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.. , 1. Report No. 2 Government Accession No. CFHR 3-8-71-161-1 4. Title and Subtitle itA Survey of Earth Slope Failures and Remedial Measures in Texas" 7. Author l s) Timothy G. Abrams and Stephen G. Wright 9. Performing Orgonizotion Name and Address Center for Highway Research The University of Texas at Austin Austin, Texas 78712 _ ...... 12. Sponsoring Agency Name and Addres. Texas Highway Department 11th and Brazos Austin, Texas 78701 15. Supplementory Notes Research performed in Highway Administration Research Study Title: 16. Ab.tract cooperation with Department of ts tabi lity of Earth S lopes It TECHNICAL REPORT STANDARD TITLE PAGE 3. Recipient's Cotolog No. 5. Report Dote December 1972 16. Performing Organization Code S. Performing Organi zation Report No. Research Report 161-1 10. Work Unit No. 11. Contract or Grant No. Research Study 3-8-71-161 13. Type 01 Report and Period Covered Interim Sept. 1970 - June 1972 14. Sponsoring Agency Code Transportation, Federal The results of a survey undertaken to identify regions in Texas where there has been a high incident rate of slope failure and to identify some of the factors responsible for these slides are presented. In conjunction with this study a review of present slope design procedures and the remedial measures employed by the Texas Highway Department for repair and maintenance of earth slopes is contained in this report. The major slope failures of significance were found to be associated with primarily excavated (cut) slopes in stiff-fissured clays and clay shales, although failures in embankment (fill) slopes constructed of high plasticity clays were also encountered in several areas. A number of remedial measures have been employed for repair of these failures, including regrading, stabilization with lime and cement additives, and various forms of restraint or buttressing structures. While exces- sive amounts of ground and surface water were usually present at most sites where slope failures have occurred, drainage of water has not been extenSively employed as a remedial or preventative measure. 17. Key Word. 18. Di,tribution Statement Soil mechanics, slope stability 19. Security Clouif. (of thi' teport) 20. Security Ciani!. (of thi s pagel 21. No. of Pages 22. Price Unclassified Unclassified 109 Form DOT F 1700.7 (S-69)

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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


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