabilizationisthe lackofsufficientworkingspacetoproperlymixthesoilwithlimeand recompactthematerialontheslope.Inanumberofcases,particularlyinthe SanAntonioarea,whereslopefailureshaveoccurredbeneathovercrossing bridges,theoverheadclearancehasbeenseverelyrestricted.Inotherin-stancestheslopewaseithertoosteeptomixthelimeproperlyonthefaceof theslopeortheavailableworkareaaboveandbelowtheslopewastoosmallto mixlimewithslidematerialremovedfromtheslope.Oneapproachtousing limestabilizationinthesesituationsistoremovetheslidematerialto anotherlocationformixingandthenreturnthestabilizedmaterialtothe slope.However,theadditionalrequirementoftransportingtheslidematerial makesthisproceduremoreexpensivethanmixinglimewiththeslidedebrisat thesiteofthefailure.TheexperienceandpolicyofHouston(District12) hasbeenthatifthematerialmustberemovedfromthesiteformixingwith 22 stabilizingagents,agenerallymoreeconomicalalternativeistoreplacethe slidedebriswithamoresuitableborrowmaterial. LimeStabilization(DeepTreatment).Deeplimestabilizationas referredtointhisreportisaccomplishedbyplacinglimeindryorslurry formintodrilledholes.Typicallythedrilledholesare8to12inchesin diameterandareplacedinstaggeredrowson5tolO-footcenters,theexact diameterandspacingoftheholesbeingdeterminedbytheeconomicsofcon-structionandthejudgementoftheengineersinvolved.Thedrillingofthese holesiscommonlydonefrom12tol4-foot-widebenchesexcavatedtoprovidea levelworkingarea.Eitheroneortworowsofaugeredholesareusedforeach bench,dependingonthecapabilitiesofthedrillingequipmentandtheengi-neer'sjudgement.Thedepthoftheholesusedforthedeeplimetreatmenthas variedfrom5to25feetbelowthedrillingsurfaceandgenerallyanattempt ismadetopenetrateseveralfeetbeyondtheknownorestimatedfailuresurface. Fromtheavailablecasehistoriesitisdifficulttoassesstheextent towhichdeeplimetreatmenthascontributedtothestabilityofafailed slopebecausethelimewasusedinconjunctionwithothercorrectivemeasures. However,it appearsthattheuseofdeeplimestabilizationhasincreasedthe stabilityofaffectedslopes.Forinstance,alimeslurrywasusedtocorrect aprogressiveslideproblemona1:1slopealongBusinessU.S.87innorthwest SanAntonio.Thelimewasplacedindeepholesaugeredintoa25-foothigh fissuredcalcareousclayslope.Inadditiontothelime,ashallowconcrete linedditchwasbuiltalongthecrestoftheslopetominimizetherunoff overthefaceoftheslope.Thesetwocorrectivemeasuressuccessfullycor-rectedwhathadpreviouslybeenanannualproblem. Ahydratedlimeslurrywasalsousedtostabilizethepreviouslydis-cussedslidenearEvantinHamiltonCountyonU.S.281.Thesoilprofileof the40-foot-highslopeconsistedofthinlimestonelayersalternatingwith thickerclaylayers.Inthelate1960's,aslideoccurredalongathincon-tinuouslayeroflimestoneasshowninFig.2.4.Aperchedwatertablewhich contributedtothefailurehaddevelopedabovethislimestonelayer.Ten benches,wideenoughtoallowatruckmountedaugertodrilll2-inch-diameter holesdowntothezoneofslippage,werecutintotheslopeparalleltothe roadway.Atotalof122holeswereaugeredandfilledwithapproximately50 tonsoflimeslurry.Onlyenoughwaterwasusedtoallowmixingandpumping 23 ofthelime.Alltheholeswerecheckedandrefilledwithslurryforaperiod ofseveralweeks.Thelimestonelayerwherethefailurehaddevelopedwasfrac-turedwithdynamitetolowertheperchedwatertable,andinoneareawhere watercontinuedtoconcentrate,ahorizontalpipewasinstalledtoremovethe seepage.Theprojectwasleftforsixmonths,duringwhichtimetheseepswere observedandtheslopesurveyedforindicationsofadditionalmovement.After thisperiodtheslopeappearedstablesogradingandfinishingwerecompleted andryegrasswasplantedovertheentireslope.Theslopehasremainedstable forthepastelevenyears. Thereappeartobeatleasttwolimitationsonthereliabilityofdeep limetreatmentforstabilizationofanactiveslide.Thefirstofthesein-volvestheuncertaintyregardingtheextenttowhichthelimemigratesintothe soilsurroundingthedrilledholes.Indense,non-fissuredclaystheextentof limemigrationintothesoilhasbeenfoundtobegenerallylessthan1inch (McDowell,1970).Infissuredclaysthedegreeoflimemigrationappearstobe somewhatgreater;however,theextentofmigrationisuncertainandprobably highlyvariable.Inaddition,boththerateoflimemigrationandtherateof strengthincreaseinthesoilareprobablyrelativelyslow,andconsequently theimmediateeffectsofdeeplimetreatmentareprobablyminimal. Asecondlimitationofdeeplimetreatmentinvolvesthepotentially adverseeffectsoflimeadditioninslurryformundereithergravityorpressure heads.Theadditionofawater-limeslurrymayincreasethepresenceoffree waterintheslopeandresultinincreasedhydrostatic(porewater)pressures, thusdecreasingtheshearstrengthofthesoilandthestabilityoftheslope. Also,ifthelimeisinjectedunderpressure,aseriousdangerexistsinthe possibilitythatcracksmaybedevelopedintheinjectedformationandasig-nificantreductioninstabilitymayoccur. CementStabilization.Theapplicationofcementforstabilizationof earthslopefailureshasreceivedonlylimitedusebytheTexasHighwayDepart-ment.Thelessextensiveuseofcementascomparedtolimeprobablyresults inpartfromthefactthatmostslopefailureshaveoccurredinpredominantly claysoilsofmediumtohighplasticity.Intheseinstanceslimewouldappear tobecomparablyaseffectiveascementatapproximatelyhalftheexpense. However,theeffectsofcementstabilizationmayberealizedmuchmorerapidly duetothemorerapidratesofstrengthgainwithtime. 1 24 InsurveyingtheremedialmethodsusedbytheTexasHighwayDepartment onlyoneinstancewasfoundwherecementwasusedtodirectlystabilizea fine-grainedsoil.ThisoccurredontheIH35-MooreStreetslidepreviously described.Instabilizingthisslide,cementwasusedtostabilizethehighly plasticclayslopeatthetoe;however,limewasusedontheremainderofthe slope. Inallotherinstanceswherecementhasbeenutilized,thecementhas beenmixedwithcohesionlessmaterialswhichhavebeensubstitutedforthe originalslidedebris.Typicallyaboutfivepercentcement,byweight,has beenusedforstabilizingthesematerials.InoneinstanceHouston(District 12)hasutilizedoystershellsasareplacementmaterial,stabilizedwith approximatelysevenpercentcement. Cementstabilizationappearstohavebeenrestrictedentirelyto directmixingtechniques,andtherearenoknowninstanceswheredeepcement treatmentorgroutinghasbeenemployed.Theprincipalrestrictionsonthe useofcementascomparedtolimearethelimitedadvantagesofcementfor treatmentofmediumtohighlyplasticclays,andthehighercostofcement. Ingeneralcementmayalsobeexpectedtosharemostofthelimitationsand restrictionspreviouslydiscussedwithrespecttolime. SoilSubstitution Soilsubstitutioninvolvesthereplacementoftheslidematerial, generallyinthevicinityofthetoeoftheslope,withamoresuitable material,usuallyeitheraclayoflowplasticityorcohesionlesssandsand gravels.Thesubstitutedmaterialmayservetoincreasethestabilityofthe slopeinthreeways: (1)byprovidingahigherstrengthandgreatershearresistance againstsliding, (2)byprovidingamoreperviousmaterialwhichwillallowwater todrainmorefreelyfromtheslopeandpreventthedevelopment ofexcessivehydrostatic(porewater)pressures,and (3)byprovidinganincreaseintheweightofthesoilatthetoe oftheslopewhichprovidesanincreasedpassiveresistance tosliding. Generallycohesionlesssandsandgravelsarethemostsuitableforthis purposeastheypossessahigherstrength,greaterpermeabilityandsomewhat higherunitweightthanclayeysoils.Apersistentlandslideproblemon u.S.59betweenShepherdDriveandGreenbriarinHoustonwascorrectedby 25 substitutingsandfortheoriginalfillmaterial.Timberpilingwasdriven nearthetopofboththenorthandsouthfillslopestoretaintheembankment whilethesoilbelowthepilingwasreplacedwithcompactedsand.Theseslopes whichhadcontinuallyfailedafterperiodsofheavyrainfallhaveremained stablesincebeingrepairedduringtheearlymonthsof1969. Somereplacementmaterialsmaybestabilizedwithcementtoimprove theirresistingcapacity;however,theuseoffine-grainedcementstabilized materialsmayresultinundesirablylowpermeabi1itiesandimpedanceoffree drainage.Thepermeabi1itiesofcementstabilizedsandsandgravelsappearto beadequate.Indexesoftherateofflow,ordrainagefactors(TexasHighway Department,1962),determinedforthesestabilizedmaterials(District12, Houston)rangedfrom1200to1500cc/hr. Wellcompactedandwellgradedgravelscanbeusedtoincreasethe unitweightofthesoilatthetoeoftheslopebythirtyorfortypercentas thesematerialsmayhavedensitiesofupto1401b/cu.ft.However,the overallincreaseinstabilityduetothisaddedunitweightisprobablyminor ascomparedtotheincreaseinstabilityderivedfromtheaddedstrengthand drainagewhichisprovidedbygravelorsandsubstitution. RestraintStructures Pilingandcast-in-p1aceconcretedrilledshaftshavebeenemployed relativelyextensivelybytheTexasHighwayDepartmentforstabilizationof earthslides.Wherepilinghasbeenused,timberhasbeenthemostcommon structuralmaterial,ratherthansteelorreinforcedconcrete.Inseveral instancesaretainingwallorsimilarstructurehasbeenaffixedtothepile headstoprovidefurtherlateralresistancetoslidingandtoprecludesoil movementaroundandbetweenthepilesordrilledshafts. DrilledShafts.Drilledshafts,whicharewidelyusedbytheTexas HighwayDepartmentforthesupportofbridgestructures,havealsobeenused asaslideretentionmeasurewithapparentsuccess.Drilledshaftswereused asacorrectivemeasurealongthenorthslopeandsouthslopeofthedepressed sectionofI.H.10andNewBraunfelsStreetinSanAntonio.Theseslides occurredinapproximately25-foothigh3:1slopesexcavatedinahighly plastic,stiff-fissuredclayshale.Thefourhundredfootlongslidewhich occurredalongthesouthslopeofthislocationwascorrectedbyusing24-inch-diametershafts,20feetinlength.Thedrilledshaftswerespacedon8-foot '. J 26 centersalongasinglelineapproximatelyone-thirdofthewayuptheslope. InadditiontotheseshaftsaI-footby4-footverticalwallwascastinplace againsttheupper4feetofshaftandanattemptwasmadetopositionthewall suchthattheslideplaneinterceptedthecenterofthewall.Thewallwas thenbackfilledwithgravelandthesloperegradedtoitsoriginalshape.This particularconfigurationofdrilledshaftsandtheadjacentconcretewallbelow gradehasbeenlocallyreferredtoasa"slidesuppressorwall." Thesecondandsmallerslidealongthenorthslopeofthesamelocation wasstabilizedwithapproximately13drilledshafts,similarinsizeandloca-tionofplacementtothoseusedonthesouthslope.However,aconcretewall wasnotusedinconjunctionwiththeshaftsatthissecondslide.Theremedial measuresusedfortheseslideswereconstructedin1968andtheslopeshad remainedstabletothemostrecent(1971)survey. Dlilledshaftswerealsousedtocorrectaslidewhichdevelopedbeneath ahighwaybridgeonU.S.183atBoggyCreekinAustin(District14).Thefirst slideatthislocationoccurredin1965alongaportionofa30-foot-highcut slopeinhighlyplasticweatheredshalealongthebankofBoggyCreek.Fifty, l8-inch-diameterdrilledshaftswereplacedon6-footcentersintwostaggered rowsapproximately6feetapart.The32-foot-longshaftswerelocatedapproxi-matelyone-thirdofthewayuptheoriginalslope.Reinforcementforeachshaft consistedofsixevenlyspacedNo.6barsandaNo.2spiralextendingthefull shaftlength.Inaddition,threeNo.9barswereplacedinthelower20feet oftheshaft,alongtheupslopeside,toprovideresistancetobending.In conjunctionwiththeplacementofthesedrilledshafts,theslopewasregraded andflattenedto3:1abovetheelevationofthelineofshafts.However,below thiselevation,theslopewassteepenedtoapproximately1.4:1asillustrated inFig.3.1.Whilethedrilledshaftshaveapparentlybeensuccessfulin haltingfurtherslidemovementsfromabove,anewslideoccurredinthesteepened portionoftheslopebelowthedrilledshaftsduringthewinterof1969-1970. Thisslide,whichexposedtheupper6to8feetoftheshafts,wasrepaired withregradingand. limestabilization. SteelPiling.InseverallocationstheTexasHighwayDepartmenthas utilizedsteelI-beamsorH-pilesforstabilizingslidemovements.Thesteel pilingiseitherdriveninplaceorinstalledinapre-boredholeandback-filledwithconcrete.Inseveralinstancesatimberplankwallhasalsobeen fittedbetweentheflangesofadjacentpilestoprovideadditionalslope OriQinal .J Fig.3.1.TypicalcrosssectionofBoggyCreeknear U.S.183insoutheastAustin 27 30' . . restraint,andwhensuchawallisused,placementofthesteelbeamsin preboredholesisgenerallypreferredbecauseoftheimprovedalignmentfor constructionofthewall. 28 SteelI-beamswereusedtocorrectaslideextendingapproximately1000 feetalongasectionofU.S.180nearBradinPaloPintoCounty.TheI-beams weredrivenon8-footcenterstoadepthofapproximately15feetbelowthetoe oftheslopeandtreatedtimberplanks(oldrailroadties)wereplacedtoform a7-foot-highwall.A3-foot-widedrainagetrench,includinganunderdrain outletpipe,wasplacedbehindthewallandbackfilledwithfiltermaterial. Whiletheabovewallwasleftexposedatthefront,asimilarretaining structurewasincludedentirelywithintheoriginalslopegradeasacorrective measureinthestabilizationofaslidealongU.S.174nearPaloAltoin JohnsonCounty.Inthisinstance,thesteelpileswereplacedinpre-boredholes adistanceofapproximatelyone-thirdofthewaydowntheslopefromthecrest. Thepilesextendedtoadepthofapproximately26feetbelowtheslopesurface andan8-foot-hightimberretainingwallwasaffixedtotheupperportionof thepiles.A4-foot-widedrainagetrench8feetdeepwasplacedbehindthe wallandbackfilledwithstandardfiltermaterialincludingapipeunderdrain. AnotherslideinFortWorth(District2)occurredinanaturalslope locatedonthelowersideofahighwayandresultedinpavementdamageextending toapproximatelytheroadwaycenterlineatthetopoftheslidescarp.This slidewasbelievedtohavebeeninitiatedbyentranceofwaterthrougha3 footdeeputilitiestrenchconstructedalongtheshoulderseveralyearsafter completionofthehighway.Inthisinstancetheslidewassuccessfullystabil-izedwitharestraintstructureconsistingofoldsteelguardrailsattached toverticalsteelI-beams.Thesteelbeamswereplacedalongtheroadshoulder inpre-boredholesandbackfilledwithconcrete.Thetopofthesteelwall waslocatedatapproximatelytheelevationofthehighwayandsomeportionsof thewallwereleftexposedonthedownslopeside. TimberPiling.Inthecorrectionofslides,timberpilinghasbeen usedbothasaprimarycorrectivemeasureandasatemporarymeasurepriorto majorremedialwork.Inalargeslide.occuringbeneaththeoverpassstructurp. forU.S.183atBoggyCreekinsoutheastAustin,timberpilingwasusedasa primarycorrectivemeasure.Theslideatthissitedevelopedina2:1slope excavatedinahighlyplastic,expansiveclayshaleandinvolvedabout200feet ofthe25-foot-high,concreterip-rappedslope.Theslidewascorrectedby , J 29 drivingtworowsoftimberpiling,onerowbeingdrivenon6-footcenters approximatelyatandparalleltothecrestoftheslopewhilethesecondrow, locatedabout14feetupslopefromthefirstrow,wasdrivenon12-footcenters. Allpilesweredriventorefusal,approximately20to30feetdeep.Incon-junctionwiththisremedialworktheslopewasregraded. TimberpileswerealsousedtocorrectaslideonI.H.35nearSuperior DairiesinAustin.Thisslidedevelopedonaslopewhichhadfailedduring aperiodofheavyrainsapproximatelytwoyearsearlierandhadbeenstabilized byinstallinganinterceptortrenchparalleltoandabovethecrestofthe slope.Thenewslidedevelopedintheupperhalfofthe21 2 : 1 ~ 20-foot-highslope.Timberpileswereinstalledon5-footcentersintwostaggered rows.InthisandotherinstanceswheretimherpileshavebeenusedbyDis-trict14forslidecorrection,therehavebeennoproblemswithsoilflowing aroundandbetweenpiles.Suchpileshavenormallybeenspacedon5or6-foot centers. Inatleastoneinstancetimberpileshavebeeninstalledinaslide totemporarilyhalttheslidemovementandtopreventmoreextensivedevelop-mentofaninitiallysmallslide.Suchactionwastakentoarrestprogressive movementsofaslidewhichdevelopedalonga520-footsectionoftheembankment slopeonU.S.59betweenShepherdDriveandGreenbriarinHouston.Theslide developedinthelowerhalfoftheslope,andinordertopreventtheslide fromprogressingupslopeandthreateningthemainhighway,12-inch-diameter timberpiles,30feetinlength,weredrivenonapproximately21/2footcenters alongtheupperacarpoftheslide(atmid-heightoftheslope).Thetipof thepilesinthisinstanceextendedapproximately16feetbelowtheelevation ofthetoeoftheslope.Thepurposeofthesepileswastostabilizetheportion oftheslopeabovetheslideforasufficientperiodoftimetopermitthe slidedebristoberemovedandreplacedwithamoreselect,stablematerial. Inseveralcaseswheretimberpilinghasbeenusedtostabilizeslide movements,additionaltimberpileshavebeenplacedhorizontallybehindand againsttheverticalpilesinordertoprovideincreasedlateralrestraintand toreducesoilmovementbetweenthepiles. Oneofthedifficultieswhichmaybeencounteredwhenusingtimber pilesisthelimitationonthedepthtowhichthepilingmaybedrivenwithout structuraldamageand"brooming"ofthepile.Thisproblemmaybeparticularly significantinareaswhereslidesareoccurringinslopesexcavatedinverystiff .J claysandshales.However,Austin(District14)hassuccessfullyand economicallyovercomethisproblembydrivingthepilesinpre-boredholes. 30 Asecondpotentialproblemarisingfromtheuseoftimberpilingasa remedialmethodisrottinganddecayofthepilematerial.Inpractically allcaseswhereslopefailureshavebeenobserved,thepresenceofwaterwas clearlynoted.Consequentlyinmostofthesecasestimberpilingwillbe subjectedtowaterandprobablyalternateperiodsofpartialwettinganddrying, aconditionwhichmayleadtorelativelyrapiddecayinthepile.Becauseof thestronglikelihoodofdecayandlossofstructuralintegrity,thetimber pilingshouldpreferablybetreatedanditshouldberecognizedthatthelong-termbenefitsoftimberpilingmaybeuncertain. CONTROLOFWATER ----Thepresenceofuncontrolledwaterappearstohavebeenoneofthe primarycausesofnearlyallearthslopefailuresinTexas.Theintroduction ofwaterintotheslope,eitherasgroundwaterorsurfacerunoff,increases thehydrostatic(porewater)pressureintheslopeandreducestheavailable shearingresistanceofthesoil.Inordertorestrictthewaterfromentering theslopeinitiallyandtoremoveanywaterwhichdoesentertheslope,the TexasHighwayDepartmenthasemployedseveralmethodsofcontrol. InterceptorTrenches.Oneoftheprincipaltechniquesforcontrolling subsurfacegroundwaterusesinterceptordrainagetrenches,whicharecon-structedbyexcavatingatrenchnearandparalleltothecrestoftheslope. Thedepthofthetrenchmayvary,depend ingonthesoilprofi Ie,depthofthe slide,locationofgroundwaterandtheproblemsandcostsassociatedwithbracing anddewateringthedrainagetrench.Thetrenchisgenerallybackfilledwitha standardfiltermaterialusedbytheTexasHighwayDepartment,andwherelarge quantitiesofflowareanticipated,drainagetileorperforated,corrugated metalpipeunderdrainsareinstalledatthebottomofthetrench.Inaddition, transversedrainsareusuallyinstalledtoremovethewatercollectedbythe interceptordrainsandtodischargethewateratthetoeorsideoftheslope. Thesetransversedrainsmaybesimilarinconstructiontotheinterceptor trenchesormayconsistonlyofatileormetaloutletpipe . Acombinedschemeofinterceptorandtransversedrainswasusedto correctaseriesofslideswhichdevelopedduringtheexcavationoftheslopes alongasectionofI.H.45onemilefromCenterville(District17).These J 31 slidesdevelopedinanapproximatelyl5-foot-thickclaystratum,boundedabove andbelowbylayersofsand.Theclaylayerhadapparentlybecomesaturated byaperchedwatertableintheuppersandstratum,andtheremovalofsupport fromtheclayduringtheexcavationofthe4:1cutslopesresultedinsliding. Aseriesoflateralinterceptordrains,averaging6feetindepthand21/2 feetinwidth,wereconstructedinseveralrowsparallelingthecrestofthe slope.Thebottomofthetrencheswerebackfilledwithapproximately21/2 feetofstandardfiltermaterial,theremainderofthetrenchbeingfilledwith imperviousmaterial.Wheretheanticipatedquantityofflowwasrelatively large,a6-inch-diameterperforated,corrugated,galvanizedmetalpipewas alsoplacedinthebottomofthetrench.Drainageofthewaterfromthe interceptordrainswasaccomplishedbyconstructionofseveraltransverse drainswith6-inch-diametermetalpipeoutletstodrainthewateroutnearthe toeoftheslope.Atotalof4500feetoffilterunderdrainand2200feetof drainagepipewereusedtocorrectthisslideatanin-placeconstructioncost ofapproximately$27,000.Theuseofinterceptordrainsinthiscasehasproven successful.Waterisstillbeingdischargedfromsomeofthetransverseoutlet drainsandnofurtherstabilityproblemshavebeenexperiencedsincethedrains wereplacedin1966. InitialslidesatMooreStreetandI.H.35inSanAntonioandonI.H.35 nearSuperiorDairiesinAustinwerealsocorrectedbyusinginterceptordrainage trenches.Bothoftheseslidesoccurredinhighlyplastic,expansiveclay slopesandwerecorrectedbyexcavatinganinterceptortrenchapproximately 15feetdeepnearthecrestoftheslope.Atbothsitesagravelunderdrain wasusedandonlytheAustinsiterequiredshoringofthetrench.Additional slidingoccurredatbothofthesesitesafterconstructionofthedrainage system;however,attheSanAntonioMooreStreetlocationthenewslidedevel-opedinanareaadjacenttotheoldslideanddidnotinvolvethecorrected area.AttheAustinsiteasmallslumpslideoccurredattheoldslidelocation, butthenewslidewasmuchsmallerinmagnitudeandwassuccessfullystabilized withtimberpiling. OtherMethodsofGroundwaterControl.Horizontaldrainsextendinginto thefaceoftheslopehaveoccassionallybeenusedtocontrolwaterfrom isolatedseeps.Generallyasmalldiameterperforatedcorrugatedmetalpipe isusedforthispurpose.Horizontaldrainswereusedinconjunctionwith othermeasurestocorrectaslideatMooreStreetandI.H.35inSanAntonio ) 32 andoneonu.s.281nearEvant.Initiallywaterdrainedfreelyfromdrains atbothsites.However,thedrainshavenotbeencheckedonaperiodicbasis, andthus,itisnotknownifthedrainsarestillremovinggroundwater. InseveralareasoftheState(Districts2,9,and18)perchedwater tables,whichhavedevelopedabovethinlimestonestratainclayshaleforma-tions,havecontributedtoseveralslides.Inatleastoneinstancethe stabilityoftheseslopeshasbeenimprovedbyfracturingthelimestonestrata withdynamite,thusimprovingthedrainagecharacteristicsoftheformation. Whereslopesarerip-rappedwithconcrete,sanddrainageblanketshave occasionallybeenusedtocollectgroundwaterseepingfromtheslopefaceand todrainsurfacewaterwhichmayentertherip-rapfromabovetheslopefaceor throughconstructionjoints.Thesedrainsareusuallyconstructedwitha minimumthicknessof6inchesandweepholesareprovidedintherip-rapto allowfreedrainageofwaterfromthesandblanket.Frequently,theweep holesplacedatthebottom,middle,andtopoftherip-raparetheonly measurestakentodrainthewaterfrombehindtherip-rap.Whilesuchdrainage beneaththerip-rapisprobablynecessary,itspresencedoesnotnecessarily precludethepossibilityoffailureoftheslopeduetoexcessivegroundwater, althoughthegroundwaterisfreelydrainedattheslopeface. ControlofSurfaceWater.Controlofsurfacewaterisanimportantand necessaryphaseinachievingapermanentandeffectivesolutiontoslope stabilityproblems.Surfacewateriscontrolledbyditches,curbing,crack filling,andslopeplanting.Concretelinedditchesplacedaboveunstable slopeareasshouldbeconstructedtoproperlyinterceptsurfacerunoffand drainthewaterawayfromtheunstablegroundandthefaceoftheslope.Where ahighwayorfrontageroadislocatedaboveaslope,theuseofcurbingalong theroadwayhasprovidedthisdrainagecontrol.However,theconstructionof pavedditchesforthesinglepurposeofinterceptingwaterabovetheslope appearstohavereceivedonlylimitedapplicationbytheTexasHighwayDepartment. InsurveyingtheslopeproblemareasinTexastwospecificslide locationswerenotedtohavereceivedinadequateconsiderationforsurface drainageabovetheslope.AttheBoggyCreekslidelocationinAustin,water wasobservedatonetimetobepondedinareasofthe groundoftheslope. Theexistingpaveddrainagechannelsinthevicinityofthecrestoftheslope allowedwatertopondatlowpointsandseepagewasoccurringthroughsomeof thejointsintheconcreteditchlining.WhiledrainagewasprOVidedat the , .. 33 BoggyCreeklocation,thedrainageappearedinadequateforcompleteandproper controlofthesurfacewateratthissite. Atasecondsite,locatedonS.H.337inPaloPintoCounty,thecomplete absenceofsurfacedrainageappearstohavebeenacontributingfactorinthe lossofstability.Theslideatthislocationoccurredalongaportionofa 70-foot-highcutslopeinashale-sandstoneformation.Intheareaoftheslide thepresenceofexcessivemoisturecouldbeclearlynotedandamoderateamount ofsurfaceerosionhadoccurredonthefaceoftherecentlycompletedslope. Inspectionoftheareaabovetheslopeclearlyshowedtheevidenceoflarge amountsofsurfacerunoffbetweenthecrestoftheslopeandtheright-of-wayline.Immediatelyabovetheslidearea,atthecrestoftheslope,surface runoffhasbecomechannelizedinanerosiongullyvaryingfrom6to10inches indepth.Whilesubsurfacegroundwaterprobablyplayedasignificantrolein initiatingtheslideatthislocation,theinfiltrationofuncontrolled surfacerunoffundoubtedlyaggrevatedtheslideproblem. Inadditiontopaveddrainageditches,surfacewatermaybecontrolled anditsentranceintotheslopemaybereducedbyfillingandsealingcracks whichformnearthecrestoftheslopeduetoshrinkageandslopemovements. Ifsuchcracksareallowedtoremainopen,theyprovideanaturalpathfor entranceofrunoffintotheslopeandthesubsequentdevelopmentofhighpore waterpressures.TheTexasHighwayDepartmenthasemployedclay,RC-2asphalt, andlimeforfillingopencracks;however,thesoiutionisonlytemporarysince newcracksgenerallydevelopwithinashortperiodoftime.IntheSanAntonio areashrinkagecracksformedinoneinstancetoadepthofapproximately20to 25feet.Theseformedinanembankmentofapproximatelythesameheight(20-25feet)constructedofahighlyplasticclayborrowmaterial.Attemptswere madetosealthecracksbypumpinganestimated1000gallonsofRC-2liquid asphaltintotheopenings.Laterinspection(aftertheoccurrenceofaslide inthesameembankment)revealedthatbelowadepthofseveralfeettheasphalt hadremainedinaliquidstateandhadfailedtocure. Sealingoftheslopesurfacewithamembranetopreventcrackingdueto shrinkagehasmetwithonlylimitedsuccess.Inanumberofinstancescracking hasbeenobservedtoextendintothepavedhighwaysurface,theexistenceof thepavinghavinglittleinfluenceonthepreventionofcracking.SanAntonio (District15)hasattemptedtocoverandsealthecrestoftheslopewitha flexibleasphaltmembrane;however,theseattemptshavegenerallybeen , ., 34 unsuccessfulbecausecrackssoondevelopinthemembraneitself. Grassandothervegetationplantedontheslopeforerosioncontrol mayalsoaidintheremovalofsomesurfacewaterandpreventdeepinfiltration ofwaterintotheslopethroughthefaceduringwetperiodsoftheyear.However, duringdryperiodstheslopevegetationmayaidintheformationofshrinkage cracks,thusaggrevatingtheproblemofsurfacewaterinfiltration,especially duringthefirstfallorwinterrains. SlopeAlteration ThestabilityofseveralTexashighwayslopeshasbeenimprovedby flatteningtheslopegradebyeitherremovingoraddingmaterial.The flatteningofaslopebyexcavationtendstoreducetheforceswhichdrivethe slidemass,whiletheadditionofcompactedsoil,principallyinthetoeregion, tendstoincreasetheforceswhichresistmovement.Thegradetowhicha slopeisflattenedhasbeendeterminedprimarilyonthebasisofexperienceand right-of-wayrestrictions.Typically,wheretherehavebeennoright-of-way restrictionstheslopehasbeenflattenedtothenexthigherintegerratio,i.e., froma2:1toa3:1slopegrade.Theslopefailuresinanaturalslopeatthe intersectionofI.H.35andU.S.81inWaco(District9)andthepreviously discussedslidesontheI.H.30embankmentwhichcrossedLakeHubbardin northeastDallasweresuccessfullycorrectedbyflatteningtheirrespective slopegrades.Thel8-foot-high,highlyplasticnaturalslopeandthe46-foot-highembankmentslopewerebothflattenedfroma2:1toa3:1grade. Theuseofslopeflatteningasacorrectivemeasuremayberestricted bythecostofadditionalright-of-way,limitedsourcesofborrowmaterial,or longhauldistancestoremoveoraddmaterial.Inaddition,whensoilis addedtothetoeregionofacutslope,caremustbeexercisedtoavoidsealing offgroundwaterflow.Disruptionofthisflowmayleadtoabuildupof hydrostaticheadwhichintimemaycausetheslopetofailagain. ConcreteRip-Rap Thepresenceofconcreterip-raplontheslopefaceappearstohave contributedtosomeslopefailureswhileinotherinstancessimilarrip-rap hasapparentlypreventedfailures.Thisapparentanomalyappearstoresultin 1 Concreterip-rapasreferredtointhisreportisacontinuousslabofconcrete generallysixtoeightinchesthickconstructedonthefaceoftheslope. .. r 35 partfromthepresenceofsurfaceandsubsurfacewaterandtheeffectwhich concreterip-raphasonthecontrolofthiswater.Thegeneralcharacteristic offailuresassociatedwithrip-rapandtheroleofrip-rapinpreventingslope failuresarediscussedbelow. FailureModes.Instabilityandfailureofconcreterip-raphave generallyoccurredintwomanners.Insomeinstancestherip-raphasmoved downslopeasagradualcreepmovement.Thismodeoffailurecommonlyproduces abulgenearthetoeoftherip-rap,sometimesextendingintothepavedshoul-deroftheroad.Thesecondmodeoffailuremaybeclassifiedascomplete failureoftherip-rapduetoashallowordeepsoilfailureintheunderlying slope. Rip-rapdistressresultingfromgradualcreepmovementshasgenerally beenconfinedtorelativelysteepslopes,usually2:1orsteeper.Problems ofthistypehavebeenfoundinHouston(District12)andtoalimitedextent inotherareasofthestate.Inoneinstance,inFortWorth(District2),rip-rapmovementsapparentlyresultedinlargelateralloadsbeingtransmittedto abridgesupportcolumn,producinghairlinetensioncracksonaportionofthe downslopesideofthecolumn.Thisproblemwasremediedbyremovingandre-placingasmallportionoftherip-raparoundthecolumntoallowforadjust-mentsoftherip-rapwithouttransferringloadtothecolumn. Theproblemofdownslopecreepoftherip-raphasgenerallybeenre-ducedbytheuseofeitherflatterslopes(District2)ortoewallsvarying from9to10inchesinthicknessand2to3feetindepth.However,insome instancestheseprocedureshavenotperformedsatisfactorilyandadditional measureshavebeentaken.District15hasinrecentinstancesadoptedtheuse ofanintermediate"key"walllocatedmidwayuptheslopetoprovideadditional anchoragefortherip-rap.Thesehavebeenusedforcutslopeswhichare2:1 orsteeperwithaslantheightofmorethan45feet.Theintermediatewall isapproximately18inchesdeepand10incheswideandthetoewallsareeither approximately18or36inchesdeepand9inchesinwidth.Thedeepertoewall isusedwitha5-inch-thickrip-rapsection,whilethesmallerwallisused with4-inchrip-rap.Rip-rapandslopemovements,whichoccurredattheinter-sectionofU.S.81andI.H.35inDistrict9,weresuccessfullycorrectedwith a3-foothighcantileveredwall,aboveground,atthetoeoftheslope.The rip-rapwasreplacedontheflattenedslope,extendingbackfromthetopof thewall. .. ,r 36 Thesecondcategoryofsloperip-rapfailureshasgenerallybeena resultofslidesinitiatingintheslopeitself.However,inmanyofthese instancesitappearsthattherip-rapmayhaveindirectlycontributedtothe failurebyimpedingthefreedrainageofgroundwaterandpermittingsurface runofftoinfiltrateandbecomeentrappedintheslope.Theimpededdrainage ofthegroundwater,observedinsandyclayseamsinthestiffclayslopewhich failedatMooreStreetandI.H.35inSanAntonio,isbelievedtohavebeen partiallyresponsibleforthefailure.Inseveralotherinstancesinthisarea similarinfluencesofrip-raparebelievedtohavecontributedtoslides. Theentranceofwaterintotheslopeandtheimpedanceoffreedrainage contributestoinstabilitybypermittingthesoil(commonlystiff,highly plasticclaysincutslopes)toswellandlosestrengthduetothedevelopment ofincreasedporewaterpressures.Entranceofsurfacewaterintotherip-rap mayoccurthroughconstructionjointsandopeningswhichformatthetopof therip-rap.Inseveralinstancesexpansionofthesoilbeneaththerip-rap, duetoeithersubsurfacemoistureorsmallamountsofsurfacewaterinfiltra-tion'hascontributedtotheformationofcracksandpartingsintherip-rap. Wheretherip-rapisusedforslopeprotectionbeneathhighwayoverpasses,the topoftherip-raphasbeenobservedtopullseveralinchesawayfromthe supportingconcretefoundationcapforthebridge,thusprovidinganatural conduitforsurfacewatertoinfiltratetheslope. Effortstoremovesurfaceandsubsurfacewaterentrappedbehind relativelyimperviousconcreterip-raphaveincludedtheuseofweepholesand theoccasionaluseofsandblanketdrains.Weepholeshavegenerallybeen placednearthebaseoftherip-rap;however,weepholeshavealsobeenplaced aboveintermediatekeywalls,andnearthetopofsomerip-rappedslopesto draingranularmaterialbackfilledbehindbridgeabutments.Rip-rapped slopefailuresinvolvingasoilfailurehavebeencorrectedinanumberof instanceswithaslidesuppressorwall(foradditionaldetails,seethe discussiononDrilledShafts). Rip-Rapas PreventiveMeasure.Whetherrip-rapincreasesorreduces thestabilityofaslopeisatthepresenttimeuncertain,andprobably, dependingonthecircumstances,rip-rapmayeitherincreaseordecreasethe stability.InSanAntonio(Districti5)therehavebeenanumberofslidesin concreterip-rappedslopesbeneathhighwayoverpassesbothbeforeandafter failuresinadjacentslopeswithnorip-rap,suggestingthattherip-raphad , 37 notimprovedthestabilityoftheseslopes.Althoughtheserip-rappedslopes weregenerallysteeper(2-21/2:1)thantheadjacentslopes(3-31/2:1),the relativelylowerstabilityoftherip-rappedslopesascomparedtotheflatter adjacentslopesmayhavebeenlargelyoffsetorimprovedbytheinclusionof drilledshaftsinthecrestoftheserip-rappedslopes,thedrilledshafts beinginstalledtosupporttheoverpassbridgestructure.Whileitisdifficult toascertainthedegreetowhichthedrilledshaftshaveimprovedthestability oftheslope,thepresenceofconsiderablesub-surfacemoistureintheslide debrissuggeststhathighporewaterpressuresmayhavecontributedmoretothe instabilityoftherip-rappedslopethandidthesteepnessoftheseparticular slopes. WhiletheexperienceinSanAntoniomaysuggestthatrip-rapcontributed toslopefailures,anapparentlycontradictoryexperienceexistsintheurban areaofFortWorthinDistrict12.Nofailuresofconcreterip-rappedslopes havebeenreportedwiththeexceptionofthesingleproblempreviouslydescribed withrespecttocreepmovementintherip-rap.Inmanyinstancesslopeswhich werenotrip-rappedhavefailedwhileadjacentslopes,havingthesameinclin-ation,butwithconcreterip-rapanchoredwitha3-foot-deeptoewall,have experiencednoproblems.Oneapparentreasonfortheeffectivenessofrip-rap inthesecasesistheabsenceofexcessivesubsurfacemoistureandtheprevention ofsurfacewaterinfiltrationbytherip-rap. Onthebasisofthelimitedamountofevidenceavailable,itappears that,wheresurfacerunoffistheprimarysourceofmoisture,concreterip-rap mayreducetheamountofinfiltrationandreduceexpansionandshrinkageinthe soil,providedthatcracksandopenjointsdonotexistintheconcrete.Thus, therip-rapmayaidinmaintainingthestabilityoftheslope.However,if subsurfacemoistureandseepagearepresent,rip-rapmayprovidelittleimprove-menttothestabilityoftheslopeandinsomeinstancesmayevenadversely effectthestability.Consequentlytheeffectivenessofconcreterip-rap probablydependstoalargeextentonthesourcesofmoistureandtheintegrity oftheconcrete.Becauseofthedifficultyinpredictinganddetermining eitheroftheseinadvance,thereliabilityofrip-rapforimprovingandmain-tainingthestabilityofearthslopesappearsuncertainatthepresenttime. .. CHAPTERIV ESTIMATINGSOILSTRENGTHPARAMETERVALUES Thedesignofareliableandeconomicalremedialmeasuredependstoa largeextentontheaccuracywithwhichtheshearstrengthoftheslopecanbe determined.Theshearstrengthmaybeestimatedfromexperience, .measuredor backcalculatedfromtheslopefailure.Whileexperienceisavaluableguide inthedesignofacorrectivemeasureitmaynotbesufficienttoexecutea reliableaswellasaneconomicaldesign.Experiencemaybesupplemented withstrengthdataobtainedfromlaboratoryorin-situsoiltests.The evaluationofthestrengthtestdatawillbeinfluencedby,amongotherfactors, theextenttowhichthesoilsamplesrepresentthein-situconditionsandby theextentthetestproceduresreproducethestressconditionsintheslope. Thecostofovercomingtheuncertaintyinobtainingandsubjectingrepresenta-tivesamplestolaboratoryandin-situtests,whichaccuratelymodelthein-situ soilconditions,wouldrepresentahighpercentageofthetotalcostfora correctivemeasureonatypicalTexasHighwayslope.Furthermore,anextensive samplingandtestingprogrammayrequiremoretimethancanbeallowedto designandinstallacorrectivemeasure. Inmanyinstances,includingthosewherecostortimeprohibits extensivesamplingandtesting,theshearstrengthcanbeestimatedbyperform-ingastabilityanalysisusingtheactualfailuresurfacetodeterminethe strengthparameterswhichwouldberequiredtogiveafactorofsafetyof unity.Thisapproachhasbeenusedtodevelopachartfromwhichanaverage cohesionvalueandangleofinternalfrictioncanbeestimatedfromaslope whichhasfailed. BackCalculationofandThevaluesofcohesion(c)andangleofinternalfriction(), correspondingtoafactorofsafetyofunity,weredeterminedforarangeof slopeconditions.Forconveniencethemostcriticalfailuresurfacewasin allinstancesassumedtobeacirculararcpassingthroughthetoeoftheslope, 38 ,, 39 avalidassumptionformosthomogeneousslopes.Theprocedureusedforthe analysesinthischapterisaprocedureofslicesbasedontheassumptions presentedbySpencer(1967).Thisprocedurehasbeenshowntobeanextremely accurateprocedurebasedonstaticequilibriumandacomputerprogramemploying thismethodwasreadilyavailableforperformingthenecessaryanalysesfor backcalculationofcandvalues(Wright,1969,1971). Indeterminingcandvalues,whichcorrespondtoafactorofsafety ofunity,itisconvenienttousethedimensionlessparameterAC'definedas A=y. H tan cc (4.1) whereHistheslopeheightandYisthetotalunitweightofsoil.Fora givenslopeangle(p)andvalueofACauniquestabilitynumber,Ncf'exists whichdefinesthefactorofsafety(F)intheform (4.2 ) ThevalueofNcfisdependentonlyonthevaluesofACandp regardlessof thevaluesofc,,Y,andR.Similarly,foragivencombinationofAC p values,auniquesetofvaluesforandtheratioyCR canbeshownto and exist forafactorofsafetyofunity. c ThevaluesofandyRcorrespondingtoF=1.0weredeterminedin thefollowingmannerforvaluesofACrangingfromato100andsloperatios (cotangent~ rangingfrom1/1to4/1.First,foreachcombinationofvalues ofACandp,thevalueofNcfwasdetermined.Next,thevalueofyCR was calculatedforafactorofsafetyofunityusingEq.4.2intheform c yH 1.0 (4.3) Finally,thevalueofcorrespondingtotheparticularAvaluebeingused c ~ wascalculatedfromEq.4.1intheform tan =A~cY R Thevaluesof'YcHandtan obtainedcouldbeplottedintermsof"-candp, andifACandp wereknownforanexistingslopewhichhadfailed,sucha plotcouldbeusedtodeterminethecohesionvalueandangleofinternal 40 frictionfortheslopeinquestion.However,Acannotbedeterminedwithout c priorknowledgeoftheshearstrengthitself. Proceeding,itcanbenotedthatinadditiontofindingauniquevalue ofNcf'ageometricallysimilaranduniquefailuresurfaceexistsforagiven setofAandp values.Forcircularfailuresurfacespassingthroughthe c toeoftheslope,itisconvenienttodescribethelocationofthecritical failuresurfaceintermsoftheratiosobtainedbydividingthexandycoor-dinatesofthecenterofthecircle(measuredfromthetoeoftheslope)by dbXcYc theslopeheight.Theresultingdimensionlesscoor~ n t e numers,iHandH' respectively,willthendefinethecenterofauniquecriticalcirclefora particularcombinationofslopeangle(p)andAC.Thevaluesofthedimension-lesscentercoordinatenumbersweredeterminedforvaluesofArangingfrom c ato100andsloperatios(cotangentp)rangingfrom1to4.Thecentersfor thecriticaltoecirclescorrespondingtotheserangesofACvaluesandslope ratiosaretabulatedinAppendixII.Thesedimensionlesscentercoordinate numbersareplottedinFig.4.1.AnegativevalueoftheratioX;inthis figureindicatesthatthecenterofthecircleislocatedinadirectiontoward theslopefromthetoe,whilepositivevaluesindicatethatthecenterofthe criticalcircleliesbeyondthetoeoftheslope. Ifthepositionofthefailuresurfacecanbelocatedforanactual slideandanequivalentcirculararccanbedefinedtoapproximatetheobserved surface,thevalueofACcanbeestimatedfromFig.4.1.Thisprocedurewould requirethatthexandycoordinatesforthecenteroftheestimatedcritical failuresurfacebedividedbytheslopeheighttoobtainthedimensionless XcYc coordinatenumbersiHandiHThevalueofACcouldthenbeobtainedby interpolationoncethecenterpointfortheestimatedcriticalcirclewas locatedonFig.4.1.Itmaybenotedthatinthisproceduretheestimated centercoordinatefromanobservedfailuremaynotlieonthecurvecorres-pondingtotheactualslopeangle.Thisdiscrepancyislikelytooccurdueto theinaccuraciesintheestimatedpositionofacircularfailuresurface,the assumptionofacircularfailuresurfaceandtheassumptionthattheshear strengthcanbecharacterizedintermsofasinglevalueofcand.However, ,. i---==-'---.--------j-----+___----+ 6.0 --t------5.0 4.0 3.0 2.0 -3.0-2.0-1.0 Xc H 0.01.02.0 Fig.4.1.Criticalcentersfortoecircleswith originofcoordinatesattoeofslope. Yc H 41 42 errorsindeterminingthevalueofACforaparticularslopefailureare probablyrelativelyinsignificantinasmuchasthevalueofACisonlyusedto c determinetherelativebalancebetweenthevalueofHand.Regardlessof cy therelativebalancebetweenthevaluesof- andanysetofvaluesback-yH calculatedfromanactualslopefailurewillyieldafactorofsafetyofunity, theobjectiveofestimatingAbeingonlytoobtainasetofvalueswhich c correspondsmorerealisticallywiththeobservedfailuresurface. TheaboveprocedurefordeterminingACwasjudgedtobesomewhat inconvenientbecauseofthedifficultyinvolvedinestimatingandfittinga circularshearsurfacetoanobservedfailure.Forthisreasonasomewhat moreconvenientprocedurewasdeveloped,basedalsoupontheexistenceofa geometricallyuniquecircularfailuresurfaceforagivencombinationofA c valueandslopeinclination.However,ratherthancharacterizethissurface intermsofthexandycoordinatesofitscenter,therelativemaximumdepth ofthecriticalsurfacewasused.Thedepthwasrepresentedastheratio(d/H) obtainedbydividingthedepth(d)ofthecriticalcircle,measurednormalto thefaceoftheslope,bytheslopeheight.Foragivenslopeinclinationand ACvalueauniquevalueofthisdepthratio(d/H)exists.Similarly,ifthe slopeinclinationandthedepthratioforthecriticalcircleareknown,the valueofAcanbefound.Thus,foraparticularslopeanddepthratioit c followsthatauniquesetofvaluesforcandcanbefoundcorrespondingto afactorofsafetyofunity. correspondingtoafactorofsafetyof rangeofsloperatiosandACvalues previouslydescribed.Thesolidlinecurvesinthisfigurewereobtainedby plotting,foreachslopeinclination,thelocusofpointscorrespondingtothe Thevaluesof c and yH tan, unity,areshowninFig.4.2forthe c yHandtancombinationswhichwereobtainedusingdifferentACvalues.For eachslopethe(d/H)ratiosforthecorrespondingcriticalcircleswerealso computedandintermediateplotsofthed/Hratioversustanweredeveloped. Fromtheseplotsthevalueoftan wasdeterminedforselectedvaluesofthe depthratioandthisinformationwasusedindevelopingthelinesofconstant depthratio(brokenlines)shownsuperimposedonFig.4.2. 0.15 c - 0.10 yH 0.05 0.00.10.20.30.40.50.6 Tan'" c Fig.4.2.ValuesofyHandTancorrespondingtoafactorof safetyofunity. -, .. 44 ApplicationofCharts ThechartillustratedinFig.4.2maybeusedtodetermineavalueof thesoilcohesionandangleofinternalfrictionfromaknownslopefailure. Theuseofthechartforthispurposecanbebestillustratedbyconsidering thefollowingexample.Supposethatafailurehasoccurredina2:1,25-foot high-slopeandthattheestimateddepthoftheslide(measurednormaltothe slopeface)isapproximately7feet.Further,letusassumethattheunit weightofsoilisapproximately110poundspercubicfoot.Thedepthratio forthisslopefailurecanbecomputedas d H 7 =- = 25 0.28 ReferringtoFig.4.2andthesolidcurvecorrespondingtoa2:1slopethe followingvaluescanbeobtainedbyinterpolationbetweend/Hratiosof0.25 and0.30: Thus, and c;- 0.015 'y'H tan";;'0.36 c= O. 015X11 0X2 5 41p s f Thecriticalcircularfailuresurfacecorrespondingtothesevaluescanalso beobtainedbyusingFig.4.1.Forthisexample,thevalueofACisdeter-minedfromtheaboveinformationasfollows: A= tan=0.36= 24 c c / 'y'H0 0 15 ReferringtoFig.4.1thedimensionlesscriticalcentercoordinatenumbersfor a2:1slopeandACvalueof24arefoundtobe -. 45 x cH 0.1 H 2.8 Thus,thexandycoordinatesforthecenterofthecriticalcircleare x=0.1X25=2.5ft. y=2.8X25=70ft. Thecenterofthecriticalcircleisthenlocatedatapoint70feetaboveand 2.5feetbeyondthetoeoftheslope. Inmostapplicationsit maybeusefultoplotthecriticalcircleon acrosssectionoftheslopeinordertojudgethereasonablenessofthe assumeddepthofslide(d).Inmanyinstancesseveraldepthsmaybeassumed toestablishthesensitivityoftheback-calculatedshearstrengthparameters tovariationsintheslidedepthoveranestimatedrange.However,itshould benotedthatallsetsofstrengthvaluessoobtainedcorrespondtoafactor ofsafetyofunity,and,thus,significanterrorsarenotnecessarilyintroduced bypoorestimatesinthedepthofslide. Inanumberofinstancestheobservedfailuresurfaceforslopesin Texashasbeenfoundnottopassthroughthetoeoftheslope,butrather throughsomepointlocatedabovethetoe,onthefaceoftheslope.Inmany oftheseinstancesthechartsillustratedinFigs.4.1and4.2mayalsobe used,providedthattheslidescarpintersectsthecrestoftheslopeinthe mannerillustratedinFig.4.3a.Forsuchacasetheslopeheightshouldbe takenasthedistance(H')showninthisfigure.WhenthechartinFig.4.1 isusedtolocatethecriticalcirclethecoordinatesobtainedfromthischart willthencorrespondtomeasureddistancesfromthetoeofthefailuresurface (PointAinFig.4.3)ratherthanthetoeoftheslope.Inthosecaseswhere thefailuresurfacedoesnotintersectthecrestoftheslope,asillustrated inFig.4.3b,thechartspresentedinthischaptermaynotbeentirelyvalid, astheirapplicationtosuchcasesremainstobeestablished. ,. (a) (b) Fig.4.3.(a)Caseforwhichchartsillustratedin Figs.4.1and4.2maybeused. (b)Caseforwhichchartsarenotapplicable. 46 SummaryandConclusions Inthischapterameansforestimatingsoilstrengthparametersby back-calculationfromanobservedfailureispresented.Thisprocedureis probablyinmanyinstancespreferabletoobtainingstrengthvaluesfromcon-ventionallaboratorytestsforseveralreasons: 47 1.Theback-calculatedvaluesprobablyreflecttoamoresignificant degreetheinfluenceofcracks,joiritsandinhomogeneitiesinthe soilprofile. 2.Theerrorsintroducedbyinfluencesofsamplesize,anistropy anddisturbanceinlaboratorysamplesaswellastheerrorsin laboratoryteststhemselvesareinparteliminated. 3.Thecostandtimeinvolvedinanextensivesamplingandtesting programarevirtuallyeliminatedbytheback-calculationprocedure. 4.Errorswhichinevitablyexistinthemethodsemployedforper-formingstabilityanalysesmaybepartiallycompensatedforwhen thesameanalysisproceduresusedforre-designofaslopeare alsoemployedtoback-calculatethestrengthvaluesusedinsub-sequentanalyses. Whiletheconceptofbackcalculatingshearstrengthsfromactual slopefailuresisnotnew,inthepastback-calculationofshearstrengthswas restrictedtodetermininganaverageshearstrengthexpressedaseitheravalue ofcohesionwithassumedequaltozerooranangleofinternalfrictionwith cassumedequaltozero(Harty,1953;Gould,1960;Skempton,1964;Hutchinson, 1969).Inaddition,plotsofcversussimilartoFig.4.2havebeenutilized toprovideaconvenientmeanstodetermineifshearstrengthparametersevalu-atedfromlaboratorytests,performedonsoilsamplesfromanactualslope failure,correspondtoafactorofsafetyofunity(CrawfordandEden,1967; Petersonetal.,1960;Singh,1970).However,ininstanceswhereplotsofc versushavebeenusedinthepast,theknowledgeoftheactualfailuresur-facehasreceivedonlylimitedconsideration.Theprocedurepresentedinthis chapterprovidesameanstoevaluatefromanactualfailureabalancedcombina-tionofcandcorrespondingtoafactorofsafetyofunity. Thevaluesofthestrengthparameters(cand)whicharedetermined usingthechartillustratedinFig.4.2,arebasedontotalstressesandno .r . . 48 directconsiderationofporewaterpressureshasbeenincluded.However,the strengthvaluessodeterminedshouldindirectlyreflectinparttheinfluence ofporewaterpressuresonthesoilstrength.Whileaprocedureforback calculatingeffectivestressshearstrengthparameterscouldalsobedeveloped, theprocedurewouldbeconsiderablymorecomplexandwouldrequireadditional parametersforcharacterizationofthegroundwaterconditions.Thedevelopment ofsuchaprocedureisbeyondthescopeofthisreportandwasnotconsidered inthepresentstudy. CHAPTERV EARTHPRESSUREFORCESFORRESTRAINTSTRUCTURE ThecorrectionofanumberofslopefailuresinTexashasbeen accomplishedbytheuseofconcreteretainingwallslocatedentirelywithin theoriginalslopegrade,asillustratedinFig.5.1.Thesestructural restraintsystems,locallyreferredtoasslidesuppressorwalls,aregenerally foundedondrilledshaftslocatedapproximatelyone-thirdthedistanceup theslopewiththewallbeingplacedsoastointersecttheknownorestimated failuresurface.Oncetheslidesuppressorwallhasbeenplaced,adrainage trenchisexcavatedimmediatelybehindthewallandbackfilledwithfilter material.Theslopeisthenregradedtoitsoriginalgrade. Theearthpressureforce(E)forwhichtheslidesuppressorwallis designediscommonlydeterminedbyusingeitheranequivalent-fluidprocedure oratrial-wedgeprocedure.Intheequivalentfluidprocedure,whichhasbeen usedbytheTexasHighwayDepartment,thebackfillisassumedtoactasa fluid.Inthisprocedurethereisno thattheshearstrengthof thesoilbeknownotherthanasapossibleguideintheselectionofanequi-valentfluiddensity.However,thereisnorationalproceduretodetermine thefluiddensity,andthus,thevalueselectedforagivensituationisbased onexperienceandhandbookrecommendations.Thefluiddensityselectedin thismannermayleadtoeitheroverlyconservativeorunconservativeresults. Thetrial-wedgeprocedure,aforceequilibriummethodbasedonplane failuresurfaces,isamorerationalapproachforestimatingthemaximum earthpressure(Bowles,1968).However,unliketheequivalent-fluidprocedure, thesoilstrengthparametersmustbedeterminedeitherbyperforminglaboratory testsorbybackcalculatingthesoilstrengthsfromtheslopefailure.When thepositionofthefailuresurfacecanbelocatedforanactualslideandan equivalentcirculararccanbedefinedtoapproximatetheobservedsurface,the totalstressstrengthparameters(cand)maybeestimatedbytheprocedure outlinedinChapterIII.Theequivalentcirculararccanbecharacterizedby thedepthratio(d/H),obtainedbydividingthedepth(d)ofthecirculararc, 49 ~ L ~DrilledShaft--SplitShaftSection UsedAdjacenttoWall Fig.5.1.Slopestabilizedwith