E&EG Journal Volume XXX, Number 1-2 full

Environmental& EngineeringGeoscience

Volume30,Number1-2,May2024

TableofContents

1DocumentingtheCollisionofaLandslideinPermafrostwithaHighwayEmbankment

MargaretM.Darrow,RonaldP.Daanen,andMeaghanC.Gould

19NeedforaMulti-SensorMonitoringApproachforEmbankmentFailures:LessonsLearnedfromthe EdenvilleDamFailure

JobinThomas,Ryanklida,ThomasOommen,SajinkumarK.S.,BinZhang,andXieHu

31ElectricalResistivityImagingofRecentLandslidesinColoradoSprings,Colorado,USA MoamenAlmaz,KevinMickus,MohamedH.Aly,andGregoryDumond

45PossibleRoleofInternalErosionintheDevelopmentofGroundFissuresaroundLakeZiway,Ethiopia YonathanAdmassuandTrufatGugsa

59TheSeptember18,2018,DebrisSlideinWarrensville,NC:ALandslideResponseCaseStudy

RichardM.Wooten,BartL.Cattanach,G.NicholasBozdog,andSierraJ.Isard

77EvaluationofPlant-BasedNaturalExtractsasCoagulantsforSurfaceWaterTreatment ZanibFida,MuhammadAshrafTanoli,QaisarMahmood,MuhammadSohailAlamgir,andDilawarSajjad

TechnicalNote

91AProposedProcess-Landform-Material(PLM)SystemforEngineeringGeologyFieldMapping PaulM.SantiandJeffreyR.Keaton

OpenAccessArticle

DocumentingtheCollisionofaLandslidein PermafrostwithaHighwayEmbankment

MARGARETM.DARROW*

UniversityofAlaskaFairbanks,P.O.Box755900,Fairbanks,AK99775-5900

RONALDP.DAANEN

AlaskaDepartmentofNaturalResources,DivisionofGeologicalandGeophysical Surveys,3354CollegeRoad,Fairbanks,AK99709

MEAGHANC.GOULD

U.S.ArmyCorpsofEngineers,AlaskaDistrict,22043rdStreet, JBER,AK99506-1898

KeyTerms: Landslide,FrozenDebrisLobe,Shear Strength,Infrastructure,BrooksRange,Alaska

ABSTRACT

Frozendebrislobes(FDLs)areslow-movinglandslidesinpermafrost.FDL-A,thelargestmonitored FDLintheBrooksRangeofAlaska,hassteadily progresseddownslopetowardtheDaltonHighway, whichistheonlyroadtotheoilandgas fieldsofthe NorthSlope.Toavoidthisencroachinglandslide,the DaltonHighwaywasrealignedfartherdownslopein 2018.Theabandonedportionofthehighwaywasleft inplace,providingauniqueopportunityforafullscale fieldexperimenttomonitortheimpactofa landslideonanengineeredstructure.In2020,weconductedasubsurfaceinvestigation,drillingandsamplingthesubsurfacesoilsandinstallinggeotechnical instrumentationwithintheabandonedhighway embankment.Here,wepresenttheintegrationof multipledatasetstoprovideadetaileddescriptionof thelandslide–embankmentcollision.FDL-Aisshearingwithinweatheredbedrock »7.6mbelowthe embankmentsurface.Ithorizontallydisplacedaportionoftheembankment »0.3masofNovember1, 2023.Usingtheinfiniteslopeapproachandstrength propertiesdeterminedfromlaboratorytesting,we estimatethatFDL-Aisimpactingthehighway embankmentandunderlyingsoilswithatleast77.8 kN/mwidthshearforce.Asthisforceoccursapproximately4mbelowthebottomoftheembankment,we postulatethatthepresenceoftheabandonedDalton

*Correspondingauthoremail: mmdarrow@alaska.edu

HighwayembankmentdoeslittletostopFDL-A

downslopeprogression.

DEDICATION

Wededicatethispapertooneofourco-authors our long-timecolleagueinFDLresearchanddearfriend Dr.Ronald(“Ronnie”)Daanen,whodiedinahelicopter crashwhileconducting fieldworkontheNorthSlopeof AlaskaonJuly20,2023.

INTRODUCTION

Landslidesaredestructiveevents,causinganestimated$3.5billionworthofdamageannuallyinthe UnitedStates(USGS,2017).Linearinfrastructureis especiallysusceptibletodamagebecause,byitsnature, itmustsometimestransversedifficultandlandslide-susceptibleterrain.Alaska’sDaltonHighway, “oneofthe northernmostroadsintheworld” (ADOT&PF,n.d.),is anexcellentexampleoflinearinfrastructurecrossing difficultterrain.TheDaltonHighwayis666km(414 mi)long,stretchingfromLivengoodinInteriorAlaska toDeadhorseontheArcticCoastalPlain(ADOT&PF, n.d.;Figure1a).Originallybuiltinthe1970stosupport theconstructionoftheTransAlaskaPipelineSystem, currentlytheDaltonHighwayfacilitatesthetransportationofgoodsandservicesfromcitieslikeAnchorage andFairbankstotheoilandgas fieldsoftheNorth Slope.UnderlainbycontinuousanddiscontinuouspermafrostandcrossingtheBrooksRangeinnorthern Alaska,theDaltonHighwayhasexperienceditsshareof geohazards,includingthawsettlement,avalanches,slush flows,andlandslides.Inthispaper,wefocusonaspecificgeohazardimpactingtheDaltonHighwayinthe

Figure1.Locationoftheresearcharea.(a)PortionofAlaskawithselectedcommunities,majorroads(inorange),theDaltonHighway(inpurple),and thelocationoftheprojectarea(redrectangle).(b)AreaofinterestintheBrooksRangewithninemonitoredFDLsandthelocationofinfrastructure. (c) FDL-A’sandsurfacemeasurementpinlocationsin2020.(d)ToeofFDL-Awithlocationsofrelevantinstrumentation;A-A 0 isthecross-sectionpresentedinFigure9,andblueandredtransparentconesindicateviewdirectionsofnorthandsouthcameras,respectively.TAPS ¼ TransAlaskaPipeline System.BasemapdataarefromADOT&PF(2020),AGC(2020),GINA(2001),and2020lidar(unpublished)andcourtesyofAPSC(unpublished).

south-centralBrooksRange,namely,frozendebrislobes (FDLs;Daanenetal.,2012;Darrowetal.,2016).

FDLsareslow-moving(CrudenandVarnes,1996) landslidesinpermafrost,typicallycomposedofsilty sandwithgravelbasedontheUnifiedSoilClassification System(USCS)followingASTMD2487-17e1(ASTM, 2017a).Inadditiontothesoilcomponent,FDLscontain somequartz-richcobblesandbouldersthatlikelyare

remnantsofresistantveinsintheheavilyfracturedmetasedimentaryrocksthatformthecatchmentsfromwhich thesefeaturesoriginate(Darrowetal.,2016).Although thesoilmatrixisice-poor,FDLscontainareasofmassiveinfiltrationice,whichformsincracksthatareopen atthesurfaceduetodownslopemovementthroughout theyear(Darrowetal.,2016).FDLsalsocontain organicmaterialandwoodydebris.BecauseFDLs

predominantlyconsistofsoil,maturespruceforests growontheirsurfaces.AsFDLsmovedownslope,their movementknocksoverthesetreesandthoseontheslope infrontofthem,andtheyentrainthatwoodydebris,as wellastheoriginalorganicmatintotheirmovingmass.

OurFDLresearchbeganin2008withtheinitialinvestigationoffourFDLs(Daanenetal.,2012).Wenowregularlymonitornineoftheselandslides(Figure1b).Our previousinvestigationsincludedmappingandtestingof rocksinthesurroundingcatchments,analysisofhistoricand currentmovementrates,changedetectionanalysisusing lidarandinterferometricsyntheticapertureradar,andthermalmodeling(Darrowetal.,2016,2017,2019;Gyswyt etal.,2017;Byrdetal.,2019;andGongetal.,2019).

ThelargestandclosestFDLtotheDaltonHighwayis termedFDL-A(Figure1c).OurinitialstudiesofFDL-A includedsubsurfaceinvestigationsandanalysisofgeotechnicalinstrumentationdata(Darrowetal.,2013; Simpsonetal.,2016).WeinformedtheAlaskaDepartmentofTransportationandPublicFacilities(ADOT&PF) aboutFDL-A’shazardousincreasingrateofmovement, whichpromptedADOT&PFtorealigntheDaltonHighway,movingit »125mdownslopein2018.Aportionof theabandonedhighwayalignmentwasleftinplaceinfront ofFDL-A,providingauniqueopportunitytoobservethe impactofalandslidewithacompactedembankment.In 2020,we alongwithpersonnelfromtheADOT&PF NorthernRegionMaterialSection(NRMS) conducteda subsurfaceinvestigation,drillingandsamplingthesubsurfacesoilsandinstallinggeotechnicalinstrumentationwithin theabandonedpartofthehighwayembankment.The objectivesofthispaperareto1)summarizeresultsofthis full-scale fieldexperimenttomonitortheimpactofalandslidewithaninstrumentedembankment,2)evaluatethe movementmechanismsatFDL-A’stoe,and3)estimatethe forceFDL-Aisimpartingtotheembankmentandsubsurfacesoils.Weintegratedatafromabove-groundandsubsurfaceinstrumentation(includinglocalclimate,ground temperatures,groundwaterpressure,andslopemovement), laboratorytesting,lidar,andrepeatphotographytobuilda conceptualmodeloftheinteractionbetweenFDL-Aand theabandonedDaltonHighwayembankment.Thispaper includesdatacollectedfromaprevious2018subsurface investigationandfromthe2020subsurfaceinvestigation untilNovember1,2023.

RESEARCHMETHODS FieldInvestigations

FDLSurfaceMovementMeasurements

In2012,weinstalledanarrayofsurfacemarkerpins onFDL-A(Figure1c),whosechangingpositionswe havedocumentedsincethattimeusingareal-time

kinematicglobalpositioningsystem(RTK-GPS).For thisproject,wecontinuedthosemeasurements,usinga LeicaVivasystem(includingGS14receiversanda CS15controller)astheRTK-GPSdevice.Withpostprocessing,thissystemisaccuratetowithin5cmin bothhorizontalandverticaldirections.

2018SubsurfaceInvestigation

OurjointUniversityofAlaskaFairbanks(UAF)–AlaskaDivisionofGeologicalandGeophysicalSurveys (DGGS)researchteamcompletedtwoboreholesin2018, eachto3.05m(10ft),usingahandheldpowerheadand augerassembly(Figures1dand2a).Thepurposeofthis investigationwastomeasurechangesinthesubsurface temperatureandporewaterpressureasFDL-A approachedandcoveredtheinstrumentlocations.Thus, welocatedeachboringclosetoboththetoeofFDL-A andastreamdrainingfromit.Intoeachborehole,we installedatemperaturesensorcableandavibratingwire piezometer(VWP) whichwepreviouslytestedandcalibratedinthelaboratory andbackfilledeachboringwith cement–bentonitegrout.Finally,wehand-duga »15-cmdeeptrenchintowhichweburiedtheinstrumentation cables,routingthemtoanautomateddataacquisitionsystem(ADAS).TheADASconsistedofadatalogger,multiplexer,solarregulator,solarpanel,enclosure,battery bankwithenclosure,andtemperaturesensorstomeasure airandsurfacetemperaturesattheADASlocation.

2020SubsurfaceInvestigation

OurUAF-DGGSresearchteam,alongwithpersonnel fromADOT&PFNRMS,performedthesubsurface investigationandgeomechanicalequipmentinstallation intheabandonedsectionoftheDaltonHighwayinmidAugust2020.Priortogoingtothe field,wecalibrated thetemperaturesensorcablesandVWPsandtestedthe componentswiredintotheADAS.Onceinthe field, usingasmalltrackexcavator,ADOT&PFNRMSpersonneldugashallowtrench(»0.4mdeep)intotheembankmentsurfaceforburialofinstrumentationcabling.All boreholeswereadvancedusingatruck-mountedCME55 drillrig,whichwascenteredoverthetrench(Figure2b). Wecompletedfour12.19-m-deepboreholes(40ftdeepas measuredfromtheembankmentsurface)usinga16.5-cm (6.5in.)outsidediameter(OD)hollow-stemauger,which allowedforsplit-spoonsamplingduringdrillingandserved asacasingtokeepeachboreholeopenduringinstrument installation(locationsareprovidedinFigure1d).Wecollectedsamplesusinga1.51-kN(340lb)hammertopound a7.6-cm(3in.)ODsplit-spoonsamplerat0.76-m(2.5ft) intervals,providingnear-continuousvisualizationofthe subsurfacematerials.Wecollectedrepresentativesamples

Figure2.Imagesfromthe2018and2020drillingprograms.(a)Drillingaboreholewiththehandheldpowerheadandaugerassembly.(b)Drill truckcenteredonthetrenchtodrillTH20-03.(c)Lookingdownatthe2-and5-MPapressureplatesandtiltmeterembeddedintotheuphillside slopeoftheembankmentatTH20-02.(d)AllADASatthesouthernendoftheprojectlocationin2022.(e)Collectinglidardatawiththebackpackcollectionconfiguration.

fromeachborehole,whichweresealedandtransportedto theFrozenSoilTesting(FROST)LaboratoryatUAF.In eachboring,weinstalledonemicro-electro-mechanical system–basedin-placeinclinometer(M-IPI),atemperaturesensorcable,andaVWPandthenbackfilledwith cement–bentonitegrout.Allcablingwasroutedtoan ADASthroughthetrenchandburied.Simultaneous withthedrilling,theUAF-DGGSresearchteamlaidout tiltmetersandpressureplatesforinstallationontheeast oruphillsideoftheembankment.Foreachborehole location,a2-MPapressureplateandtiltmeter(both affixedtoasinglesteelplate)wereinstalledvertically intoaslotdugintotheedgeoftheembankment.The exceptionwasatTH20-02,whereanadditional5-MPa pressureplateonitsownsteelplatewasinstalledimmediatelyadjacenttothe2-MPapressureplateandtilt meter(Figure2c).Itwasduringthiseffortthatwediscoveredthatthetiltmetercableswereprovidedtothe specifiedlengthinfeetinsteadofmetersandthuswere

tooshorttobeinstalledwheredesired.Toaccommodate theshortcables,wemovedtheADASassociatedwith the2018subsurfaceinvestigationintothemiddleofthe embankmentandduganadditionaltrenchtoroutethe remaining2018instrumentationcables,plusthetilt meterandpressureplatecabling,tothiscentralADAS. Later,insummer2022,weexposedtheburiedcables, extendedthemthroughsplicing,routedtheextended cablestothesouth,andmovedthisADASoutofthe wayofFDL-A’simminentimpactwiththeembankment (Figure2d).

In2020,weinstalledtwoADASsatthesouthendof theembankmentsection,oneforthetime-lapsecamera systemandonetoserveasthemainADASforthe majorityoftheinstrumentation.Thetime-lapsecamera system(NupointSystemsInc.)wassetuptotakeone photographperdayofthetoeofFDL-A,whichwas transmittedtoUAFviaasatelliteuplink.Wemounted twoadditionalstand-alone fieldcameras(Reconyx)to

Table1. Summaryofgeomechanicalinstrumentationinstalledin2018and2020.

YearInstalledInstrumentManufacturerSpecifications

2018VWPGeokon4500,350-kPapressurerange;installedat3mbgs

TemperaturesensorstringGeokonAccuracy 6 0.2°C;thermistorsspacedat0.25,0.5,1,2,and3mbgs 2020VWPGeokon4500B(slimmodel),700-kPapressurerange

Temperaturesensorstring re-usedforthisinstallationandfailedtoworkproperly;seetextforexplanation M-IPIMeasurandSAAV,10-mlongwith200.5-m-longsegments

PressurecellGeokon4815(forgranularsoil);4at2MPa,1at5MPa;armoredcableuntilcenterline, transitioningtoregularcable

TiltmeterGeokonNEMA4enclosure;biaxialmodel;armoredcableuntilcenterline,transitioning toregularcable

acquireimagesofthelandslidecollisionfromdifferent angles.ThemainADASconsistedofadatalogger,solar regulator,solarpanel,batterybank,andthenecessary instrumentationtoserveasaweatherstationfortheprojectarea,includingmeasurementsofairtemperature, precipitation,andsnowdepth.ADAScomponentswere mountedinsideanenclosureordirectlyontoarugged tripodthatwasanchoredintotheembankmentsurface. Table1isasummaryofthegeomechanicalinstrumentationinstalledduringbothdrillingprograms.

LaboratoryTesting

WetransportedallsamplestotheUAFFROSTLaboratoryforstandardtests,includingmoisturecontent(ASTM D2216-19,ASTM,2019),Atterberglimits(ASTMD431817e1,ASTM,2017b),unitweight(ASTMD7263-21, ASTM,2021a),andparticle-sizedistributionconsistingof hydrometertests(ASTMD7928-21,ASTM,2021b),specificgravitytests(ASTMD854-14,ASTM,2014),and sieveanalyses(ASTMD6913M-17,ASTM,2017d).We classifiedsamplesusingtheUSCS(ASTMD2487-17e1, ASTM,2017a).Wewereabletocollectthreesplit-spoon samplesofthebedrockmaterialinTH20-04.Thehigh moisturecontentandweatheredconditionofthebedrock samplesallowedustopreparetestspecimensfordirect sheartests(ASTMD6528-17,ASTM,2017c).

LidarDataAcquisitionandChangeDetectionAnalysis

WeconductedlidarscanningoftheFDL-Atoeareain May2021andJune2022,usingaRiegleVUX-LRintegratedintoafullydynamiclidarsystembyPhoenix LiDAR.Weusedabackpacktotransportthelidarsystem, whichallowedhigh-resolutionscanningofthesmalltoe area(Figure2e).ThesystemissetupwithRTK-GPSfor accuratelocationinformationat10pointspersecond. BetweenRTK-GPSreadings,thesystemreliesona NorthropGrummaninertialmomentumunitatarateof 100timespersecond.Thecombinationofthesetwo devicesmakesitpossibletoknowthelocationandorientationofthedevicewithanaccuracyofabout1cm.We

produceddigitalelevationmodels(DEMs)ataresolution of10cmfromeachlidarscan.Weusedthesedatasets,as wellasapreviouslycollected2020dataset,toconduct thechangedetectionanalysis.WealsousedtheDEMsto producelongitudinalprofilesacrosstheDaltonHighway embankmentandupthetoeofFDL-A,whichwereused toproducescaledconceptualmodelsofmovement.

RESULTS

LocalClimate

Figure3acontainsairtemperaturedatafromthetoe ofFDL-AandfromColdfoot,Alaska(approximately 70.8kmsouthoftheresearchsite;Figure1a).Forthe4 years(2019–2022)ofcompletetemperaturedatafrom theFDL-Aresearchsite,themeanannualairtemperature(MAAT)was 5.0°C,whereasfortheColdfoot location,theMAATwas 4.0°C.ThewarmerColdfoot airtemperaturesalsoareapparentintheairthawing index(ATI)andairfreezingindex(AFI)summarizedin Table2.Despitetheslightdifferenceinairtemperature betweenlocations,weusetheColdfootdataasaproxy forFDL-A,aswedonothaveacompleterecordofprecipitationorsnowdepthfromtheresearchsitedueto intermittentequipmentmalfunctions.Figure3bandTable 3containsnowdepthandprecipitationdataforColdfoot bywateryear(awateryearbeginsonOctober1andends onSeptember30;USGS,2016).Thewinterof2020/21 (orwateryear2021)isthemostnotableduringthisperiod ofobservation,asitexperiencednearlyhalfthetypical snowfall,whichalsostartedaccumulatingaboutamonth laterthannormal(seeboldvaluesinTable3).Thislack ofsnowfallismanifestedinotherdataattheFDL-A researchsite,whicharediscussedlater.

HistoryofFDL-A’sMovementRate

Figure4isasummaryofFDL-A’srateofmotion, includingratesobtainedfromanalysisofhistoricimagery andrecentRTK-GPSmeasurements(refertoDarrow

Figure3.Climateandwaterpressuredata.(a)AirtemperaturesfromColdfoot,Alaska(snowtelemetry[SNOTEL]site958;USDA,n.d.)andthetoeof FDL-A.(b)CumulativeprecipitationandsnowdepthforColdfoot.SelectedVWP(c)waterpressureand(d)temperaturedatafromthetoeofFDL-A.

etal.,2016,formoreinformationonthehistoricimagery analysis).Alineartrendisthebest fitforthehistoricrate ofmotion,withan R2 of0.95(blueseries,Figure4). WhentherecentRTK-GPSratesareadded,thebest fitis nolongerlinear(orangeserieswith R2 of0.62)but becomesexponential(redserieswith R2 of0.82).The RTK-GPSmeasurementsdemonstrateapeakinFDL-A’s movementratein2020,followedby3yearsoflower annualmovementrates.Weattributethisslowingtothe below-averagesnowfallduringthewinterof2020/21. Thelackofinsulationprovidedbythesnowhadtheeffect ofcoolingthesubsurface,whichincreasedtheshear

strengthofthefrozensoilandslowedthedownslope movementinsubsequentyears.Weobservedasimilar below-averagesnowfallduringthewinterof2012/13, whichresultedincoldergroundtemperatureswithinthe lobeandlowerstrainrates(Darrowetal.,2017).

EmbankmentandSubsurfaceAnalysis

SubsurfaceSoilsandLaboratoryTestingResults

Figure5containsschematicsofthefourboreholes, includingasummaryofthelaboratorytestingresults.

Table2. SummaryofATIandAFIforthetoeofFDL-Aandfor Coldfoot,Alaska(SNOTEL958;USDA,n.d.).

Calendar Year FDL-AColdfoot

Wheredrilled,theembankmentrangedfrom3.05to 3.66m(10–12ft)thick,consistingofmoist,brown,nonplasticsiltygravelwithsandtosiltysandwithgravel. Thefoundationsoilstypicallyconsistedofbrown,nonplasticslightlyorganicsiltysandwithgraveltosilty gravelwithsand.Thesesoilsweremoisttowetwhere unfrozen.Weinterceptedgray,whitemicaschistbedrock inallboreholes,varyingbetween4.27and9.75m(14.0 and32.0ft)belowtheembankmentsurface.Thehighly weatheredbedrockwasmoisttowetwhereunfrozen.We interceptedthepermafrosttable6.04–7.62m(19.8–25.0 ft)belowthegroundsurface(bgs)inTH20-01through TH20-03;TH20-04wasunfrozen.Weattributethisdepth topermafrosttothelackofwintermaintenanceandthe insulatingeffectsofsnowaccumulationontheabandoned embankment’ssurface.Awatertablewasperchedonthe permafrosttableinTH20-01andTH20-02.WealsointerceptedawatertableinTH20-04.

Weconducteddirectsheartestsontwosetsofthree unfrozensamplesoftheweatheredwhitemicaschist collected4.9–5.2mbgsinTH20-04(samplelocations notshownonFigure5).Thesesampleswerecollected fromtheunfrozenlayerabovethepermafrosttableand testedatabove-freezingtemperatures.Althoughthesampleswerenotorientedinacardinaldirectionwhentaken inthe field,thefoliationwithintheschistwas

Table3. SummaryofprecipitationdataforColdfoot,Alaska (SNOTEL958;USDA,n.d.).

Water Year Total Precipitation (cm)

Average Snowfall Depth (cm) Peak Snowfall Depth (cm) Dateof First Snow Dateof Last Snow 201846.538.2111.810/5/20175/19/2018 201957.236.9149.910/14/20185/22/2019 202053.636.9114.310/5/20195/15/2020 202156.9 20.181.310/25/2020 5/11/2021 202247.833.6104.19/20/20215/13/2022 202365.527.188.910/5/20225/16/2023

Dataaresummarizedbywateryear.Themostnotablewinterduring thisperiodofobservationisinbold.

approximatelyperpendiculartothelongaxisofthesample,orapproximatelyparallelwiththegroundsurface. Thefollowingisasummaryoftheaverageresults obtainedfromthetests:dryunitweightof16.76kN/m3; gravimetricmoisturecontentof6.4percent,frictionangle of30.8degrees,andcohesionof8.79kPa.

GroundTemperature

Duetohardwareconflictsandprogrammingissues, wewereunabletoobtainlong-termdatafromthetemperaturecablesinstalledintheborings,despitemultiple attemptstotroubleshootthesystemwiththeaidofthe vendor.Becauseofinterferencewithrecordingotherdata fromthesite,wedisconnectedthetemperaturecablesin 2022.Fortunately,theM-IPIscontaintemperaturesensors ineachoftheirinstrumentedsegments.Althoughuncalibrated,thesesensorscanserveasabackupfortemperaturemeasurements(DarrowandJensen,2012).Asan example,Figure6containstemperatureprofilesfor TH20-01formonthsforwhichwehavecompletedata. At1mbgsinTH20-01,theembankmenttemperature rangedbetween 5°Cand15°C.Thedepthofzero annualamplitude(ZAA)wasat6.6mbgsin2020and 2021,deepeningto7.1mbgsin2022(theinterpretation ofZAAdepthislimitedtothespacingofthesensors withintheM-IPI).ThedeepestthawatTH20-01occurred inSeptemberorOctober,dependingontheyear.

PorewaterPressure

Figure3ccontainstheporewaterpressuremeasurementsfromboth2018VWPs(eachat3.0mbgs)and fromthesouthernmostVWP(TH20-04at8.0mbgs)as

Figure4.IntegrationofhistoricandrecentmovementratesofFDL-A.

Figure5.Schematicsofthe2020boreholes,presentedfromsouthtonorthtomatchtheorientationofcross-sectionA-A 0 (Figures1dand9). Depthsareinfeettomatchhowtheywerelogged(metersareprovidedontherightforreference).

examplesofthe2020installations.TheVWPinstrumentsalsocontainthermistorbeads,allowingtemperaturemeasurementsattheinstallationdepth(Figure3d). Typicaldataindicatedtwopeaksinwaterpressure,the firstoccurringwiththeendofsnowmeltinmid-tolate Mayandthesecondoccurringwiththeincreaseinlate summerprecipitationinlateJuly.TheTH18-01installationfailedonFebruary24,2020,afteritwasoverrunby thetoeofFDL-A.Followingthebelow-normalsnowfall duringthewinterof2020/21,theVWPinTH18-02 frozeinearlyApril2021,causingaspikeandsubsequentdropinwaterpressure.FDL-AcoveredtheTH1802locationinJuly2021,afterwhichtheVWPthawed; however,asthefreezingmayhavedamagedthediaphragmwithintheVWP,thedatamaynolongerbe accurate.TheotherVWPsdidnotexperiencefreezing. Peaktemperaturesoccurredinearlytomid-September at3.0mbgsinTH18-01andinlateSeptembertolate Octoberat8.0mbgsinTH20-04.

PressurePlateandTiltMeterMeasurements

Figure7containsgraphsofthetemperaturesrecorded bythetiltmetersandpressureplates(Figure7a)andthe pressureplate(Figure7b)andtiltmeter(Figure7c) readings.Asallofthesedeviceswereburiedwithin0.6 m(2ft)ofthesurface,themeasuredtemperaturesdemonstratedseasonal fluctuations.Thetemperaturesfrom pressureplate1(PP1)andtiltmeter1(Tilt1;both installednearTH20-01)wereabout2.4°Cwarmerduringthewinterof2022/23andabout4.2°Ccoolerin thesummerof2023thantheotherreadings.Weattribute thistothetoeofFDL-Acoveringthispartofthe embankmentinthelatesummerof2022,whichwe observedinthe fieldinJuly2022.

Thepressureplatesarecomposedofstainlesssteel plateswithde-airedhydraulicoilasaworking fluid

(Geokon,2020).Thepressurereadingsarecorrectedfor temperature;however,weattachedthepressureplatesto steelplatestomakethemmorerobustagainsttheimpact ofFDL-A.Thesteelplatesmaybeaddingadditional temperaturesensitivitytothepressureplatereadings (Figure7b),asallreadingsincreasedwithsub-freezing temperaturesandthendecreasedastheembankment thawedinthespring.Thewinterincreaseinpressurealso maybeattributedtopressureexertedbytheembankment soilsastheyunderwenttheseasonalfreeze–thawcycle. ThePP1readingsincreasedmorethantheothersduring thewintersof2021/22and2022/23,andalthoughthe readingsalsodroppedwiththespringthaw,theycontinuedtoincreasethroughthesummer.AlthoughthePP4 readingsalsoincreasedduringwinterof2022/23,we knowfromvisualobservationsthatFDL-AisnotimpactingtheTH20-04location.Oneofthetiltsensorsatthe TH20-01location(Tilt1A;Figure7c)alsostartedto deviatefromtheotherreadingsonAugust2,2022.These changesaremostlikelyduetotheimpactofFDL-A.Furtherobservationsduringthelandslide–embankmentcollisionarerequiredforafullexplanationofthepressure plateandtiltmeterdata.

M-IPIMeasurements

Figure8acontainsplotsoftheM-IPImeasurements forthefour2020boreholes(locationsprovidedinFigure 1d)intheA0direction(i.e.,thedirectionofgreatest movement downslope).Wedetectedmovementin TH20-01,TH20-02,andTH20-03immediatelyuponthe firstreadingonSeptember6,2020,whilethetoeof FDL-Awasstillmorethan10maway.Figure8bisa graphoftherateofhorizontaldisplacementforselected depthsalongeachofthefourM-IPIs.Inthisanalysis, forTH20-01throughTH20-03,weselectedthesensorat thedepthexperiencingthemostchange;forTH20-04,

Figure6.TemperatureprofilesfromTH20-01.

Figure7.Pressureplateandtiltmetermeasurementsovertheprojectmeasurementperiod.(a)Temperaturesfromallpressureplateandtiltmeter devices.(b)Pressureplatereadings.(c)Tiltmeterreadings.

wechoseamediandepthwithintheembankment. Throughtheperiodofobservation,therateofdisplacementinTH20-04hasbeensteady,atanaveragerateof 0.013mm/d.Aftertheirinstallation,thenorthernthree M-IPIsdemonstratedsimilarratesofdisplacement,peaking atanaveragerateof0.223mm/d.Unfortunately,thereare missingdatafromthewintersof2020/21and2021/22due tohardwareconflictsandprogrammingissues;however, theperiodsofexistingdatasuggestthatTH20-01startedto divergewithagreaterhorizontaldisplacementrateinJune 2021andhascontinuedthistrendthroughouttheobservationperiod.ThenorthernthreeM-IPIsalldemonstrated peaksinmovementratesometimebetweenmid-October andmid-Novemberandtheslowestmovementratesin mid-MaytoearlyJune.Thesedataareingoodagreement withpreviousanalysisofstrainratedatafromaM-IPI installedwithinFDL-Ain2012,fromwhichapredictive formulaindicatedamaximumvelocityinthefallanda minimumvelocityinearlyspring(Darrowetal.,2017).

Figure9isschematicillustratingthesimplifiedsubsurfaceandincludesthecumulativedisplacementofthe M-IPIsastheyarepositionedalongtheembankment;in thisschematic,allcumulativedisplacementsaredrawn tothesamehorizontalscale.Thesedataindicatethatthe baseofFDL-Aisshearingintotheweatheredbedrock (TH20-01)andbelowthepermafrosttable(TH20-01 throughTH20-03).ThesedataalsoindicatethatFDL-A hasdisplacedtheareaaroundTH20-01approximately 0.32mhorizontally,withthisM-IPIdemonstrating

aboutthreetimesmorehorizontaldisplacementthan TH20-02orTH20-03.TheM-IPIinTH20-04demonstratesaslowandsteadydownslopemovementofthe embankmentmaterial,indicatingthatFDL-Ahasnotyet startedtoimpactthisportionoftheembankment.The subsurfaceinterpretationinFigure9alsoindicatesthat therearesomevariationsinthebedrocksurfacebeneath theembankment.Forexample,weinterprettheTH2003areaasapaleo-drainage,asthisareawasalsooccupiedbythemoderndrainagebeforeFDL-Amodifiedits path.Resultsfromothersubsurfaceinvestigationsinthe area(e.g.,Simpsonetal.,2016)indicatedthatthebedrocksurfaceistypically3.7mbgs,similartotheTH2001location.

LidarDataandChangeDetection

Wetookthedifferencebetweenthe2021datasetand ourdatasetcollectedin2020(notpartofthisproject) andthedifferencebetweenthe2022and2021datasets, producingDEMsofdifference(DoDs);seeFigure10. Weassumedspatiallyuniformerrorsandappliedthe squarerootofthesumofsquarestocalculatetheerror oftheDoDs(Wheatonetal.,2010),whichwas0.43m. BothDoDsweremaskedtoexcludedifferencesofless than0.5mtobeconservative.Someerrorsstillremain intheDoDs;forexample,the2021–2020differencing indicatedmovementwherethereshouldbelittle(e.g., greenareaswestoftheembankmentinFigure10a).This

Figure8.M-IPIdatafortheperiodofanalysis.(a)CumulativedisplacementmeasurementsintheA0direction(i.e.,directionofgreatestmovement)forselecteddates.Boreholesarepresentedfromsouthtonorthtomatchthecross-sectionA-A 0 orientation(Figures1dand9).Notethe changeinthecumulativedisplacementscalebetweenTH20-01andremainingboreholes.(b)Ratesofhorizontaldisplacementforselecteddepths. Cum.Displ. ¼ cumulativedisplacement.

maybeduetoacquiringthe2020lidarafterleaf-out inthespring,asthispartoftheembankmentiscoveredbydenseshrubs.The2021datasetcontained pixelatedareasthatproducedartifactsandanapparentelevationgain(greenareaindicatedbypurple arrowinFigure10b)alongtheeasternpartofthe embankment,whichiserroneousbasedonvisual observationsofthesite.

Despitetheseerrorsandartifacts,theDoDsalsoillustratedramaticchangesinthepositionofFDL-A,with verticalchangesupto8.2masthetoeadvancedtoward theembankment.The2021–2020DoDalsocaptured sediment fillingasmallbasinattheinletofaculvertthat usedtoprovidecross-drainageforthestreamdraining fromFDL-A(indicatedbytheredarrowinFigure10a). Asanothermeansofchangedetection,thelocationsof FDL-A’stoe,asmappedusingtheRTK-GPSdevice from2020until2023,areprovidedinFigure10c.These curvesillustratetheproximityofthetoetoTH20-01by July2023.

DATAINTEGRATIONANDDISCUSSION

BasedontheM-IPIdata,FDL-Awasshearingapproximately4mbelowthebottomoftheDaltonHighway embankmentattheTH20-01locationbeforethisproject startedin2020andwhilethelobewasstillmetersaway. Figure11illustratesthemovementprocessesevidentin March2021.Thedailyphotograph(Figure11a)indicated anoverallsteepeningofthetoe.Duringatriptothesite thatmonth,wenotedashearplanethatdaylightedin FDL-A’stoe,about3mabovethegroundsurfaceinfront ofthelobe(Figure11b).Figure11cisascaledconceptual modelofthetoeofFDL-A;thegroundsurfacewas derivedfromthe2020lidarDEM.Wedeterminedthe thicknessofFDL-AnearitstoeusingTH22-02;although thisboreholewasdrilledforadifferentprojectandisnot discussedinthispaper,weincludeitspositionheretoconstraintheconceptualmodel.Basedon fieldobservations andtheM-IPIdata,wehypothesizethatadécollement

Figure9.Schematicinterpretationofthesubsurfacealongcross-sectionA-A 0 (asindicatedinFigure1d),withcumulativedisplacementofthe fourM-IPIssuperimposed.Thedisplacementisperpendiculartothecross-section(i.e.,90degreesfromwhatisshown,or “intothepage”);the schematichas4.5timesverticalexaggeration.vert. ¼ vertical;horiz. ¼ horizontal;cum.disp. ¼ cumulativedisplacement.

formsthebaseofFDL-A,whereitsmassshearsalongthe bedrocksurface(Figure11c).Aseriesofminorthrust faultsoriginateatthedécollementanddaylightwithin thetoeslope;theyalsoformtheinterfacebetween FDL-Asoilandtheoriginalgrounddownhillofit. OnlythedécollementwasevidentintheTH20-01MIPIdatainMarch2021,approximately7.6mbelowthe embankmentsurfaceandabout1.3mbelowthebedrocksurface(Figure11d).

BeginningSeptember19,2021,inthedailyphotographsweobservedamoundofsoilrisingfromthe groundsurface(redovalinFigure12a).Thismound, locatedbetweenFDL-AandTH20-01,wasobviousduringatriptothesiteinOctober2021(Figure12b).Our camerainstalledatthenorthendoftheFDL-Asite recordedtheformationofthesoilmound(redovalin Figure12c),aswellasFDL-Apushinganear-surface layerofsoilinfrontofittowardtheembankment(northernextentofshearedsoilsindicatedbytheorangearrow andcurveinFigure12c).Wehypothesizethatthe moundformedassoilwasforcedupwardduetorotationalmovementandbucklingasFDL-Aapproachedthe embankment(Figure12d).Duringthefallof2021,the M-IPIatTH20-01continuedtodemonstratesteady

horizontaldisplacementatthedécollementlocatedwithin theupperbedrock(Figure12e).

Basedon fieldobservations,andRTK-GPSmeasurementsofFDL-A’stoeandthetoeoftheembankment, FDL-Astartedtooverlapthesideslopeofthehighway embankmentinJuly2022.Thenextnoteworthychange inthewestwardprogressionofFDL-Awasthebacktiltingoftreesthatbecamevisibleinthedailyimageson March1,2023(Figure13a).Thissuggestsashallowrotationalfailureofthetoe,possiblyalongoneofthethrust faultsdevelopedduringthefallof2021.Duringa field visitinJune2023,wenotedthepresenceofscarpsand down-droppedblocksinthevicinityoftheback-tilted trees,whichisadditionalevidenceofrotationalfailure (Figure13b).Figure13cillustratesthelocationofthe shallowrotationalfailurerelativetothethrustfaultsand décollement,thelatterofwhichcontinuedtobedemonstratedbytheM-IPIinTH20-01(Figure13d).

BySeptember2023,therewereseveralpulsesofsoil slumpingdownthetoe,causingthetreestorotateforwardtowardtheembankment(Figure14a).Theremoval ofthiswetandloosesurfacelayeragainexposedthe minorthrustfaultsdaylightinginFDL-A’stoe(indicated byorangelinesinFigure14b).Figure14cillustratesthe multipletypesofmovementnowobservedwithinthe

Figure10.ChangedetectionatthetoeofFDL-A.DoDsfor(a)2020–2021and(b)2021–2022.(c)LocationsofFDL-A’stoeprogressionfor 2020–2023,andpersistentcracksintheembankmentasmeasuredinMay2023.(d)Exampleofonecrack(person’slegsforscale;photograph takenJuly26,2023).Basemapdataare2020and2022lidar(unpublished).

toeofFDL-A.TheM-IPIreadingsinTH20-01(Figure 14d)indicateddownslopeleaningofthedevicebeginningabout2mbelowtheembankmentsurface.We interpretthisasFDL-A’smass finallyimpactingthe embankmentatandabovetheoriginalgroundsurface, whichiscorroboratedbythepressureplateandtiltmeter readingsnearTH20-01(Figure7)andthroughvisual observations. Certaindeformationfeaturesdemonstratedduringthe collisionofFDL-Awiththeabandonedpartofthe

Figure11.FDL-AmovementandinteractionwiththeabandonedDaltonHighwayembankmentinMarch2021.(a)Dailyphotographofthe embankmentandFDL-Atoe(takenMarch20,2021).(b)PhotographoftheshearplanedaylightedinthetoeofFDL-A(takenMarch20,2021). (c)SchematicofmovementprocesseswithlocationsofTH20-01andTH22-02.(d)M-IPIdataforTH20-01forMarch2021.vert. ¼ vertical; horiz. ¼ horizontal;Cum.Displ. ¼ cumulativedisplacement.

Figure12.FDL-AmovementandinteractionwiththeabandonedDaltonHighwayembankmentinSeptember2021.(a)Dailyphotographofthe embankmentandFDL-Atoe(takenSeptember19,2021)withanupliftedsoilmoundindicatedbytheredellipse.(b)Photographofthesoil moundatthetoeofFDL-A(takenOctober2,2021).(c)Annotatedphotographfromthenorthcamera,indicatingthesoilmoundandshearingof surfacesoil.(d)SchematicofmovementprocessesforSeptember2021.(e)M-IPIdataforTH20-01forSeptember2021.vert. ¼ vertical; horiz. ¼ horizontal;Cum.Displ. ¼ cumulativedisplacement.

DaltonHighwayembankmentareanalogoustopro-glacialpushmoraines.Forexample,theareaofnear-surface soilshearingcapturedbythenorthcamerainOctober 2021(Figure12c)issimilartoaforelandwedge,andthe soilmoundformedduringthesametimemayrepresent distalfoldingaheadofthethrustfaultswithinthetoe (Bennett,2001).Likewise,themajorshearzoneatthe baseofFDL-Acanbeinterpretedasadécollement,above whichFDL-Ashearsdownslopeandbelowwhichnosignificantdeformationoccurs(AberandBer,2011).The resultsofthedirectsheartestsoftheunfrozenweathered whitemicaschistindicateamoderatefrictionangleof 30.8degreesandlowcohesionof8.79kPaalongfoliation.Forcomparison,GonzálezdeVallejoandFerrer (2011)listvaluesof20–30degreesand2–15MPafor frictionangleandcohesion,respectively,forfreshschist alongfoliation.Recognizingthatthesesampleswere testedunderthawedconditionsanddonotnecessarily reflectthestrengthofthefrozenbedrock,thislowcohesionsupportsthehypothesisthattheweatheredbedrock representsaweaklayerwithanunfavorablefoliationorientationalongwhichFDL-Ashearsdownslope. Basedonthelimitedsubsurfacedatausedtoproduce theconceptualmodels,wehypothesizethatthebedrock surfaceisparallelwiththegroundsurfaceattheresearch

site.Thisgeometry,aswellasFDL-A’sshearingwithin aweakzoneintheupperbedrock,lendsitselftoinfinite slopeanalysis(Cornforth,2005;Duncanetal.,2014). UsingthegeometrydefinedintheschematicinFigure 15aandassumingnogroundwater,theresistingshear force(S)canbecalculatedusing:

S ¼

/ (1) where c iscohesion, r isgeostaticstress,and / isthe frictionangle(Cornforth,2005).Forthiscalculation,we assumea1by1msoilcolumn(b equals1).Eq.1representsthetotalshearforce.Todeterminetheportionof theshearforceresistingthemovementofFDL-A (SFDL),wemustresolvethegeostaticstressintoitsprinciplecomponentsandsubtractthecomponentoriented inthedownslopedirection(i.e.,thelastterminEq.2):

FortheTH20-01location,thereis3.66mofembankmentwithamoistunitweightof21.95kN/m3,basedon typicalmaximumunitweights,optimummoisturecontent

cb cosb þ b cos br tan

SFDL ¼ cb cosb þ b cos br tan / b r sin b (2)

Figure13.FDL-AmovementandinteractionwiththeabandonedDaltonHighwayembankmentinMarch2023.(a)Dailyphotographofthe embankmentandFDL-Atoe;thearrowpointstoback-tiltedtrees(takenMarch1,2023).(b)Photographofascarp(indicatedinorange)uphillof theback-tiltedtrees(takenJune22,2023).(c)SchematicofmovementprocessesforMarch2023.(d)M-IPIdataforTH20-01forMarch2023. vert. ¼ vertical;horiz. ¼ horizontal;Cum.Displ. ¼ cumulativedisplacement.

ofsoilfromlocalmaterialsites(Maxwell,2016),and95 percentcompaction.Recognizingthattheembankmentis notcontinuousacrosstheentireslope,wetreateditasa striploadandreducedthisverticalstressusinganinfluencefactorandtheprincipleofsuperposition(Osterberg, 1957).Figure15bisaschematicofthepositionofTH2001withintheembankment;weassumedauniform embankmentthicknessforthesecalculations.Thevalues A1, B1, A2,and B2 weredeterminedfromlidardata.We reducedtheverticalstresscausedbytheembankmentat

thedepthofshear(indicatedbyareddotinFigure15b) bydividingtheembankmentintotwoportions,andcalculatingtheterms

,and

.Usingthetwo pairsofterms(i.e.,either

and

or A

and B2 z ), wedeterminedinfluencefactorsforeachportionofthe embankmentusingtheOsterberg(1957)chart,addedthe influencefactorstogether,andthenmultipliedtheresult totheverticalstressfromtheembankment,resultingin 71.5kPa.Theembankmentisunderlainby2.59mofsilty

Figure14.FDL-AmovementandinteractionwiththeabandonedDaltonHighwayembankmentinSeptember2023.(a)Dailyphotographofthe embankmentandFDL-Atoewithsurfaceslumpingontothehighwayembankment(takenSeptember9,2023).(b)Photographofminorshear zones(indicatedinorange)daylightingwithinthetoeofFDL-A(takenJune20,2023).(c)SchematicofmovementprocessesforSeptember 2023.(d)M-IPIdataforTH20-01forSeptember2023.vert. ¼ vertical;horiz. ¼ horizontal;Cum.Displ. ¼ cumulativedisplacement.

sandwithanassumedmoistunitweightof19.75kN/m3 (Codutoetal.,2011),inturnunderlainby1.35mof weatheredwhitemicaschistbedrockabovetheshearsurfacewithaunitweightof17.83kN/m3.Thisresultsina totalgeostaticstressof146.7kPaatthedepthofshear (7.6m).Forthe b angle,weusedthelidardatatocalculateaslopeangleof7.0degreesadjacenttoFDL-A.For c and /,weusedtheresultsfromthedirectsheartestson thewhitemicaschistsamples(c of8.79kPaand / of 30.8degrees).Theseassumptionsyieldedan SFDL of77.8 kN/mwidth.SinceFDL-Aismoving,weassumeafactor ofsafetyof1,meaningthat77.8kN/mwidthistheminimumvalueofthedrivingforceaswell.Thissimple approach,withallofitsassumptions,providesanorderof magnitudefortheimpactfromFDL-A.

Aspreviouslyindicated,theM-IPIinTH20-01 recorded »0.3mofhorizontaldisplacementbetween September6,2020,andNovember1,2023.Thedisplacementisnotyetdiscerniblethroughclosevisual inspectionofthedailyphotographsorthroughthe DoDs,butthishorizontaldisplacementismanifestedas cracksintheembankment(Figure10candd).In2022, wenoticedpersistentcracksintheembankmentsurface andmappedtheirpositionsusingtheRTK-GPSdevice inMay2023.Itispossiblethatthesecracksformedthrough thawsettlement,asthisportionoftheembankmentisno

longermaintained;however,theirlocationsandorientations alsosuggestthattheycouldbeduetohorizontalshearing fromFDL-A.ByJuly2023,thenorthcamerawasindangerofbeingoverrunbyFDL-A’stoe.Wemovedthis cameratothewestoftheembankmentinalocation wherewehopetocaptureevidenceoftheembankment’s horizontaldisplacement.

FDL-A’sannualizedmovementrateforJuly2022to July2023was7.96m/yr.ThedatapresentedinFigure4 indicatedareductioninthemovementratesince2020, whichweattributemainlytocoolingofthelobedueto lowandlatesnowfallduring2020/21.Thishypothesisis supportedbyasimilarslowinginnearlyallmonitored FDLsafterthatwinter.Thisreinforcespreviouswork indicatingthatFDLmovementiscloselytiedtoitsinternaltemperature,whichiscontrolledbyairtemperature andprecipitation,especiallysnow(Darrowetal.,2017). UnliketheotherFDLs,FDL-Aalsohasbeeninteracting withthehighwayembankment,whichmayserveasan additionalimpedimenttoslow althoughnotstop its downslopemovement.Continuedmonitoringwillresolve thisquestion.

CONCLUSIONS

ResultsfromourinitialresearchindicatedthatFDL-A wasanimpendingthreattotheDaltonHighway.Based ontheseresults,ADOT&PFrealignedthehighwayto avoidthegeohazard,leavinginplaceasectionofthe abandonedembankment,whichweinstrumentedin 2020.Theintegrationofmultipledatasetsobtained throughthisfull-scale fieldexperimenthasallowedusto produceadetaileddescriptionofthecollisionofalandslideinpermafrostwithanengineeredembankment. Althoughwehavemade fieldobservationsofthissite foroveradecade,thisprojectmadeitpossibletounderstandthenuancesofmovement.Forexample,whilestill metersawayonthesurface,FDL-AwasalreadyimpactingtheDaltonHighwayembankmentbyshearingwithin theupperbedrockat7.6mbelowtheembankmentsurfaceinoneoftheboreholelocations.FDL-Amade “official” contactwiththehighwayembankmentinJuly 2022andhashorizontallydisplacedaportionofthe embankmentby »0.3masofNovember1,2023.Using theinfiniteslopeapproachandstrengthpropertiesdeterminedfromlaboratorytesting,weestimatethatFDL-A isimpactingthehighwayembankmentandunderlying soilswithatleast77.8kN/mwidthshearforce.Asthis force,manifestedbythemeasuredhorizontaldisplacement,occursatdepthbelowthebottomoftheembankment,wepostulatethatthepresenceoftheembankment willdolittletostopFDL-A’sdownslopeprogression. AnalysisofthedatapresentedheresupportsthehypothesisthatFDL-A’sannualizedmovementrateistiedtoground

Figure15.Schematicsof(a)infiniteslopeand(b)verticalembankmentloading,withrelevantsymbology.

temperatureandthussensitivetochangesinairtemperature andprecipitation(namely,snow).Wealsoreconfirmthat FDL-A’speakmovementrateoccursinlatefall theperiod ofdeepestthaw,asmeasuredintheembankment.ManyfeaturesobservedatthetoeofFDL-Abetween2020and2023 aresimilartopushmorainesproducedinapro-glacial environment.Alloftheseobservationscanbeincorporated intomodelingofthelandslidetodetermineappropriate andeffectivemitigationmeasures.Withcontinuedmonitoring,wehopeto1)obtainproofofconceptforthetilt meterandpressureplateinstrumentation,2)observehorizontaldisplacementwithintheembankmenttoverifythe causeofcrackinginthesurface,and3)determinehowthe presenceoftheabandonedDaltonHighwayembankment affectsthemovementcharacteristicsandlong-termmovementratesofFDL-A.

Inseveralplacesinthisarticle,wereferredto changesobservedindailyimagesfromthecamerasat theresearchsite.Asanimatedsequencesprovideabettermeansforvisualizationthanstaticimages,theFDL website(UAF,n.d.)providestime-lapsemoviesfrom thenorthandsouthcamerasthatshowtheprogression ofFDL-A’sapproachandcollisionwiththeabandoned DaltonHighwayembankment.

ACKNOWLEDGMENTS

ThisresearchwassupportedbyaPacificNorthwest TransportationConsortiumgrant(UWSC10217),an ADOT&PFgrant(ADN45-2-1065),andongoingsupport fromtheAlyeskaPipelineServiceCompany(APSC).We thankJ.Schwarberforhertruegritduringthe2018 field work,andP.Presler,J.Simpson,P.Lanigan,andS.Parker fortheirexpertiseandenthusiasminthe fieldduringthe 2020geotechnicalinvestigation.WethankD.Cronmiller forguidingustorecentworkonpushmorainesandM.Bray, Y.Shur,B.Leshchinsky,andtheanonymousreviewersfor improvingthispaperthroughtheirthoroughreviewsand suggestions.

Disclaimer

Theviews,opinions, findings,andconclusionsreflectedin thispaperaretheresponsibilityoftheauthorsonlyanddonot representtheofficialpolicyorpositionoftheADOT&PF, APSC,U.S.ArmyCorpsofEngineers,orotherentity.

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OpenAccessArticle

NeedforaMulti-SensorMonitoringApproachfor EmbankmentFailures:LessonsLearnedfromthe EdenvilleDamFailure

JOBINTHOMAS

DepartmentofGeologyandGeologicalEngineering,UniversityofMississippi, Oxford,MS38677,USA

RYANKLIDA

DepartmentofGeologicalandMiningEngineeringandSciences,Michigan TechnologicalUniversity,Houghton,MI49931,USA

THOMASOOMMEN*

DepartmentofGeologyandGeologicalEngineering,UniversityofMississippi, Oxford,MS38677,USA

SAJINKUMARK.S.

DepartmentofGeology,UniversityofKerala,Thiruvananthapuram,Kerala 695581,India

BINZHANG

FacultyofGeo-InformationScienceandEarthObservation,UniversityofTwente, 7511AEEnschede,TheNetherlands

XIEHU

CollegeofUrbanandEnvironmentalSciences,PekingUniversity,Beijing100871,China

KeyTerms: EmbankmentFailure,EdenvilleDam, SyntheticApertureRadar,PersistentScatterer Interferometry,SoilMoistureIndex

ABSTRACT

TheEdenvilleDam,anearthenembankmentin Midland,MI,failedonMay19,2020,after3daysof elevatedrainfall(3.83–8.0in.;9.7–20cm),which caused floodingdownstreamofthedam.DaminfrastructuresimilartothatattheEdenvilleDamisnot uncommonintheUnitedStatesandhencerequires periodicmonitoring.Apre-failureanalysiswasconductedinthedamtoexploretheapplicationofremote sensingindammonitoring.Persistentscattererinterferometry(PSI),aformofsyntheticapertureradar (SAR)interferometry,wasusedtoanalyze97Sentinel1imagestomeasuretheline-of-sight(LOS)displacement priortothefailure.ThePSIanalysisshowedhardlyany

*Correspondingauthoremail: toommen@olemiss.edu

signsofdeformationatthefailurelocationandimplied stableembankmentswithvelocitiesrangingfrom3.1 to 5.4mm/yr.Thesoilmoistureindex(SMI)derived using14Landsat-8imagesbetween2016and2020 suggestedthepresenceofsubsurfaceseepageanda potentialfailurezone.Thisstudyillustratestherequirementformulti-sensorremotesensing–basedapproaches forcontinuousdammonitoringandanalysis.

INTRODUCTION

OnMay19,2020,theEdenvilleDaminMichigan, midwesternUnitedStates,failedduetostaticliquefaction instabilityafteraheavyspellofrainfallspatiallyranging between3.83in.(9.7cm)and8.0in.(20cm)over2 days.TheEdenvilleDam,constructedbetween1923and 1925,wasdesignedfor floodcontrolandhydroelectric powergeneration(Pradeletal.,2021;Franceetal., 2022).Thefailureresultedinthewaterimpoundedbythe dam(WixomLake)surgingdownstreamintoSanford Lake,ultimatelyleadingtotheovertoppingandfailureof

Thomas,Klida,Oommen,K.S.,Zhang,andHu

theSanfordDamonMay20,2020.Theseeventsresulted intheinundationofthe floodplainsoftheTittabawassee andTobaccoRivers,includingthecityofMidland,MI.

The floodtriggeredtheevacuationof11,000residents andcausedanestimated245millionU.S.dollarsinpropertyandinfrastructuredamages(Pradeletal.,2021; FEMA,2022).Unfortunately,damfailuressuchasthese arenotuncommonintheUnitedStates(Graham,2009).

TheCommitteeonFailuresandAccidentstoLargeDams oftheU.S.CommitteeonLargeDamsreportedthatnaturalerodiblematerialsformtheembankmentsfornearly 80percentofthelargedamsintheUnitedStates(Redlingeretal.,1975).Thesestructuresarehighlylikelytobe subjecttodeformationandfailure,particularlyunder extremehydrologicalconditions(Wuetal.,2011).

Onameanbasis,roughly10damfailuresoccurredin theUnitedStatesannuallybetween1848and2017,and recentdecadesshowanincreaseinthenumberoffailures (i.e.,24peryear)(NationalPerformanceofDamsProgram,2018).Yearsofneglectingaginginfrastructurehave leftmanydamsintheUnitedStatesinfairtopoorconditions.AccordingtotheonlinedaminventoryoftheDam SafetyProgramoftheMichiganDepartmentofEnvironment,GreatLakesandEnergy(EGLE),Michiganhas 2,545damswithvaryingdimensions,where10percentof allthedamshave “high” or “significant” hazardpotential, includingtheEdenvilleandSanfordDamsbeforetheirfailures(EGLE,2023).TheMichiganSectionoftheAmericanSocietyofCivilEngineers(ASCE)(2023)listeddams inMichiganwithanoverallevaluationofa “C ” intheir 2023infrastructurereportcard,andatthetimeofthe report,twothirdsofthedamswereolderthantheir50year designlife.Constructedin1924,theEdenvilleDamwas designedbeforethemoderngeotechnicalengineeringpracticesofProfessorKarlTerzaghiwerewidelypracticedand builtbeforethequality-controlassessmentofearth fill compactionwasdevelopedbyRalphProctorinthe1930s (Pradeletal.,2021).Investigatingthedamfailure,the Embankments,Dams,andSlopesCommitteeofASCE’s Geo-Institutepostulatedpossiblefailuremechanisms, including:(1)staticliquefactionduetotheriseinreservoir waterelevation,resultinginlossofstrength,and(2)slope instabilityduetothecorrespondingincreasedpore-water pressures(Pradeletal.,2021).

Damembankmentsaresubjecttodeformationandpotentialfailure,mostlylinkedtoinadequatedesignand/orconstruction,operational flaws,andhigh-magnitude floods (Fosteretal.,2000;Graham,2009).Suchfailuresposesignificant floodanddebrisriskstopeople,property,andinfrastructuredownstreamofthestructure.Forinstance,on October4,2010,theembankmentfailureoftheNo.10red mudreservoirinHungarycaused10fatalitiesandsubstantialeconomiclosses(GrenerczyandWegmuller,2011). Similarly,theFundãoDaminBrazilcollapsedon

November5,2015,killing19peopleandspillingabout 32.6millioncubicmetersofminingwaste,causingsevere environmentalissues(Muraetal.,2018).InJanuary2019, thetailingsdamoftheBrumadinhoironmineinBrazil failed,resultinginadeathtollofmorethan250(SilvaRotta etal.,2020).Hadtheinstabilityoftheseembankmentsbeen detectedatanearlystage,appropriatemeasurescouldhave beentakentopreventstructuralfailure.Indeed,periodic embankmentmonitoringisacriticalcomponentinpreventingtheimpactsofsuchunforeseenevents.Furthermore, agingdaminfrastructureincreasestheneedforperiodic daminspectionsandmonitoringtomitigatetherisksassociatedwithdamfailures.Nevertheless,damsafetyhugely reliesonadvancementsindammonitoringtechniques.

Traditionaldaminspectionandmonitoringmethods requiresignificanttimeandhumanresourcesinthe field, andthemonitoringdatasets,suchasglobalpositioning system(GPS)data,elevation,andpiezometerreadings, offeronlypointinformation.Ontheotherhand,remotely senseddataoffertheabilitytosupplement insitu monitoringorreplace insitu/fieldmeasurementswherethey areabsent.Remotesensingcan:(1)acquirecontinuous dataovertheentireembankment(asopposedtodiscrete, point-sourcedatafrom insitu methods),(2)assessthe structuralconditionandprecursorsofstructuralfailures inanon-invasivemanner,preventinganypotentialnegativeimpactonthestructure,and(3)continuouslymonitor thedamwithminimalhumanresources.Interferometric syntheticapertureradar(InSAR)techniquesarewidely appliedformonitoringearthendamstability,where multi-temporalanalysisofthesyntheticapertureradar (SAR)imagesfacilitatesaccuratemonitoringofstructural deformation.Oneoftherobustmethodsispersistentscattererinterferometry(PSI),whichusespermanent/persistentscatterers(PS)withhighphasestabilityoverlong timeintervals(Ferrettietal.,2001,2011).However,the performanceofPSIsignificantlydependsonnumerous factors,suchasthesensor,numberofSARimages,PS density,anddeformationcharacteristics(Crosettoetal., 2016).Numerousresearchershavedemonstratedthe applicabilityandcompetencyofvariousInSAR-based approachesformonitoringthedeformationofembankmentsandsubsidenceworldwide(e.g.,Grenerczyand Wegmuller,2011;Zhangetal.,2015;Othmanetal., 2019;Besoyaetal.,2020;andMalteseetal.,2021).

Oneofthemajorcausesofearthenembankmentfailuresisuncontrolledsaturationandincreasedseepagedue tointernalerosion.Manyresearchershaveillustratedthe significanceofvariousnon-invasivemonitoringmethods toassessthestructuralhydrologicalcondition,particularlybasedonremote-sensingdata(SilvaRottaetal., 2020;Zumretal.,2020;andLinetal.,2021).Amongthe variousapproaches,thestudiesofSilvaRottaetal. (2020)andLinetal.(2021)combinedsoilmoisture

Figure1.(a)LocationofEdenvilleDaminMichigan’sLowerPeninsula.(b)Locationofthe fivedamslocatedalongtheTittabawasseeand TobaccoRivers.EdenvilleDamislocatedupstreamoftheconfluenceofthetworiversandimpoundsbothrivers.

analysisusingthermalandopticalimagerywithSAR analysis.TheprimarycauseofthefailureoftheBrumadinhotailingsdamwasseepageerosion(piping),which wasevidentinthetemporalvariabilityofthesoilmoistureindex(SMI)intheyearspriortofailure(SilvaRotta etal.,2020).Similarly,Linetal.(2021),whileanalyzing theSMItimeseriesbetween2013and2020,noteda decadalincreaseinsoilmoisturealongtheeasternpartof thecollapsedarea.ConsideringtheapplicabilityofSAR techniquesandmulti-spectralimagingfordeformation studies,inthisstudy,weutilizedPSIandSMItoconduct apre-failureanalysisoftheEdenvilleDam.

THEEDENVILLEDAM:BACKGROUND

TheEdenvilleDam(Figure1)isoneof fivedams alongtheTittabawasseeandTobaccoRiversinthecentralregionofMichigan’sLowerPeninsula.Located alongtheborderofGladwinandMidlandCounties,the EdenvilleDamimpoundstheTittabawasseeandTobacco Riversjustnorthoftheirconfluence.TheTobaccoRiver flowsintotheTittabawasseeRiver,downstreamofthe EdenvilleDamandupstreamoftheSanfordDam.The EdenvilleDamconsistsofaseriesofearthenembankments,spillways,andapowerhousespanningapproximately6,600ft(2,012m)acrosstheTittabawasseeand TobaccoRivers.Thereservoirimpoundedbythedamis WixomLake.Figure2illustratesthegeographicallayout ofEdenvilleDam.

TheTobaccosectioniscomprisedoftheTobaccoSpillway,thewestembankment,andthewestcrest.Thewest embankmentisapproximately1,300ft(396m)longand beginswestofMichiganHighwayM-30,extendswestwardtotheTobaccoSpillway,andthencontinuestothe westcrest.Thewestcrestisapproximately900ft(274m) longandextendswest-northwestfromthewestembankmenttothewestabutment.Theportionofthewest embankmentthatlieswestoftheTobaccoSpillwayand thewestcrestisacontinuousembankment.However,they wereseparatedinthisstudyduetovaryingembankment heights.Thewestembankmenthasaheightrangingfrom 32to47ft(9.8to14.3m),whilethewestcresthasan approximateheightof12ft(3.7m).Theupstreamand downstreamslopesoftheembankmentswereinitiallyconstructedataratioof2.5H:1V(horizontaltovertical)and 2H:1V,respectively(Franceetal.,2022).Portionsofthe downstreamembankmentslopehavebeen flattened,and bermshavebeenadded.Surveydataindicatethatportions ofthedownstreamslopesaresteeperthanthedesignratio.

TheTittabawasseesectioniscomprisedofthesoutheastembankment,eastembankment,andeastcrest.The southeastembankmentbeginssouthoftheEdenville Spillwayandextendssoutheast,approximately650ft (198m)totheeastabutment.Theheightofthesoutheast embankmentrangedbetween52ft(15.8m)immediately southofthespillwayto30ft(9.1m)towardtheabutment.TheapproximatefailurelocationisshowninFigure2,andtheheightofthefailurelocationwasbetween 30and32ft(9.1and9.8m)(Franceetal.,2022).The

Thomas,Klida,Oommen,K.S.,Zhang,andHu

Figure2.EdenvilleDamsitelayoutimage.HighwayM-30,showninthecenteroftheimage,dividesthedamintotheTobacco(western)and Tittabawassee(eastern)sections.(BackgroundaerialimageiscourtesyofNationalAgricultureImageryProgram[NAIP],imagecapturedonJuly 16,2016andaccessedfromtheU.S.GeologicalSurvey[USGS]EarthExplorerwebsite: https://earthexplorer.usgs.gov.)Theapproximatefailure locationisshowninred.

eastembankmentandeastcrestarecontinuousembankments,buttheywereagainseparatedinthisstudydueto varyingembankmentheights.Theeastembankment extendsfromtheEdenvilleSpillway1,300ft(396m)to theeastcrest.Theeastcrestextends1,450ft(442m) fromtheeastembankmenttoM-30.Theaverageheights oftheeastembankmentandeastcrestare40and15ft (12.2and4.6m),respectively.SimilartotheTobacco section,theupstreamanddownstreamslopesofthe embankmentswereoriginallyconstructedataratioof 2.5H:1V(horizontaltovertical)and2H:1V,respectively (Franceetal.,2022).Surveysconductedafterthe flood showthatsomesectionsofboththeupstreamanddownstreamembankmentsaresteeperthantheinitialdesign ratio(Franceetal.,2022).

TheEdenvilleDamwasbuiltbetween1923and1925 tomanage floodwatersandgeneratehydroelectric power(Pradeletal.,2021;Franceetal.,2022).Thedam hadtwooperatinglevelsofthereservoir,normaland winterpoollevels.Thenormalpoolelevationwas675.8 ft(205.98m),andthewinterpoollevelwas672.8ft (205.07m)(Pradeletal.,2021).Dailyvariationinpool elevationwasnottoexceed0.9ftand0.7ft(0.27and 0.21m)forthenormalandwinterpools,respectively. Thedamwasconstructedofloose,poorlygradedsand withoutaclaycore.Theonlysub-drainsystemconsisted

ofclaydraintileswithoutflowstoadrainageditch, whichranparalleltothetoeoftheembankment. Accordingtohistoricalphotographs, fillwasplaced en masse ratherthaninliftsusedinmodernearthconstructions.Fillwasdumpedviatraincar,andcompaction effortsontheplaced fill(i.e.,rollers)werenotrecorded inthehistoricaldocumentation(Pradeletal.,2021; Franceetal.,2022).Thedamfoundationconsistedofglacialtillandoutwashsands,whichweredepositedduring theretreatoftheSaginawLobeoftheLaurentideIce Sheet.Post-glacialerosionanddepositionproduceda seriesof fluvialterracesalongtheTittabawasseeand TobaccoRivers(DorrandEschman,1970;Larsonand Schaetzl,2001;andKehewetal.,2012).Thevariationin theelevationofthefoundationterracesresultedinvariationinthedam fillthicknessthroughoutthestructure.

TheEdenvilleDamfailedonMay19,2020,at approximately5:30pmESTaftera3dayelevatedrainfallevent.Atotalof9.7cmofrainfallwasrecordedfor May17–19attheEdenvilleDam(Pradeletal.,2021). OnMay18,bothspillwayswereoperatingatmaximum capacity.Adepressionalongthecrestofthesoutheast embankment,attheapproximatefailurelocationshown inFigure2,wasnotedbyresidentsandbystandersabout 30minutesbeforefailure.Atthetimeoffailure,itwas estimatedthattheWixomLakereservoirelevationwas

Figure3.(a)Pre-failureand(b)post-failureimageryoftheEdenvilleDam.Theapproximatefailurelocationwherethedepressionwasobserved isshowninred.(Pre-failureimagery:NAIPimage;post-failureimagery:ArcGISworldimagery.)

5.5ft(1.68m)higherthannormaloperatinglevels (approximately1.5ft[0.46m]belowthecrestelevation ofthesoutheastembankment).Thefailurewidened throughouttheeveningofMay19,andWixomLake

wasemptied.Figure3presentsapre-andpost-failure comparisonoftheEdenvilleDam.Itwasalsoreported thatthedownstreamembankmentslopewasnotuniform andwasover-steepenedinareas,andabulgingfeature

Thomas,Klida,Oommen,K.S.,Zhang,andHu

Table1. Parametersusedforprocessinginterferometricsynthetic apertureradar(InSAR)time-seriesanalysis.

ParameterValueParameterValue weed_standard_dev(rad)1.2unwrap_time_win180 unwrap_method3Dunwrap_grid_size10 scla_deramp ‘ y ’ scn_time_win180 scn_wavelength50

waspresentina2017lightdetectionandranging (LiDAR)survey.Aslopestabilitymodelshowedalow factorofsafety(1.08)forarotationalslidewhenmodeledwiththereservoirlevelatthetimeoffailure.

MATERIALSANDMETHODS

InSARAnalysis

Thedeformationoftheearthenembankmentofthe EdenvilleDamwasinvestigatedusingSentinel-1SAR data.TheSentinel-1missionoftheEuropeanSpace Agency(ESA)includesaconstellationoftwopolarorbitingsatellitesutilizingC-bandSARimaging.In total,97single-lookcomplex(SLC)SARscenesin interferometricwide(IW)swathmodeandfromascendingorbitswereacquiredbetweenDecember2,2016, andMay3,2020,fromtheESA.Thecomplexvalueof SLCscenescanbeinterpretedasphaseandamplitude components,inwhichthephasepresentstheranging distancefromtheSARsensortotheground,andthe amplitudesuggeststhebackscatteringaftertheelectromagneticwavesinteractwiththegroundsurface. Grounddeformationcanbemeasuredbycomputingthe phasedifference,i.e.,rangingdistancedifference.Sentinel-1scenesobtainedduringthewinterseasonwere excludedduetosnowcoverandthusvolumetricdecorrelationofSARsignals.Followingtheco-registration andresamplingofalltheSARscenestoagivenreferencescene(acquisitiononNovember22,2018,inthis study),interferogramsweregeneratedbyconjugatemultiplicationofselectedtwocomplex-formatSARscenes. Theamplitudedispersionindex(ADI)(Ferrettietal., 2001)wassettobe #0.7tosortoutthePStargetwith comparativelystableamplitudespanningthetime,e.g., man-madestructuresandrocks(Table1).

Long-wavelengthphasenoiserepresentedasaphase rampwasestimatedandremovedforeachinterferogram.TheStatistical-Cost,Network-FlowAlgorithm forPhaseUnwrapping(SNAPHU)algorithmandagrid sizeof10mwereappliedtounwraptheinterferometric phases(ChenandZebker,2001).Thedisplacement phasewasgeneratedbyremovingphaseartifactsdueto topographicerror,orbiterror,andtroposphericdelay.A moderatetemporalwindowof180dayswasappliedto

fi lteroutthetemporallyhighfrequencyturbulenttroposphericdelayduringdisplacementinversion.Eventually, wederivedspatiotemporalgrounddeformationatthe locationofPStargetsthroughleast-squareestimations.

SoilMoistureIndex(SMI)

TheSMIisanindexusedtomodeltherelativesoil moisturecontentbasedonthedryandwetedgesofthe scatterplotbetweenlandsurfacetemperature(LST)and normalizeddifferencevegetationindex(NDVI).The SMIvaluesrangefrom0to1,with0representingrelativelydriersoiland1representingrelativelywettersoil (Zhimingetal.,2004;Mallicketal.,2009;Paridaetal., 2013;andYuanetal.,2020).SMIwascalculatedasfollows(Zhimingetal.,2004):

where a and b aretheslopeandintercept,respectively, ofthedryandwetedges.

TheboundaryconditionsoftheLST-NDVIspaceare thedryandwetedges,or LSTmax and LSTmin ,respectively. Theseboundinglayersarethetheoreticalmaximumand minimumofSMIvaluesforthegivensceneandarelinear. Tomodelthedryandwetedges,linearregressionwas used.TheNDVIandLSTdatacorrespondingtoeach scenewereprocessedindividually.Tomodelthedryedge, theLST-NDVIplotwassubsetalongtheNDVI x axisto representtheboundaryslopesforboththedryandwet edges.Withinthesubset,theLST/NDVIscatterplotwas dividedintobinsofwidth0.01alongtheNDVIaxis.The meanofthe10highestLSTvalueswithineachbinwas usedto fitthelinearregressionmodel(Mallicketal., 2009;Paridaetal.,2013;andYuanetal.,2020).Tomodel thewetedge,ahorizontallinewas fittedtotheLST-NDVI scatterplot.Forallscenesanalyzed,thewetedgewas nearlyhorizontal(slope , 0.01);therefore,thewetedge wasmodeledasahorizontallinewithaslopeof0(Mallicketal.,2009).Similartomodelingthedryedge,the fivelowestLSTvaluesineachbinwererecordedformodelingthewetedge.Further,themeanofthelowestLST valueswas fittedtothehorizontallinearmodel.Valuesin theLST/NDVIspaceoutsidethedryandwetedgeswere reclassifiedtoeitherdryorwetedges(LSTmin or LSTmax ).

TheU.S.GeologicalSurvey(USGS)managesLandsat multi-sensorimagearchivesthatarerestructuredintoaformalcollectionstructuretoensureauniformdataquality standard.TheLandsatCollection2representsthesecond

Figure4Averagevelocity(mm/yr)betweenDecember2,2016,andMay3,2020,measuredusing97Sentinel-1imageswithPSIprocessing.

significantupdatetothisreprocessingeffort,comprising calibrated,quantized,andscaleddigitalnumberswithsubstantialimprovementsinabsolutegeolocationaccuracy (Crawfordetal.,2023).FourteenCollection2Level1 Landsat8imageswereobtainedbetweenApril23,2016, andMay4,2020,fromtheUSGSEarthExplorer(https:// earthexplorer.usgs.gov).ImagesbetweenNovemberand Februarywereexcludedduetosnowcoverandseasonal vegetationdormancy.Sceneswithcloudcoverwithinthe speci fi edextentwereexcludedfromtheanalysis.The LSTandNDVIlayersweregeneratedusingtheERDAS IMAGINEsoftware.ThespatialresolutionofLandsat data(multi-spectralOperationalLandImager[OLI]and resampledThermalInfraredSensor[TIRS]thermalbands) is30m;therefore,thespatialresolutionofSMIis30m. AtimeseriesoftheSMIwasderivedfromtheLandsat8 images.AstandarddeviationmapoftheSMItimeseries wasalsoderivedtoanalyzetheregion’stemporalvariation ofmoisturecontent.

RESULTS

InSARAnalysis

ThemeandeformationvelocityfromthePSImeasurementsfromDecember2,2016,toMay3,2020,forthe

region,isshowninFigure4.Themeandeformation velocityoftheregionvariedbetween 5.35mm/yrand 3.27mm/yr.Themeandeformationvelocitywasmeasuredintheline-of-sight(LOS)direction(approximately N80°E,40°tonadir),wherepositiveandnegativevalues werePStargetsmovingtoward(i.e.,upward)andaway (e.g.,downward)fromthesatellite,respectively(Vassilevaetal.,2017).Threeareasofinterest(AOIs)were identifiedtocomparethemovementalongthedamoutline:thewestembankment,theeastembankment,andthe southeastembankmentwherethefailureoccurred.The threeembankmentsincludedintheAOIshadadifference inelevationfromthereservoirtothetoeofthedam between8and12m.Thesoutheastembankmentcontainingthefailurelocationbeginsimmediatelysoutheastof theEdenvilleSpillwayandextendssoutheasttotheeasternmostabutment.Themean(standarddeviation)deformationvelocityforthePSImeasurementsofthewest, eastandsoutheastembankmentswere 1.1mm/yr(1.8 mm/yr), 0.1mm/yr(1.5mm/yr),and 0.5mm/yr(1.1 mm/yr),respectively(Table2).ThisvariabilityinInSAR resultedfromtheheavilyvegetated floodplainenvironmentandcoarsespatialresolutionrelativetothenarrow embankmentsystem.Theaveragecumulativedisplacementsoftheeast,west,andsoutheastembankments showednomajordifferencesandwere 21.3mm, 47.5

Thomas,Klida,Oommen,K.S.,Zhang,andHu

Table2. Summaryofthepersistentscattererinterferometry(PSI)analysisoftheembankmentsoftheEdenvilleDam. SoutheastEastWest NumberofPS4411657

mm,and 33.2mm,respectively.MostPStargetsinall threeAOIshaddeformationvelocitiesthatfellwithintwo standarddeviations.Althoughthesoutheastembankment isconsideredtorepresentanunstableareaofthedam,as thefailureoccurredwithinthisAOI,insufficientnumbers ofPStargetswereresolvedwithintheapproximatedfailurelocation.

SoilMoistureIndex(SMI)

TheSMIrepresentingrelativemoistureinthestudy areawasderivedfromtheLandsat8imagesbetween April23,2016,andMay4,2020(Figure5).ThestandarddeviationoftheSMItimeseriesisshowninFigure 6.TheSMIsceneswerenotevenlydistributedovertime duetothelackofcloud-freeimages.Basedonthestandarddeviation,theareaswiththegreatestvariabilityin theentiresceneweretheagricultural fieldsshowninthe southeastcornerandthetwo “bull’s-eye” featuresalong thewesternedgeofthescene,withvaluesrangingfrom 0.20to0.30.TheSMIvaluesfortheEdenvilleDam rangedbetween0.5and0.9.Asexpected,thehighest SMIvalueswereobservedatoraroundWixomLake andtheTobaccoandTittabawasseeRivers,whilethe lowestSMIvalueswereobservedintheurbanareas,the landareabetweentheeastandwestsidesofWixomLake, andtheagricultural fieldstothesoutheastoftheEdenville Dam.Areasdownstreamofthesoutheastembankment alsoshowedrelativelylowertemporalvariabilityinthe SMI.TheSMIscenesinAprilandMayappearedtohave lesscontrast(“blurrier”)betweenvaryingland-usetypes, i.e.,urban,agricultural,forested,andwater.

DISCUSSION

TheresultsofthePSIanalysisoftheEdenvilleDam didnotprovideanysignificantobservationssupporting thedeformationofthesoutheastembankmentandapparentdisplacementatthefailurelocationovertheperiod ofstudy.Althoughthesoutheastembankmentrecorded anegativedisplacement,themagnitudewasnotsignificant(withintwostandarddeviations)andwascomparablewiththeeastandwestembankments,representing

stableconditions.Thelackofevidenceofnoticeable deformationofthesoutheastembankmenthighlightsthe limitationsofInSARtechniquesinvegetated floodplains orwhenthedeformationmagnitudeisbeyondthecapabilitiesoftheInSARtechnique.Theoretically,themaximumrelativedeformationratemeasurableforSentinel-1 istensofcentimetersperyearinadistanceofonepixel spacing,butitvarieswiththenoiselevelofthedataand thespecificphaseunwrappingtechniqueusedtoresolve phaseambiguities(Crosettoetal.,2016).

TheSMIanalysisfortheEdenvilleDamillustrates thevariationsinrelativesoilmoisturecontentfrom April2016toMay2020.Consistentsoilmoistureconditionsaretobeexpectedinanearthdamstructureconstructedwithengineereddrainage.Relativelyhigher SMIvaluesareanticipatedintheareasnearthedamand spillway,whichisevidentinFigure6.However,alinear stretchofconsistentsaturation(orhigherSMIvalues) trendingsouthwestwasobservedinmostoftheimages. Therelativelylowerstandarddeviationofthiszone (Figure6)impliesconsistentsaturationofthezone,suggestingthepossibilityofperennialsubsurfaceseepage. Itisworthnotingthatthepost-dambreakriverchannel developedthroughthissaturationzone.Hence,weargue thatthesaturatedzoneperpendiculartotheembankment reflectsthepresenceofapotentialconduitmechanism forseepageandinternalerosionfortheembankment failure.Pradeletal.(2021)identifiedsixterracelevels inthevicinityofthedam,wherethesecondandthird terracelevelspassbeneaththedamattheapproximate failurelocationandcorrespondtoachangeinelevation alongthebaseofthedam(»6.6ft; »2.0m)anda changeinunderlyinggeology.Theassociationbetween differentterracetreadsandglacialstratigraphyimplies thattheembankmentpredominantlyrestsonglacialtill ofvaryingagesandphysicalcharacteristics.However, incertainareas,thesub-damstratigraphyislikelycomposedofglacialoutwashand fluvialsands.Sincethe damwasconstructedacrosstheseterraces,resultingin lateralvariationsinthethicknessoftheembankment andgeologicalsubstrates(Pradeletal.,2021),thesedifferencesintheterracescouldlikelyhavefacilitatedconsistentseepage.Aspreviouslynoted,Pradeletal.(2021)

Figure5.SMItimeseriesforthe14Landsat8imagesobtainedbetweenApril23,2016,andMay4,2020.

reportedthatthefailedpatchoftheembankmentcorrespondstothetopographicstepbetweenthesecondand thirdterracesurfaces.BasedontheresultsoftheInSAR andSMIanalysis,weproposeamodelforthefailureof theembankment.Thesoutheastembankmenthadconsistentsubsurfaceseepage(zoneofsaturation)betweenthe secondandthirdterracelevels,reachingtheTittabawasseeRiverroughly750mdownstreamoftheEdenville Spillway.Theelevatedreservoirlevelsduetothestorm eventcouldhaveenhancedthepore-waterpressureand seepage,formingafailurezone.Thefullreservoirlevel

oftheWixomLakealsocausedstaticliquefactionof earthen fillmaterials(Pradeletal.,2021),andthesefactors,incombination,facilitatedaconventionalrotational slopefailure.Thelackofevidenceonthedeformationof theembankmentbytheInSARmethodsalsoreaffirms thishypothesisasstaticliquefactionoccursveryquickly.

Theoperatingrulecurvesforthereservoir(i.e., WixomLake)weretomaintainwaterlevelsbetween þ0.3ft(0.09m)and 0.4ft( 0.12m)ofthenormal poollevel,exceptduring floodsandwinteroperations (Franceetal.,2022).Drawdownforthewinterpool

levelbeganafterDecember15andlasteduntilJanuary 15.WixomLakewouldreturntonormalpoollevelonce surfacetemperaturesreached39°F(3.9°C).Withconsistentpoollevelsandadequatetimewhenchangingpool levelstoavoidrapiddrawdown,itisreasonableto assumethatthephreaticsurfacewouldremainconsistent throughouttheyearifproperdrainagewereinstalled. Rainfallcouldchangethephreaticsurface;however, properlyengineereddrainagewouldallowrainfallinfiltration.ThedrainagesystemfortheEdenvilleDamconsistedofclaytilesinstalledatthetimeofconstruction (Pradeletal.,2021;Franceetal.,2022).Originaldesign drawingsdidnotshowthetiles;however,a2012survey ofthedam’sunderdrainnotedthattheclaytileswere installedperpendiculartothefaceoftheembankment, wereuniformlyspaced,andweredischargedtoadrainageditchthatranparalleltothetoeoftheembankments. Thesurveyalsopointedoutthatalongthesoutheast embankment,claytilesweremissingfromthecenterof theapproximatefailurelocation,andsomeobstructions inthetileswerealsoobserved(Franceetal.,2022).The lackoftilescouldhaveresultedinahigherphreaticsurfacethaninareaswithgreaterclaytiledensity.

AlthoughInSAR-basedmethodshavebeendemonstratedtobepreciseandhighlyeffectiveinmonitoringthe deformationofearthenembankmentanddamfailures(e.g., GrenerczyandWegm€ uller,2011;Gamaetal.,2019;and SilvaRottaetal.,2020),thepre-damfailureanalysisofthe EdenvilleDamusingPSIdidnotofferanysignificantsigns ofmovementacrossthesoutheastembankment.However,

theSMI-basedanalysisfacilitatedtheidentificationof potentialfailurezoneswithperennialseepage.Hence,this studyundoubtedlysuggeststherequirementforamulti-sensor-basedapproachforperiodicmonitoringofdams.A majorlimitationoftheSMIanalysisisthelackofuniform temporaldistributionduetocloud-coveredscenes.Ofthe morethan50Landsat8imagesobtainedbetweenApril 2016andMay2020(excludingwintermonths),only14 wereabletobeusedintheanalysisduetocloudcover.The coarsespatialresolutionoftheSMIdatasetsisanotherlimitationofthisstudy.Thecurrentadvancementsindronebasedmulti-spectralimagingwillprovideasignificant opportunitytoovercomethelimitationsoftherelianceon satellite-basedSMIestimates.Inthe 2019Brumadinho damfailureanalysis(SilvaRottaetal.,2020),thespatial extentofthetailingsdamwaslargeenoughthattheuseof 30msatellitedatawasappropriateandcouldshowvariationsinmoisturecontent.Hence,arecommendationfor futureanalysisofspatiallysmallembankmentsistouse drone-baseddatasets,asgroundsamplingdistancesare muchsmaller,thereforeallowingtheobservationofsoil moisturevariationsingreaterdetail.Adrone-basedSMI analysiswouldalsoimprovetemporaluniformityasthe dronecanoperateirrespectiveofthecloudcover.

SUMMARYANDCONCLUSION

Inthisstudy,weinvestigatedthepotentialmanifestationsoftheEdenvilleDamfailure(Michigan,midwesternUnitedStates)onMay19,2020,usingbothPSIand

Figure6.SMIstandarddeviationfortheEdenvilleDamarea.Thedamoutlineisshownastheblackdashedportion.Theapproximatefailure locationisoutlinedinred.

SMIanalyses.Thepre-failureanalysisofthesoutheast embankmentofthedamusingPSIprovidednoclear evidenceofdisplacementanomalies.Further,therateof deformationandcumulativedisplacementofthefailed (southeast)embankmentwerecomparablewiththestable conditions(eastandwestembankments).Thetime-series analysisofthesatellite-derivedSMIhighlightedthepotentialfailurezoneasachronicsubsurfaceseepagezone (zoneofsaturation)betweenthesecondandthirdterrace levels,whichissupportedbythepost-failurechannel development.Basedontheresultsoftheanalysis,weproposedamodelforthefailureoftheEdenvilleDam:The elevatedreservoirwaterlevelduringthestormeventmight havecausedstaticliquefactionofearthen fillmaterialsand increasedthepore-waterpressureandseepage,potentially creatingazoneofweakness,andthesefactors,incombination,likelyledtoaconventionalrotationalslopefailure andfailureoftheembankment.Thus,thisstudyhighlights thesignificanceofmulti-sensor-basedremote-sensing (multi-spectral,thermal,andradar)approachesintheperiodicmonitoringofcriticalinfrastructure,suchasdams.

ACKNOWLEDGMENTS

The firstauthoracknowledgesthepartialsupportforthis workfromMichiganSpaceGrantConsortium(MSGC) Fellowshipgrantno.80NSSC20M0124.

DISCLAIMER

Theauthorsdeclarenoconflictofinterest.

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ElectricalResistivityImagingofRecentLandslides inColoradoSprings,Colorado,USA

MOAMENALMAZ

DepartmentofGeosciences,340N.CampusWalk,216GearhartHall, UniversityofArkansas,Fayetteville,AR72701

KEVINMICKUS*

DepartmentofGeography,GeologyandPlanning,901S.NationalAve., MissouriStateUniversity,Springfield,MO65897

MOHAMEDH.ALY GREGORYDUMOND

DepartmentofGeosciences,340N.CampusWalk,216GearhartHall, UniversityofArkansas,Fayetteville,AR72701

KeyTerms: ElectricalResistivity,InverseModels, Landslides,Colorado

ABSTRACT

Landslidescancausesignificantinfrastructuredamage andthousandsofdeathsaroundtheworld.Colorado Springshasexperiencednumerousdamaginglandslidesthathavenotbeenpreviouslygeophysically investigatedduetoitsgeologicalandclimaticconditions aswellasincreasedurbandevelopmentintolandslideproneareas.In2015,heavyrainfallcausedtheSkyway andBroadmoorBluffslandslidesthatdestroyed30 homes.Inthisstudy,thesetwolandslideswerestudied usingtheelectricalresistivitymethodtoevaluatethe natureofthelandslides.Two-dimensionalinversemodelsconstrainedbygeologicalandboreholedataindicate thatbothlandslidesoccurredwithintheweatheredportionoftheCretaceousPierreShale.Thedepthtothe landslidefailuresurfacerangedbetween3to6mand 6to8mfortheSkywayandtheBroadmoorlandslides, respectively.Bothlandslideshavecharacteristicsofa rotational-typelandslide,butthreemodels(twofor Skyway,oneforBroadmoor)donothavesmooth failuresurfaces.ThePierreShaleischaracterized bysignificantlylowerresistivitythantheoverlyingclay. Thissuggeststhelandslidefailurewaspromoted byhighmoisturecontentwithinthePierreShale. Therefore,theSkywayandBroadmoorareasneed tobemonitoredforfuturelandslidesduringhighprecipitationevents.Additionalgeophysicaldataare neededtofullydeterminethenatureandextentofthe landslidesinColoradoSprings.Ourresultspresentthe

*Correspondingauthoremail: kevinmickus@missouristate.edu

firstsubsurfacemodelsoftheSkywayandBroadmoor landslides.Cityplannerscanusethesemodelsascritical dataformitigatingfuturelandslides,whichcanbeused forfutureurbandevelopment.

INTRODUCTION

ThestateofColoradohasbeenaffectedbylandslides sincehistoricaltimesbecauseofitssteepterrainandgeologicalandclimaticconditions(Ivesetal.,1976).TheoccurrenceoflandslidesinColoradohasincreasedinmodern timesasaresultoflanddevelopmentintoregionsthatare susceptibletolandslides(Ivesetal.,1976).Theselandslides havecausednumerousdeathsanddamageintheorderof millionsofdollarsperyearthroughdestroyingtransportationlines,homes,buildings,andblockedwaterways(Ives etal.,1976;Carrara,1979).InColorado,themainfactors triggeringlandslidesareprecipitation,geologicalconditions,constructionactivities,andfastsnowmeltonland thathassteepslopes(Wieczoreketal.,1989).Whereas landslidesoccurinnumerouslocationsinColorado,El PasoCountyincentralColoradoandespeciallytheeasternslopeoftheFrontRangeoftheRockyMountainsare consideredhigh-risklocationsforlandslides(Figure1) duetotheirunstablegeology(friablerockanddipping layers)andafastpopulationgrowth(Depalo,2018).

ColoradoSpringsisthemajorurbancitywithinElPaso County,whichislocatedintheeast-centralportionofColorado;itoccupiesanareaof505km2 (Figure2).ThewesternpartofColoradoSpringsisapopularchoiceinwhich tolivebecauseoftheviewsoftheRockyMountainFront Range(Henryetal.,2017).Thispopulationgrowthhasled todevelopmentintohigh-landslide-riskregions,which,in turn,increasestheprobabilityofalandslideoccurrence (Squire,2006;Henryetal.,2017).

fiedfromDepalo,2018).

Thecityanditssurroundingregionhaveasemiaridclimatewithanaverageannualprecipitationof420mm(Figure3).Ofimportancetolandslidedevelopment,isthe summermonsoonalrainthatenterstheregionmainlyfrom theSouthwestUnitedStates(Henryetal.,2017).These monsoonalrainsproducesudden,heavyrainsthatcanact aslandslidetriggers.Inadditiontothemonsoonalrain,a varietyofclimaticeventstriggeredseverallandslidesin the1990s(Squire,2006).In1993,landslidesoccurredduringaheavyraineventthatdamagedtheBroadmoorgolf course,andin1995,threehomesweredestroyedbya heavysnowfalleventthatcausedseverallandslidesinthe southwesternpartofthecity.In1999,heavyrainscaused landslidesthatresultedinaround$80millionindamages (Squire,2006).Additionally,severalexceptionallyheavy raineventsin2015reactivatedoldlandslidesurfacesand causednewlandslidesthatdestroyed30homes(Squire, 2006;Henryetal.,2017).

Theconnectionbetweenlandslidemechanismsand precipitationcanbeseenbymonthlyprecipitationdata (January2014toJune2015)(Figure4)thatprecededthe SkywayandBroadmoorlandslidesthatoccurredin 2015.TherainfalldataindicatethatboththeSkyway andBroadmoorlandslideareaswereaffectedbyheavy rainfall.BeforelandslidesoccurringinMay2015,the landslideareasexperiencedheavyprecipitationsixtimes in2014inApril,June,July,August,September,and

OctoberandonetimeinApril2015.Overa15-month period,thelandslideareasbecamemoresaturatedand morevulnerabletolandslideoccurrences.

ColoradoSpringshasgrownbeyondthedevelopedmain cityandintothesurroundingareas,especiallyonhillslopes onthewesternsideclosertotheRockyMountains(Henry etal.,2017),wherethegroundconsistsofunstableshale, claystone,andolderlandslidedeposits(White,2017).This developmenthasdecreasedthestabilityofthehillslopeareas andmadethisregionvulnerabletolandslides.Consequently, thewesternsideofthecityhasexperiencednumerouslandslidesduringthelast43years.Afterthe2015landslide events,thecityofficialsandthehomeownersmettodiscuss waystostudytheselandslidestominimizefuturedamage, andtheyrecognizedtheSkywayandBroadmooractive landslidezones(Henryetal.,2017).

Thereareseveralmethodstoinvestigateandmonitor thenatureandlocationoflandslides,includingboreholes, geodeticmethods,remotesensing,andgeophysicalmethods(Perroneetal.,2014;Zeybeketal.,2015;Zhaoand Lu,2018;andWhitleyetal.,2019).Althoughallofthese methodsareneededtofullyinvestigatethepotentialfor landslidemovement,geophysicalmethodsareidealfor determiningboththelateralextentanddepthoftheregions susceptibletolandslidemovements.Mostgeophysical methods,includingelectricalresistivity(ER),electromagnetics,seismicreflection,seismicrefraction,surfacewaves,

Figure1.LandslidehazardratingsofColoradothatareusedinlocalhazardmitigationplans(modi

ground-penetratingradar,andmicrogravity,canprovide informationononeormoreofthesubsurfaceparameters thatdefinelandslidemovements.Theseparametersinclude watercontent,claycontent,materialdensity,velocityvariationsinmaterialcomposition,andslipsurfaces(Whitley etal.,2019).Oneofthemostusedgeophysicaltechniques istheERmethod,whichcanalsobeusedtoestimatethe waterandclaycontentofthesubsurfaceandimageany potentialslipsurfaces.Additionally,ERdataarerelatively easytocollectandcanalsobecollectedrepeatedlyover timetocontinuouslymonitoraregionthatispronetolandslidehazards(Crawfordetal.,2018;Huntleyetal.,2020).

Inthisstudy,weusedERbecauseitisoneofthemost suitablegeophysicalmethodsforshallowexplorationof landslides(Telfordetal.,1990;Reynolds,1997).Many studiesuseERtostudylandslidespartlybecausethefactorsthatcontrolbothERandlandslidesarealmostthe

same(CarisandvanAsch,1991;Schmutzetal.,2000; Lapennaetal.,2003;andHuntleyetal.,2020).Someof thesefactorsarerockandsoiltypes,rockandsoilthicknesses,typeofbedrock,andwatercontent(Crawford etal.,2018).Electricalresistivityissensitivetothenature ofthesubsurfacematerials,especiallyclaycontent,water content,waterconductivity,andtheweatheringofrocks. Consequently,ERcanprovidevaluableinformationabout thelocationofthefailuresurface,thegeometryofthe landslidesurface,themoisturecontent,andtheboundary interfacesbetweenvarioussoilsandrocktypes(McCann andForster,1990;Ganceetal.,2016;CrawfordandBryson,2018;andCrawfordetal.,2018).ThederivedER modelsarethe firstgeophysicalstudyonthelandslides withinthesusceptibleregionofColoradoSpringsarea, Colorado,andwillaidcityandcountyplannersindeterminingfutureurbandevelopment.

Figure2.LocationofColoradoSprings,showingthestudyareas,highways,drainagenetwork,andlandslidesoverlainonatopographicmap. InsetshowsthelocationofElPasoCountyinColorado.

GEOLOGICALSETTING

ColoradoSpringsisadjacenttotheFrontRangeofthe RockyMountains.ThegeologicalunitswithinandsurroundingtheColoradoSpringsareaconsistmainlyofQuaternary torecentdepositsoverlyingPaleozoictoMesozoicsedimentaryrocks,andthesesedimentaryunitsareunderlain byProterozoicgranites,granodiorites,andmetamorphic rocks(CarrollandCrawford,2000;Siddowayetal.,2013; andWhite,2017).TheProterozoicrocksandthemajority ofthePaleozoicandMesozoicunitscropoutwithinthe FrontRange,whereaseastoftheFrontRange,thegently slopingtopographyisdominatedbyoutcroppingofQuaternarytorecentsurficialdeposits.Anumberofrockoutcrops eastoftheFrontRangeincludetheCretaceousPierreShale,

andthisunitformsthebedrockforthelandslidesthatare thefocusofthisstudy(Figure5).

Thesurficialdepositsaretypically .1.5mthick,but inplaces,theycanbethinner(CarrollandCrawford, 2000).Thesesurficialdepositsincludeavarietyofcolluvium,eolian,andalluviumdepositsthatformterraces, fans,andstreamdeposits.Ofimportancetothisstudyare thecolluviumdeposits,whichcontainmainlysilt,sand, gravel,andclayandalsoincludetalusandlandslide deposits(CarrollandCrawford,2000;Southerland, 2020).Thelandslidedepositsincluderecentlandslides andolderHolocenetoPleistocenelandslides.The olderlandslidedepositsaregenerallyinactiveand includetranslationallandslides,rotationallandslides, andearth fl ows.Themajorityoftherecentandolder

Figure3.PrecipitationmapoftheColoradoSpringsarea,ElPasoCounty,Colorado.

Figure4.Monthlyprecipitationdatafor(Jan2014–June2015)forSkywayandBroadmoorlandslideareaswith0.25° 3 0.25°resolutionfromthe GlobalPrecipitationClimatologyCentrewebsite(Schneideretal.,2017).

landslidesoriginatedintheweaker,i.e.,lesscompetenteastdippingunits,whichincludetheCretaceous PierreShale,theGlenEyrieShalememberofthe PennsylvanianFountainformation,andtheclay-rich layerswithintheCretaceousLaramieandPaleocene Dawsonformations(White,2017).However,afew smallrecentlandslidesformedwithintheProterozoic granodiorite(CarrolandCardwell,2003).

ThePierreShaleformation,amaincomponentofthe landslides,wasformedinashallowmarineenvironment duringtheLateCretaceouswhentheWesternInterior SeawaycoveredcentralandeasternColorado(Noblett, 1994;Southerland,2019).ThePierreShalemainlycontainsbrowntogreenishclayandalsocontainsanupper sandypartinadditiontothinbentonitelayersandsome fractures filledwithsulfosalts(CarrollandCrawford, 2000).WeatheringofthePierreShaleproducesabundant smectiteandbentonite(clay)layers,thataresusceptible toshrinking,swelling,andheaving(Squire,2006).These propertiesmakethePierreShaleunstablewithalow cohesivestrength.Theclaylayersareespeciallysusceptibletolandslideformation(CarrollandCrawford,2000). ThePierreShaleoccursthroughoutColoradoSpringsand ismainlyfoundinitswesternpartonslopesrangingfrom 15°to45°.Inthisregion,thedipoftheshaleisnearlythe sameasthetopographicslope(Figure6).Inadditionto theerosionofthematerialsoverlyingtheconsolidated PierreShaleformation,theshaleitselfhasthepotentialto expandwhenitgetswet,whichincreasestheprobability oflandslideoccurrences(Squire,2006).

Structurally,theColoradoSpringsareaisdominated bythecentralFrontalRange,atopographichighthatformed duringtheLateCretaceousLaramideorogeny(Siddoway etal.,2013).ThisMesozoicorogenygeneratedabasement

upliftinwhichtheRampartRangefaultzonebroughtProterozoicgranitoidrocksuptothesurface(Siddowayetal., 2013).Aseriesofback-thrustfaultsandfrontal-thrustfaults emplacedPaleozoicsedimentaryunitsadjacenttoMesozoic sedimentaryunitstothewestofColoradoSprings.The PierreShaleliesina flexuralbasintotheeastoftheFront RangeupliftandgraduallydecreasesinslopeeastofColoradoSprings(Figure6;Siddowayetal.,2013).

LANDSLIDESINCOLORADOSPRINGS

WithinthegreaterColoradoSpringsarea,therearetwo landslidezonesthatwereactivein2015:theSkywayand theBroadmoorBluffs.TheSkywaylandslidezoneisarelativelynewzoneincontrasttotheothertwozonesthat containreactivatedlandslides.Precipitation,lithology types,soiltypes,andhighslopesgreatlyin fl uencedthe amountofmovementandtheareaaffectedbythe2015 landslides(Henryetal.,2017).BoththeSkywayand theBroadmoorlandslidesarelocatedwithintheregion ofhighestprecipitationinColoradoSprings(Figure3). The2015landslidesweredevastatinganddestroyed30 homes(White,2017).Figure7showsvariousfeatures (e.g.,scarp,toe,propertydamage,etc.)relatedtotheSkywayandtheBroadmoorlandslides.Mostofthe2015landslidesarestillactiveandneedfurtherstudy.

SkywayLandslide

TheSkywayneighborhoodhadnotpreviously experiencedanylandslidespriorto2015whenthe Skywaylandslidesuddenlyoccurred.Additionally, thelandslidesusceptibilityhadnotbeendetermined

inthisneighborhoodatthetimeofbuildingofthehomes (Henryetal.,2017).TheSkywayneighborhoodwasbuilt alongthebase,side,andtopofamesawithsandandgravel depositscoveringthetopofthemesa.

ThereisasteepslopebetweenhomesonZodiac Driveatthebottomofthemesaandthehilltophomes onConstellationDrive(Figure8).OnJuly4,2015, theSkywaylandslidewas fi rstobservedonConstellationDrivebyahomeownerwhonoticeda fi ssurein thelandscape.Then,alandslideformedwiththe heightoftheheadscarpbeing »6.7m.Thetoeofthe SkywaylandslidedestroyedseveralhomesonZodiac Drive(Figure7).

BecausetherehadbeennolandslideswithintheSkyway landslideareapriorto2015,therewaslittle knownabout thesusceptibilitytolandmovement.Thus,theoccurrenceof the2015landslidewassomewhatofasurprise(Henry etal.,2017).However,WhiteandWait(2003)state thatthePierreShale,whichliesbetweenthevalley fl oorandthemesatopanddipsinthesamedirectionof theslope,makesthisareasusceptibletolandslides.

BroadmoorBluffsLandslide

ParalleltoBroadmoorBluffsDrive,whichliesonaridge withintheBroadmoorGlenSouthdevelopment(Figure9) inColoradoSprings,alandslideheadscarpdevelopedin 2015(Figure9).TheheadscarpoftheBroadmoorBluffs Drivelandslidecutthroughsixofsevenhomes,destroying them.Someresidentsmovedoutbecausetheirhomeswere splittingintotwosides(Figure7C).Ageologicalhazard studyperformedbeforetheconstructionofhomeson BroadmoorBluffsDriveshowsthatbothsidesofBroadmoorBluffsDrivehadexperiencedlateralgroundmotion withinthelast20years(Henryetal.,2017).Unfortunately,mosthomeswereconstructedonrecentlandslide deposits(Terracon,1999),andtheshearfailuresurface existedundermanybuildinglots.

BecausetheBroadmoorBluffsareahadexperienced landslidesbefore,thereismoreboreholedatathanthe Skywaylandslidearea.CTLThompson(1997)foundthat theclaystone/shaleinterfacederivedfromsoil-boringlogs betweentheCheyenneMountainZooandthebaseofthe

Figure5.GeologicmapoftheColoradoSpringsarea,ElPasoCounty,Colorado.

Figure6.Geologiccross-sectiononthesouthwesternpartofthecityattheCheyenneMountainFront,ColoradoSprings,ElPasoCounty, Colorado(CarrollandCrawford,2000;White,2017).

oldBroadmoorSkiAreathatrangedfrom4.2to10.4m (Table1).Additionally,fortheBroadmoorGlenSouth neighborhood,theboreholedatafoundthatthedepthof theclaystonerangesfrom1.2to18.3m(Table2)(Chen,

1987;Terracon,1998,2005a;andCTLThomson,2005a).

FortheBaron’sRidgeandBroadmoorResortcommunities,theclaystonedepthrangesfrom3.1to7.6m(Table3) (CTLThompson,2005a,2008a,2008b;Terracon2005b).

Figure7.PhotosoftheSkywayandtheBroadmoorlandslidesandthedamagetheycausedafterthe 2015heavyprecipitation.(A)Skywayheadscarpata destroyedhouse.(B)DestroyedinterioratthetoeoftheSkywayslide.(C)Breaking floorofahomeattheSkywayslide.(D)Toeandsag-pondofSkyway slide(afterHenryetal.,2017).(E)thecurrentSkywayheadscarp.(F)ThelastpartsofthedestroyedhouseattheSkywaylandslideshowninFigure7A. (G) ThetopoftheSkywayheadscarp.(H)Thelocationsoftheresistivityprofiles,whichshowthedifficultyofspreadingthecablesattheBroadmoorlandslide.

Figure8.LocationmapoftheSkywaylandslidezoneinColoradoSprings.Blackcircle-thelandslidezone,redcircle-locationofadestroyed homebecauseofthelandslide.ThetwowhitelinesshowthelocationoftheresistivityprofilesSKW1andSKW2.

Thedepthofthefailuresurfacerangedfromlessthan3m toabout12m(Henryetal.,2017),andtheslopeofthe underlyingclaystoneofthePierreShalevariedfrom5to .30percent(Terracon,1999).

ELECTRICALRESISTIVITY

Electricalresistivityisageophysicalmethodthatis commonlyusedtoinvestigatetheinternalmakeupoflandslides(Whitelyetal.,2019).Thisgeophysicalmethodis usuallynoninvasive,easilydeployable,andrelativelyquick; itallowsfortheinvestigationoflargeareasandisrelatively inexpensivecomparedwithboreholemethods.ERsurveys areconductedinthreemainstages:dataacquisition,data processing,anddatainterpretation.

Electricalresistivityworksbyinjectingadirectcurrent throughtwoelectrodesspacedasetdistanceapartandmeasuringtheelectricalpotentialdifferencebetweentwoother electrodes.BecausetheERmethoddependsonOhm ’s Law,thephysicalparameterdeterminedistheelectricalresistance,andbecausethecurrenttravelsthrough avolume,thephysicalparameterisknownasresistivity(ohm-m).BecausetheEarthisnothomogenous,the measuredparameterisreferredtoasapparentresistivity.

Theapparentresistivityvaluesdependontheelectrical propertiesofthesubsurfaceandonhowthedataare collected.MostERdataarecollectedinalinearfashionwiththedistancebetweenthecurrentandvoltage electrodesvarying.Therearemanydifferentelectrode configurations(arrays),amongwhichtheWenner,Schlumberger,anddipole –dipolearraysarethemostcommon (Telfordetal.,1990).

ERInstrumentationandDataCollection

TheERsurveywasperformedusingamulti-electrode GFInstrumentsAutomaticResistivitySystem(ARES). ThisERsystemiscomposedof64steelelectrodes;multielectrodecables;andabattery-powered,computer-based receiver.Thesteelelectrodesareinsertedintotheground andarefastenedtothemulti-electrodecablestoinjectcurrentintotheground.Eachmulti-electrodecablehaseight electrodes,andeachcableisconnectedtoanothercableto formthedesiredprofilelength.Thecomputer-basedreceiver containsaprogrammableswitchboxthatautomaticallyperformstheelectricalresistivitysurveysaftertheuserassigns thevariousfactors,includingthearraytype,numberofelectrodes,electrodespacing,lengthoftheprofile,dataerror,

Figure9.LocationmapoftheBroadmoorlandslidezoneinColoradoSprings.Blackcircle-thelandslidezone,redcircle-thelocationof five destroyedhome.ThetwowhitelinesshowthelocationoftheresistivityprofilesBRM1andBRM2.

andnumberofreadingstakenateachelectrode(stackingto reducenoise;Berglund,2012).Beforecollectinganydata, theinstrumentcalculatesthecontactresistanceateachelectrodeandinformstheuserofanyelectrodethatistwostandarddeviationsaboveorbelowthemeancontactresistance.

TheERsurveyautomaticallycollectsdatabyselecting thepotentialandcurrentelectrodepairstogeneratethe resistivitydata.Themainunitstorestheresistivitydatato belatertransferredtoacomputerforprocessingpurposes. ToinvestigatetheSkywayandBroadmoorlandslides,the Schlumbergerarraywasusedasitislessaffectedbyurban noisesources.Severalelectrodespacingswereusedbased

Table1. Claystone/shaleinterfacederivedfromsoilboringlogs retrievedfromtheinvestigationforBaron’sRidgeandBroadmoor ResortCommunityDevelopment(TH-15,TH-23)andtheBroadmoor GolfCourse(A-2,A-4,andC-20)(CTLThompson,1997). SpecimenDepth(m)

ontherequireddepthtoreachthebottomofthelandslides. Smallelectrodespacings(1and1.5m)wereusedtostudy thethinnerSkywaylandslide,anda3-melectrodespacing wasusedforthethickerBroadmoorLandslide(Table4). Increasingtheelectrodespacingincreasesthepenetration depthbutreducesthelateralresolution.

Topographyalongeachprofilewasalsoconsideredas topographicvariationscaninfluencetheresistivitymeasurements,andtheseelevationsmustbeincludedinmodelingtoobtainreliablemodels.Thetopographyalongeach profilewasdeterminedusingTopconAT-G7GlobalPositioningSystem(GPS)unitandastadiarodtomeasurethe

Table2. DepthtoweatheredclaystoneinslopestabilityanalysesconductedfortheBroadmoorGlenSouthneighborhood(Chen,1987; CTLThomson,2005a;Terracon,1998,2005a).

IndividualHome vs.MultipleLots

Depthto Claystone(m)

SlopeofClaystone LayertoNearest(%)

Individualhome11.6–12.832

Individualhome18.35

Commerciallot1.2–3.113

CheyenneMountainRanch10.7–12.2Slopefailedat6° dipor10.5%grade

Table3. DepthtoweatheredclaystoneinslopestabilityanalysesconductedfortheBaron’sRidgeandBroadmoorResortCommunities (CTLThompson,2005a,2008a,2008b;Terracon2005b).

LocationDepthtoClaystone(m)

HavershamDrive5.5 HavershamDrive3.1–4

IvyBrookLane4.3–5.8

ChildeDrive6.1

CharlesGrove6.1–7.6

relativeelevationalongeachprofileevery10m.ATrimble GPSunitwasusedtocollectgeospatial(latitudeandlongitude)informationattheendpointsofeachprofileandatall othersignificanttopographicvariationsalongeachprofile. TheseGPSdatawerecollectedwithaverticalaccuracyof 0.5m.Repeatmeasurementswereperformedwhenastandarddeviationforasinglepointwashigherthan10percent.

ERDataProcessing

Theresistivitydatafromeachprofileweredownloaded toacomputerusingtheGFInstrumentssoftwareandconvertedintoaformatreadablebytheAGIEarthImager2-D (two-dimensional)modelingsoftware.Thetopography datawerealsoenteredintotheAGIEarthImagersoftwareforuseinthemodelingprocess.Therawresistivity datawere fi rstviewedinpseudo-sectionformat.Thisis donetoseeifanyanomalousvaluescomparedwiththe surroundingvaluesarehighorlow.Thesoftwareautomaticallyremovessingularspikes,andtheusercanmanuallyremoveanyanomalousvaluesthatstillexist.Thedata qualityforthemajorityoftheprofileswaspoortogood, dependingonthenumberofanomalousvaluesthathadto beremoved.Poordatawereduetothepartsoftheground thatconsistedofloosegravelandrocks(Figure7H).This madethecontactresistanceshighandmadeitdifficultto injectcurrentintotheground.Thisoccurreddespitevariouseffortstoovercometheseconditions,suchaspouring salinewaterontoeachelectrode.Thedataforoneprofile weresopoorthattheprofilecouldnotbeusedinthisstudy, whereastheremainingprofileshadenoughdatatocreate 2-Dmodels.

Table4. Electricalresistivityprofilesforeachlandslide.

Landslide Profile Name Length (m) ElectrodeSpacing(m)Description

SkywaySKW1641Paralleltoslope failure

SKW294.51.5Transverseto slopefailure

BroadmooreBRM11893Paralleltoslope failure

BRM21893Paralleltoslope failure

Thedataalongeachprofilewereinvertedfora2-D resistivitymodelusingthesoftwareofLoke(1999).The inversionmethodusedwasaquasi-Newtonoptimization withsmoothnessconstrainedleastsquares.Theoptimization methodstartswitha2-Dresistivitymodelanditeratively makesadjustmentstoreducethedifferencebetweenthecalculatedandobservedresistivitydatauntilavaluelessthan theassignedspecificdataerrorisreached.Weapplieda 10percentdataerroroneachoftheprofiles,andallthe resistivitymodelsincludedthetopographyalongeach profile.Multiplestartingmodelsusingavarietyofresistivityvaluesandinversionparameterswereevaluatedto determinewhichfeaturesinthemodelwereconsistently requiredbythedata.Eachmodelrequiredbetween5and 17iterationstoreachapointatwhichtheroot-mean-squared (RMS)valuecouldnotbeloweredanyfurther.Forallthe finalmodels,thenumberofiterationstoachieveareasonablemodelwas fiveandseven.Foriterationsbeyondseven, theprogramwastryingto fittheregionswithpoordata qualityandtheregionswithgooddataquality,andthe modeldidnotchangeappreciably.Evenintheregionswith poordata,themodelsdidnotchangenoticeablywith increasediterations.Theinterpretationofthemodelsisconcentratedintheregionswithgooddataqualityasmostmodelshaveareaswithpoordataqualityinwhichreasonable modelscouldnotbeobtained.TheselatterareasareresponsibleforthehighRMSerroronsomeofthemodelsand, thus,arenotinterpreted.Eachmodelhassufficientregions ofgooddataqualitytomakeinterpretationsoftheelectrical resistivitymakeupofthelandslides.Giventhenon-uniquenessoftheresistivitymethod,thelackofdepthandphysical parameterconstraints,andtheattentionpaidtotheregions withgooddataquality,the finalmodelsrepresentageologicallyreasonableresistivitymodelofthesubsurfacealong eachprofileintheregionswithgooddataquality.

RESULTSANDDISCUSSION

Theelectricalresistivitymodelscanprovideconstraints onthelandslidetype,thelithologicalvariations,thefailure zones,andthemoistureconcentration.Themodelswere interpretedwithknowledgeofthelocalgeologyandnearby boreholedata,andwhencombinedwiththeresistivity measurements,a finalinterpretationofeachlandslidewas reached.Thisprocedurehasbeensuccessfulinnumerous otherresistivitystudies(e.g.,McCannandForster,1990; CarisandvanAsch,1991;Schmutzetal.,2000;and Ganceetal.,2016).Tworesistivityprofileswerecollected overtheSkywaylandslide(SKW1andSKW2,Figure8), andtwopro fi les(BRM1andBRM2,Figure9)were collectedintheBroadmoorareathatrecentlyexperiencedlandslides.BRM1andBRM2weremeasured paralleltotheslopefailureandparalleltoeachotherwith thedistancebetweenthembeingapproximately30m.

Inaccessibilitypreventedcollectionofdataparalleltothe slippagefaceoftheBroadmoorlandslide.

SKW1Profile

TheSKW1modelofthelandslideisparalleltoitsslippageface,which,therefore,aidsinidentifyingallportions ofthelandslide.ThelengthoftheSKW1profileis64m, which,giventhesubsurfaceresistivities,allowedforapenetrationdepthofapproximately20m.Theelevationdifferencebetweentheendsoftheprofileisaround13m.The SKW1profilebeginsbeforetheheadscarp,thencrosses theheadscarpandthefailurezoneuntilitreachesthetoe ofthelandslide(Figure8).The firstportionoftheprofile (1to22m)waslocatedbeforethefailurezoneandisnoted byhighresistivityvalues.Theentirelandslideisbetween 22and64malongtheprofile(Figure8).Theheadscarp ofthelandslideisfrom22to28m,andthelandslidetoe rangedfrom46to64m(Figure8).

Ahigh-resistivityzoneunderthestableareaisinterpretedtoberesistantsanddeposits(Figure10).This layeroccursatthesurface,extendsto22malongthe modelprofileandisupto5mthick.Thelowerresistivityareaoccursunderneaththehigh-resistivityzoneand isinterpretedtobeporousclaystonebasedonthelocal geology(Henryetal.,2017).Thedepthoftheclaystone layerrangesfrom5to8m,whichagreeswiththeboreholedatainTables2and3.Thislowerresistivityarea extendsalongmostofthemodelwithathicknessrangingfrom2.5to5m.Thelowestresistivitylayeroccurs undertheclaystone,anditisdeterminedtobethePierre Shale.Thedepthtotheclaystone/shaleinterfaceranges from5to12m,whichmatchestheboreholedatain

Table1.However,poordataqualityintheNWportion oftheprofileprecludesadefiniteinterpretationinthis area.Notably,thePierreShaleoccursalongthewhole profile,andithasthelargestthicknessofalltheprofiles intheSkywayarea.Thefailuresurfacebeginsatthe headscarpandextendsbelowthegroundlevelbetween thehigh-resistivitylayerandtheclaylayer.Thefailure surfaceliesbelowtheclaystoneportionofthePierre Shaleandappearsasasmoothsurfaceunlikeitsappearanceintheothermodelprofilesinthestudyarea.The claystoneandshalelayershadthelargestthicknesscomparedwiththeotherrocktypesandprobablycontaina highamountofmoistureasrevealedbythelowresistivity valuesandmayhavepromotedlandslidedevelopmentin thisarea.Thehigh-resistivityanomalyabovetheslopefailuresurfaceisinterpretedtobedryrockdebrisdeposited duringpreviouslandslideevents.

SKW2Profile

TheSKW2modelis94.5mlong,anditisatanangle totheslopefailure(notparalleltothefailureslope) becausetheroughterrainofferedlimitedaccess(Figure 8).TheSKW2profilebeginsaftertheheadscarpwithin thefailurezoneandthencrossestherestofthelandslide (Figure8).The finalmodelshowsasmallhigh-resistivityzonethatoccursontheNWportionofthemodeland maybeasanddepositduetoitshighelectroderesistance (Figure11).Underthiszone,twothicklayersoflower resistivityarepresent,andtheyareidentifiedtobeclaystoneandshalebasedonthelocalgeology(Henryetal., 2017).Thisprofileisdominatedbythelow-resistivity layersattributedtothewater-saturatedPierreShaleand

Figure10.2-DelectricalresistivitymodelandinterpretationofprofileSKW1(Figure7).

Figure11.2-DelectricalresistivitymodelandinterpretationofprofileSKW2(Figure7).

hassmallhigh-resistivitylayersbecausetheprofilehas beenentirelymeasuredwithinthefailurezone.Thismade itunfeasibletodeterminethetotalextentofthelandslide. Thefailuresurfaceisinterpreted,basedontheresistivity modeland fieldobservations(e.g.,Henryetal.,2017),to varyindepthbetween3and8m.Thefailuresurfaceis thenlocatedwithintheclaystoneatthetopofthePierre Shaleandisnotasmoothsurfaceasiscommoninlandslides(Crawfordetal.,2018).Thisprofiledoesnotshow theheadscarp,andasemi-rotationalslopefailuresurface maynotbeveryclear,butitshowstheexpectedlocations ofpotentiallandslidesduetothehighthicknessofclaystoneandshale.Consequently,landslidesinthisareacan belikelytriggeredbyhigh-precipitationevents.

BRM1Profile

SettinguptheresistivitycablesintheBroadmoorlandslideareawasdifficultbecausetheareahassincebeen coveredwithmanysmalltreesthatmadeaccessdifficult. Thus,onlytworesistivitypro fi leswerecollectedinthis area(Figure9).ProfilesBRM1andBRM2areparallelto thedirectionoftheslopefailure.Collectionofanytransverseprofileswasnotpossibleduetolackofsafeaccess. Thelengthofthetwoprofilesis189meachwithelectrodespacingsof3m(Figures12and13).Bothprofilesare beneficialtolandslidecharacterizationastheyshowthe sidesofthelandslides.

TheelevationoftheBRM1profilerangesfrom1,893m onthesouthendoftheprofiletoaround1,863monthe northendoftheprofile(Figure12).BRM1beginsnexttoa roadandnearthelocationofahomethatwasdestroyedby apreviouslandslide(Figures9).Theprofilecrossesthestableareaonthesouthendoftheprofile,theheadscarp,the failurezones,andthelandslidetoe.Asmallhigh-resistivity

sanddepositoccursonthetopnearthesouthendofthe profile.Thelowerresistivityclaystoneisfoundbelowthe high-resistivitylayerandisfollowedbyunderlyingshale andclaystonelayers.Claystoneisinferredtwiceinthis profilebecauseofthehigherdepthofthisprofile,which reachesupto40m,comparedwiththeSkywaymodels, whichimagedtoaround20mindepth.ThePierreShale layerisabundantatdepthinthestudyarea(White,2017) andimagedasalow-resistivityregionalongthewholeprofile.Thedepthtothefailuresurfaceisbetween6and8m, slightlydeeperthanseenontheSkywaymodels(Figures 10and11).Thefailuresurfaceisrelativelysmooth.Similar totheSkywaymodels,thefailuresurfaceoccurswithina higherresistivityregion(claystone)abovethelowresistivity ofPierreShale.ThethicknessofthePierreShaleisaround 23minthisstudyarea.Inadditiontothehighthickness ofshale,ithasahighmoisturecontent,whichisindicated bythepresenceofacreekthatcarriessurfacewater.On theotherhand,thethicknessoftheclaystonelayeris around7m,anditisnotaswidespreadalongtheprofile comparedwithshale.

BRM2Profile

TheelevationoftheBRM2profileissimilartothe BRM1.TheBRM2profileisalsonexttoaroad,butitwas carriedout »30mfromprofileBRM1andthedestroyed home(Figure9).The finalmodel(Figure13)showsbasicallythesamelandslidefeaturesastheBRM1model(Figure12).Themodelhassanddepositsandclaystone,which isfollowedatdepthbyshaleandclaystone.Also,the PierreShaleiswidelyspreadintheBroadmoorarea (Henryetal.,2018),andthismodelhasthethickest occurrenceofthePierreShaleintheBroadmoorarea. Thefailuresurfaceislocatedatadepthbetween6and

Figure12.2-DelectricalresistivitymodelandinterpretationofprofileBRM1(Figure8).

Figure13.2-DelectricalresistivitymodelandinterpretationofprofileBRM2(Figure8).

8mbelowthesurface.Thefailuresurfaceresides,as intheothermodels,withintheclaystoneunitsontop ofthePierreShale.Thedepthtotheclaystoneranges from8to12m,whichmeetswiththeboreholedatain Tables2and3.Theinterpretedfailuresurfaceseems tobesmootherthanimagedinmodelBRM1althoughthe lowresolutionofthedatamakesthisdifficulttovalidate. Again,thePierreShalehaslowresistivitiesandprobably hashighmoisturecontent,contributingtogroundfailure. Theareaishighlyvulnerabletolandslidesinthefuture becauseofthehighthicknessoftheshalelayerandits highmoisturecontent.

Thefourmodels(Figures10–13)indicate,basedon geologicalstudies(e.g.,White,2017;Henryetal.,2018), thatbothlandslidesoccurredwithinthePierreShale.However,theactualfailuresurfacewasintheweatheredupper portion(claystone)ofthePierreShaleandnotthemain unititself.ThePierreShaleinallthemodelsappearsasa low-resistivityunitinwhichthelowresistivityiscaused byahighmoisturecontent.Themodelsindicatethatthe landslidesareofarotationaltype,andexceptforBRM2, thefailuresurfaceisnotasmoothsurface.Thismaybe duetovariationsofmoisturecontentseeninthePierre Shale.Thelengthofthelandslidesvariedbetween30and 50m.Thedepthtothefailuresurfacerangedbetween3 and6mfortheSkywaylandslideand6and8mforthe Broadmoorlandslide.Giventhatthesurveywasperformedinthedrytimeoftheyear(May),thelikelyhigh moisturecontentduringwetseasonshouldbeaconcern forfuturelandslidesinthesetwoareas.Iflandslidesoccur ineitherareaagain,modelsindicatethattheareaaffected mayberelativelysmallintermsofdepthtoapotential failuresurface,butanylandslidemaycausedamagefor 30to60mperpendiculartotheheadscarp.Thesetwo areasshouldbeespeciallymonitoredforlandmovement duringanylargerprecipitationevents.

CONCLUSIONS

TheSkywayandBroadmoorlandslidesinColorado Springswereinvestigatedbytwo-dimensionalelectrical resistivitysurveystodeterminethegeophysicalcharacteristicsofeachslide.TheColoradoSpringsregionis especiallysusceptibletolandslidedevelopmentdueto climatic,geologic,andurbandevelopmentconditions. Thepresentedmodelsarethe firstgeophysicalstudiesto investigatetheselandslidesandwillformthebasisfor futurestudies.Foreachlandslide,twoprofileswereobtained usingtheSchlumbergerelectrodearraywithdifferentlengths andelectrodespacings.Thedatawereinvertedtodetermine anelectricalresistivitysubsurfacemodelandtheninterpreted toaidindeterminingthethickness,length,watercontent, andlocalfailuresurfaceforeachlandslide.The finalmodels indicatethatthelandslidesinbothareasoccurredwiththe

weatheredportions(claystone)ontopofthePierreShale. Thefailuresurfaceoccurredatshallowerdepths(3to6m) fortheSkywaylandslidethanfortheBroadmoorlandslides (6and8m).Thelandslidesseemtobeofarotationaltype, butonlymodelBRM2hadarelativelysmoothfailuresurface.ThePierreShalehadlowresistivityvalues,whichare interpretedtobecausedbyhighmoisturecontent,andthis highamountofmoisturewaskeytotheformationofthe landslides.GiventhatthePierreShalealreadymighthavea highmoisturecontent,theSkywayandtheBroadmoorareas shouldbemonitoredforpotentiallandslideswheneverthere arehigh-precipitationeventsinthearea.Themodelsprovide decisionmakerswithconstraintsonthepotentialsizesof futurelandslidesinbothareas.Itisrecommendedtoperform additionalERsurveysinadditiontoothergeophysicalstudies,includingseismicrefraction,verylowfrequencyelectromagnetics,andmultichannelanalysisofseismicwavesin theColoradoSpringsareabeforeanyfuturedevelopmentsto fullyunderstandthesubsurfaceandtoknowmoreaboutthe rocktypesandlayerconfigurationthatmaytriggerlandslides inthefuture.

ACKNOWLEDGMENTS

Wearegratefulforthelocalgeologyadvicewereceived fromProfessorChristineSiddowayfromColorado College.Thisstudywasconductedusingelectrical resistivityequipmentfromMissouriStateUniversity.The datafromthisresearchcouldnothavebeencollected withoutthehelpofDonaldWormingtonfromMissouri StateUniversityandBenjaminOpiyo,RyanStrickland, andJahangirAlifromtheUniversityofArkansas.We thankthecityofColoradoSpringsforprovidingpermits thatallowedforthisstudytohappen.

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OpenAccessArticle

PossibleRoleofInternalErosioninthe DevelopmentofGroundFissuresaroundLake Ziway,Ethiopia

YONATHANADMASSU*

DepartmentofGeologyandEnvironmentalScience,JamesMadisonUniversity Harrisonburg,Virginia22807,USA

TRUFATGUGSA

SchoolofEarthSciences,AddisAbabaUniversityPOBox1176,AddisAbaba, Ethiopia

KeyTerms: GroundFissures,InternalErosion, Piping,Pseudokarst,LakeZiway

ABSTRACT

Ground fissuresarelinearorcurvilinearcracksthat arecommonintheMainEthiopianRift,whichispart oftheEastAfricanRiftSystem.Theyareaunique typeofgeohazardaffectingroadsandrailwayinmany partsoftheriftvalley.ThevicinityofLakeZiway isunderlainbylacustrine,pyroclastic,and/orvolcaniclasticsedimentsthatarecutbynumerous ground fi ssures,whichrunfor2–3kmwithupto 1 –3-m-wideopenings.Plausiblecausesofground fissureevents,asdescribedinpreviousstudies,include extensionalmovementduetoactiverifttectonics, hydro-compaction,andpiping.However,theexact mechanismoftheirevolutionispoorlyunderstood. Basedon fi eldobservationsandsedimentcharacterization,weproposeastrongcasefortheroleplayedby internalerosionorpipingofpumicedepositsinthe developmentofground fissures.Internalerosioncauses theformationofsubsurfaceconduits,whichgrowin sizeandeventuallycollapse,formingground fissures. Typicalsediment-erosion–relatedgeomorphologicfeaturessuchasdisappearingstreams,sinkholes,blind gullies,andpipingmoundshavebeenobserved.These featuresindicatethepresenceofanetworkofpipes thatconnectthegroundwaterwithsurfacewaterina mannerverysimilartokarsthydrology.Pumicedepositsinthestudyareaarefoundtobethemostsusceptibletointernalerosionleadingtoground fissuringdue totheirbimodalgrainsizedistributionandultralow density(averagespecificgravity 5 0.6).

INTRODUCTION

TheMainEthiopianRift(MER)isalinearbeltofextensionaltectonicstructuresmarkedbynormalfaultsystems. TheMERisalsoriddledwithground fissures,whichare commonly1–3-m-widecracksthatmayextendfor2to 3km(Figure1).Therehavebeenveryfewreportsoftheir impactoninfrastructureinEthiopiasincethe1950s (GouinandMohr,1967).Asfaw(2000)arguedthat ground fi ssuresshouldbeconsideredasextensional geologicstructuressimilartonormalfaults.Mohr(1967), Asfaw(1998),andYirguetal.(1997)proposeda directrelationshipbetweenground fi ssuresandearthquakes.Mostpreviousresearchhaslinkedground fissuresto rifttectonicsbasedontheirorientationbeingparallelwith theNE-SWriftaxis(Asfaw,1998,2000;Yirguetal.,1997). DespitetheirwidespreadoccurrenceintheEthiopian riftvalleyandtheirgeohazardpotential,ground fi ssureshavenotreceivedenoughattentionintermsof geologicalandengineeringinvestigations.

Ground fissureshavealsobeenreportedandstudied inmanyotherpartsoftheworld.Theearliestreportwas fromthePicachoMountainsinArizona,southwestern UnitedStates,wherea15km fissureopenedfollowinga heavyrainfallaroundtheyear1927(Leonard,1929). Moreground fissureeventsandsubsequentstudieswere undertakenmainlyinArizonaandothersouthwestern U.S.states(Carpenter,1993;Galloway,1999).Mostof the fissuresthatforminthesouthwesternUnitedStates arelong,linearfeaturesorparallelanastomosingcracks thatshowmainlyhorizontalmovementwithaminorverticalcomponent(Carpenter,1993).Therearealsoseveral reportsofground fissuringfromChina’sShanxigraben systemandtheNorthChinaPlain,whereformationof ground fissuresisattributedtopre-existingfaultscoupled

withexcessivegroundwaterwithdrawal(Leeetal.,1996; Pengetal.,2016,2018,2020).Arecentgiantground fissure(approximately50ft[15m]deepand65ft[20m] wide)fromtheKenyanpartoftheriftvalleyoccurredin 2018,cuttingtheMaiMahiu–Narokroad.Thisground fissurereceivedworldwideattentionbythepressandmedia becauseitwaswronglythoughttobeasignofsplittingof theAfricancontinent.Priortotheincident,severalother ground fissureshadbeenreportedfromtheKenyansegmentoftheEastAfricanRiftSystem(EARS),specifically intheLakeNakuruarea(NgecuandNyambok,2000).In westernSaudiArabia,ariseinthenumberofground fissuresassociatedwithdifferentialcompactionofloosesedimentsduetoexcessivegroundwaterwithdrawalhasbeen reported(BankherandAl-Harthi,1999).

Thestudyarea(boundedby7°56 0 10.12 00 Nand8° 03 0 11.95 00 Nlatitudeand38°38 0 22.49 00 Eand38° 45 0 03.64 00 Elongitude)isinthevicinityofLake Ziway;thisareaisunderlainbyunconsolidated pyroclastic/volcaniclasticandlacustrinesediments, whichareriddledwithseveralground fi ssuresthat threatenthestabilityofthenewlybuiltMeki-Ziway freeway.Theobjectiveofthisstudywastoinvestigatethemechanisms(sur fi cialortectonicprocesses) thatpromotetheformationanddevelopmentof ground fi ssuresbasedondatacollectedfromthe environsofLakeZiway(Figure2).LakeZiwayisoneof fourmajorlakes,alsoincludingShala,Abaya,andLangano,thatwereconnectedtoforman »2,960km 2 paleolakeinthelateQuaternaryoccupyingtheMER (Gillespie,1983).TheMERispartofEARS,whichis majortectonicfeaturemarkingazoneofextensionalong theboundarybetweentheNubianandSomaliPlates. TheextensionaltectonicsintheMERhasproducedtwo

mainsetsofNE-SW –trendingboundaryfaultsand NNE-SSW–trendingfaults,whicharesub-orthogonalto theE-Wextensionalfaults(Cortietal.,2013).Lake Ziwayisfedbytwomajorstreams(KetarandMeki) anddrainedbytheBulbulaRiver.Theaveragerainfallis837mm(Alemayehu,2010),andthelake receivesanapproximategroundwaterin fl owof 220,700m 3 /dandout fl owof40,047m 3/d(Ayenew, 2001).Groundwaterrechargeismainlyfromthehighlandstotheeastandwestofthelake.Aquifersarerepresentedbylacustrine/alluvial/volcaniclasticsedimentsand fracturedvolcanicunits(FentawandMihret,2011).

PossibleGeneticModelsforGroundFissure Development

Ground fissuresaffectingsedimentscanbetheresult ofsurficialprocessessuchasinternalerosion,decayof organicmatter,hydro-compaction(rapidsettlementofloose soiluponadditionofwater),faultmovements,and/orhorizontalseepagestress(Ayalew,2004).However,mostpreviousworksinEthiopia(GouinandMohr,1967;Asfaw, 1982,1998;andAyalew,2004)reportthattheorientationof ground fissuresissubparalleltotheNE-SWriftaxis,suggestingatectoniccontrolontheirgenesis.Fromcursory observationsintheZiwayarea,onecannoticeasignificantnumberofground fissuresthatdonotconformwith theNE-SWorientationoftheriftaxis.Theubiquityof ground fissuresandtherarityofearthquakeactivityinthe Ziwayareamakeitdifficulttodirectlylinktectonicswith theground fissures.Ayalew(2004),basedonamorecomprehensivestudyintheMuletiarea(»100kmsouthof LakeZiway)inthecentralpartoftheEthiopianriftvalley, stronglydisagreedwiththeconnectionbetweenground fissuresandearthquakeactivityandproposedtwomechanismsasprobablecauses:(1)thepossiblepresenceof

Figure1.(a)Ground fissureinLakeZiwayareaand(b)GoogleEarthimageshowingan0.8-km-longcurvilinearground fissure.

unrelievedstressthatcouldspreadtothesurfaceupon beingsubjectedtoheavyrainfall,whichwouldreduce tensilestrength,leadingtotheformationof fi ssures, and(2)rainfall-inducedhydro-compactionofloosely packedalluvialsoils,whichcoulddensifyandbuildtensile stressthatwouldeventuallyproducecracks.Theeffectof groundwaterwithdrawalhasbeenextensivelyinvestigated byShengetal.(2003),whosuggesteddifferentialaquifer deformationduetogroundwaterextractioncausingextensionalstressalongplanesofweaknesssuchasfaultplanes andburiedbedrockedges.IntheZiwayarea,however, thereislittlegroundwaterextractionthatcouldaccountfor ground fissuredevelopment.

Analternativemodelthatmayexplainground fissure developmentintheZiwayareaisthegeomorphologic expressionsofinternalerosion,alsoknownaspipingof sediments.AccordingtoBernatek-JakielandPoesen (2018),internalerosionofsoilscanproducesurfacefeaturessuchassinkholes,blindgullies,andpipingmounds (Figure3).Thesefeatures,aswellasdisappearingstreams, arealsowellevidentinGoogleEarthimageryintheZiway area,suggestingtheinfluenceofinternalerosion(Figure4). Internalerosionofsoils/sediments,alsoknownassuffusion ofinternallyunstablesoils/unconsolidatedsediments,leads totheformationofopenconduitsorpipesthatserveaspassagewaysforturbulentgroundwater flow.Duetoseepage

Figure2.LocationmapofLakeZiwayarea.

forces,internallyunstablesedimentslose finergrainsizes throughporespacescreatedbylargergrains.Continued internalerosionformssubsurfaceconduitsthatcancollapse, initiallyformingsinkholes,whichthencoalescetoform ground fissures.Overtime,ground fissureswidenaswalls collapsetobecomeblindgullies(Figure5).Internalinstabilitydependsonthegrainsizedistributionandthemagnitudeofseepageforces.Itisthereforenecessaryto characterizethepipingpotentialofthemainlyunconsolidatedpyroclasticrocksunderlyingtheZiwayarea

basedonthehighhydraulicgradientandbimodalgrain sizedistribution.

Soil/SedimentSusceptibilitytoInternalErosion

Internalerosionistriggeredwhengroundwaterseepage forcesexceed insitu soil/sedimenteffectivestress,leading totheformationsubsurfacepipes.Seepageforcesarea functionofhydraulicgradientandsaturateddensityofsediments/soils.Thecritical hydraulicgradientvalueatwhich effectivestressofsoil/sedimentbecomeszeroandpiping

Figure4.GoogleEarthimageryshowing(a)disappearingstreams,(b)circularsedimentmound,and(c)blindgullyintheLakeZiwayarea.

Figure3.Geomorphologicfeaturesassociatedwithinternalerosion.

Figure5.Developmentofablindgullyfromcontinuederosionofa subsurfacepipe.Thepipegrowsinsizeduetocontinuederosion,forming initiallyaground fissureandeventuallyaburiedvalley.

occursis »1formostsoilsandsiliciclasticsedimentswith specificgravityof »2.65.Atverticalhydraulicgradients largerthan1,soilparticleswouldlosestrengthandbegin to flowandinternallyerode.Thecriticalhydraulicgradient (ic)iscalculatedbytheequationbelow(Terzaghi,1939):

ic ¼ð!sat !wÞ=!w; (1) where !sat isthesaturatedunitweightofsoil/sediment, and !w istheunitweightofwater.

Internalerosioncanalsooccurathydraulicgradientvalueslessthan1,particularlyforsoils/sedimentsthatlack certaingrainsizefractions,exhibitingabimodalgrain sizedistribution(SkemptonandBrogan,1994).Suchsoils areknownasgapgradedbecausetheyconsistoflarge grainsizeswithoutintermediatesizes,allowing finergrains tobewashedthroughthelargeporespacescreatedbythe coarserfractions.Well-gradedsoilsarethereforemorestable asporespacesareprogressively filledby finerfractions.

Thedegreeofgapgradationcanbeevaluatedbyvisual inspectionofcumulativegrainsizedistributioncurvesto

Figure6.(a)Cumulativegrainsizedistributioncurveshowingcurve forgap-gradedsoilsand(b)KenneyandLau’s(1985)methodof measurementofgap-gradeddistribution.

identifyifagivensoil/sedimentismissingcertaingrain sizeranges.Cumulativegrainsizedistributioncurvesfor gap-gradedsoilstypicallyhave flatsection(s)onthecurve wheregrainsizesaremissing(Figure6),asopposedto theuniformgentleslopesofwell-gradedsoils.Toevaluatethepresenceof flatsectionsindicatinggap-gradeddistribution,KenneyandLau(1985)proposedtheuseofthe H/F value,where H isthedifferenceinpercentofgrains passingbetweenarbitrarygrainsizesofonediameter (1D)and4D,and F isthepercentofgrainscorresponding tograinsize D (Figure6).Several H/F valuescanbecalculated,buttheminimumisusedtoidentifygap-graded

distributions.Soils/sedimentswith H /F ,1areconsideredasgapgradedandthereforesusceptibletointernal erosion.OthergraphicallydeterminedquantitativemeasuresofpipingsusceptibilityincludeTerzaghi’s filtercriteriarule(Terzaghi,1939).Terzaghi’s filtercriteria originallywasdesignedtoselectsoilsthatwouldactas filtersblockingthetransportof finesfromearthdams. Thecriteriawasmodifiedtoassesspipingpotential withinagivensoil/sedimentbyseparatingthegrainsize distributioncurveintocoarseand finegrainsizefraction distributioncurves.Iftheratioofthegrainsizecorrespondingto15percent(d15)passingofcoarsefraction tothegrainsizecorrespondingto85percent(d85)

passingofthe finerfractionisgreaterthan4,then finer grainsizescanbeerodedthroughporespacescreatedby thecoarserfraction,andhenceinternalerosionispossible(SkemptonandBrogan,1994).Additionally,the coefficientofuniformity(CU),whichmeasurestheuniformityofthegrainsizedistribution,isalsousedto characterizeinternalinstability.

CU ¼ d 60=d 10; (2)

where d60 isthegrainsizecorrespondingto60percent passing,and d10 isthegrainsizecorrespondingto10percentpassingonthecumulativegrainsizedistributioncurve.

Figure7.Geologic/geomorphologicmapofLakeZiwayarea.

High CU values(.10)mayindicateawell-gradedgrain sizedistribution,whichisalsotypicalforinternallyunstablesoils/sediments(Kovacs,1981).

Toexplorethepresenceofgeomorphologicalevidence forinternalerosionandsusceptibilityofsedimentstothe processofpiping,theresearchmethodologyincludedGoogle Earthimageryinterpretation, fieldinvestigation,grainsize analysis,andprecipitation/earthquakedatainterpretation.

METHODSANDRESULTS

GoogleEarthImagery

Ground fissuresweremappedbytracingtheirextent onGoogleEarthimagery,whichprovideshigh-resolution imagesthatwereunavailablewhenmostofthepreviousworksonground fissureswerepublished.Itwasalso possibletoconstrainthetimeoftheiropeningusinghistoricalGoogleEarthimagery,whichprovidedhigh-resolution aerialphotographsdatingbacktoearly2010s.Planetscope imageswith3mresolutionfrom www.planet.com were usedtoconstraintimingonsomeground fissures.

OnGoogleEarthimageries,ground fissuresappearas linear/curvilinearopenings,whichcanbecontinuousfor upto2–3kmper fissure.Insomecases,linearvegetation tracksarealsocommonmanifestationsofground fissures. TheirorientationwasdeterminedusingGoogleEarth’s “ruler” tool.TheorientationsofGround fissuresshoweda moredominantN-Sorientationaswellasasigni fi cant E-Wtrend(Figure7).Some fi ssuresappearedtohave polygonalgeometry,andothersshowedbranchingrelationships,whereshorter fi ssuresconnectedperpendicularly withlonger,moreextensive fissures.FromhistoricalGoogleEarthimagery,ground fissuresappearedtogrowover time,insomecasesbyasmuchashundredsofmetersina

fewyears.Thehistoricalimagesalsoshowedthat fissuring startedoutassinkholes,whicheventuallyconnectedto formlinearcracks(Figure8).Itwasdifficulttotellifthe growthwasgradualorsuddenjustfromGoogleEarth imagery,whichdoesnothaveshorttemporalresolution andonlydatesbacktothe2010s,especiallyforthehighresolutionimages.However,usingtheavailablehistorical imagery,the(2016–2017)timeperiodshowedwidespread fissuredevelopment(Figure7).

FieldInvestigation

Imageinterpretationwasfollowedby fieldmappingof thestudyarea ’sgeologyandgroundtruthingofground fissuresandgeomorphologicalfeaturesinterpretedfrom GoogleEarthimagery.During fi eldinvestigation,a geologicmapandrepresentativestratigraphicsections wereproduced(Figure7).Ground fi ssuresinterpreted fromGoogleEarthimageryweregroundchecked.Geologic contactsandgeomorphologicfeaturesweremapped. Samplesfromthepyroclasticandlacustrinedeposits werecollectedforgrainsizecharacterization.

Groundtruthingofimage-interpreted fissuresshowed thatGoogleEarthimagerywasareliabletooltomap fissureswithopeningsasnarrowas ,1mwidth.Field investigationshowedthatground fissurescanbe1–3m widewithnoverticaldisplacementcomponentandoften runforupto2–3km.Theirdepthisverydifficulttoestimate,butmostwereestimatedtobeatleast10–20m deep.ThegeologyoftheZiwayareaismarkedbyextensiveunconsolidatedpyroclasticsedimentsinterlayered withlacustrinedeposits(Figure7).Thelakeandits environsarewithinfault-boundedcalderadepressions, whichprovidedaccommodationspaceforsediment accumulationandformationofthelake.Thepyroclastic depositsappeartobederivedfromnearbyvolcanoes,

Figure8.Sinkholesformingasprecursursofground fissureson(a)ground-levelphotoand(b)GoogleEarthimagery.

observedintheupperlayerofgravelpumice.

andthelacustrinedepositsaretheresultofshrinkingof alargepaleo-lakethatprecededLakeZiway(Gillepsie etal.,1983).Thepyroclasticdepositsconsistofunconsolidatedsedimentsrangingfrom fineashtogravelsizedpumicedeposits.Itispossiblethatsomepyroclasticmaterialmayhavebeenreworkedbywateraction. Weldedtuffwiththickne ssesrangingfrom10to30cm andlacustrinedepositswerealsoobserved.Arepresentativestratigraphyasexposedonstreamsectionsshows, fromtoptobottom,weldedtuff,unconsolidatedsand-sized pyroclasticdeposits,unconsolidatedgravel-sizedpumice, siltyandclayeylacustrinedeposits,andunconsolidated sand-sizedpyroclasticdeposits.

Therearenodrillingdatatoestablishthedepthto bedrock,butitispossiblethatsedimentthicknessmay pinchoutwestwardtothebasinboundary.Theonlybedrockmappedisinthesouthwesterncorner,whererhyoliteoutcropsasafootwallofanormalfault(Figure7). Fieldmappingalsoidentifiedgeomorphologicalfeatures

includingdisappearingstreams,blindgullies,andsedimentmoundsthatareindicativeofaprocessbywhich surfacewaterisconnectedtothesubsurfaceviaopenconduitssimilartokarsttopography(Figure7).Most fissures lieinthemiddleofopen fields,buttheyarealsoobserved developingalongthedrainageditchesadjacenttofoot paths(Figure9).

LaboratoryAnalysis

Sedimentsamplesweresievedinthelaboratoryto characterizesediments’ susceptibilitytointernalerosion. Atthestudyarea,pyroclastic/volcanoclasticandlacustrinesedimentsamplesforgrainsizedistributionanalysiswerecollectedonastreamsectionofMekiRiver (Figure10).Samplesweresievedusingsievesizes22.4, 11.2,8,4.75,4,2,0.85,and0.423mm.Saturateddensityanddrydensitywerealsomeasured.

Aplotofpercentbyweightretainedbyeachsieve showedthattwosievesizes(2mmand4.75mm) retainedhighamountsforthecoarse-grainedpumice (SC2-1A,B,C),indicatingabimodaldistribution,which wasfurtherconfirmedbylow H/ F values(Figure11; Table1).Thecumulativedistributioncurvealsoshowed thatthecoarse-grainedpumicelayerhas flatsections, indicatinggap-gradeddistribution(Figure11).Saturated anddrydensitieswerealsodeterminedinthelaboratory. Thedryandsaturateddensityvaluesofthepumicelayer wereverylow,and,correspondingly,thecalculatedhydraulicgradientswerealsoverylow(Table1).Thepumicelayer

Figure9.SamplingsiteontheMekiRiversection.Pipesare

Figure10.Ground fissureformingalongthedrainageditchofafoot path.

Figure11.(a)Weightpercentageretainedbysievesand(b)cumulativegrainsizedistributioncurvesofsamplesfromtheMekiRiversection.

exhibitedbothgap-gradeddistributionandextremelylow criticalhydraulicgradientvalues,makingithighlysusceptibletointernalerosion(Table1).

EarthquakeandPrecipitationData

Tostudythelinkbetweenground fissuredevelopment andearthquakeandhighrainfallevents,earthquakeevents datingbackto1950andrainfalldatasince1980were obtainedfromtheU.S.GeologicalSurvey(https://earthquake .usgs.gov/earthquakes/map/)andtheClimateResearchUnit GriddedTimeSeries(CRUTS)database(https://www.uea .ac.uk/web/groups-and-centres/climatic-research-unit/ data),respectively.Table2showsearthquakeevents within100kmfromthestudyareasince1950.One

particularearthquake,29.5kmfromLakeZiway, appearstohavebeenthemostspatiallyandtemporally relatedtoground fi ssuresthatoccurredsincethe2010s.

TheCRUTSprecipitationdatabaseisacompilationof interpolatedmonthlyclimateanomalies(Harrisetal., 2020).BasedonCRUTSprecipitationdata,theyear 2016receivedthelargestamountofrainat1,278.5mm, followedby1983(1,215mm),1996(1,207.7mm),and 2021(1,200.6mm)(Figure12).

DISCUSSION

Themechanismofground fi ssuringinthecentralrift valleycanbecomplicated.Accordingtoprevious

Table1. Summaryoflaboratory-determineddensityvaluesanddescriptorsofgrainsizedistribution.

SampleNo.DryDensity(g/cm3)SaturatedDensity(g/cm3)CriticalGradient CUH/Fd15/d85

SC2-0A0.441.070.071.7571

SC2-10.431.020.0242.20.33

SC2-1A0.311.110.1110.90.53.33

SC2-1B0.291.010.1312.90.53.33

SC2-1C0.311.030.03400.443.33

SC2-3a0.421.080.0853.252

SC2-3b0.551.160.16

SC2-51.021.430.4315.20.64

SC2-60.581.160.161.8932

CU ¼ coefficientofuniformity; H/F ¼ H/F valueproposedbyKenneyandLau(1985); d15/d85 ¼ percentofgrainsizepassingspecifiedfraction (15or85percent).

work,themostfavorablecauseisrelatedtoextensional tectonics(Asfaw1982,1998;Yirguetal.,1997).Ayalew ’s(2004)moreextensiveanalysisproposedacombinationofreleaseofelasticstrainduetoheavyrainfall andhydro-compaction.Althoughplausible,itisdifficulttoshowthepresenceofelasticstraininunconsolidatedsediments.Thisstudyshowedthatthe orientationsofground fi ssuresarenotincomplete agreementwiththegeneralE-W–directedextensionof theriftvalley.Animportantobservationmadeinthe fi eldwasthatthetwosidesofground fi ssuresdonot havethesameoutlineasonewouldexpectfroman extensionalcrack,andalsonoverticaloffsettypicalof extensionalfaultswasobserved.Mostground fissure eventsdonotcoincidewithseismicevents,asground fi ssuringismuchmorefrequentthanearthquakeactivities.Outoftheearthquakeeventssince1950,only eightwerewithin100kmradiusfromthestudyarea, andnonewaswithin30km.However,oneevent,from January27,2017,at »37kmfromthestudyarea,was suspectedtohaveplayedaroleinthedevelopmentof thenumerousground fi ssuresthatformedwithinthe 2016 –2017timeframe.Toverifythis,satellite imageswithknownacquisitiondateswereobtained fromPlanetScope( www.planetscope.com ).ThePlanetscopeimage(3mresolution)showninFigure13demonstratesthatoneofthe2016–2017 fissuresexistedin

December2016,whichiswellbeforetheearthquakehad happenedonJanuary27,2017.

Fieldinvestigationofgeomorphologicfeaturesand sedimentgrainsizeanalysisshowedthatpipingcaused byinternalerosionofultralow-densitypyroclastic depositsisthemostplausiblemechanism.Instriking similaritytoareassubjecttointernalerosiondescribed byBernatek-JakielandPoesen(2018),geomorphologic featuressuchasdisappearingstreams,blindgullies,and circularsedimentmoundsarewellevidentinthestudy area.Subsurfaceconduitscanlinksurfacewaterwith groundwater,causingstreamstodisappear.Theconduits couldserveassubsurfacecontinuationsofsurfacedrainageandalsoreappearonthesurface,depositingsedimentsintheformofcircularsedimentmoundsorin somecasesformingcircularponds.

Thepresenceofgeomorphologicfeaturesindicative ofinternalerosionandthepresenceofultralow-density pyroclasticsedimentsareproofthatinternalerosioncan easilybetriggeredatverylowcriticalhydraulicgradientsthataremuchlowerthantheidealcriticalgradient of1.Inadditiontotheirlowdensity,thegravel-sized pumicedepositsshowlargevariationsingrainsize,as shownbygrainsizedistributioncurvesand H/ F values, promotingtheirlikelihoodtointernallyerodeandmakingthemthemostsusceptibletointernalerosion.ObservationsalongtheMekiRiversectionshowedopen

Table2. Earthquakeoccurrencesince1950within100kmdistancefromLakeZiwayarea(https://earthquake.usgs.gov/earthquakes/map ).

ApproximateDistancefrom ZiwayArea(km)EarthquakeMagnitudeDate(month/day/year)EpicenterLocation 36.64.57/4/200632kmEofZiwayTown 375.31/27/201729kmSofZiwayTown 374.98/14/198526kmNEofButajiraTown 64.64.23/31/20159kmWofWerabeTown 674.412/29/201432kmNEofShashemeneTown 775.32/13/199313kmSofWenjiTown 97.651/20/199511kmNofHawassaTown 994.410/17/201114kmNofHosainaTown

Figure12.PrecipitationdatacompiledfromCRUTS(ClimateResearchUnitGriddedTimeSeries):(a)monthlyand(b)annual.

conduitsthatarestrata-boundwithinthe4–5-m-thick gravellypumicelayer(Figure10).Thefactthatground fissuresfollowheavyrainfallisthereforeduetoincreased groundwater flowinsubsurfaceconduits,whichincreases

theerosionofconduits,leadingtoroofcollapse.Although itneedsthoroughinvestigation,theabsenceofground fissuresindrierareasunderlainbysediments(Asfaw,1998) isfurtherproofthatground fissuringiserosioncontrolled.

Figure13.PlanetscopeimagecapturedinDecember2016showing thepresenceofa2016–2017ground fissurebeforetheearthquake eventonJanuary27,2017.

AlongtheMekiRiversection,collapsedpipesobserved ontheriversectioncanbeseeninplanviewinitiating ground fissuredevelopment(Figure14).Linearground fissuresdevelopassubsurfacegroundwaterconduitsor pipesgrowinsizeandeventuallycollapse.Continued erosionwillwidenground fissuresandturntheminto blindgullies.Asimilargeneticmodelwasproposedfor thedevelopmentground fissuresintheLakeNakuruarea ofKenya,whereburiedfaultsserveassitesofsubsurface flow,causingerosionofoverlyingsediments,whichsubsequentlycollapsetoformlinearground fissures(Ngecu andNyambok,2000).ThewidelypublicizedKenyan fissurefrom2018followedheavyrainfall,andnopreceding seismicactivitywasrecorded.Ground fissuringactivity inthe2016–2017timeperiodintheZiwayarea,asconstrainedbyhistoricalGoogleEarthimagery,canbedirectly relatedtotheyear2016,whichrecordedthelargestamount ofrainfallinover40years.Basedonconversationswith locals,thetimingofseveral fissuresintheareaoccurredas overnighteventsinMayandAugustof2016following heavyrainfall.Inadditiontopipingduetointernalerosion, anadditionalplausiblemechanismforthedevelopmentof ground fissuresissurfaceerosion,asobservedalongdrainageditches,causingdeeperosionthateventuallymanifests asground fissures.

Arelevantgeomorphologicalmodelthatbestexplains theformationofdisappearingstreams,blindgullies,and

ground fissureswasdescribedaspseudokarst,which produceskarst-likefeaturesinsoils/sediments(Kempe andHalliday,1997).AccordingtoHaliday(2007),one ofthemechanismsforpseudokarstdevelopmentispipingofpoorlyconsolidatedsediments,formingpiping caves,funnel-shapedsinks,anddryvalleys.Pipingin soilscanbecausedby(1)soilwithverticalpermeability differencesthatcauselateralwatermovement,(2)a steepslopethatmaycausehighhydraulicgradient,and (3)thepresenceofdispersivesoils(Wilsonetal.,2017). Therefore,ground fissuresintheareassurroundingLake Ziwayappeartobecontrolledbythepresenceofpumiceouslayersthatarehighlysusceptibletointernalerosionandpromotepseudokarstformation.Heavyrainfall usuallyprecedesground fissuringandactsasatriggeringfactor.Earthquakeeventscanalsopotentiallyhavea triggeringeffect.Theproposedpresence/patternofsubsurfacegroundwaterconduitsshouldbeinvestigated usinggeophysicalmethodssuchaselectricalresistivity andground-penetratingradar(GPR).

Althoughnotbasedonthoroughinvestigation,areas ofhighground fissureoccurrenceappeartobespatially linkedtocircularcalderabasinsandanearbylake.This spatialassociationistruefortheZiway,Gedemsa,and MuletiareasinthecentralEthiopianriftandLake NakuruinKenya.Thismaybeduetothehigherhydraulicgradientwithinthesebasinsandthefactthatthelakes canprovidelargeaccommodationspacefordeposition ofinternallyerodedsediment.Theimportanceofpumice layersbeingsusceptibletointernalerosionishighlighted inthisstudy.Interestingly,studiesintheMuletiareaof EthiopiaandLakeNakuruofKenyareportthepresence ofthickdepositsofpumiceinareasaffectedbyground fissures(NgecuandNyambok,2000;Ayalew,2004).As forpreferredlocations/alignmentsofground fissures, furtherresearchshouldbedonetoounderstandifthere arerolesplayedby(1)buriedfaults/fracturezones, whichmayplayapassiverolebyservingaspreferential groundwater flowconduits,promotingthedevelopment ofsubsurfacepipes,and(2)oldburiedchannels,which mayserveasconduitsofconcentratedgroundwater flow, promotinginternalerosion.Extensional fi ssuresin bedrock,asdescribedinAcocellaetal.(2003),may beunderlyingtheunconsolidatedsediments,providing sitesofconcentratedgroundwater flow,andpotentially explainingtheground fissuresthatareorientedconcordanttotheE-Wriftextension.

Theresultsandconclusionsofthisworkshouldbe limitedtoground fi ssuresformingintheriftvalley underlainbylow-densityvolcanicsedimentssuchas pumice.Itwouldbeanover-simplifi cationtoextend theroleofinternalerosiontoallground fi ssuring observedintheMER.Forinfrastructureengineers, ground fi ssure –relatedstudiesshouldfocusonhowto

Figure14.(a)PipecollapseontheMekiRiversection.(b)Traceofthecollapseinplanviewshowinginitiationofground fissuredevelopment.

predictground fi ssuringandidentifyingengineering solutionstoslowtheprocessandprotectcivilstructures.Amodelrelatingground fi ssuringeventswiththe presenceofinternallyerodiblesedimentsandrainfall amountsshouldbeformulated.Suitablegeophysical methodstomapsubsurfaceconduitsshouldbeidentifi edtoproactivelydesignengineeringsolutionstostop orslowinternalerosion.Possibleremediesmayinclude increasingrunoffwhiledecreasingin fi ltration,and designinggeotechnical fi ltermechanismsatsiteswhere ground fi ssuringislikelytoaffectinfrastructure.

CONCLUSIONS

Thefollowingconclusionscanbedrawn:

(1)Ground fissuresprimarilyaffectingsedimentsin theLakeZiwayareadonotappeartohaveany significantassociationwithextensionaltectonics andassociatedseismicactivity.

(2)Astrongargumentcanbemadethatground fissuresaffectingsedimentarydepositsintheZiway

areaaretheresultsofcollapsingsubsurfacegroundwaterconduits.Thesubsurfacegroundconduits appeartoresultfrominternalerosionofgravelly pumicelayers,whicharegapgradedandhaveultralowdensity.

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KEMPE ,S. AND HALLIDAY,W.R.,1997,Reportonthediscussionon pseudokarst.In Proceedingsofthe12thInternationalCongress ofSpeleology, Volume6:InternationalUnionofSpeleology,La Chaux-de-Fonds,Switzerland.

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PENG,J.;QIAO,J.;SUN,X.;LU,Q.;ZHENG,J.;MENG,Z.; AND ZHAO,J., 2020,Distributionandgenerativemechanismsofground fissures inChina: JournalofAsianEarthSciences,Vol.191,pp.104–218.

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TheSeptember18,2018,DebrisSlidein Warrensville,NC:ALandslideResponseCaseStudy

RICHARDM.WOOTEN

NorthCarolinaGeologicalSurvey(ret.),24TwoOaksDrive,Fletcher,NC28732

BARTL.CATTANACH G.NICHOLASBOZDOG

NorthCarolinaGeologicalSurvey,2090U.S.Highway70,Swannanoa,NC28778

SIERRAJ.ISARD

CityofAsheville,WaterResources,70CourtPlaza,Asheville,NC28801

KeyTerms: Landslide,BlueRidgeMountains,North Carolina,UncrewedAerialSystems

ABSTRACT

Adebrisslideandsubsequent fireonSeptember 18,2018,destroyedabankbuildinganddamaged propertyinWarrensville,NorthCarolina.Theslide pushedapropanetankandanelectricalgenerator againstthebank.Inthatcollision,anelectricalarc causedthepropanetoexplode,whichproduceda fireball.Employeesinthebanknarrowlyescaped withminorinjuries,andslidedebrisburiedpartof anunoccupieddrive-throughlane.NorthCarolina GeologicalSurveygeologistsrespondedtoarequest fromtheAsheCountyEmergencyManagerand investigatedtheslideareatoassessslopestabilityfor publicsafety.Aidedbyuncrewedaerialsystem imagery,wedocumentedslopeanddebrisslidefeaturesandgeologicmaterialsandconstructedageologiccrosssectionthroughthe1,400m2 debrisslide. Weconcludedthatrecordabove-normalprecipitationfollowedbyrainfallfromtheremnantsof HurricaneFlorenceduringSeptember15–18,2018, inducedtheslide,andthatgeologicconditionsand hillsidemodificationsoverpriordecadeshadpredisposedtheslopetofailure.Colluvialdepositsderived fromshearedamphibolitebedrock,andcurvedtrees withinandadjacenttothe2018slideindicatedpreexistingmarginallystablehillslopeconditions.The 2018slideincludedtheareaofa2000slidethat involvedacutslopeconstructedduringearliersite development.Stabilityanalysesshowedthatthe describedconditionsledtoanunstableslopeatthe timeofthe2018slide.Asepticleach fieldinvolvedin

*Correspondingauthoremail: Richard.M.Wooten@gmail.com

the2018slidewasdamaged;however,itisuncertain ifitwasacausalfactorintheslide.

INTRODUCTION

Landslidesarethemostcommongeohazardinthe southernAppalachianMountainsofwesternNorthCarolina. Debris flowsanddebrisslides,dominantamonglandslide processeshere,aretriggeredbyrainfallonsteep,soilmantledslopesoftheBlueRidgePhysiographicProvince (Wootenetal.,2016,andreferencestherein).Inaddition tocounty-widelandslidehazardmapping,respondingto landslideeventsisaprimaryfunctionoftheNorthCarolina GeologicalSurvey(NCGS)geohazardsteam.Landslide dataandcounty-widelandslidehazardmapsareavailable onaninteractiveWebmap(http://mapviewer.landslidesncgs .org),whichissupplementedbyalandslidehazardsWebsite(http://landslidesncgs.org),bothdevelopedinpartnershipwiththeNationalEnvironmentalModelingandAnalysis Center(NEMAC)attheUniversityofNorthCarolina–Asheville.Asofthiswriting,county-widelandslidemapping forAsheCountyispending,andcurrentlytwodocumented landslidesareshownontheaforementionedNCGSlandslideWebmapviewer.OncelandslidemappingiscompletedforAsheCounty,thenumberofdocumentedlandslides therewillundoubtedlyincrease.

Since1990,theNCGShasrespondedtoeventsinvolvingover360landslidesintheBlueRidgeMountainsof NC(Figure1),includingthoseresultinginfatalities,injuries,destroyedanddamagedhomes,andthreatstoregional infrastructure(Baueretal.,2012;Wootenetal.,2017, 2022).Responsestotheserequestsforassistanceonlandslideeventsfromlocal,state,andfederalgovernmentagencies,thepublic,andconsultantshaveentailedcommunicating with firstresponders,thepublic,andthemedia,andgiving expertwitnesstestimonyincasesinvolvinglitigation.Aprimarylandslideresponseobjectiveistoprovidestakeholders

Figure1.MapsshowingthelocationsofWarrensville,NC,andNCGSlandslideresponsesfrom1990toJuly2023bycategory.Landslide responsecategories:emergency responsetimecritical,involveslossoflifeorseriousinjury,severedamage,andposesanimmediatethreatto publicsafetyorproperty; urgent responsetimeimportant,andposesapotentialthreattopublicsafetyorproperty; routine responsetimenot criticalandposesalowpotentialthreattopublicsafetyorproperty,orresponsemaybeforinformationalpurposesonly.Insetmapbottomright: longitudeandlatitudecoordinatesforthe2018debrisslidemainscarp;mapcontourinterval ¼ 50m. Wooten,Cattanach,Bozdog,andIsard

withtimely,unbiased,scientificinformationtohelpprotect publicsafetyandproperty.Akeyresponsefunctionistohelp increasesituationalawarenessforemergencyresponders. Post-landslideeffortsincludeassistancewithdamage assessments,contingencyplansifunstableslopesremain athreat(e.g.,Wittetal.,2012),anddocumentationtosupportrecoveryandmitigationfunding.Asinlandslide responseprogramsinotheragencies(DeGraffetal.,2007; Reidetal.,2021),anoverarchingobjectiveistoreducethe riskfrompast,ongoing,andfuturelandslides.Importantly, theresponsesandsubsequentinvestigationsbytheNCGS areneitherintendedtobenorcarriedoutasforensicinvestigationsforthepurposesofestablishingresponsibilityor liabilityofthepartiesinvolved.TheNCGSisnotaregulatoryagency;therefore,guidanceandrecommendations offereddonotcomewithanyregulatoryrequirements.

OnSeptember19,2018,NCGSgeologistsconducted areconnaissance fieldinvestigationoftheslopefailure thatoccurredonTuesday,September18,2018,inWarrensville,NC.Thedebrisslide(asdefinedbyCrudenandVarnes,1996)andsubsequent firedestroyedtheLifeStore Bank,locatedat4951N.C.Highway88West,Warrensville,AsheCounty,NC,anddamagedproperties adjacenttothebank.Theheadscarpofthedebrisslide islocatedatapproximatelylat36.45858°Nandlong 81.51667°W(Figure1).

TheAsheCountyEmergencyManagementDepartment reportedthattheemergencycalltotheir911dispatchcentercameinat3:03p.m.EDTonTuesday,September18, 2018.Firstrespondersdeterminedthatalandslidepusheda propanetankagainstthebackofthebuilding.Anelectrical arcfromanauxiliarygeneratorintheimpactareaignited

Figure2.FiredestroyedtheLifeStoreBankafterthedebrisslidestruckthebuilding.The911callreportingthe firewasat3:03p.m.(15:03 hours)onTuesday,September18,2018.PhotoisfromavideoexcerptprovidedbyAsheCountyEmergencyManagementthatwastakenbya passerbywhowitnessedthe fireshortlyafterthedebrisslideoccurred.Viewlookingsouth.

thepropanefromtherupturedtank,whichproducedthe fireballobservedbywitnesses(Figure2).Bankemployees insidethefrontofthebanknarrowlyescapedsafely,with onepersonsustainingminorburns.Awitnesstotheslide whowasoutsideattheadjacentpropertytothesoutheast reportedthattherewasa “whooshingnoise” whentheslide movedrapidly,pushedametaloutbuildingoffitsfoundation,andburiedthedrive-throughlanebehindthebank (Figures3and4).

Thepurposeoftheinvestigation,asrequestedbythe AsheCountyEmergencyManagementDepartment,was toassessthestabilityoftheslopewithrespecttopublic safetynearthedebrisslideandadjacentproperties.Our verbalcommunicationswithstakeholders,andsubsequent reporttoAsheCountyEmergencyManagement(Wooten etal.,2018),aimedtoprovidegeneralguidance,convey theobservationsand findingsoftheinvestigation,anddocumentthecircumstancesandconditionsleadingtotheslope failure,sothatsimilarsituationscanberecognizedand avoidedinthefuture.Inthispaper,wesummarizethe2018 investigationandreport,andwediscusstheWarrensville debrisslideinthecontextoftheperiodofrecordrainfall andincreasedlandslideactivityduring2018–2020inwestern NorthCarolina.

STUDYMETHODS

OnSeptember19,2018,weconductedareconnaissancelevelinvestigationofslopeanddebrisslidefeaturesand geologicmaterialsexposedinthevicinity,andweconstructeda field-developedgeologiccrosssectionthrough thedebrisslide.Fieldclassificationsofsoilandrock

materialsandstationlocationswererecordedandgeolocatedusingglobalpositioningsystem(GPS) –equipped fi eldcomputers.Uncrewedaerialsystem(UAS)imageryofthedebrisslidetakenbytheNorthCarolinaCooperativeExtensiononSeptember19,2018,was georegisteredinageographicinformationsystem(GIS) andusedtohelpmapthedebrisslideandrelatedfeatures intheofficephaseoftheinvestigation(Figure5).Observationsweremadeofpre-slidesiteconditionsusing1993, 1998,2005,2010,and2014(Figure6A)vintagesof orthophotography.Inour2018study,weassessedsite topographyusing “ legacy” lightdetectionandranging (LiDAR)data(Figure6A)collectedin2004at6mpixel resolution(NCDPS,2005).In2019,quality-level1(QL1) LiDARdatacollectedforNorthCarolinain2017became available(NCDPS,2020).The2004and2017datasets werereducedtobare-earthdigitalterrainmodels(DTMs) with6mand0.5mpixelresolution,respectively.The 2004LiDARdatausedinthe2018reportweresupplementedbythehigher-resolution2017LiDARdata,which showedslopeconditionspriortothe2018debrisslide. Thiscasestudyalsoincludesastabilityanalysis.

SITEBACKGROUND

DevelopmentHistory

Astructureatthelocationofthebankbuildingisnot shownonthe1966U.S.GeologicalSurvey1:24,000scale topographicmapoftheWarrensville7.5minutequadrangle;however,abuildingatthebanklocationisvisiblein 1993andlaterorthophotography.Basedonthetopography shownontheWarrensville7.5minutequadrangle,andthe

Wooten,Cattanach,Bozdog,andIsard

Figure3.Toeofthedebrisslidedepositwhereitovertoppedtherock andwirebasketretainingwall,buriedthedrive-throughlane,and pushedthepropanetankandthegeneratorintothebackofthebank building.Slidedebriscoversthepropanetankinthisphotograph. Viewlookingsoutheast.September18,2018,photocourtesyofAshe CountyEmergencyManagement.

presentslopeconfigurationsatthesiteandadjacentproperties,sitegradingforthebuildingandthedrive-through lanelikelyresultedincutslopesexcavatedintothehillside, seenasthesteepslopesimmediatelybehindthebuilding sitesinFigure7.Asepticleach fi eldwithin fi ltration benchesandpumpstationsbuilt(dateunknown)into thehillsideforusebytheadjacentrestaurantareclearly visiblein2017LiDARdata(Figure7).Effluentfromthe adjacentrestaurantwaspumpeduphilltotheleach field. Thelayoutsandrespectivedatesofsepticsystemsatthe sitecouldnotbereliablydeterminedfromsepticpermits providedbyAsheCounty.

Thepropertyownerrecalledthattheslopebehindthe bankbuildinghadfailedin2000,afterwhichtheexisting rock-filledwirebasketretainingwallwasconstructedat thetoeoftheslope.Thesteepslope(remnantscarp)interpretedtocoincidewiththesourceareaofthe2000slope failureisvisiblein2005orthophotographyand2017 LiDARdata(Figure7).Theareaofthe2018debrisslide appearstoincludetheareaofthe2000slopefailure.A propanetanklocatedontheuphillsideoftheretaining

Figure4.Geologistsstandonthetoeofthedebrisslidedepositnear thesouthcornerofthebankbuildingwhereitburiedthedrivethroughlaneanddamagedthetellerwindow.Groundwaterseepage fromtheslidedeposit flowsontothepavement.Viewslookingsouthwest.September18,2018,NCGSphoto.

wall,anddownslopefromtheinferredareaofthe2000 failure,isvisiblein2005,2010,and2014orthophotography(Figure6A).Thepavedareabetweentheretaining wallandthebankbuildingservedasadrive-throughlane forthetellerwindowonthesoutheastsideofthebank.

GeologicandTopographicSetting

WarrensvilleislocatedintheBlueRidgePhysiographic ProvinceandwithintheNeoproterozoictoMiddleOrdovicianAsheMetamorphicSuite,consistingofthicksequences ofmetaclasticrocksinterspersedwithamphibolite(metamorphosedbasaltic flowsordikes)andultramaficbodies (possibleophioliticcrust).AsheMetamorphicSuiterocks werecomplexlydeformedandmetamorphosed(upper amphibolitefacies)duringPaleozoicorogenesis(Misra andConte,1991;RaymondandAbbott,1997).Amphibolitewastheonlyrocktypeobservedinthevicinityofthe debrisslide.Thedebrisslideoccurredonnortheast-facing slopeswherethenortheasternextentofThreeTopMountain hasbeendowncutbyBuffaloCreek,atributarytotheNorth ForkNewRiver(Figure1).PeakelevationatthenortheasternextentofThreeTopMountainisapproximately 3,180ft( »969m),andtheelevationnearthetoeofthe slopeatthedebrisslideisapproximately2,700ft ( »823m),resultingin480ft(146m)oftopographic relief.Topographiccontoursindicatethatintermittent stream fl owisprimarilywithinnortheast-drainingdraws tothenorthwestandsoutheastofthedebrisslideandthe septicdrain field(Figure6A).Hillslopesontheinterfluve betweenthesedrainagesareslightlyconvexabovethe debrisslideandgenerallyplanarinthefootslopearea wherethedebrisslideoccurred.

Figure5.AerialviewoftheSeptember18,2018,debrisslideandthedamagedbankbuildinginageoregisteredSeptember19,2018,UASimage. Yellowpolygon ¼ debrisslideoutline;whitearrow ¼ generaldirectionofslidemovement;blue-greenline ¼ locationofcrosssectionA-A 0 ; A15andA23 ¼ cross-sectionreferencestationlocations;blue-greentriangles ¼ otherSeptember19,2018,datapoints.Mappedfeatureswere geospatiallylocatedrelativetoGPSlocationsof fieldpoints,2014orthophotography,andgeoregisteredSeptember18,2018,UASimagery.UAS imageiscourtesyoftheNorthCarolinaCooperativeExtension.NumbersinparenthesesarelocationsforFigures9–11,respectively.Notethe crewwithabuckettruckrepairingutilitylinesdownedbytheslide.

CASESTUDY

DebrisSlideDamage

Thepurposeoftheinvestigationdidnotincludea completedamageassessment,anditwasunsafetoenterthe buildingorcloselyinspectitsexteriorforsignsofdamage fromthedebrisslide.Weattributedthefollowingdamageto thebuildingexteriortobefromthedebrisslideimpact (excludingthatcausedby fire):(1)Theslidedebrisburied aportionofthedrive-throughlanebehindthebanktoa depthofapproximately6ft(1.8m)anddamagedthedrivethroughtellerwindow;(2)theslidedebrispushedthegeneratorthroughalowersectionoftherearwall;(3)slideimpact damagedthemetalstairwayinthebackofthebuilding;and (4)andtreesdownedbythedebrisslidepossiblydamaged thebank’sroof.Theseobservationsindicatedthatdirect damagetothebuildingfromthedebrisslideimpactwas likelyreparable;however,theensuing fi reessentially destroyedthebankbuilding.

Theslidedisplacedanddestroyedportionsofthesepticleach fieldbenches(Figures7,8,9,and10).Damaged

remnantsofthesepticsystemcomponentswereobservedin themainscarp(Figure9)andotherareasaffectedbythe debrisslide.Otherdamagefromthedebrisslideincludedthe following:(1)Theslidedebrisovertoppedtheretainingwall behindthebank,whereitseverelydamagedtheupper rock-filledwirebasket;(2)slidedebrisadvancedpastthe southeastsideofthebankbuilding,coveringaportion ofthedrive-throughlaneandparkinglot;(3)utilitypoles weredamaged,andutilitylinesweredowned;(4)ametal outbuildingonanadjacentpropertywaspushedoffits foundationanddamaged;and(5)runoffandgroundwaterdrainagefromtheslideresultedinon-site sedimentation.

SiteGeology

UnconsolidatedQuaternaryslopedepositsofunknown agescomprisethecolluvialdepositsfrompastslope movementactivityobservedinandaroundthedebris slide(Figures6A,8,and10),likelyoriginatingfrom soilcreep,debrisslides,anddebris- fl owprocesses.In general,thesedepositsconsistofgravel-toboulder-sized

Figure6.(A)Aerialviewoftheareain2014orthophotographypriortothe2018debrisslidewithbedrockstructuraldataatoutcroplocations. Yellowpolygon ¼ 2018debrisslideoutline.LocationsBandCcorrespondtooutcroplocationsin figurepartsBandC,respectively. TopographiccontourswerederivedfromaLiDARDTMwith6mpixelresolution.Contourinterval ¼ 20ft(6.1m).(B)Amphibolitebedrock outcrop.ArrowpointstoNW-strikingfractureface.Viewlookingsouthwest.(C)Subangularamphibolitebouldersinafootslopedebrisdeposit overlietheamphiboliteoutcropatphotoright.Dashedwhitelineapproximatescontactofamphiboliteoutcropandthedebrisdeposit.Thedebris deposithereconsistsofamphiboliteboulders,cobbles,andgravelandsoilmatrixoriginatingfrompastprehistoric(?)slopemovements.View lookingsouthwest.September19,2018,NCGSphotos.

Wooten,Cattanach,Bozdog,andIsard

Figure7.Groundslopeoftheareasuperimposedontoashadedreliefmapderivedfroma0.5m2017LiDARDTMshowingpre-2018slideslope conditions.Yellowarrows ¼ generalmovementdirectionsofthe2000and2018slopefailures.The2017LiDARDTMbecameavailablein 2019,afterthe2018debrisslideinvestigationandresponse.

amphiboliterockfragmentsinvariousweatheringstates intermixedwithsoilmatrixrangingfromclayey,sandysilt tosiltysand.Proportionsofsoilmatrixrelativetorockfragmentsvary,resultinginmatrix-supportedandclast-supported soiltextures.Briefdescriptionsofthethreecolluvialsoilunits encounteredintheimmediatevicinityofthedebrisslideare givenongeologiccrosssectionA-A0 (Figure8).Exposures ofin-placebedrockwerenotobservedwithintheperimeter

oftheslide,indicatingthattheslidemassdidnotinvolve,or expose,in-placebedrock.Althoughtheagesofthesedeposits areunknown,thebrowntoyellowMunsellhueand claycontentofthelowercolluvialunitsuggestitmay beasoldasLatetoMiddlePleistocene(MillsandAllison, 1995;Mills,2005)andanalogoustosimilardepositsderived fromamphiboliteoftheAsheMetamorphicSuiteinneighboringWataugaCounty(Mills,1998).

Figure8.Schematicgeologiccross-sectionA-A 0 throughtheSeptember18,2018,debrisslidemeasuredonSeptember19,2018.SeeFigures5, 7,9,and10forcross-sectionlocations.Survey:clothtape,Bruntoncompass,clinometer,GPS-equipped fieldcomputer;supplementedbya LiDARDTMwith6mpixelresolution.Note:Thepre-2018failuregroundsurfacederivedfromthe2017LiDAR(seeFigure9)updatesthepre2018failuregroundsurfaceestimatedinthe fieldasshownintheoriginal2018report.Alldistances,elevations,andlocationsofgeologicunits areapproximate.SoilclassificationisingeneralaccordancewithAmericanSocietyforTestingandMaterials(2002).Rockclastweathering descriptors:STS ¼ stainedstate;PDS ¼ partlydecomposedstate;CDS ¼ completelydecomposedstate(fromWilliamson,1984).

TheNationalResourcesConservationService(NRCS) SoilSurveyGeographicDatabase(SSURGO)(NRCS, 2023)showsEvardstonyloam(Esf)withintheEvardsoil seriesmappedinthesourceareaofthedebrisslide.The overallparentmaterialorigingivenfortheEvardseriesis felsictomafic,igneous,andhigh-grademetamorphicrocks, suchasmicagneiss,hornblendegneiss,andamphibolite.Amphiboliteistheprimaryparentmaterialforthe soilobservedintheareaofthedebrisslide.

Bedrockoutcropsobservedbothupslopeandalong aroadcuttothesoutheastofthedebrisslideconsist of fi nelylayered,shearedamphiboliteoftheAshe MetamorphicSuite(Figure7).Weatheringproducts oftheamphibolitebedrockarethesourceofthecolluvialsoildepositsdiscusseda bove.Theorientation ofthestrikeoflayering(foliationandmyloniticfoliation)isgenerallyNEwithdipsthatrangefrom43° to72°towardtheSE.Withdipdirectionsnearly90° fromtheNEdirectionofslidemovement,foliationis notfavorablyorientedtoactasslope-parallelslide planes.Joints(fractures)withintherocksoccurpredominantlyintwosets.OnesetstrikesWSWwith dipsaround30°towardtheNW,anditisunfavorably orientedtofacilitateNE-directedslidemovement.A secondsetstrikesNNWanddipsaround80°toward theNE,anorientationfavorableforrockblockstodetach andaccumulateinthedownslopedebrisdepositspresent atthesite.

Wedidnotobservesurfacewater flowingintothe landslidearea.Smallerosiongulliesfrompriorsurface runoffwerepresent,butnotinareasthatappearedto

draindirectlyintothedebrisslide.Thepropertyowner hadreportedthatarockcliffupslopefromthedebris slide “hadwaterconstantlyrunningoutofit.” Wedid notobservewater flowingfromarockslope,but field traversesmaynothavecoveredthatlocation.

DebrisSlideCharacteristics

FieldmappingandgeoregisteredUASimageryindicate theareaofthedebrisslideencompassesabout15,000ft2 (»1,400m2).Thein-placevolumeofearthmaterialon thehillsidebeforetheslidemoved,andsubsequently involvedintheslide,wasestimatedtobeontheorderof 3,700yd3 to4,200yd3 (2,829–3,211m3).Themaximum lengthoftheslidefromthemainscarptothetoeofthe slidedebrisdepositwas »175ft(53m),withamaximum widthof »105ft(32m).Anominalgroundslopeof approximately30°sestimatedforthepre-failuredebris slideuppersourcearea.Thegroundsurfaceprofilederived fromthe2017LiDARDTMupdatedthepre-2018failure groundsurfaceestimatedinthe fieldasshowninthe2018 report(Figure8).The2017groundprofilerevealedanominalgroundslopeofabout45°inthe2000slidesource area(Figure10).

Thedisplacedmassofthedebrisslide(slidedebris) wasmadeupofintermixedcolluvialslopedepositsfrom pastslopemovementactivity,designatedhereastheupper, middle,andlowercolluvialunitsoncrosssectionA-A 0 (Figures8 –10).Themiddlecolluviumunitappeared tocomprisethebulkofthedisplacedmass.Aportion ofthesliderupturesurfacecoincidedroughlywiththe

Figure9.Viewofthemainscarpofthedebrisslidealongcross-sectionA-A 0 .Inset:Viewoftheupper(UCV)andmiddlecolluvium (MCV)unitsexposedinthemainscarp.Weatheredamphibolite clastsprojectfromthescarpsurface.Yellowarrowpointstodamagedremnantsofthesepticsysteminfiltrationchamber.Measuring tapeis0.5in.(1.3cm)wideforscale.Viewlookingsouthwest. September19,2018.NCGSphotos.Location9inFigure5.

contactbetweenthemiddleanduppercolluvialunits.

Curvedtreesonthedisplacedslidemassprovidedevidence thatsomeoftheslopewas,orhadbeen,undergoingslow movement(creep)priortotheestimatedrapidtoveryrapid movement(»ft/stom/srates)onSeptember18(Figure11).

Curvedtreespresentontheslopesadjacenttothe2018slide alsoindicatedpastand/orongoingsoilcreep;however, obviousscarpsortensioncracksbeyondtheimmediate perimeterofthe2018slideareawerenotobserved.Indicationsofunequivocalprecursorscarpsorotherground-displacementfeatures(e.g.,displacedsepticinfi ltration benches)werenotevidentonthe2017LiDARDTM upslopefromtheinferredsourceareaofthe2000slope failure(Figure7).

Moistsoilwaspresentintheportionsofthemain scarpthatexposedremnantsofthesepticleach field.We didnotdetectactiveseepageorsepticodorsinthese areas,althoughtheabsenceofodoralonedoesnotprecludetheinfluenceofsepticwaterontheslide.Seriesof groundwaterdischargeareaswereactiveinazonenear

Figure10.Viewlookingdownslopealongcross-sectionA-A 0 toward thedestroyedbankbuilding.Geologistisstandingonslidedebris. Viewlookingnortheast.September19,2018,NCGSphoto.Location 10inFigure5.

themiddleofthedebrisslidedownslopefromstation A22oncrosssectionA-A 0 (Figure8).Seepagealsocontinuedfromthetoeofthedebrisslidedepositinthearea excavatedtoremovethepropanetank,andfromthetoe oftheslide(Figure4).Septicodorswerenotdetectedin theseseepageareas.

AntecedentRainfall

TheNationalWeatherService,Blacksburg,VA,Weather ForecastOfficeprovidedrainfalldatatodocumenttheprecipitationconditionspriortotheSeptember18,2018,debris slide.Recordabove-normalrainfallbeganinMay2018with asharpincreaseinAugustthatresultedintheareabeing 13.3in.(»334mm)abovenormalwithelevatedmoisture conditionspriortothepassageoftheremnantsofHurricane FlorenceonSeptember15–18,2018(Figure12).

SoutheastofWarrensville »260mi(»418km),HurricaneFlorenceproducedhistoricamountsofrainfallofup to30to50in.(762–1,270mm)alongcoastalandinland areasofNorthandSouthCarolina(Figure13),resulting inrecordpeakstreamflowsandcatastrophic floodingin theseareas(Feasteretal.,2018).Hadtheheaviestrainfall

Wooten,Cattanach,Bozdog,andIsard

Figure11.Geologistsstandonslidedepositblockscomprisedofsoil androotmatnearthetoeofthedebrisslide.Curvedtrees(reddashed lines)transportedbythedebrisslideareevidenceofslopecreepprior totheSeptember18,2018,debrisslide.Doublewhitearrowindicates verticaldisplacementalongtheright(southeast)lateralscarpofthe debrisslide.Viewlookingsoutheast,downslopetotheleft. September19,2018,NCGSphoto.Location11inFigure5.

impactedwesternNorthCarolina,theremostcertainly wouldhavebeenhistoriclevelsof floodingandlandslides intheBlueRidgeMountains,perhapsevensurpassingthe levelsofdamagefromthe1916and1940tropicalcyclones (Wootenetal.,2016).

The3dayrainfalltotalsfromHurricaneFlorenceand latershowersendingTuesdaymorningonSeptember18, 2018,recordedatthreeraingaugestationsnearWarrensville(Figure14)areasfollows:5.0in.(127mm)atClifton,NC;4.64in.(»118mm)atLansing,NC;and8.09 in.(»206mm)attheAsheCountyAirport(KGEV),inJefferson,NC.ThebulkoftherainfallfromFlorencerecorded atthesestationsbeganthemorningofSeptember15and diminishedintheearlyhoursofSeptember17,subsiding approximately33to39hourspriortothe911callreporting thedebrisslideat3:03p.m.(15:03hours)onSeptember18. LocallyheavyshowerspassedovertheWarrensvilleareaon theeveningofSeptember17,withrainfallextendinginto theearlymorninghoursofSeptember18.Althoughrainfall hadceased »9to10.5hoursbeforethedebrisslide(Figure 15),groundwaterdischargecontinuedfromthetoeofthe debrisslide(Figure4)andatseverallocationsat,ornear, theexposedrupturesurfaceoftheslide(Figure8),indicatingpost-stormdrainageoftheslope.

FindingsandGuidancetoStakeholders

OurreporttotheAsheCountyEmergencyManagement Department(Wootenetal.,2018)madethefollowinggeneral findings.(1)TheSeptember18,2018,debrisslidewas likelytriggered(induced)byrainfallfromtheremnantsof HurricaneFlorencecoupledwiththeabove-normalrainfall forAugust.(2)Thepropanetankwasinavulnerable

Figure12.Chartofrainfallaccumulation(in./mm)anddeparturefromnormalforJanuary1toSeptember30,2018,showingrecordrainfallfor thistimeintervalatraingaugeJefferson2E,NC.Recordedabove-normalvaluesareshownforSeptember13,2018,priortorainfallfrom HurricaneFlorence,andforSeptember18,2018,withrainfallfromHurricaneFlorencerainfallpriortothedebrisslide.RefertoFigure14for raingaugestationlocation.Datasource:NationalOceanicandAtmosphericAdministration(NOAA)OnlineWeatherData(NOWData), https:// www.weather.gov/wrh/Climate?wfo¼rnk ,accessedSeptember6,2023.

Figure13.ObservedprecipitationtotalsfromHurricaneFlorenceendingTuesdayat2:00p.m.EDT,September18,2018.Multi-sensorrainfall estimateswereproducedbytheNOAA-NWS,EasternRegionHeadquarters.

locationupslopeoftheretainingwallbetweenanunstable slopeandthebuilding.Hadthepropanetanknotbeen involvedinthedebrisslide,thendamagetothebuilding fromthedebrisslideprobablywouldhavebeenreparable. (3)Thescarpareasoftheslideareover-steepenedandare consideredunstable;therefore,thescarpsandslopesadjacenttothemaresubjecttofutureslopefailures.(4)The debrisslidedepositconsistsofloose,unstablematerial, especiallywhenthewatercontentofthedepositiselevatedsuchasfromgroundwaterdischarge,rainfallinfiltration,andsurfacewaterrunofffromupslopeareas.(5)The fieldreconnaissancedidnotrevealanysignsofimpending slopeinstabilityabovethebuildingsontheadjacentproperties.Curvedtreesobservedontheslopesadjacenttothe slide,however,indicatedpastorongoingsoilcreep.Slope modificationstotheseareascouldincreasethepotential forfutureslopefailures.

Basedontheinvestigationand findings,the2018report madethefollowingrecommendations.(Notethatagency policyrequiresthatNCGSgeologistsdonotmakespecific engineeringrecommendationstostabilizeormitigateunstableslopes;inthesecases,stakeholdersarereferredtoqualifiedgeotechnicalengineers.)(1)Involvementofaqualified engineertoadvisedemolitionofthebankbuildingisrecommended.Loadsfromtheslidedebristhatremainagainstthe buildingcouldresultinshiftingandaffecthowthebuilding collapsesduringdemolition.Workingbetweentheslide debrisandthebuildingcouldbehazardous,especiallyifthe slidematerialiswet.(2)Theslopesaffectedbythedebris slideareover-steepenedandunstableandcouldposeathreat topublicsafety,particularlyduringorafterperiodsofheavy

rainfall.Theslopesshouldbestabilized,orthehazard shouldbemitigatedinsomeway,undertheguidanceofa qualifiedgeotechnicalengineer.(3)Ahazardmitigation effortshouldinvolveaslopestabilityanalysisbyaqualifiedgeotechnicalengineer.Iftheuseoftheleach fieldfor septicsystemistobecontinued,thenthehydrologicand slopemodificationeffectsofthesepticsystemshouldbe incorporatedintothestabilityanalysis.(4)Continueduse oftheexistingleach fieldforthesepticsystemshouldbe certifiedbyaqualifiedenvironmentalhealthprofessional. (5)Slopesintheareaofthedebrisslideshouldbemonitoredforanyindicationsoffurthermovementorinstability.

SLOPESTABILITYANALYSIS

AlthoughtheNCGSuseslimitequilibrium–basedslope stabilitymodelsforcounty-widedebris-flowsusceptibility modeling(Baueretal.,2012;Wootenetal.,2016),sitespecificslopestabilityanalysesarebeyondthescopeand resourcesoftheNCGS’slandslideresponsefunctions. Thiscasestudy,however,presentsanopportunityforaretrospectivestabilityanalysis.Intheabsenceofsoiltestdata andgroundwatermonitoringdataforthesite,ouranalysis aimstoconstrainsiteparametersatthetimeoffailure andtoprovideaconceptualframeworkforsimilarconditionsinhillslopesunderlainbycolluvium.

Weevaluatedthestabilityoftheslopeusingacomputer programthatutilizesthemethodsoflimitequilibriumanalysespresentedbyMorgensternandPrice(1965)andsimplifiedversionofBishop(1955).Wemodeledtwoslope stabilityscenarios:onedepictingconditionsafterthe2000

Figure14.MapofraingaugestationlocationsneartheWarrensvilledebrisslide.Rainfalltotalsgivenforeachstationareforthebulkofrainfall fromHurricaneFlorenceandsubsequentrainfallprecedingtheSeptember18,2018,debrisslide.RefertoFigure12fortheJefferson2Erainfall datachartandFigure15fortheClifton,Lansing,andAsheCountyAirportrainfalldatacharts.Mapcontourinterval ¼ 50m.

failurebutpriortothe2018debrisslide,andonerepresentingconditionsatthetimeofthe2018debrisslide. The fi eld-developedcrosssection(Figure8)servedas thegeologicconceptualmodelusedtode fi nematerials andtheirdistributionsfortheanalyses.Weusedthe2017 LiDARDTM(Figure7)toapproximatetheslopeprofile afterthe2000slopefailureintheareaaffectedbythe2018 debrisslide.Weperformedaback-calculationanalysisof soilshearstrengthbyvaryingstrengthparameterstoarrive atarepresentativemodelofmetastableconditionsinthe over-steepenedslopesafterthe2000failurewithnomodeledphreaticsurface.Wethenappliedaphreaticsurfaceto thismodeltoproduceinstability(i.e.,afactorofsafetyof 0.99)andrepresenttheslopefailureconditionsin2018.

Table1summarizesthematerialpropertiesusedin theanalyses.Tosimplifytheanalysis,wecombinedthe upperandmiddlecolluvialsoilunitsintheslopestability modelandarrivedataneffectivefrictionangleof32°and aneffectivecohesionof100lb/ft3 (4.8kPa)toaccountfor rootreinforcementonthemixedforestslopewithshrubs andgrasses.Forthelowercolluvialunit,wearrivedatan effectivefrictionangleof34°andnocohesion.Halesetal. (2009)reportedameanvalueof159lb/ft3 (7.61kPa)for rootcohesionmeasuredintestpitswithmixedforestspecies intheNorthCarolinaBlueRidge;however,rootbiomass androotcohesionvaluesgenerallydecreasetolessthan125 lb/ft3 (6kPa)inthedepthrangeof1.3to3.6ft(0.4–1.1m) andarenegligibleatdepthsof5.25ft(1.6m).Thus,root reinforcementwasconsideredbeinconsequentialinthe lowercolluvialunit,whichisgenerallyatdepthsgreater than8ft(2.4m).Forcompleteness,weassignedmaterial propertiesfortherockandwirebasketretainingwalland amphibolitebedrock;however,varyingthesepropertieshad noeffectontheresultsofthestabilityanalysis.

Weconstrainedthecirculararcfailuresurfaceusedin themodelwiththreepointsdeterminedfrom field

Tohelpconstrainestimatesofstrengthparametersfor thecolluvialsoilunitsusedintheback-calculationanalysis,wereliedonshearstrengthvaluesandunitweightsfor colluviumfromdebris-flowsourceareasandhillslopehollowsfrom13sitesintheNorthCarolinaBlueRidgethat weredeterminedfromconsolidatedundrainedtriaxialtestingofShelbytubesamples.Testvaluesforeffectivefrictionangles(ueff)rangedfrom29°to39°.Colluviumwas generallycohesionless;however,wherepresent,effective soilcohesion(Ceff)rangedfromto20to144lb/ft2 (0.96to 6.9kPa).Publishedvaluesofthesesoiltestdataaregiven inWootenetal.(2012)andHalesetal.(2009).

Figure15.Cumulativerainfall(in./mm)chartsforHurricaneFlorenceandafterwardsthatprecededtheSeptember18,2018,debrisslideatstation locationsnearWarrensvilleshowninFigure14.Redarrowspointtothetimemarkerlineat3:03p.m.(15:03hours)onSeptember18,2018,whenthe debrisslidewasreportedtoAsheCountyEmergencyManagementdispatch.Approximatetimes(hours)fromtheendofpeakrainfall(epr)andtheendof rainfall(er)thatprecededthedebris flowareshownforeachstation.(A)Clifton,NC.(B)Lansing,NC. https://wunderground.com,accessedSeptember 2018.(C)AsheCountyAirport(KGEV),NC. https://mesonet.agron.iastate.edu,accessedNovember2023.Dataanddatasourcesarecourtesyofthe NOAA-NWSWeatherForecastOffice,Blacksburg,VA.

Material

Wooten,Cattanach,Bozdog,andIsard

Table1. Summaryofmaterialpropertiesusedintheslopestabilityanalyses.

Effective Cohesion, Ceff

EffectiveFriction Angle, ueff (degrees)

MoistUnitWeight, cmoist (lb/ft3)

SaturatedUnitWeight, csat (lb/ft3)

1.Rock-filledwirebasketretainingwall035°(sliding)130(20.42kN/m3)__

2.Upper-middlecolluvialsoilunits100lb/ft3 (4.8kPa)32°90(14.14kN/m3)110(17.28kN/m3)

3.Lowercolluvialsoilunit034°95(14.92kN/m3)122(19.16kN/m3)

4.Amphibolitebedrock040°(planardiscontinuity)180(28.3kN/m3)__

observationsportrayedinthe field-developedcrosssection. Thephreaticsurfaceappliedinthemodeltoproduceinstabilitywassetslightlyabovethecontactbetweenthelower andmiddlecolluvialunits,whichcoincidedwithourobservationsofgroundwaterdischargeatornearthiscontact.

Theresultsofthestabilityanalysesarepresentedin Figure16.Afactorofsafetyof1.126representsmarginallystableconditionswithmoistsoilconditions,withno appliedphreaticsurface(Figure16A).Theadditionof themodeledphreaticsurfaceproducedafactorofsafety of0.996,whichdepictsfailureconditionsforthe2018 debrisslide(Figure16B).

DISCUSSION

Rainfallhaslongbeenrecognizedasatriggeringmechanismforlandslidesworldwide(Wieczorek,1987;Polemio andPetrucci,2000,andreferencestherein)includingthe southernAppalachianhighlands(Wieczoreketal.,2004; Wootenetal.,2016,andreferencestherein).Weconclude thattheSeptember18,2018,debrisslidewaslikelytriggered(induced)byrainfallfromtheremnantsofHurricane Florencecoupledwithabove-normalrainfallforthearea beginninginMay2018thatledtoelevatedantecedentsoil moistureconditions(Figure12).Thesharpincreasein

above-normalrainfallduringAugust2018waslikelya compoundingfactor.StudiesinwesternNorthCarolina haveshownthatduringperiodsofextendedrainfall,lag timesinexcessof30dayscanberelevanttomoisture conditioningofsoilpriortoalandslide-triggeringrainfall event(Nippgenetal.,2016;Milleretal.,2019).

AlthoughmostofmountainouswesternNorthCarolina receivedrainfallfromHurricaneFlorence(Figure13),and theregionhadalreadyexperiencedabove-normalrainfall, nolandslidesrelatedtoHurricaneFlorenceotherthanthe Warrensvilledebrisslidehavebeendocumentedbythe NCGS.Presumably,therequisiterainfallthresholdswere notattainedtoproducenumerousslopefailures,asinother stormeventsinwesternNorthCarolina(Wootenetal.,2017, 2022;Milleretal.,2021a,2021b).Althoughpeakrainfall fromHurricaneFlorencerecordedattheAsheCountyairportwasapproximately5in.(127mm)over18hours,which exceededthegeneralthresholdof5in.over24hoursused asarule-of-thumbestimateforlandslide-triggeringrainfall inwesternNorthCarolina(EschnerandPatrick,1982;Wootenetal.,2007),theCliftonandLansingstationsfellbelow thisthresholdatapproximately5in.(127mm)and4.25in. (108mm)over24hours,respectively.The2000and2018 Warrensvilleslopefailuresinvolvedanexcavatedslope,and insomecases,rainfallthresholdsneededtoinducefailures

Figure16.Crosssectionsusedinthestabilityanalyses.FS ¼ factorofsafety, r ¼ radiusofcirculararcfailuresurface.StationA15coincides withthesamestationlocationinFigure8atthetopofthemainscarpofthe2018debrisslide.(A)Analysisrepresentingmetastableslopeconditions(FS ¼ 1.126)afterthe2000failureandpriortothe2018failurewithnophreaticsurface.(B)Analysisrepresenting2018failureconditions (FS ¼ 0.996)withaphreaticsurfaceapplied.ResultingfactorsofsafetyinbothcasesarethesamefortheanalysismethodsofMorgensternand Price(1965)andthesimplifiedmethodofBishop(1955).RefertoTable1formaterialproperties.Groundprofilesweresimplifiedtoeliminate computationalerrors.

Figure17.(A)Cumulativerainfalldeparture(CRD)fromthemeanandmonthlyprecipitationplotof2003–2020rainfalldatafromAsheville,NC (NOAA,2021)shownwithselectedlandslideeventsinwesternNorthCarolinafortheperiod.Landslide-producingrainfallfromtheMay18, 2018,storm,SubtropicalCycloneAlberto,andHurricaneFlorence(Warrensvilledebrisslide)coincidedwiththeonsetofrecordabove-normal rainfallinwesternNorthCarolinaduring2018–2020.(B)Numbersoflandslidesandcumulativelandslidesforthesameperiod.Numbersof annuallandslides,CRDanalysis,andmonthlyprecipitationindicateperiodsofbelow-normalandabundantrainfall,withabundantrainfallcoincidentwithlandslideevents.FigureismodifiedfromWootenetal.(2022).

ofmodifiedslopessuchasthisarebelowthoseneededto inducefailuresonunmodified,forested(i.e.,natural)slopes (Wootenetal.,2017).

Infiltratingrainfallcanleadtoincreasesinsoilporewaterpressure(anddecreasesinmatricsuction),which decreasesoilstrength,therebyleadingtoslopefailure (LuandGodt,2013).Shallowdebris flowsanddebrisslides (generallylessthan6ft[2m]deep)canbetriggeredwhen positiveporepressuresfrominfiltratingwaterovercome soilstrength(notnecessarilyalongdiscreteslipsurfaces), whereasmovementofdeeperlandslidescanbeinducedby risinggroundwaterlevelsthatcreatepositiveporepressuresonslipsurfaces(VanAschetal.,1999).Antecedent moistureconditionspriortolandslide-triggeringeventsare importantconsiderationsindevelopingrequisiterainfall thresholdsfordifferinghydrometeorologicalconceptual modelsappropriateforshallowdebris flowsversusdeeper landslides(BogaardandGreco,2018).

Thelagtimesof »33–39hoursfromtheendofpeak rainfalland »9–10.5hoursfromtheendofrainfalluntil theWarrensvilledebrisslideoccurredcontrastwithother documentedcasesinNorthCarolinawheredebris flows initiatedatornearthetimeofpeakrainfallrates(Wooten etal.,2017,2022).GroundwaterdischargeonSeptember 19withintheWarrensvilleslidescar(zoneofdepletion)

wasobservednearthecontactofthemiddleandlowercolluviumunits(Figure8),indicatingthatincreasedporewaterpressuresinthatzonealongthatslipsurfaceatabout 12to14ft(»3.7–4.2m)belowthepre-slidegroundsurfacetriggeredtherapidslidemovement.Ourstabilityanalysisofslopeconditionsatthetimeofthe2018failure (Figure16B)areconsistentwiththisinterpretation.The lagtimebetweenrainfallandslidemovementcouldbe explainedbythetimerequiredforrainfallinfiltratinginto therechargeareaupslopetoresultincriticalpore-water pressuresalongtheslipsurfaceatdepthinthefootslope debrisdeposits.Fromlong-termmonitoringoftheAlani-Paty landslide,Reid(1994)determinedthataftertheonsetofrainfall,deeperporepressuresnearthebasalslidesurfaceshowed adelayed(althoughattenuated)responseincontrasttoshallowporepressures.Fromthepublicsafetyperspective,the Warrensvilledebrisslidedemonstratesthathazardsfrom rapidtoveryrapidslidemovementcanoccurover2days afterpeakstormrainfallandhoursafterrainfallceases.

RainfallandLandslides2018–2020

TheMay18,2018,stormeventthattriggeredover240 debris flowsinPolkCounty,NC(Wootenetal.,2022), markedtheabruptbeginningofa2yearperiodofincreased

landslideactivityinwesternNorthCarolinaoverthepreviousfouryears(2014–2017).Thecumulativerainfall departurefromthemeananalysisforwesternNorthCarolina(Figure17)showsthatrainfallconditionssteadily increasedfromearly2018to2020,withacommensurate overallincreaseinlandslidefrequency.Inhindsight,the September2018Warrensvilledebrisslide,attributedhere toelevatedantecedentrainfallcoupledwiththatfrom HurricaneFlorence,occurredat4monthsintothisthis 2yeartrend.Similarincreasesinlandslideactivity wereseenin2004,2009–2010,and2013,coincidentwith periodsofextendedabove-normalrainfall.Withinthese periodsofabove-normalrainfall,individualextremerainfalleventstriggerednumerousrapidlymovingdebrisslides anddebris flows(Milleretal.,2019,2021a,2021b). Cumulativerainfallduring2018–2020reactivatedlarge, slow-movingdebrisslidesandweathered-rockslides,with primeexamplesbeingtheBright’sCreekandHowardGap landslidesinPolkCounty,andtheBuffaloCreekslidein RutherfordCounty,whichcontinuetothreatenprivate propertyandinfrastructure(Wootenetal.,2019,2022).

Theshiftfromrecorddrought(2016–2017)torecord rainfall(2018–2020)asseeninwesternNorthCarolina (Figure17)isacontinuationoftheoverallpatternofincreased droughtseverityandmoreextremeprecipitationdistributionsthatLaseteretal.(2012)identifiedinclimatedata from1935to2010attheCoweetaHydrologiclaboratory insouthwesternNorthCarolina.Theseextremeprecipitationpatternsmayalsoinfluencethetransitionfrommetastableorslow-movinglandslidesandslopestorapid accelerationandcatastrophicfailureanalogoustothat documentedfortheMudCreeklandslideinCalifornia (Handwergeretal.,2019)andasexperiencedinthe Warrensvilledebrisslide.Giventhepredictedincr ease inprecipitationextremeswithchangingclimatelinked tolandslidemovement(Handwergeretal.,2019)coupled withfutureincreasesinstormintensityandfrequency(e.g., Preinetal.,2017),acommensurateincreaseinlandslide activitycanbeexpected.

SUMMARYANDCONCLUSIONS

AseriesoffactorsthatcontributedtotheSeptember 18,2018,debrisslidetookplaceoveraperiodofyears thatpredisposedtheslopetofailureandincreased losses.Colluvialdepositscanbemarginallystableinan undisturbedor “natural” condition,andslopemodificationssuchasexcavationscandecreasethestabilityof colluvialdepositsinwesternNorthCarolina(Wooten etal.,2017)andelsewhere(FlemmingandJohnson, 1994;Turner,1996).Colluvialdepositsfrompastslopemovementactivityhavethepotentialforslopeinstabilitywhendisturbed,asevidencedherebythereported failureofthecutslopein2000.Theretainingwallbuilt

afterthe2000slopefailuredidnotadequatelyprotect thebankfromthedebrisslide.Curvedtreespresenton thedisplacedmassoftheslideareevidencethatthe slopewas,orhadbeen,movingslowlybycreepandwas alreadymarginallystable.Groundwaterdischarging withinthedebrisslidenearthelowercolluvialunitindicatedthatporepressureswithinthecolluvialsoilaquifer likelyhadadestabilizinginfluenceontheslope,asshown inourstabilityanalyses.Assessmentofthedegreeto which,ifany,theslopemodificationsandleachaterelated tothesepticsystemcontributedtotheMay18,2018,slope failurewasbeyondthescopeofourinvestigation.Although thesepticleach fieldwasconstructedinavulnerablelocationanddamagedbytheslide,itwasuncleariftheleachate moisturefromitwasacontributingfactortotheslopefailure.Damagetothesepticsystempresentedaneconomic hardshiptotheownerwithnegativeimpactstotherestaurantitserved.

Locatinggastanksandtransmissionlinesinvulnerable locationsonornearslopessusceptibletofailurewithout theappropriateprotectionmeasuresisunwise.OnMay30, 2018,duringrainfallfromsubtropicalstormAlberto(Figure 17),twolandslide-relatedfatalitiesoccurredinWatauga County,NC(WootenandWitt,2018).TheWataugaCounty Sheriff ’sOfficereportedasequenceofeventsinwhicha landslidedamagedalow-pressuregasregulator,causingan explosionthatdestroyedonehome,killingtheoccupants (WataugaDemocrat,2018).Inthe2018Warrensville case,thepropanetankwasplacedinavulnerablelocation upslopeoftheretainingwall,therebybeinglocated betweenanunstableslopeandanoccupiedbuilding. Thedebrisslidestruckthepropanetank,andtheensuingexplosiondestroyedthebuilding.

Fortunately,noliveswerelost,andnoseriousinjuries resultedfromtheWarrensvilledebrisslide.Certainly,the circumstancescouldhavebeentragicifcustomershad beeninthedrive-throughlanebehindthebank,orif employeesorcustomersinsidehadnotbeennearthefront ofthebankatthetime.

Landslideeventslikethesecanresultinlossesand negativeimpactstocommunitiesthatlastforyears.The damagedbankbuildingbelowthe2018slidescarisvisible in2021and2022orthophotographyhostedontheAshe CountyGISWebsite(https://gis.ashecountygov.com/maps/ default.htm).AnAugust31,2023,sitevisitrevealedthatthe heavilydamagedandabandonedbuildingstillstands,and remnantsofthedebrisslidedepositandretainingwallare overgrown.Anymajoractionstakentomitigatetheslope failurehazardtherewerenotreadilyapparent.Anongoing challengeistorecognizethepotentialnegativeimpactsof seeminglyindependent,butinterrelated,land-usepracticesin mountainousterrainthatmaytakeyearsordecadesfortheir cumulativeeffectstoresultindamaginglandslides.Landscape changesassociatedwithurbanizationcanhaveadecades-long Wooten,Cattanach,Bozdog,andIsard

effectonlandslidesusceptibility,whereurbanizedareasare generallymoresusceptibletolandslides(Rohanetal., 2023).Understandinginteractionsbetweenlanduseand landslidesandapplyingthisknowledgetopromotesmart developmentinmountainousterrain(SidleandOchiai, 2006)canhelptoreducelandslidelosseslikethosethat continuetoimpactthetownofWarrensville,NC.

ACKNOWLEDGMENTS

StephenKeighton,NationalWeatherService,Blacksburg, VA,WeatherForecastOfficegraciouslyprovidedtherainfall datatodocumenttheprecipitationconditionsleadingupto theSeptember18,2018,debrisslide.ImageryfromUAS flightsbytheNorthCarolinaExtensionservicewasan invaluableresourceforassessinganddocumentingthedebris slide.Logisticalsupportandinformationprovidedby theAsheCountyEmergencyDepartmentaregratefully acknowledged.KennethB.Taylor,NorthCarolina StateGeologist,wasinstrumentalinguidingtheNCGS landslideresponseprograminclosecoordinationwith stateandlocalemergencymanagers.Wethankthree reviewersfortheircomments,whichresultedinsubstantial improvementstothemanuscript.

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EvaluationofPlant-BasedNaturalExtractsas CoagulantsforSurfaceWaterTreatment

ZANIBFIDA*,MUHAMMADASHRAFTANOLI,andQAISARMAHMOOD

DepartmentofCivilEngineering,COMSATSUniversityIslamabad,Abbottabad Campus,22060Abbottabad,Pakistan

MUHAMMADSOHAILALAMGIR

DepartmentofWorks,AbbottabadUniversityofScienceandTechnology,Havelian Campus,22500Abbottabad,Pakistan

DILAWARSAJJAD

DepartmentofCivilEngineering,UniversityofLahore,IslamabadCampus, Islamabad,Pakistan

KeyTerms: GreenTechnology,NaturalCoagulants, WaterTreatment,SustainableEnvironment

ABSTRACT

Thisstudyinvestigatedtheuseofnaturalextractsas bio-coagulantsforwatertreatmentandpurification.The utilizationofnaturalmaterialsrepresentsimportantprogressinsustainableenvironmentalinitiatives,andtheir applicationisdirectlyassociatedwithimprovedquality oflifeduetotheirlowtoxicity,cost-effectiveness,andnaturallyrenewableproperties.Therefore,threeplantspecieswereidentified,andtheireffectivenesswasevaluated inthelaboratoryonsurfacewatersamples: Moringaoleifera(MO),Cicerarietinum(CA), and Pinusroxburghii (PR).WatersampleswerecollectedfromtheRiver Kunhar,GarhiHabibullah(Mansehra)forlaboratory analysis.Acombinedtreatment(coagulationand filtration)wasfoundtobemoreefficientcomparedtodirect filtrationorcoagulationalone.Thehighestturbidity removalof99.17percent,solidsreductionof93.21percent,hardnessreductionof67.35percent,andtotalcoliformremovalof86.13percentwereachievedforthe filteredwatersamplestreatedwith MO. CA providedthe maximumchloridereductionof54.85percentcompared totheothertwo.Theseextractsslightlyaffectedthe acidity,alkalinity,pH,andconductivityofwatersamples. However,aslightincreaseinbiologicalandchemical oxygendemandwasfoundintreatedwater.Among thethreenaturalcoagulants, MO causedasignificant increaseinorganicmatter.Thisproblemcanbeavoided byusingthepurifiedformofactivecoagulatingagents ratherthancrudeextract.Thisresearchshowstheeffectivenessof MO forsurfacewatertreatmentandalso

*Correspondingauthoremail: zanibfida@cuiatd.edu.pk

illustratesthegreatpotentialof CA and PR aspromisingbio-coagulantsforthetreatmentofpollutedsurfacewatersamples.

INTRODUCTION

InPakistan,about70percentofthepopulationobtains waterfordrinkingpurposesfromgroundwateraquifers. Surfacewaterisanothermainsourceofwaterfordrinkingandotherdomesticpurposes(Daudetal.,2017). Mostoftheworld’spopulationdoesnothaveaccessto safeandcleandrinkingwater.ThePakistanNational ConservationStrategyreportshowedthatwater-related diseasesrepresent40percentofcommunicablediseases. InPakistan,60percentofinfantdeathsarecausedby water-relateddiarrhea,whichisthehighestproportionin Asia(Daudetal.,2017).Wateravailabilityhasdeclined dramaticallyfrom5,000m3/yrpercapitain1951to1,100 m3/yrpercapitain2005,anditwillfallto800m3/yrper capitaby2025(QureshiandAshraf,2019).

Freshwaterresourcesandtheenvironmentareseverely threatenedbecauseofurbanization,industrialization,and overpopulationstresses(Ensinketal.,2004).Bigcitiesare constantlyproducinghugeamountsofeffluentwaterthat needpropermanagementanddisposal(GhoshandSingh, 2005).Only2percentofthecitiesinPakistanhavethe advantageofwastewater-treatmentfacilities(Clemettand Ensink,2006).Thedeficienciesintreatmenttechnologies leadtohealthaswellaseconomicconsequences(Fuente etal.,2020;McClellandetal.,2022).

InPakistan,90percentofthetotaluntreatedwastewater isdischargedintolocalwaterbodies.Themajorpollutants influencingthewaterqualityincludepathogensandheavy metals(AlAkeel,2013),aswellasnon-metallicsubstances,fertilizers,pesticides/herbicides,hydrocarbons, variousorganicsolvents,andexplosives(Tanvir,2010).

Highturbidityisanotherpollutantprevalentinlower areasofPakistan(TahirandAslam,2010).

IndenselypopulatedcitieslikeKarachi,Lahore,Rawalpindi,Peshawar,Faisalabad,Sialkot,andGujrat,drinking waterishighlypollutedduetovariousactivitiesandcannot berecommendedforhumanconsumption(Bhuttaetal., 2005).Athigheraltitudes,thereisnoprovisionforthesafe disposalofthegeneratedhouseholdwastewaterandsolid waste.Inmostcases,suchkindsofdomesticliquidandsolid wastesaredischargedintofreshwaterstreams,whichserve aspotentialsourcesoffreshwatersupplytodownstream localities(Fergusonetal.,2007).Thus,thehighlevelof drinkingwatercontaminationhasledtoanincreaseindissolvedorganiccarbon(DOC),whichactsasavectorforthe transportofbacteriaandviruses.Consequently,consumption ofthispollutedwateristheleadingcauseofvariousacute diseases,includinghepatitis,tuberculosis,diarrhea,andcholera,especiallyininfants(Soomroetal.,2011).

Differentphysico-chemicalmethods,suchasdistillation, sedimentation, filtration,chemicaloxidation,andchlorination,havepreviouslybeenemployedfortheremovalof pollutantsfromdifferentwatersystems.Forthepreservationofthehealthandwell-beingofhumans,thetreatment ofrawwaterbeforeconsumptioniscruciallyessential. Coagulation/flocculationisoneoftheunitprocessesamong thedifferentphysico-chemicalprocessesusedforwater treatment.Itsapplicationincludestheremovalofdissolved chemicalspeciesandturbidityfromthewaterbyusing conventionalchemical-basedcoagulants,namely,alum (Al2[SO4]3·18H2O),ferricchloride(FeCl3),andpolyaluminumchloride(PAC)(Yin,2010).Althoughtheefficacyof thesechemicalsascoagulantsiswellrecognized,thereare neverthelessdisadvantagesassociatedwiththeuseofthese coagulants:Whiletheyarerelativelycheap,theyarestill quiteexpensivefordevelopingcountries(Konkoboetal., 2021);fromanenvironmentalpointofview,theirusesignificantlyproducesmetallicresidues(Krupinska,2020; Fouadetal.,2021);and finally,theiruseexposeshumansto apossibleriskofcontractingAlzheimer ’sdisease,because thealuminumresiduesthatremaininthewateraftertreatmentaresuspectedtoberesponsibleforthatdisease(Kandimallaetal.,2016;Zhangetal.,2019).

Itis,therefore,imperativetoreplacethesechemical coagulantswithnaturalusefulsubstitutestocounterbalance thedrawbacks.Currently,muchofthisworkisbeingdone ontheuseofplant-basednaturalcoagulantsforwaterand wastewatertreatment(Yin,2010).Thesenaturalextracts actasadsorbents,coagulants,and flocculantsforthetreatmentofsurfacewater.Theyalsoactasnaturalantimicrobialandantibacterialagents(KaziandVirupakshi,2013). Theydonotproduceanytoxiceffectsintreatedwater (Konkoboetal.,2023).TheydonotrequirepHandalkalinityadjustmentsanddonotcausecorrosionproblems

likealum.Theyproducerelativelysmallervolumesof sludge,whichcanbeusedasbio-fertilizersinruralareas. Itisquiteacheapermethodandcanbeusedinareas wherenotreatmentfacilitiesareavailable(Kaziand Virupakshi,2013).

Forthispurpose,severalnaturalplantshavebeenstudied scientifically.Theseextractshavebeenderivedfromdifferentpartsofplants,includingtheseeds,roots,leaves,bark, andfruits(Yongabi,2010).Forexample,seedsof Jatropha curcas (physicnut)werefoundtobeveryusefulforthe treatmentofwastewater. Parkiabiglobossa (locustbean seed)wasusedinnorthernNigeriaforturbidwaterclarification. Citrusaurantifolia (limejuice)isbasicallyacidicin natureandthustoxictoseveralmicroorganisms(Yongabi, 2010).Ithasbeenusedasadisinfectantindifferentwatertreatmentsystems. Moringaoleifera ( MO)isatropical plantthatshowedstrongcoagulationactivityduetothe presenceofdimericcationicproteins(Yongabi,2010). Parkinsoniaaculeate and Vignaunguiculata areplantspecies inwhichcationicproteinswerefoundtobeactivecoagulatingagents(Blix,2011).Previousstudiesreportedthatseeds of Cicerarietinum ( CA)havebeenusedasanaturalcoagulantforthetreatmentofsurfacewateraswellastannery wastewater(Asrafuzzamanetal.,2011;KaziandVirupakshi,2013). Pinusroxburghii ( PR),beingastrongadsorbent, hasbeenusedfortheremovalofchromiumfromsynthetic andindustrialwastewater(Ahmadetal.,2005).Theutilizationofplant-basedcoagulantsforturbidwaterpurification datesbackseveralmillennia(Sanghietal.,2002),anddifferenttypesofplantspeciesarecurrentlybeingexplored forwater-treatmentfacilities.Therealsoexistnumerous studiesfocusedontheusageofthesematerialsfortreating industrialwastewater(Sanghietal.,2002;SanghiandBhattacharya,2005;andVijayaraghavanetal.,2011).

Withthewidespreadenvironmentaldeteriorationand thepresentdepletionofEarth’snaturalresources,theapplicationofthesecoagulantsrepresentsapromisingideain the fieldofsustainableenvironmentaltechnology.Therefore,thisstudyinvestigatedtheuseoflocallyavailable plants,theextractsofwhichcouldbeusedasbio-coagulants,andevaluatedtheirefficacybycoagulation/flocculationfollowedbyslowsand filtrationintheprocessofraw waterpotabilization.Filtrationisbasicallyatechnology thathasbeenusedforpotablewaterpurification.Itisaprocesswellsuitedforsmall,ruralcommunities,sinceitis simple,reliable,andinexpensive,anditdoesnotrequirea highdegreeofoperatorskillorattention.Nochemical additionisrequiredforproper filtrationoperation.Particle removalisaccomplishedprimarilythroughbiologicalprocessesthatprovidetreatment.Slowsand filters(SSF)are notbackwashedlikerapidrate filters(ThomasandKani, 2016).Gravity flowkeepstheoperatingcostslowand allowsSSFtobeusedinremotelocationswithoutaccess

topower;thatiswhyanSSFcolumnwasusedinthis researchwork(Bielefeldt,2011).

StudyObjectives

InnorthernareasofPakistan,peopleareboundtoconsumerawsurfacewaterthatiscontaminatedbecauseof industrialandmunicipalwasteeffluents.Thehighmortalityratebecauseofwater-bornediseasesismainlyattributedtotheconsumptionofpollutedwaterthatneedsto betreated.Thus,thefocuswastoconserveandsafeguard localwaterbodiesbyusingnaturalplantextractsfor watertreatment.Thisstudyfocusedon MO, CA,and PR.

Themainobjectivesofthisstudywere:

(1)toidentifylocalplantspeciesthatcanbeusedas coagulantsandcoagulantaidsforwatertreatment ofKunharRiversurfacewater;

(2)tooptimizethedosageofplant-basedcoagulants usedinthecoagulationprocess;and

(3)toevaluatetheeffi cacyofplant-basedcoagulants intheremovalofturbidityandothercommon pollutants.

MATERIALSANDMETHODS

Throughouttheprojectduration,varioustypesof materialswereused.Thedetailsofthematerials’ preparationandproceduresaredescribedinthissection.

WaterSampling

Therawwatersamplesusedinthisresearchworkwere takenfromtheRiverKunhar,GarhiHabibullah(Mansehra).Alocationmapofthestudyareaandriverfrom whichthewatersamplesweregatheredisshowninFigure 1.Watersampleswerecollectedinplasticairtightbottles andbroughttothelaboratoryforfurtheranalysis.New watersampleswerecollectedeverysecondtothethirdday.

SampleCharacterization

Aftercollectingsamples,alltheanalyticalparametersforriverwatersampleswereanalyzedperstandard methods(Carranzo,2012)intheEnvironmentalEngineeringlaboratory,COMSATSAbbottabad(A224), within24hours.

CollectionandPreparationofPlantMaterials

Moringaoleifera

Samples

Inthisresearchwork,theleavesof MO wereusedfor water-treatmentpurposesbecausetheactivecoagulating agentinthesaidplantisproteins.Previousstudies revealedthattheproteincontentof MO leaveswasfound

tobehigher(Sultana,2020)ascomparedtotheseedsof thisplant(Sivakumar,2013).Leavesof MO werecollectedandwashedthoroughlytoremovedirtanddust particles.Theleavesweredriedundershadefor10days untilaconstantweightwasachievedbecauseshade-dried leavescontainthehighestproteincontentascomparedto sun-driedleaves(Deepaetal.,2021).Thedriedleaves werethenpulverizedandsievedtogetthenaturalextract of MO inpowderedformhavingaparticlesizeofless than100 lm(VermaandNigam,2014).

Cicerarietinum Samples

CA seedswerecollectedfromthelocalmarketof Abbottabad.Theseedsampleswerecleanedtoremove grit,dirt,andbrokengrainsandthenpackedinairtight jarsatroomtemperature.Afterthat,theseedswere groundtoa finepowderhavingaparticlesizeofless than100 lmandstoredinairtightcontainersforwater orwastewater-treatmentanalysis(Sharmaetal.,2013).

Pinusroxburghii Samples

Barkof PR wascollectedfromtheMansehrahills. Thesebarksampleswerecarefullyremovedwithastainless-steelpenknifeatanaverageheightofabout2m abovetheground(Baslaretal.,2009).Theywerethoroughlywashedandthendriedundershadefor10days. Thedriedsampleswerethengroundintoa finepowder (Khanetal.,2012)andsievedtogetaparticlesizeof 60–150 lm(Ahmadetal.,2005).

Color

Thepowderobtainedfrom MO leavesisgreenincolor. However,thepowderobtainedfrom CA and PR issalmonwhiteanddarkbrownincolor,respectively.Apictorial representationofthepowderedformofthethreenatural coagulantsisgiveninFigure2.

ExperimentalSetup

OptimizationofDosageofNaturalCoagulantsbyJar Testing

Ajartestisthemostwidelyusedmethodtodetermine theefficacyofnaturalplantextractsbecauseitiseasyto perform.Theexperimentwascarriedoutasabatchtest. Thestandardassemblyofthejartestingapparatusconsistsofsixbeakerswitha1Lcapacityeach.Here,thejar testapparatusshowninFigure3consistedoffourbeakers havingacapacityof100mLbecausethisassemblysetup wastheonlyoneavailableintheEnvironmentalEngineeringlaboratoryforlaboratoryanalysis.Eachjarwas filledwith100mLofrawwatersamplewithidentical

turbiditylevels.Theinitialstirringratewassetto150 rpm(Konkoboetal.,2023).Differentamountsofselected coagulantswereaddedtothreeofthejars.After3minutes, thestirringratewasloweredto40rpm,andthiswasmaintainedfor15minutes.Then,thestirrerswerestoppedcompletely,andthejarswereleftfor1hourofsedimentation (Asrafuzzamanetal.,2011).Thejarwiththehighest

claritywasselectedtoassessthecoagulantactivityinterms ofturbidityreduction.Thewatersamplewasthenwithdrawnfromthetopofthejartoevaluatewater-quality parameters.Theamountofcoagulantthatgavethelowest residualturbiditywasconsideredtheoptimumdosage.The effectofdosagesofnaturalcoagulantsonturbidityreductionwasalsostudied(Satterfield,2005).

Figure1.Locationmapofstudyarea.

Fida,Tanoli,Mahmood,Alamgir,andSajjad

Filtration

Aftercoagulation,watersampleswerepassedthrough aSSFcolumnhavingaheightofsandabout1/3ofthe totalheightofthecolumn.Thetotalheightofthecolumn was3.0m.Figure4showsasketchofatypicalSSF. Throughthe filter,SSFsystemsarenormallyoperated withagravityfeedofwater,designedfor filtrationrates of2to7m3/m2 filterarea/d(0.083–0.3m/hr).The flow ratewasmaintainedatavalueof0.3m/hr(1.5L/15min). Thesandwaspackedinabedwithatypicaldepthof0.6 to1.4m.Thesandbeddepthwasmaintainedat1min thisresearchwork.Belowthesandbed,thereisasupportinglayer,whichistypicallygravel.Thegravelsupportistypicallythreetosixlayers,withthe finestgrain sizeunderthesandandthecoarsegrainsizedirectlyover theunderdrain.Eachlayerwasgenerally10cm,witha totaldepthof30cm(Bielefeldt,2011).Afterpassing throughthe filtercolumn,thesampleswereagainanalyzedtoevaluatethequalityofthewater.

AnalyticalProcedures

Toassessthecoagulationactivityofthenaturalextracts andtheefficiencyofthesand filters,physico-chemicaland bacteriologicalparameterswereanalyzedperstandardmethods(Carranzo,2012).Thephysico-chemicalparameters includedpH,turbidity,conductivity,totalsolids,acidity,

alkalinity,hardness,dissolvedoxygen(DO),biologicaloxygendemand(BOD),chemicaloxygendemand(COD),and chlorides.Thebacteriologicalqualityofwatersampleswas determinedbythetotalcoliformtest.

RESULTSANDDISCUSSION

Physico-ChemicalCharacteristicsofRiver(Kunhar) Water

Thephysico-chemicalcharacteristicsofrawwater sampleswereanalyzedintheEnvironmentalEngineering Laboratory,COMSATSAbbottabad.Thewatersamples weredirectlytakenfromtheKunharRiver.Alltheanalyticalparametersweremeasuredbeforeperformingthejar test.TherawwatercharacteristicsaregiveninTable1.

OptimizationofDosageofNaturalCoagulants

Turbidityisanextremelyusefulandaestheticparameter affectingtheappearanceandacceptabilityofdrinking watertoconsumers.Sinceturbidwaterscontainsuspended andcolloidalparticles,theyarenormallytreatedbycoagulation-flocculationfollowedbya filtrationprocess.This

S.No.AnalyticalParametersObservedValues

Figure3.Jartestingapparatus.

Figure4.Typicalconfigurationofslowsand filter(SSF).

Figure2. Moringaoleifera, Cicerarietinum,and Pinusroxburghii powder.

Table1. Characteristicsofrawwater.

Table2. Optimizationofdosageofnaturalcoagulants.

water-qualityparameteritselfdoesnotalwaysrepresenta directrisktopublichealth;however,itcanindicatethe presenceofpathogenicmicro-organismsandbeaneffectiveindicatorofhazardouseventsthroughoutthewater supplysystem.Itcanbeusedasabasisforchoosing betweenalternativesourcewatersandforassessingthe performanceofafewcontrolmeasures,includingcoagulationandclarification, filtration,disinfection,andmanagementofdistributionsystems.Thatiswhyajartestwas performedto findtheoptimumdosageofcoagulantsfor maximumturbidityreduction.Itwasobservedthattheturbidityofrawsurfacewaterdecreasedbyincreasingthe coagulantdosageuptoacertainlimit.

Table2showsthattheresidualturbidityofwatersampleswasmeasuredatdifferentdosagesofnaturalcoagulants.Thespecificamountofcoagulantthatgavethe lowestresidualturbiditywasconsideredtobetheoptimumdosage.For MO leaves,itwasfoundtobe140 mg/L,whichresultedinthelowestresidualturbidityof 22.07 6 2.62nephelometricturbidityunits(NTU).

Similarly,theoptimumdosageof CA wasfoundtobe 305mg/Lwithresidualturbidityof30.49 6 1.94NTU; for PR,theoptimumdosagewasfoundtobe65mg/L withresidualturbidityof45.31 6 7.22NTU.Theconcentrationwasquitelowbecausetheextractwasdark brownincolor,andhigherconcentrationsinwatermay increasetheturbidityoftreatedwater.

Thetabulardataalsorevealedthatabovetheoptimum dosage,theturbidityofwatersamplestendedtoincrease progressively.Afterdeterminingtheoptimumquantityof coagulants,treatedwatersampleswereanalyzedformajor water-qualityparameters.Thesesampleswerethenpassed throughaSSF.Thequalityofthese filteredsampleswas againassessedtocomparetheefficacyofbothmethods.

EffectsofCoagulationandFiltrationonWater-Quality Parameters

InfluenceonTurbidity

Itwasobservedthattheinitialturbidityofrawwater wasfoundtobe241 6 7.81NTU.Aftertreatmentwith

plantextracts,itsvaluewasreducedto22.07 6 2.62to 45.31 6 7.22NTU.Inallthe figures,MTWrepresentsthe valuesforwatersamplestreatedwith MO,CTWrepresents thevaluesforwatersamplestreatedwith CA,andPTW showsthevaluesforwatersamplestreatedwith PR

Therawwatersamplesaswellasthesetreatedsampleswerethenpassedthroughslowsand filters.The finalturbidityvaluereachedupto1.99 6 0.76to6.99 6 0.80NTU.Thesevaluesfallwithinthepermissiblelimit ofWorldHealthOrganization(WHO)guidelines,i.e.,5 NTU.Figure5showsthedecreasingtrendofturbidityat differenttreatmentstages.Inthe figures,FRWshows thevaluesfor filteredrawwatersamples,FMTWisfor filteredwatersamplestreatedwith MO,FCTWisfor filteredwatersamplestreatedwith CA,FPTWisfor filteredwatersamplestreatedwith PR,andDFrepresents thewatersamplesobtainedthroughdirect filtration.

Coagulation-flocculationisthemostwidelyusedtechniquetoremoveturbidityandotherpollutantsfromwater. Inrawwatersamples,initialturbiditywasfoundtobe 241 6 7.81NTU,whichwasbeyondthelimitperthe requirementsofWHO.Inthecurrentstudy,thestudied naturalextractsshowedgoodefficiencyintermsofturbidityremoval.Previousstudiesindicatedthat MO was

Figure5.Turbidityreductionofriverwaterbynaturalcoagulants and filtration.

foundtobehighlyeffectiveinthereductionofturbidity, havingaremovalefficiencyofabout90–99percent.It wasreportedthattheseextractsweresolubilizedinwater andremainedhighlyactiveduringthecoagulationprocess(Katayonetal.,2006).Duetotheadditionofnatural coagulants,suspendedparticleswereattractedtowardthe activecoagulatingagentbymeansofchargeneutralizationandinter-particlebridging.Flocksizewasincreased, andthesettlementofthe flocksoccurredrapidly.Overdosingofthecoagulantledtothesaturationlevel,andthe turbidityofthesamplesincreasedprogressivelythereafter asthenumberofparticleswasnotsufficienttoformmore inter-particleadsorptionsites(Mangaleetal.,2012).Inthe caseof PR,itwassuggestedthatsuspendedimpuritiesare enmeshedinthepolymericstructureofthetanninby meansofabridgingmechanism.Beltranshowedthatatannin-basedcoagulantprovidedeffectiveresultsinsurface waterclarification(Beltran-Herediaetal.,2011).

After filtration,theturbidityofthewatersampleswas furtherreduced,reachingavalueof1.99 6 0.76to6.99 6 0.80NTU.Ithasbeenreportedthatwhenwatersamples arepassedthroughaSSF,largerparticlesaretrappedin thetoplayersofsand.Smallerparticlesthatareleftin sandmediaareconsumedbymicro-organismsattached tosandgrains.Thus,cleanwaterthatpassedthrougha sand fi lterisconsideredsafeforhumanconsumption (Abudi,2011).

InfluenceonpH

AslightincreaseinpHvalueswasobservedinthecase of MO and CA.Theinitialvaluewasfoundtobe7.366 6 0.135.ThepHofthewatersamplestreatedwith PR decreased,havingavalueof7.216 6 0.077.NoadditionalalkaliswererequiredtomaintainpH fluctuations. ThepHvaluesatdifferenttreatmentstagesareshownin Table3.Afteranalysis,itwasobservedthatpHvalues slightlyincreasedwhensamplesweretreatedwith MO and CA. Sinceaproteinwasfoundtobeanactivecoagulatingagentintheseextracts,itisinterpretedthatthe basicaminoacidsintheproteinsacceptedahydrogenion fromthewater,resultinginthereleaseofahydroxyl groupandturningthesolutionbasic(Mangaleetal., 2012).

Inthecaseof PR,tanninwasfoundtobeanactive coagulatingagent,whichconsistsoflargepolyphenol compounds.Polyphenolcompoundsactasgoodhydrogendonors(Yin,2010).Thus,itwassuggestedthatthe pHofthesesamplesslightlydecreasedduetothepresenceofhydrogenions.

However,apartialdecreasewasfoundinthepHof watersamplesafterbeingpassedthroughtheSSF.The pHvalueswerefoundintherangeof7.010 6 0.144to 7.281 6 0.099,whichfallswithinthepermissiblelimit

6 8.675

Table3. In fl uenceonotherwater-qualityparameters.

l S/cm)283

3Totalsolids(mg/L)480 6 13.75228 6 12.089231 6 9.88230.51 6 13.28532.59 6 8.5943.19 6 9.6848.61 6 7.59959.21 6 6.599

4Hardness(mg/LasCaCO 3 )310 6 25.49200 6 15.69245 6 13.88250 6 12.19101 6 16.01126 6 14.69129 6 18.56160 6 20.69

5Alkalinity(mg/LasCaCO 3 )110 6 13.53120 6 9.53116 6 6.88106.21 6 7.51110 6 8.33105.6 6 6.5495.21 6 6.99104.53 6 12.36

6Acidity(mg/LasCaCO 3 )248 6 8.175200 6 5.67213.23 6 5.99237 6 8.64185.48 6 4.31192.9 6 3.77216.87 6 5.01230 6 7.12

7Dissolvedoxygen(mg/L)9.65 6 0.3438.92 6 0.0599.45 6 0.2999.54 6 0.2799.38 6 0.1929.69 6 0.2549.71 6 0.2019.93 6 0.019

8COD(mg/L)25 6 6.07931.5 6 5.2329.87 6 4.9928.65 6 5.43323.625 6 6.0121.98 6 3.96720.76 6 4.97818.75 6 6.012

9Chlorides(mg/L)100 6 4.34778.01 6 4.5668.96 6 4.6893.51 6 2.9752.19 6 3.5646.56 6 2.68856.789 6 4.08965.219 6 3.154 RW ¼ rawwater;MTW/CTW/PTW ¼ watertreatedwith Moringaoleifera , Cicerarietinum ,or Pinusroxburghii extracts,respectively;FMTW/FCTW/FPTW ¼ fi lteredwatertreatedwith Moringaoleifera , Cicerarietinum ,or Pinusroxburghii extracts,respectively;DF ¼ direct fi ltration;COD ¼ chemicaloxygendemand.

ofWHOguidelines.Theefficiencyofmostcoagulants likealumisdependentuponthepHofthewater,so somealkalisareaddedtomaintainpHlevels.Inthecase ofnaturalmaterials,noadditionalalkalisarerequired. PreviousstudieshaveindicatedthatSSFhasasignificanteffectonthereductionofphysio-chemicalparameters(Nancyetal.,2014).

InfluenceonConductivity

Fromrawwateranalysis,theinitialvalueofconductivitywasfoundtobe283 6 6.99 lS/cm.Whenextracts wereaddedtorawwatersamples,theirvaluewasslightly increasedduetothepresenceofdissolvedionsinthe water.Amongtheseextracts, PR significantlyincreased thevalueofconductivityupto345 6 6.912 lS/cm.After filtration,thesevalueswerefurtherreducedandwere observedintherangeof268.49 6 8.675to329.7 6 8.678 lS/cm,asillustratedinTable3.

Aftercoagulation,theconductivityofwatersamples slightlyincreased.Thisbasicallyrecordstheionicspecies presentinthewater.Itwasobservedthat,ononehand, chlorideionswerereduced,but,ontheotherhand,other ioniccontentwasincreased.Thisincreaseinioniccontent waslikelythecauseoftheincreasedconductivityofthe watersamples(Katayonetal.,2006).Previousstudies reportedthattheseextractsonlyslightlyaffectedtheconductivityoftreatedwater(Arnoldssonetal.,2008).However,after filtration,ioniccontentwasreducedtosome extent.So,valuesofconductivitywerealsoreduced (Nancyetal.,2014).

InfluenceonTotalSolids

Turbidityinthewatersamplesismainlyduetothe presenceoftotalsuspendedsolids.Sincetotalsolidsare acombinationofbothsuspendedanddissolvedsolids, forrawwatersamples,aninitialvalueoftotalsolids wasobservedat480 6 13.752mg/L.Afterbeingtreated withplantextracts,totalsolidsconcentrationwas reducedtotherangeof228 6 12.089to230.51 6 13.285mg/L.After filtration,thesolidscontentinwater sampleswasfurtherreducedtovaluesintherangeof 32.59 6 8.59to59.21 6 6.599mg/L(Table3). MO was foundtobemoreefficient,havingasolidsremovalefficiencyof93.21percent,asillustratedinFigure8a.

Totalsuspendedsolidsinwaterareconsideredtobe themaincauseofturbidity.Aftercoagulation,thesesolidswereadsorbedonthesurfacesofnaturalpolymers. Thisimprovedthesizeofthe flocks.The flockswere removedeasilybythesedimentationprocess(Mangale etal.,2012).The flocksthatfailedtodisappearafter sedimentationwereremovedby filtration.Thevaluesof totalsolidsdidnotexceedthestandardvalueofWHO.

Theconcentrationoftotalsolidswasreducedafter filtrationasthesand filtermediaactedasastrainerinwhich solidswerecaptured.Fadaei(2015)revealedthatSSFs arefoundtobehighlyeffectiveinmakingwaterphysicallyandbiologicallysafe.

InfluenceonHardness

Hardnessisthepropertyofwaterthatpreventsthe leatheringactionofsoap.Itismainlyduetothepresence ofcarbonates,bicarbonates,sulfates,andchloridesin thewater.Theinitialvalueofhardnesswasfoundtobe 310 6 25.49mg/LasCaCO3 MO reducedthevalueof hardnessupto67.35percentascomparedtotheother twoextractsrepresentedinFigure8b.

Thenaturalcoagulants,beingpolyelectrolytes,reduced hardnessbyadsorptionandinter-particlebridging.Ithas beenreportedthatduringcoagulation,aprecipitationreactionconvertssolublehardness-causingionsintoinsoluble compounds,thusreducingthehardnessofwatersamples. Thehighervalueofhardnessindicatesthatitcontainsa largernumberofhardness-causingionssuchascalcium, magnesium,etc.(Mangaleetal.,2012).

InfluenceonAlkalinity

Alkalinityisthetermthatreferstothebufferingcapacity ofwatersamples.IfthepHofawatersampleincreases, thenalkalinitywillalsoincrease.Fromtheexperimental work,thealkalinityofwatersamplesincreasedinthecase of MO and CA.However,aslightdecreasewasobserved inthewatersamplestreatedwith PR.Theinitialvaluewas observedas110 6 13.53mg/LasCaCO3.Afterthe final treatment,thesevalueswerefoundintherangeof95.21 6 6.99mg/LasCaCO3 to110 6 8.33mg/LasCaCO3,as showninTable3.TheWHOguidelinevalueforalkalinity is200mg/LasCaCO3.

Alkalinitywasfoundtoslightlyincreaseduetothe presenceofahydroxylgroupinwaterinthecaseof MO and CA.Inthewatersamplesbeingtreatedwith PR,this valuewasfoundtodecreaseduetothepresenceofprotonsinpolyphenolcompounds.However,after filtration, thesevalueswereslightlyreducedbecausetheions causingalkalinitywereentrappedinthestickylayer formedonthesandsurface(Nancyetal.,2014).

InfluenceonAcidity

Acidityisduetothepresenceofhydrogenionsin water.Forrawwater,theinitialvaluewasobservedtobe 248 6 8.175mg/LasCaCO3.Ifalkalinityisincreased, aciditywillbereduced.Inthecaseofwatersamples treatedwith PR,aslightdecreasesinpHandalkalinity wereobserved,whileaciditywasfoundtoincreasei.e.,

Figure6.Biologicaloxygendemandofriverwaterbeforeandaftertreatment.

237 6 8.64mg/LasCaCO3 ascomparedtoothervalues. Thesevalueswerefurtherreducedafterthe filtrationof treatedwatersamples.

Thesenaturalmaterialscontainlowerwater-soluble proteins,whichexhibitpositivecharges.Whenacrushed formof MO and CA wasaddedtothewater,theprotein contentinitproducedpositivecharges,whichattracted predominantlynegativelychargedparticlesandmaintainedtheaciditylevelofthewatersampleswithinlimits. Inthecaseof PR,phenolicgroupsintanninactedas goodhydrogendonors,whichmayhavehelpedtomaintaintheaciditylevels(Yin,2010).

After filtration,theionsresponsiblefortheacidityof thewatersampleswerecaughtintheporous filtermedia. Thus,theacidityofthewatersampleswasreduced.Previousstudieshavereportedinterestingtrendsinacidity reductionwhenwatersampleswerepassedthroughsand filtermedia(Nancyetal.,2014).

InfluenceonDissolvedOxygen(DO)

DOistheamountofoxygendissolvedinwater.The valueofDOtendstodecreaseiforganicmatterispresentinwatersources.Aftercoagulation,itwasobserved thatthevalueofDOdecreasedfrom9.65 6 0.343mg/L to8.92 6 0.059–9.54 6 0.279mg/L.Thesevalueswere slightlyincreasedwhensampleswerepassedthrougha sand filter(Table3).

Aftercoagulation,theDOvaluewasreducedasorganic matterfromthenaturalcoagulantswasreleasedintothe watersamples.However,after filtration,theDOvalue slightlyincreasedasporoussandmediaallowedmore

oxygentobedissolvedinthewatersamples.Thepassage ofwaterthroughthesandcolumnservedasasourceof aerationandtherebyincreasedtheDOofwatersamples.

InfluenceonBiologicalOxygenDemand(BOD)

BODisanindicatorofbiodegradableorganicmatter. Priortotreatment,theBODvaluewasobservedtobe 14.97 6 1.67mg/L.Asthesematerialsareorganicin nature,whenusedascoagulants,someorganicmatter wasreleasedintothewatersamples.Thatiswhythe BODvalueincreased.However,afterbeingpassed throughsand filters,thesevalueswerereducedslightly andwerefoundtobeintherangeof10.64 6 1.408–12.96 6 1.719mg/L(Figure6).

InfluenceonChemicalOxygenDemand(COD)

CODistheamountofoxygenrequiredforthedecompositionofbothbiodegradableandnon-biodegradable organicmatter.ItsvaluesarealwayshigherthanBODvalues.Fromtheexperimentalwork,itwasobservedthat MO,althoughbeingmoreefficient,causedasignificant increaseintheCODvalueascomparedto CA and PR. For rawwatersamples,theinitialCODvaluewasfoundtobe 25 6 6.079mg/L.However, MO increasedtheCODvalue upto31.5 6 5.23mg/L,whichwasfarbeyondthepermissiblevalue(lessthan5mg/L)statedbyWHO.Isolation andpurificationofactivecoagulatingagentsmayreduce thevaluesofCODindrinkingwater.Itwasfoundthat thesevaluestendedtodecreaseafterthe filtrationprocess.

Thesenaturalextractscontainedasignificantamountof organicmatterintheformofBODandCODasreported inpreviousstudies(Blix,2011).Aftercoagulation,some oftheorganicmatterwasreleasedintothewatersample, causinganincreaseinCODandBODvalues.These parametersdonothelptoidentifythespecificorganiccompoundsfoundinthewatersamples(Katayonetal.,2006).

However,after filtration,thesevaluesweresomewhat reduced,supportingpreviouslyreportedobservations showingthatasandbedallowedmoreoxygenforthe conversionoforganicmatterintoinorganicproducts (Elbanaetal.,2012).

InfluenceonChlorides

Priortotreatment,theinitialvalueofchloridecontent wasfoundtobe100 6 4.347mg/L.Theconcentration ofchloridesinwatersampleswasreducedaftercoagulationand filtrationprocesses.Amongthese, CA was foundtobehighlyeffective,havingachlorideremoval efficiencyof54.85percent(Figure8b).

Itwasobservedthatthevalueofchloridecontent decreasedafterbeingtreatedwiththenaturalextracts. Thiswasduetothefactthatcationsfromthenatural extractsattractedthenegativelychargedchlorideions presentinthewaterandneutralizedthem.However, filtrationfurtherimprovedthechlorideremovalefficiency. Mangaleetal.(2012)reportedthechloridereduction trendbypolyelectrolytes.

InfluenceonTotalColiforms

Totalcoliformsinwaterindicatefecalcontamination andthepresenceofothermicro-organismslikeviruses,

bacteria,protozoans,etc.Figure7showsthatthetotal coliformswerereducedfrom2,040 6 36.75colonyformingunits(CFU)/mLto700 6 34.97–820 6 32.98 CFU/mLaftercoagulation.However,theirnumberwas furtherreducedafterthe filtrationofthetreatedsamples. MO showedahighercoliformremovalefficiencyof 86.13percent.Theseextractsshowedstrongantimicrobialactivities,asrepresentedinFigure8d.

Thesearetheorganismsusedtoindicatefecalcontaminationinwater.Coliformsaretheleadingcauseofwaterbornediseases.Fordrinkingwater,thisvalueshouldnot begreaterthanzero.Inthecurrentstudy,theinitialvalue ofcoliformswasfoundtobe2,040 6 36.75CFU/mL, whichwasfarbeyondtherequirementsofWHO.These extracts,beingantimicrobialagents,reducedthevalueof totalcoliforms,butthisvaluestilldidnotmeetWHO guidelinevalues.In MO samples,theactiveantibacterial agentwasfoundtobeglucosidalmustardoil(Mangale etal.,2012).Somepreviousresearchstudiesrevealedthat PR alsoexhibitedantibacterialactivityagainst Escherichiacoli andsomeotherbacteria,buttheactiveagent wasstillunidentified(Pariharetal.,2006).Itwasalso reportedthat CA exhibitedstrongantibacterialactivity against Escherichiacoli, Escherichia fecalmaterial,and othermicro-organisms.Peptidesandproteinsalongwith phenoliccompoundsareresponsibleforitsantibacterial activities(Kanetal.,2010).

Filtrationhadapronouncedeffectoncoliform removal.Ithasbeensuggestedthatwhenthewateris passedthroughaSSF,themicro-organismsbecome attachedtothesandgrains,andastickylayerisformed. Theclearwaterobtainedisrecommendedforhuman consumption(Elbanaetal.,2012).Theseextractsdonot causeanyhealthhazardsasrevealedbytoxicological studies(Kouletal.,2022).

Figure7.Influenceontotalcoliformsbeforeandaftercoagulationand filtration.

Summary

Basedonallthese findings,itwasprovedthatacombinedtreatment(coagulationand filtration)offersthe bestperformanceefficiencyfortheremovalofpollutants inwatersystems.

Physicalqualityparametersincludeturbidity,conductivity,andtotalsolids.Thevaluesofturbidityandtotalsolids werereducedaftertreatment.Mostoftheremovalwasevidentatthe filtrationstage.Valuesofconductivitywere slightlyincreased,butinthecaseofdirect filtration,its valuewasfoundtobedecreased,withapercentageremoval of5.127percent.ThistrendisdepictedinFigure8a.

ThechemicalqualityparametersincludepH,alkalinity, acidity,hardness,andchlorides.Aftercoagulation,values ofpH,alkalinity,andaciditywereslightlyincreased,but theydecreasedafterthe filtrationprocess.Nosignificant reductionwasfoundinthesevalues.TheremovalefficiencyforchemicalparametersisgiveninFigure8b.

TheorganiccharacteristicsthatareusedforthequantificationoforganiccontentincludeBODandCOD.It wasfoundthatthesevaluesincreasedaftercoagulation

butsomewhatdecreasedafter filtration.Forthesevalues, thereductionefficiencywasfoundatthe filtrationstage. ThistrendisrepresentedinFigure8c.

Totalcoliformsaretheorganismsusedtoassessthe biologicalqualityofwater.Thenumberofcoliforms wasreducedaftercoagulation.Theirnumberwasfurther reducedwhenwatersampleswerepassedthroughSSFs. Theefficiencyofcoagulationand filtrationintermsof coliformsremovalisrepresentedinFigure8d.

Finally,itisnotedthatbothwater-treatmenttechniques (conventional/combinedanddirect filtration)arehighly efficientintheproductionofdrinkingwaterwithhigh qualityfromtheviewofphysico-chemicalcharacteristics. Theconventional/combinedtreatmenttechniquebasically usesacombinationofcoagulation,sedimentation, filtration,anddisinfectiontoprovideclean,safedrinkingwater tothepublic.Worldwide,thecombinedmethodisthemost widelyappliedwater-treatmenttechnologyandhasbeen usedsincetheearly20thcentury.Ontheotherhand,direct filtrationinvolvesthe filtrationofwatersampleswithout undergoingcoagulationand flocculation.

Figure8.Removalefficiencyfor(a)physicalparameters,(b)chemicalparameters,(c)organicparameters,and(d)biologicalparameters.

Fida,Tanoli,Mahmood,Alamgir,andSajjad

Our findingshowedthatthecombinedtreatmentoffers thebestperformanceefficiencyintermsofturbidity,total solids,hardness,chlorides,andcoliformremovalascomparedtodirect filtration.Thus,thecombinedmethodwas proventobehighlyeffectiveandmorepotentascomparedtodirect filtrationorcoagulationalone.

CONCLUSIONSANDRECOMMENDATIONS

Conclusions

Inaneraofincreasingenvironmentalissues,natural coagulantsprovideanenvironmentallyfavorablealternativeforwater-purificationpractices.

Forsurfacewatertreatment,threemethodologies havebeenemployed;theyarecoagulation,direct fi ltration,andcombinedtreatment(coagulationand fi ltration).Itwasconcludedthatthecombinedtreatmentis moreeffectiveinimprovingdrinkingwaterquality. The fi lteredwatersamplestreatedwith MO gavethe maximumturbidityreductionof99.17percent,solids reductionof93.21percent,hardnessreductionof67.35 percent,andtotalcoliformremovalof86.13percent. Mostofthepollutantremovalwasnoticeableatthe fi ltrationstage. CA providedamaximumchloridereductionof54.85percentascomparedtotheothertwo extractstested.Theseresultsarequiteencouragingfor futurework.

Amongthesenaturalextracts, MO wasfoundtocause asignificantincreaseinorganiccontentbecausethe organicmatteroriginatingfromtheleavescanbereleased intothewaterduringthetreatmentprocess.Thisorganic mattermayresultinproblemsofodor,color,andtaste andthedevelopmentofmicro-organismsuponstorage. Basedonour findings,itwasconcludedthat MO leaves showedahigherefficacythan CA and PR.Theonlyproblemistheproductionoforganicmatter.Purificationof activeproteinsbyionexchangecanreducethevalueof CODintreatedwater.Thisresearchshowstheeffectivenessof MO forsurfacewatertreatmentandalso illustratesthegreatpotentialof CA and PR aspromisingbio-coagulantsforthetreatmentofpollutedsurface watersamples.

Recommendations

RecommendationsforResearchers

Futureworkshouldbeplannedtofurtherexplorethis areabyundertakingextensivelaboratorystudies.More plantextractsandnaturalmaterialsneedtobetriedfor water-remediationpurposes.

MO leaves, CA seeds,and PR barkcanbeusedas coagulantaids.Itshouldalsobenotedthattheefficiency ofalummaychangeiftheseplantsareusedascoagulant

aids.Theycanalsobeusedforthetreatmentofmunicipal,industrial,andtannerywastewater.

Toremoveorganiccontentfromtreatedwater,other purificationmethodsshouldbeinvestigated.Purification andisolationofactivecoagulatingagentsshouldbe donetoreducetheorganiccontentinwater.

RecommendationsforIndustry

Indevelopingcountries,theuseofplantsascoagulatingagentscombinedwithsand filtrationcanoffersignificantimprovementstodrinkingwaterquality.

Chlorinationcangivethebestresultsifnaturalplants andchlorinemethodsarecombined,andthewaterso producedwillbefreefrompathogenicmicro-organisms. Thereisaneedtodeveloplocalizedwater-purification systemssothatruralareascanusenaturalmaterialsto improvedrinkingwaterquality.

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,Vol.XXX,No.1-2,May2024,pp.77

AProposedProcess-Landform-Material(PLM) SystemforEngineeringGeologyFieldMapping

PAULM.SANTI*

ColoradoSchoolofMines,1500IllinoisStreet,Golden,CO80401

JEFFREYR.KEATON

WSPUSA,1230WWashingtonStreet,Tempe,AZ85281

KeyTerms: Mapping,Surficial,Geomorphology, Landform

INTRODUCTION

SinceWilliamSmithpublishedhisfamous1815geologic mapofGreatBritain,geologicmapshavebecomeanindispensableandversatiletooltounderstandtheEarth.From these “primary” datamaps,usersareabletogenerate “derivative” maps(Dramisetal.,2011)tounderstandmineraland petroleumdeposits,topredictrockunitsandconditionsin thesubsurface,andtoestimateengineeringgeologicpropertiesinareaswheresurficialmaterialsdominate,amongother things.Arecentarticleevengoessofarastosuggestthe valueina “ properties fi rst ” mappingstyleinsteadofa “ classi fi cation first” style(Kirkwood,2022).Inparallel, primaryengineeringgeologicmapsrecordspatialdistributionsthatarethebaseforderivativemaps,suchasEarthrelatedhazards,geotechnicalmaterialsandproperties, expectedbehaviorsofshallowmaterials,andrangesof groundresponsestonaturalandhuman-inducedimpacts.

ThiswastheimplicitgoalofKeaton(1984),whointroducedtheGenesis-Lithology-Qualifier(GLQ)system,incorporatingthemappingelementsfromGalster(1977)to producemapsthataresimple,capturegeologicdatawith engineeringutility,andarebroadlyapplicableandversatile.TheGLQsystemwasrecommendedforengineering geologymappinginCompton’s(1985) GeologyintheField SincepublicationoftheGLQsystem,theauthorshaveused thissystemondozensofengineeringgeologyprojectsand taughtthesystemtonearly1,000students.Basedonthis experience,werealizetheusefulnessofengineeringgeologymapsandthebene fi tsofanupdateandrevisionto addressinconsistenciesandenhanceelementsthatmayhave beenconfusingtomappinggeologistsornon-geologistusers.

Thevalueandutilityofmapsthatdisplayprimarydata inengineeringgeologyarethattheycapturemoreinformationthanaderivativemap,withdatapresentedmore

*Correspondingauthoremail: psanti@mines.edu

objectivelyforconstituentmaterialsandkeycharacteristics.Similartoclassicgeologicmaps,thecreator inherentlyinjectsgenerallyunintendedpersonalinterpretationsandbiases:atsomepoint,adecisionneedstobe madeonthecategoryorformationatalocation,andtwo differentmappersmighthavediverseopinionsorbe guidedbyvariousmappingobjectives.However,theprimarydatamapdoesnotinterpretthesignificanceorcommentonthepropertiesofthemapunitinquestion.Primary engineeringgeologydatamaps,suchasgeology maps,donothaveoverlapinpolygons:thatis,each spotonthemap fi tsintoonlyonecategory,which wouldbeaspeci fi cgeologicformationinageology maporaspeci fi cprocess-landformcombinationina primaryengineeringgeologymap.Derivativemaps donotnecessarilyhavethisrequirement.Forexample,asinglelocationinahazardmapcouldbesubject toseveralhazardssimultaneously.

Accordingly,themainpurposesofthisTechnicalNote are(1)tobrieflyreviewexistingengineeringgeologysystems,(2)todemonstratetheneedforaconsistentand broadlyapplicableprimarymappingmethodology,and (3)topresentandexplaintheProcess-Landform-Material (PLM)mappingclassification.

ENGINEERINGGEOLOGYMAPPINGSYSTEMS

Alargebodyofliteratureofmappingrelatestoengineeringgeology.Someexamplesthatcommentontheprocess includeDearman(1991),Grif fi ths(2016),andThomas (1990).ChaminéandFernandes(2023)provideadetailed citationlistofbestpracticesandexamplesofappliedgeologicmapsforengineeringpurposes,aswellasdiscussion oftechniquesformapping.Toalargedegree,thisliterature demonstratesmappingsystemsthatblendderivative elements,suchashazardanalysis,zoning,risk,andterrain characteristics,alongwithprimarydataelements,suchas surficialmaterials,landforms,andsoiltypes.

Anotherapproachtomappingengineeringgeologystarts withgeomorphologicmaps.Forexample,Hearn(2019)

Table1. Process-Landform-Material(PLM)mappingsystem.Forsoil(non-lithified)materials,theuserstartsatthetopoftheleftcolumn,indicatingthedominantprocessforanobserveddeposit,followedbydesignationofthelandformtypeandthenmaterialgrainsizesandestimateof depositthickness.Underlyingmaterialsmaybedescribedusingthesameprocedure.Forrockmaterials,theuserstartontherightcolumn,indicatingtherocktypeandthedegreeofweatheringandfracturing,alongwithanestimateofunitthickness.

UnconsolidatedMaterialsBedrockMaterials

P(l)m(t),whereMM(ww)(ff)(t),where P ¼ processsymbol;singlecapitalletterMM ¼ conventionalgeologicshorthandforbedrocktype;twocapitalletters (l) ¼ landformsymbol;oneormorelowercaselettersinparenthesesww ¼ degreeofweatheringinparentheses,ifavailable m ¼ materialsymbol;oneormorelowercaselettersff ¼ degreeoffracturinginparentheses,ifavailable (t) ¼ thickness,ifknown;includingfeetormetersymbolinparentheses(t) ¼ thickness,ifknown;includingfeetormetersymbolinparentheses

ProcessSymbols(“P” Above) (LandformSymbols; “l” Above)BedrockType(“MM” Above)

A 5 alluvial(runningwater) (f) ¼ fan (fp) ¼ present floodplain (te) ¼ terrace (p) ¼ pedimentdeposit (s) ¼ streamdeposit

R 5 residual(nomovement) (sa) ¼ saprolite (wp) ¼ weatheringprofile

C 5 colluvial(slightmovement) (sw) ¼ slopewash (cr) ¼ creepdeposit

S 5 slide(significantmovement) (fl) ¼ debris flowfan (da) ¼ debrisavalanche (sl) ¼ slump (ls) ¼ lateralspread (tr) ¼ translational (ro) ¼ rotational (to) ¼ topple (fa) ¼ fall(rockorsoil) (ta) ¼ talusconeorchute (ra) ¼ rockavalanche

G 5 glacial (ot) ¼ outwashterrace (k) ¼ kame (es) ¼ esker (em) ¼ endmoraine (gm) ¼ groundmoraine (lm) ¼ lateralmoraine (rg) ¼ rockglacier

P 5 periglacial (pg) ¼ patternedground (ge) ¼ gelifluction (so) ¼ solifluction

L 5 lacustrine

(le) ¼ lowenergy (b) ¼ beach (de) ¼ delta (ma) ¼ marsh (va) ¼ varves (pl) ¼ playa

M 5 marineandcoastal (b) ¼ beach (de) ¼ delta (et) ¼ estuary (ma) ¼ marsh (sp) ¼ swamp (ti) ¼ tidelands

E 5 eolian (d) ¼ dune (l) ¼ loess (st) ¼ strippedplains

V 5 volcanic (a) ¼ ash (la) ¼ lapilli (pu) ¼ pumice (cl) ¼ clinker (vc) ¼ volcaniclastic (lh) ¼ lahar

K 5 karst (si) ¼ sinkhole (uv) ¼ uvala (po) ¼ polje (to) ¼ towerkarst

F 5 fill(humanmade) (u) ¼ uncompacted (e) ¼ engineered

Note:A/C ¼ interbeddedalluvialandcolluvialmaterials; A-C ¼ uncertaintyofprocess.

Sedimentary

SS ¼ sandstone

SH ¼ shale

LS ¼ limestone

DO ¼ dolomite

CG ¼ conglomerate

BR ¼ breccia

ST ¼ siltstone

CH ¼ chert

CS ¼ claystone

DT ¼ diatomite

CE ¼ cementedsoils

Igneous

GR ¼ granite

BA ¼ basalt

GA ¼ gabbro

DI ¼ diorite

FE ¼ felsite

SY ¼ syenite

TU ¼ tuff

BR ¼ breccia

RH ¼ rhyolite

AN ¼ andesite

IG ¼ undifferentiated

Weathering(“ww” Above)

xw ¼ extremelyweathered (soil-like)

hw ¼ highlyweathered (weakenedthroughout)

mw ¼ moderatelyweathered (penetrativediscoloring)

sw ¼ slightlyweathered (discoloredfractures)

uw ¼ unweathered

Metamorphic

QT ¼ quartzite

AR ¼ argillite

SC ¼ schist

GN ¼ gneiss

GS ¼ greenstone

HO ¼ hornfels

PH ¼ phyllite

MA ¼ marble

SL ¼ slate

SE ¼ serpentine

ME ¼ undifferentiated

Humanmade

CC ¼ Portlandcementconcrete

AC ¼ asphaltconcrete

PA ¼ undifferentiatedpavement

FractureSpacing(“ff” Above)

nf ¼ narrow(,2cm)

nmf ¼ narrowtomoderate(2–6cm)

mwf ¼ moderatetowide(6–20cm)

wf ¼ wide(20–60cm)

vwf ¼ verywide(60–200cm)

ewf ¼ extremelywide(.200cm)

Note:Absenceofsymbolsignifieseithernonoteworthycharacteristicor undifferentiatedcharacteristics.

MaterialSymbols(“m” Above)Examples

c ¼ clay

m ¼ silt

s ¼ sand

g ¼ gravel

k ¼ cobbles

b ¼ boulders

r ¼ rockrubble

t ¼ trashordebris

e ¼ erraticblocks

p ¼ peat

o ¼ organics

d ¼ diatomaceous

Note:Mostabundantorsignificantmaterialsymbolappears first, followedbylessabundant;cm/ms ¼ interbeddedsiltyclayand sandysilt;m-b ¼ allmaterialsfromsilttobouldersarepresent.

C(sw)msg(0.6–1.3m)

L(le)ms/sm(3.3m)

R(wp)mcs(0.3–0.6m)

G(em)c-k(12m)

GR(uw)(vwf)

Colluvialslopewashdepositscomposedof gravellyandsandysilt0.6–1.3mthick,overlying3.3mofinterbeddedlacustrinelowenergydepositsofsandysiltandsiltysand

Residualweatheringprofileofsandyand clayeysilt0.3–0.6mthick,overlyingglacial endmorainecomposedofalltexturesfrom claytocobbles,possibly12mthick,over unweathered,verywidelyfracturedgranite

usesacasestudyapproachtohighlightexamplesofmaps thathaveastronggeomorphologiccomponent,suchasby superimposinggeomorphologicfeatures(e.g., “colluvium,” “mudslide,”“ rockdeposit ” )ontoageologicmaporby includingdetailedtextdescriptionsoffeaturesonthemap itself.Theauthorcitestheneedforconsistencyinincorporationofgeomorphologicinformationonmaps,hampered bythelackofawidelyacceptedmappingapproach(aproblemalsonotedbyGriffiths,2016).SmithandGriffiths (2011)offeradetailededitedvolumeoftrendsandmethodsingeomorphologicmapping,techniques,andcasestudies.ThegoalofthisTechnicalNoteistocapturemany ofthevaluableaspectsofthevariousmappingmethodsandcombinethemwithotherprimaryengineeringgeologicinformationintoasinglesystemthatcan havewidespreadapplicability.WecallthisPLM classi fi cation.

THEPLMSYSTEM

TheessenceofthePLMmappingsystemisshownin Table1.SimilartotheGLQsystem,itcanbeapplied tobothunconsolidated(soil)andconsolidated(rock) materials.Itinherentlyfocusesonsur ficialdepositsbut hasthecapabilitytoincludeinformationontheunderlyingmaterials.ThePLMsystemincorporatesseveral modifi cationsoftheGLQsystembyclarifyingsomecomponents,improvingtheintuitiveaspectsofthecategorization,andaddingquantitativeanddescriptiveinformationto theclassification.

First,wesuggestrenamingtheoriginalGLQtermswith onesthatrelatemoredirectlytogeomorphology,sousers whoaremorefamiliarwithsurficialmaterialsorgeomorphologicmappingwillseetheparallels.Inthiscase, “Genesis” becomes “Process,”“Qualifier” movesforwardinthe orderandisreplacedby “Landform,” and “Lithology” is replacedby “Material,” abroadertermthatmorereadily encompassessoilmaterial,aswellasrock.TherevisedsystembecomesPLM.

Next,wehaverevisedthealluvial,colluvial,and slidecategoriestoreduceconfusionbybetterindicatingtherateofmovementandinfl uenceofrunning water.Asageneralprinciple,weusetheterm “ alluvial, ” followingtheconventionthattheterm “ fl uvial ” (trueriverinedeposits)isasubsetof “ alluvial ” (which alsoincludesfan,terrace,andpedimentdeposits).This simpli fi cationavoidspotentialconfusionofusingboth fl uvialandalluvialterms.

Wehaveremovedthe “modifier” termforsoils,indicating “cemented,”“expansive,” andsoon,asitdoesnotseem tohavewidespreaduse.

Wehaveaddedmoreprocessandlandformcategories undertheUnconsolidatedMaterialscolumntobetterencompasstherangeofgeomorphiclandforms.Erosional

Figure1.ExampleofaPLMmapfromtheAnimasRiverValley nearSilverton,CO.BasemapcreatedbyEsch(personalcommunication,2020).Notethatunlabeledgraypolygonsrepresenteroded alluvialanddebris- flowchutes.RefertoTable1forexplanationof symbols.

landforms(e.g.,cirques,arêtes,incisedvalleys)arenot included,butasforotherfeatures,usersarefreetodevelop categoriesthatmightapplytospecificsites.

UndertheBedrockMaterialscategory,wehaveadded detailedandquantitativedescriptorstotheweathering andfracturespacingterms(fromDearman,1995)toimprove consistency.Thegeologicstrengthindex(Marinosand Hoek,2000)orotherquantitativeindexparametersfor rockcouldbeaddedasdesiredbytheuser.

Finally,wehaveorganizedthesystemintoasingletable (Table1)tobemore fieldfriendly,soausercanworkdown eithertheUnconsolidatedMaterialsorBedrockMaterials column,asneeded.

Figure1isanexamplemappedareanearSilverton, CO.Inthisexample,thereareunitlabelsforvolcanic bedrock,taluscones,debris flowfans,alluvialfans,alluvial floodplains,uncompacted fill,lateralmoraines,and colluvialslopewash.

CONCLUSIONS

Therearemanyexamplesinthetechnicalliteratureof geomorphicandengineeringgeologicmapping,accompaniedbystrongargumentsfortheutilityofthissortof map.However,thereisnotaconsistentsystemtoprovide primaryengineeringgeologymaps,thatis,mapsthatprovide objectiveinformationfromwhichderivativemapscanbe developedforspecificpurposessuchasEarth-relatedhazards,geotechnicalmaterialsandproperties,expectedbehaviorsofshallowmaterials,andrangesofgroundresponses. TheproposedPLMsystemisintendedto fillthisneed.The PLMsystemisanupdateandrevisiontothepreviouslypublishedGLQsystem(Keaton,1984),benefittingfromalmost 40yearsofuseandfeedback.ThePLMsystemisnowable toconsistentlyclassifyengineeringgeologyandgeomorphic informationinasingletable.Further,itisreadilyexpandable toaccommodateanyparticularuserneed;thus,itisinherentlyamenabletofuturechange.

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