E&EG Journal Volume XXX, Number 4

Environmental& EngineeringGeoscience

NOVEMBER2024 VOLUMEXXX,NUMBER4

Environmental& EngineeringGeoscience

Volume30,Number4,November2024

TableofContents

173QuarriesasaSourceofNitratePollutioninKarstAquifers:CaseStudy,theEdwardsAquifer,Texas JamesDavidDoyle

193DeterminingGroundwaterTableinSlopesWithHorizontalDrainsandDesignFrameworkforLength andSpacing

MihiraLakruwan,AkiyoshiKamura,andMotokiKazama

211

229

UsingtheTotalApparentSpacingApproachtoOptimizeFoundationDrainOrientations

WilliamM.RomanandJohnN.Berezniak

Developmentofa3DGeologicModelUsedintheSeismicHazardandLiquefactionHazardAnalysisof MadisonCounty,Tennessee

ValarieJoynerHarrison,HamedTohidi,RoyVanArsdale,ChrisCramer,andDavidArellano

UsingLandslidestoLocatetheEpicenterfortheHistoric1882EarthquakeinColorado JustinMarchlewiczandPaulSanti

BookReview

Introduction to Waste Management: A Textbook Thomas Oommen

QuarriesasaSourceofNitratePollutioninKarst Aquifers:CaseStudy,theEdwardsAquifer,Texas

JAMESDAVIDDOYLE*

200CedarPark,CanyonLake,TX78132

KeyTerms: EdwardsAquifer,Nitrate,Aggregate, Quarries,KarsticAquifer

ABSTRACT

TheSanAntoniosegmentofthekarsticEdwards Aquiferprovidesanexampleoftheimpactofaggregate quarriesonnitrateloadsinkarstaquifers.Previous publishedreportshavenotevaluatedaggregatequarriesasapossiblecauseofnitrateconcentrationsinthe EdwardsAquiferabovenormalbackgroundlevelsand insteadattributenitratetorecenturbandevelopment directlyovertheaquifer’srechargezone.Quarriesare knowntobesourcesofnitratepollutionofgroundwaterresultingfromuseofammoniumnitratefuel–oil (ANFO)mixturesasexplosivesinminingoperations. Ammoniumnitrateishighlysolubleandleachesreadily intotheformationbeingmined.Largequantitiesare used,upto30percentofwhichisnotconsumedby blasting,thusconstitutingapotentialgroundwater pollutant.GroundwaterdatafromtheTexasWater DevelopmentBoardandstreamdatafromtheTexas CommissiononEnvironmentalQualityshowthat urbanizationcannotexplainthegeographicdistributionandhistoricalchangesofnitrateintheaquifer. Thegeographicdistributionofnumerouslimestone quarries ﬁtswiththedata,andnitrateconcentrations intheEdwardsAquiferhaverisensinceANFOuse asanexplosivebeganonalargescale,whichindicates quarriesareamajorcontributorofnitratetothe aquifer.Water-qualitymeasurementsofnitrateinthe EdwardsAquifermostlyremainatalevelsafefor humans.However,somefreshwateraquaticorganismsareharmedbyprolongedexposuretolower nitratelevels.Recentnitratemeasurementsshowthat mostmeasurementshavereachedalevelthatmight beharmfultosomegroundwaterspecies.

INTRODUCTION

TexasleadsallAmericanstatesintheproductionof crushedstone.TheTexasHillCountryislocatedinthe southeasterncornerofthekarsticEdwardsPlateauand isamajorproductionareaforlimestoneaggregate(Kyle

*Correspondingauthoremail: bangeo1@hotmail.com

andElliot,2019).State-wide,limestoneaggregateproductionhasincreasedsteadilyovertheyears.Forexample,from2010to2018,annualcrushedlimestone productioninTexasincreasedfrom104millionto161 millionmetrictons(NationalMineralsInformationCenter,2021).Air,water,andnoisepollutionfromquarries hasbecomeanincreasingconcern,particularlywhere quarriesareconcentratedalongtheInterstateHighway 35corridor(Figure1;Chasnof,2021).Priorstudiesof nitrateconcentrationintheEdwardsAquiferknownto theauthorhavenotconsideredquarriesasapotential source.Thepurposeofthisstudyistousepublicdatato assesswhetherquarriesaresigniﬁcantcontributorsof nitratepollutiontotheSanAntoniosegmentofthe EdwardsAquiferasrepresentativeofkarstaquifersin general.

Background

EdwardsAquiferhydrologicalzonescompriseacontributing(ordrainage)zonethatdeliverssurfacewater toarechargezonewheretheEdwardsLimestoneisat thesurface,andaportionofthewater ﬂowingfromthe contributingzonepassesintothesubsurfaceviafracturesandkarstfeatures.Inarelativelynarrowtransition zone,youngerstrataareatthesurface,butwatercan reachtheEdwardsAquiferthroughfaultsandfractures. TheEdwardsAquiferisunderlainbytheTrinityAquifer,whichalsoextendsnorthoftheEdwardsAquifer rechargezonewheretheTrinitygroupisatthesurface (SharpandGreen,2022).TheEdwardsTrinityAquifer occursinthenorthwesternpartofthestudyarea(Figure 1).Asidefromstreamsthatcrossfromthecontributing zonetotherechargezone,theEdwardsAquiferis sourcedfromrainfallrunoffontherechargezone,from cross-formation ﬂowwiththeTrinityAquiferwherethe EdwardsandTrinityaquifersarejuxtaposedbyfaultsin theBalconesFaultZone,andfromtheEdwardsTrinity PlateauAquiferinUvaldeCounty(Lindgrenetal., 2004).Intheartesianzone,theEdwardsLimestoneis overlainbyyoungerimpermeableformations(Schindel, 2019;TCEQ,2019).Quarriesaremostlysitedonthe rechargezone,whereasthecontributingzonehasafew smallquarries.NearthetownofUvalde,quarriesappear tobetargetinglocalEdwardsLimestoneoutcropssouth oftherechargezone(Figures1–3).

Figure1.Quarrylocationsshownwithreferencetomajoraquifers(TWDB,n.d.a)andzonesoftheEdwardsAquifer.LocationsofEdwards AquiferzonesarefromTCEQ(2019)andEAA(2014).

Theaggregateindustrymostlyusesammoniumnitrate fuel–oil(ANFO)mixturesasexplosives.ANFOmixturesvarysomewhatbuttypicallyareastoichiometric compositionof94.5percentammoniumnitrateand5.5 percentfueloil.AmmoniumnitrateisasaltthatdisassociatesinwatertoNH4þ andNO3 anddissolves readily(Brochu,2010).Nitrifi cationconvertsammoniumtonitratenitrogen(AmericanWaterworksAssociationandEconomicandEngineeringServices,Inc., 2002).LossofANFObyleachingtotheformation beingminedisvariableandin fl uencedbyseveralfactors,includingspecifi cationsoftheexplosive,natureof thesitebeingmined,designofboreholesandexplosive patterns,andlengthoftimebetweenloadingboreholes anddetonation(Brochu,2010).Evenslightamountsof waterinboreholescandissolveenoughANFOtopreventdetonation(KonyaandKonya,2019).Asidefrom beingleachedfromboreholes,about28 –30percentof ANFOthatisusedisnotconsumedinblasting (Alberts,2016).Consequently,nitratepollutionof groundwaterfromminingisarecognizedproblemin somepartsoftheworld(GolderAssociates,2014; Alberts,2016).

Somestates,e.g.,NewHampshireandVermont,regulatetheminingprocesstominimizethepotentialfor groundwatercontaminationandrequiregroundwater monitoringduringthelifeofaquarry(Kernan,2010; AgencyofNaturalResources,2014).TheTexasCommissiononEnvironmentalQuality(TCEQ)requiresa WaterPollutionAbatementPlan(WPAP)forconstructionontherechargezoneandhasestablishedbestmanagementpractices(BMPs)forquarriestomeetwaterqualitystandardsfortheEdwardsAquiferthatarenot applicabletootheraquifers(Barrett,2005;Barrettand Eck,2012).TheBMPsaddresshandlingofsensitive karstfeatures,preventionofspills,andcreationofberms toisolatethesite.However,themannerofconducting blastingoperationsisnotaddressed,andmonitoringof wellsisnotrequired.

Toavoiddirectlyintroducingcontaminantstothe EdwardsAquifer,oneoftheBMPsrecommendedbythe TCEQistomaintainthebaseofquarryoperationsat least7.6m(25ft)abovetheaquiferlevelexpectedfor comparativelywetyears,deﬁnedashaving110cm(45 in.)ormoreofrainfall.Theheightabovetheaquiferis basedonanestimateddepthofinducedfractures

Figure2.QuarrylocationsareshownwithreferencetooutcropsofstratigraphicunitscorrelativetotheEdwardsandTrinitygroups.SurfacegeologyisfromtheUSGS(2022).RechargeandtransitionzoneoutlinesarefromTCEQ(2019).

beneathshotholes.Awet-yearaquiferelevationestimateforthequarrysitemustbebasedonawaterelevationmeasurementfromanon-sitewellorcloseoffset well.Therecordedwaterelevationintheon-sitewell mustbeadjustedusingawellinthesamecountywith historicalwatertableelevationdataandbydetermining thedifferencebetweenthewaterelevationsonthe

observationdateintheon-sitewellandthewet-yearelevationinthereferencewell(BarrettandEck,2012). AccordingtotheTexasAggregatesandConcrete Association(TACA),washingofaggregateisaroutine activity(TACA,2020),andthe ﬁnesareaccumulatedin settlingponds.Pondsmayusealiner,butduetothe scaleofwaterhandling,operatorsmaycreatepondsby

ﬁedwithpermissionfromEAA(2020).

Figure3.Generalizedcross-sectionillustratingthezonesoftheEdwardsAquifer.Modi

sealingvoidsandfracturesonthepond ﬂoorandpermeablebedsonthewallsliningthepond.Somewaterloss totheformationisexpected(BarrettandEck,2012).

TheTexasWaterDevelopmentBoard(TWDB)maintainsagroundwaterdatabase(GWDB)thatincludes water-qualitymeasurementsfromwellsamples(TWDB, n.d.b)thatarepredominantlyreportedinmilligramsper literas “nitrateasNO3” butmaybereportedas “nitrate asN.” Theratioofthetwomeasures(TWDB,n.d.c) usedforconversionisasfollows:

nitrateasNO3 ¼ nitrateasN 3 4 427

Inthispaper,nitratereferstonitrateasNO3.

Backgroundnitratelevelsaretypically4.4mg/L(1.0 mg/LnitrateasN)foraquifersingeneral(Dubrovsky etal.,2010)andfortheEdwardsAquiferinparticular (DepartmentoftheInterior,FishandWildlifeService, 2012).TheU.S.EnvironmentalProtectionAgency (EPA)setthemaximumcontaminantlevel(MCL)for nitrateinpublicwatersuppliesat40mg/L(10mg/L nitrateasN)forhumanconsumption(EPA,2017).Above theMCL,risksareprimarilytoinfants,whomaydevelop methemoglobinemia(bluebabysyndrome),whichcanbe fatal.Inaddition,forpregnantwomen,drinkingwater wherenitratelevelsareabovetheMCLhasbeenlinkedto lowbirthweights(Swistock,2022).Somefreshwater aquaticspecieshavenitratetoleranceslowerthanthe MCL.InTexas,theecologicalscreeningbenchmarkfor nitrateadoptedbytheTCEQforprotectingfreshwaterlife is13mg/L(TCEQ,2022b).Othershaveconcludedthata maximumnitratelevelfrom8mg/L(Monson,2022)to9 mg/L(Carmagoetal.,2005)wouldbeanappropriatelimit forprotectingsensitivefreshwaterspecies.

PreviousInvestigations

Bushetal.(2000)reviewedwater-qualitydatafor South-CentralTexasfortheperiodfrom1996to1998. AlthoughtheynotedthatnitratelevelsintheEdwards Aquiferwereelevatedrelativetothoseofmostother aquifers,includingtheunderlyingTrinityAquifer,they thoughtitwasrelativelyunaffectedbyhumanactivity. Regardingsurfacewater,theyfoundthatnitrateconcentrationswerehighestclosetowastewatertreatment plants,whicharemostlyovertheartesianzoneofthe EdwardsAquifer,wheretheaquiferisconﬁnedabove byimpermeablestrata(Figure3).Althoughthecontributingandrechargezoneshadbeguntoexperiencepopulationgrowth,theydescribedthoseareasasmostly rangelandanddidnotgointodetailaboutwhatmay havecausedtheaquifer ’selevatednitratelevel.

ReedyandScanlon(2017)reviewednitrateconcentrationsinallaquifersinTexaswithafocusonunderstandingnitratehotspotsexceedingtheMCL.Nitrate

sourceswereconsideredtobeeithernatural(atmosphericdeposition,nitrogen ﬁxationbylegumes,nitriﬁcationoforganicmaterial,andgeologicalsources)or anthropogenic(fertilizers,animalfeedingstations,septic tanks,andsewagesystems).Theyreviewedpublicwater supplysystems(PWSs)andwater-qualitysamplesfrom wells.FortheEdwardsandTrinityaquifers,nitratewas foundtobegenerallybelowtheMCL.However,regardingpercentagesofsamplesabovetheMCLfornitrate, theEdwardsTrinityAquiferwasamongthefourhighest aquiferstheyanalyzed,withthehighestvaluesfallingin thenorthernpartoftheaquifer.

Musgroveetal.(2016)evaluatednitratelevelsinthe BartonSpringsandSanAntoniosegmentsofthe EdwardsAquifer,whichareseparatedbyapotentiometricdivide.Theyusedsamplestakenin2011and2012 fromstreamsandfromwellsintheSanAntoniosegmentinBexarCountyandconsideredhistoricaldata fromComalSprings.ComalSpringsdataconsistedof sevensamplesfromthe1930sand1940s,onesamplein 1974,and27samplesfrom2000to2012.Althoughthey notedadoublingofnitratevaluesbetweenthe1940s and2000,thedatatotellwhetherthechangewasgradualorabruptwerelacking.TheauthorsstatedthatagriculturallandusesareminorincentralTexasand indicatedthatthesecouldnotaccountforelevated nitratelevelsintheEdwardsAquifer.Theyalsoindicatedthatstreamsamplescannotaccountforelevated nitratevaluesintheEdwardsAquifer.Instead,theypostulatedthaturbanizationsince2000overtherecharge zoneinnorthernSanAntoniocontributeshighlevelsof nitrogenspeciestotheaquiferandthatorganicnitrogen undergoesnitriﬁcationintheaquifer.

SullivanandGao(2016)addressednitrateconcentrationsinsurfacewatersinthedrainagebasinsofCibolo CreekandDryComalCreek,whicharemostlyinBexar, Comal,andKendallcounties.Thestudyareaincludes bothportionsofthecontributingzone,wheretheGlen RoseFormationoftheTrinitygroupisatthesurface,and therechargezone,wheretheEdwardsgroupisatthesurface.ASoilandWaterAssessmentTool(SWAT)model wasdevelopedforsurfacewatersinthedrainagebasins. SullivanandGao(2016)considerednitrateinputsfrom agriculture,urbanization,wildlife,andatmosphericdeposition.Thestudyassumedthesamebackgroundnitrate concentrationforbothEdwardsandTrinityaquifersbased onamedianvalueof8.19mg/L(1.85mg/LnitrateasN) reportedfortheEdwardsAquiferbyMusgroveetal. (2010)for1996–2006.NitratemeasurementsintheGlen RoseportionoftheTrinityAquiferwerenotdirectlyconsidered.Fromthecalibratedmodel,theauthorsconcluded thatthetwomostimportantnitrateinputswerefrom atmosphericdepositionfollowedbylivestock.

Sullivanetal.(2019)useda ﬁnitedifferencegroundwatermodel(MODFLOW)toaddressnitrateconcentrationsintheEdwardsAquiferfor2001–2010.Forthe studyareaofSullivanandGao(2016),nitraterecharge valuesfortheEdwardsAquiferwerebasedonthe SWATmodelresults.Elsewhere,theyweresettoaveragevaluesfortheEdwardsAquifer.Crossﬂowfromthe TrinityAquiferwasmodeledas9.34mg/L(2.11mg/L nitrateasN)basedonanaveragevaluederivedfrom TWDBdata.Theauthorsdeemedthestudysuccessfulin predictingnitratelevelswithin1,000moftherecharge zone.

RESEARCHMETHODS

Quarrylocations(Figure1)werecompiledfromseveralsources(TCEQGISTeam,2020;TexasHearstData VisualizationTeam,2021).Inaddition,quarrieswere outlinedfromsatelliteview.Usingopen-sourcegeographicinformationsystemsoftware,anareawascomputedforeach.

WelldataintheGWDBincludeaquifercodesthat weremostlydeterminedbyTWDBgeologists(TWDB, n.d.c).Theaquifercodeswerethebasisforselecting datafromtheGWDB.Nitratemeasurements,forparametercode71851, “NitrateNitrogen,Dissolved,Calculated(mg/LasNO3),” wereobtainedfromtheGWDB fortheEdwardsAquifer.Inaddition,measurementsfor parametercode618, “NitrateNitrogen,Dissolved(mg/L asN),” wereconvertedtonitrateasNO3.Thesewere reviewedforgeographicvariationandhistoricalchanges. NitratemeasurementsfromtheTrinityandEdwardsTrinityaquiferswerealsoevaluatedtoassesstheirpotentialfor contributingnitratetotheEdwardsAquiferviacrossﬂow. Somewater-qualityanalysespriorto1940maynotbereliable(WelderandReeves,1964),soallaquifersmeasurementspriorto1940wereexcludedfromthisstudy.Also excludedweresamplemeasurementsdesignatedinthe TWDBdatabasewithareliabilitycodeof14, “reliability unknownornotrecorded.”

Water-qualityanalysesincludingmeasurementsofdissolvedorganicnitrogenwereobtainedfromtheGWDB. ForEdwardsAquifersamples,totalnitrogenwasdeterminedfromdissolvedorganicnitrogen,nitrate,nitrite, andammonia.SamplemeasurementsrecordedasmilligramsperliterasNwereconvertedtomilligramsperliter asNO3.Dissolvedoxygenmeasurementswereobtained fromtheGWDBasavailable.

Nitratemeasurementsforparametercode620, “Nitrate Nitrogen,Total(mg/LasN),” fromsurfacewater-quality datawereobtainedfromtheTCEQ(TCEQ,2022a,2024) andconvertedtonitrateasNO3.Forsurfacewater-quality datawithmeasurementsofdissolvedorganicnitrogen, totalnitrogenwascalculatedandconvertedtomilligrams

perliterasNO3.Dissolvedoxygenmeasurementswere obtainedfromtheTCEQdatabaseasavailable.

TheEdwardsAquiferAuthority(EAA)records water-qualitymeasurementsfromwellandsurface locations.Inresponsetoarequestforallnitratemeasurementscollected,theEAAprovideda ﬁ leofmeasurementstakenfromDecember2004toJuly2023, whichincludedbothsurfaceandgroundwaterdata. SomeEAAdataareintheGWDB.TheEAAusesa uniquenomenclatureforsamplesitesthatisnotcrossreferencedtothestatewellnomenclature(EAAOpen RecordsCenter,2024,personalcommunication).Due tothedif ﬁ cultyofdeterminingtheoverlapbetween EAAandGWDBsources,theEAAdatawereevaluatedseparately.

Agriculturalactivitiesassociatedwithfarmingand livestockmaybeasourceofnitratecontaminationof groundwater(ReedyandScanlon,2017).Croplandharvestedandcattle-and-calvesinventorytabulatedinagriculturalcensusessince1950werereviewedasindicators offarmingactivity(U.S.DepartmentofAgriculture, 2024).Agriculturalcensusdataarereportedbycounty. Tohelpdeterminewhetheragriculturalactivityhas playedasigniﬁcantroleinexplainingnitratelevelsin theEdwardsAquifer,distributionoffarmlandbasedon datafromtheMulti-ResolutionLandCharacteristicsConsortium(MRLC)wasexamined(MRLC,2019).Locationsofconcentratedanimal-feedingoperations(CAFOs) relativetotherechargezonewerealsodetermined(CarnegieCorporationandUniversityofIowa,2017).

Musgroveetal.(2016)concludedthaturbanization overtherechargezoneistheprimaryfactorbehindelevatednitratelevelsintheEdwardsAquifer.Aspartof assessingthat,populationchangebycountywasdeterminedbytheU.S.CensusBureau.Thatdata,together withlandusemaps,allowcomparisonofthelocation andtimingofpopulationchangetothelocationandtimingofrisingnitratevalues.

SpeciﬁclimestoneaggregateproductiondataforindividualquarriesortheHillCountryasawholearenot available.Anexceptionisasinglementionofoneday’s useofexplosivesattheServtexPlantinGardenRidge, Texas(Chasnof,2021).FortheServtexexample,explosiveusageperunitvolumeminedwascalculatedfor comparisontotypicalindustryusage.

Toevaluatewhetherminingcontributessigniﬁcant amountsofnitratetotheEdwardsAquifer,industry informationontypicalexplosiveuseandunexploded residualsisappliedtoanexampleofaquarriedareaon therechargezone.Thecalculationisbasedontherock volumeremovedfroman8.09 3 105 m 2 (200acre)portionoftheCemexBalconesNewBraunfelsQuarryin ComalCounty.Thevolumeisderivedfromelevation contoursdigitizedfromgeoreferencedU.S.Geological

Figure4.ForpartoftheCemexquarry,contoursweredigitizedfrom USGStopographicmapsfortheNewBraunfels,WestTexas,quadranglefrom(A)1958and(B)2022.Contoursarelabeledinfeet abovemeansealevel.

Survey(USGS)topographicmapsoftheNewBraunfels, WestTexas,quadranglemadein1958(priortoquarrying)and2022(USGS,n.d.),asshowninFigure4.Digitalelevationmodels(DEMs)weremadefromboth topographicmapsandusedtocalculatethevolumeof rockremoved.

RESULTS

Atotalof80quarrieswereidentiﬁed.Ofthese,16are onthecontributingzone,andtheremaining64fallalong therechargeandtransitionzones(Figure1).Ofthe16 quarriesonthecontributingzone,10are4.05 3 104 m 2 (10.0acres)orless,andthelargestis4.38 3 105 m 2 (108 acres).Areameasurementsofcontributingzonequarries totaled1.74 3 106 m 2 (429acres).The64quarriesfalling alongtherechargezonerangedupwardto6.7 3 106 m 2 (1,730acres),andthemedianvaluewas2.28 3 105 m 2 (56.3acres).Areameasurementsofrechargezonequarriestotaled4.02 3 107 m 2 (9,920acres).

Historically,miningengineersuseda “powderfactor” toestimatetheamountofexplosiveperunitvolume. Thepowderfactorforamedium-hardrocktypewould be0.4–0.5kg/m3 (DynoNobel,2010).Forcomparison, theServtexPlantinasingledayused5,900kg(13,000 lbs)ofexplosivestoremove2 3 107 kg(20,000tons)of rock(Chasnof,2021).Ifthetypicalvalueofunexploded residual(28percent)isapplied,thatsuggeststhatfor oneday,1,700kg(3,640lbs)ofunexplodedANFO wouldbeavailabletoleachintotheaquifer.Material thatistoo ﬁne-grainedtobecommercialmayconstitute 20–30percentofthevolumemined,asindicatedina WPAPfortheMartinMariettaMaterialsNewBraunfels Quarry(Forster,2010).If25percentofaminedvolume isleftbehind,theexplosiveusageattheServtexPlant translatestoapowderfactorof0.6–0.5kg/m3 foralimestoneporosityrangeof15–25percent(Table1).

Soilthicknessisnegligibleintheareaofthe8.09 3 105m2 (200acre)portionoftheCemexBalconesNew BraunfelsQuarry(Pelletieretal.,2016).Therockvolumeremoved,calculatedbysubtractingthe2022DEM fromthe1958DEM,is1.99 3 107 m 3 (7.03 3 108 ft3). Applyingapowderfactorof0.45kg/m3 andavalueof 28percentforunexplodedresidualwouldresultin2.5 3 106 kg(5.5 3 106 lbs)ofANFO,or2.4 3 106 kg(5.3 3 106 lbs)ofammoniumnitrate,leftbehind(Table2).Since nitratefromANFOcanbeintroduceddirectlytogroundwaterandammoniumissubjecttonitriﬁcation(Musgrove etal.,2016),thepotentialamountofnitrateafternitriﬁcationtotals3.7 3 106 kg(8.2 3 106 lbs).

U.S.CensusBureaudatashowacontrastinpopulationgrowthbetweenBexarCounty,alongwithComal

Table1. Explosiveuse(powderfactor)estimatefortheServtexplantinGardenRidge,Texas. *

*BasedonChasnof(2021).

† Usinglimestonematrixdensity ¼ 2.710gm/cc(AsquithandGibson,1982).

‡ Using25percentvolumeofnon-commercial ﬁnes(Forster,2010).

Table2. Potentialnitrateresidualfromquarrying8.09 3 105 m 2 (200acres)inComalCounty.

† (kg)ResidualANFO‡ (kg)Ammoniumnitrate

*Basedon1958and2022USGStopographicmapsfortheNewBraunfels,WestTexas,quadrangle(USGS,n.d.).

† Using0.45kg/m3 (DynoNobel,2010).

‡ Using28percentnotconsumedinblasting(Alberts,2016).

§ Using94.5percentofANFObyweight(Brochu,2010).

andHayscounties,andMedinaandUvaldecountiesto thewest.BexarCounty’spopulationincreasedfrom 1,185,394in1990to2,009,322in2020,whichis20percentmorethanitsincreaseintheprevious40years.From 1990to2020,ComalCounty’spopulationincreasedfrom 51,832to161,482,whereastheHaysCountypopulation increasedfrom56,225to241,066.Forthesametime period,thepopulationinMedinaandUvaldecounties increasedfrom27,312to50,739and23,340to24,564, respectively.

TherechargezoneoccursinBexar,Comal,Hays, Medina,andUvaldecounties.SinceMedinaandUvalde countiesareupgradientinthe ﬂowdirectionofthe EdwardsAquifercomparedtothemorepopulouscountiestotheeast(Figure5),nitratemeasurementsmade since1990fromwellsintheEdwardsandTrinityaquifers weredividedintoeastandwestareas(Figure6AandB).

Backgroundnitrateconcentrationsareexpectedtobe4.4 mg/Lorless(Dubrovskyetal.,2010),sonitratevalues wereplacedinseparatecategoriestoemphasizechanges inthelowend.Measurementsinthefourthcategory(5–8 mg/L)andaboveindicatenitrateconcentrationsabove background.Thethreehighestcategoriesmarktherange inwhichnitratevaluesareharmfultosomefreshwater aquaticanimals(8–13mg/L),inexcessoftheTCEQ benchmarkforfreshwaterorganisms(13–40mg/L),and ofconcernforhumanconsumption(.40mg/L).Nitrate measurementsfromtheEAAfortheperiodfromDecember2004toSeptember2023areshownforcomparison (Figure6CandD).

Exactdatesofoperationofthequarriesarenotavailable,sothemaximumnitratevaluerecordedforeach wellfrom1990tothepresentwasdetermined.Forthe Edwards,Trinity,andEdwardsTrinityaquifers,Figure7

Figure5.Generalized ﬂowpatternwithintheEdwardsAquifer.ModiﬁedwithpermissionfromEAA(2019).

Figure6.For eachwell,nitratemeasurementssince1990aregroupedintooneofsevencategoriesforGWDBmeasurementsfortheEdwards andTrinityaquifers.Barchartsshowthepercentageofeachcategoryfor(A)theeasternareafortheEdwardsAquifer(1,289samplesfrom Bexar,Comal,Guadalupe,andHayscounties)andTrinityAquifer(742samplesfromBexar,Blanco,Comal,Kendall,andHayscounties)and (B)thewesternareafortheEdwardsAquifer(532samplesfromMedinaandUvaldecounties)andTrinityAquifer(318samplesfrom Bandera,Kerr,Medina,Real,andUvaldecounties).AlsoshownarethecategoriescorrespondingintheeasttotheEdwardsAquiferaverage (7.6mg/L)andmedian(7.6mg/L)andtheTrinityAquiferaverage(4.5mg/L)andmedian(1.0mg/L)andinthewesttotheEdwardsAquifer average(8.7mg/L)andmedian(8.0mg/L)andtheTrinityAquiferaverage(1.9mg/L)andmedian(0.13mg/L).Forcomparison,nitratedata since2004providedbytheEAAweregroupedintosevencategories.Thepercentageofeachcategoryisshownfor(C)theeasternareaand (D)thewesternarea.

displaysthecategoryofeachwell’smaximumvaluein mapview.

FortheEdwardsandTrinityaquifersonthecontributingzones,observationsinthetwocategoriesabove 13mg/Lnitratecompriseasmallpercentageofthe total(Figure6).Figure8showscategoriesinmap viewforwellswithmaximumnitratevaluesabove13 mg/L.Aclusterofwellsnorthofthecontributingzone neartheTrinityAquifer ’sborderwiththeEdwards TrinityAquifer(Figure1)havevaluesabove13mg/L (Figure8).

Forthetwoareas,Figure9illustrateschangesby decadeinnitrateconcentrationwithintheEdwards AquiferbyshowingthechangingpercentagesofthevariouscategoriesdeﬁnedinFigure6.ThemedianmeasurednitratevaluefortheEdwardsAquiferisalso shownforeachdecade.

Nitratemeasurementsareavailableforstreamsamplesbeginningin1970(TCEQ,2022a,2024).Figure 10AandBcomparenitratemeasurementsfromsurface watersince1990toEdwardsAquifergroundwater samplessince1990separatelyforeastandwestareas. Inaddition,totalnitrogenfromstreamsamplesiscomparedtototalnitrogenfromgroundwatersamplesseparatelyforeastandwestareasforthesameperiodin Figure10CandD.

Agriculturalactivitiesassociatedwithfarmingand livestockmaybeasourceofnitratecontaminationof groundwater(ReedyandScanlon,2017).Croplandharvested,astabulatedinagriculturalcensusessince1950, isusedasanindicatoroffarmingactivity.Censusdata arereportedbycounty(Figure11).Locationofcropland andCAFOsrelativetotherechargezoneisimportant forunderstandingwhethertheyhaveplayedasigniﬁcant roleinexplainingnitrateelevationsintheEdwards Aquifer(Figure12).

FortheentireareaoftheEdwardsTrinityAquifer,6,473 nitratemeasurementsmadesince1940wereobtainedfrom theGWDB.Fromnitratemeasurementstakensince1990, themaximumvaluewasdeterminedforeachof1,117 wellsandassignedtooneofthesevencategories(Figure 13A).Forcountiesinorclosetothestudyarea,thepercentageofeachnitratecategorywasdeterminedfrom758 measurementsin420wells(Figure13B).Inaddition,the changeinmediannitratevalueswasdeterminedforeach decadeforthosecounties(Figure13C).

DISCUSSION

TheServtexPlant(Chasnof,2021)providedthelone examplefromthestudyareaofusageofANFOper

Figure7.CategorycorrespondingtothemaximumvalueofnitrateforeachwellintheGWDBdatasince1990fortheTrinity,Edwards,and EdwardsTrinityaquifersisshowninmapview.TheMCLfallsinthenitratecategory(.40mg/L),andthecategory(8–13mg/L)isthelevelat whichsomefreshwaterorganismsmaybeharmed.

minedrockvolumeandwasfoundtobeconsistentwith usagetypicalforindustry(Table1).

ApplyingtypicalindustryvaluesforANFOusageand unexplodedresidualofANFOtothevolumeofrock removedfromthe8.09 3 105 m 2 (200acre)portionof theCemexBalconesNewBraunfelsQuarry(Figure4; Table2)illustratesthelargevolumesofnitratethat quarryoperationsmaycontributetotheaquifer.Quarry areaisnotadirectindicatorofvolumemined,because volumeofaparticularminedependsontopographyand quarrybaseelevation,butthetotalareaminedalongthe rechargezoneis50timestheareaoftheCemexexample, whichcanbeusedtoillustratetheorderofmagnitudeof potentialnitratecontaminationfromquarryingovertime. Incontrast,the16smallcontributing-zonequarries,scatteredacrossalargerarea,wouldhavearelativelyminor impactonnitrateconcentrationingroundwater.

BMPsforquarriesfromtheTCEQstipulatethatkarst featuresdeemedlikelytobeconnectedtotheaquifer

andcapableofallowingrapidinﬂowofwaterbesealed orisolatedusingbuffersuntiltheycanbemined(Barrett andEck,2012).Examplescitedofsensitivekarstfeaturesincludesolution-enlargedfractures,caves,and sinkholes.Togetherwithrequiringaminimumelevation forthebaseofaquarryat7.6m(25ft)abovethewetweatherelevationoftheunderlyingaquifer,theBMPs areintendedtopreventpollutantsfromreachingtheaquifer.However,theyareunlikelytobesuccessful.Surface watercrossingtherechargezone ﬂowsintotheEdwards Aquiferprimarilythroughkarstfeatures,manyofwhich arenotreadilyapparent(Schindel,2019).Johnsonetal. (2010)demonstratedthroughadyetracestudythatwater deliveredtoasurfacesitewithnorecognizablekarstfeaturescouldbedetectedinmonitoringwells.

Ifcorrectlyimplemented,stoppingaquarryingoperation7.6m(25ft)abovetheestimatedgroundwaterlevel mayshieldtheaquiferfromdirectlyintroducingpollutantstotheaquiferthroughfracturesinducedbyblasting.

Figure8.Wellsforwhichthemaximumvalueofnitrateconcentrationobservedsince1990areinthetoptwocategories(aboveTCEQ’secologicalscreeningbenchmarkof13mg/L)areshowninmapview.

However,fracturesinducedduringquarryingwould likelyenhancethepassageofpollutantstothenatural fracturesystemwithinthequarry.Inaddition,thequarryingoperationmayincreasethelikelihoodofnitrate reachingtheaquiferbyreducingthedistanceofthesurfacetotheunderlyingaquifer.

Aggregatewashingisaroutinepractice,andprocess waterisdirectedtosettlingpondswithinaquarry (TACA,2020).Pondsaremonitoredforatleast6 monthstoensuretherateofwaterlosstotheaquifer doesnotexceedanallowedamount(BarrettandEck, 2012).SincethehighlysolubleANFOresiduewould likelybedissolvedinthewashingprocess,anysuch waterlossisanothermeansfornitratetoreachthe underlyingaquifer.

Formostofthecontributingzone,theGlenRoseFormationoftheTrinitygroupisatthesurface(Figures1and2), andrechargeismostlyfrominﬁltrationofprecipitation (Jones,2011).Streamscrossingthecontributingzonehave bothgainingandlosingreaches.Inparticular,wherethe

upperGlenRoseisatthesurface,streamlosstotheunderlyingaquifermayberapid(Greenetal.,2011).Using water-qualitymeasurementssubmittedsince1990tothe TWDB,Figure6comparesthelevelofnitrateforthe EdwardsandTrinityaquifers.TrinityAquifermeasurementsaredistinctlylowerthanthoseoftheEdwardsAquiferforboththeeasterncounties,whichhavehadrapid populationgrowthsince1990,andthoseinthewesternpart ofthestudyarea,whichhavenot.Thedatacollectedbythe EAAshowthesamecontrast.Wherecross-faultexchange betweentheTrinityandtheEdwardsaquifersisoccurring, itismostlikelywiththeGlenRoseFormation(Johnson etal.,2010;Schindel,2019).Measurementsfromthe GWDBfromwellsinbothareastakensince1990thatwere recordedasGlenRosehavelownitratevaluesrelativeto theEdwardsAquifer(Figure14).Themapviewofthe maximumrecordednitratevaluessince1990furtherillustratesthecontrastbetweentheTrinityandtheEdwards aquifers(Figure7).Together,Figures6,7,and14show thatcrossﬂowfromtheTrinityAquifertotheEdwards

Figure9.FortheEdwardsAquifer,changesbydecadeinthemedian valueandrelativeproportionsofnitratecategoriesfor(A)theeastern area(Bexar,Comal,Guadalupe,andHayscounties)and(B)the westernarea(MedinaandUvaldecounties).

Aquifercannotaccountforthelevelofnitrateinthe EdwardsAquifer.

ThemapviewinFigure7isusefulprincipallyin illustratingtheoverallcontrastinnitratelevelsforthe twoaquifers.Sufﬁcientinformationislackingtointerpretindividualmeasurements.Thatwouldrequire knowledgeofthenatureandtimingofapotentialsource ofnitratenearby,inadditiontothetimingofthewater sample.Basedonstudiesthatmeasurerapidchangesin aquiferparametersfollowingrechargeevents,maximum recordedvaluesinkarstaquifersareoftenwellbelow actualmaximumsduetothehighanisotropyofkarst aquifers.Thismakestheactualmaximumdifﬁcultto recordevenbystandardperiodicmonitoring(George Veni,2022,personalcommunication).Evenmonthlyor

quarterlysamplingofmonitoringwellsisunlikelytoadequatelyrepresentthecontaminantcharge(Worthington, 2011).NitratevaluesinFigure7arenotfrommonitoring wellsbutinsteadareforallwellsintheTWDBdatabase. FortheEdwardsAquifer,46percenthaveonlyonemeasurementand71percenthavethreeorfewermeasurements.FortheTrinityAquifer,thosevaluesare59and91 percent,respectively.Only12EdwardsAquiferwellshad asmanyas20samples,andineachcase,thesamples weretakenonceayear.FortheTrinityAquifer,themost observationsinasinglewelltotaled12.

FortheEdwardsAquifer,observationsinthetwocategoriesabove13mg/Lnitratecompriseonlyasmall percentageofthetotal(Figure6BandD).Highvalues areclusterednearthequarries,andallbuttwoarewithin 10kmofaquarry(Figure8).Althoughindividualmeasurementsmaybehighorlowdependingonthetiming ofthepotentialsourceandsampleandtheposition,clusteringofhighvaluesreﬂectsproximitytosourcefor wellssituatedfavorablywithinthefracturesystem.

Nitratemeasurementstakenoverthecontributingand rechargezonesinstreamsarelowrelativetothe EdwardsAquifer(Figure10).BMPsforquarriesdictate thatbermsandothermeasuresbeusedtoisolatequarries frombothstormwaterinflowandwateroutflow(Barrett,2005),withtheresultthatquarriesarenotlikelyto directlycontributenitratetostreamsviarunoff.As showninFigure7,nowastewateroutfallsarelocated overtherechargezone,butsomearepresentonthecontributingzoneincludingthreethatareclassifiedasmajor (EPA,2023).Wastewateroutfallsandotheraspectsof increasingpopulationonthecontributingzonecausethe elevationofstreamnitratelevelsrelativetotheTrinity Aquifer(Figures6,10,and14)andlikelytheslightelevationofnitrateintheeastcomparedtothewestGlen Roseportionsoftheaquifer(Figure14).However, nitratelevelsinstreamsarelowerthanintheEdwards Aquifer,andcomparisonoftotal-nitrogenmeasurements betweenstreamsamplesandEdwardsAquifersamples showsasimilarpatterntonitrate.Evenifnitrificationof organicmaterialtakesplaceintheEdwardsAquifer,the total-nitrogenmeasurementsindicatethatsurfacewater crossingtherechargezonecannotaccountfornitrate levelsintheEdwardsAquifer,aspreviouslydetermined byMusgroveetal.(2016).

Assessingsourcesthatpotentiallydelivernitrateto theEdwardsAquiferrequirescomparisonforeachgeographicdistributionofthepotentialsourcetoelevated nitratemeasurementsintheaquiferanddetermination thatapotentialsourceisincreasingcoincidentwiththe timingofanyriseofnitrateconcentrationintheaquifer. Agriculturalactivitiesareananthropogenicsourcethat hasthepotentialtocontaminategroundwater(Reedy andScanlon,2017).Inthestudyarea,farmlandand

Figure10.Fortheentirestudyarea,nitratecategoriesweredeterminedforstreamsamplemeasurementssince1990.Comparisonsaremade between(A)1,289EdwardsAquiferwellsamplesintheeastareaand4,647streamsamplesintheGuadalupeandSanAntonioriverbasinsand (B)532EdwardsAquiferwellsamplesinthewestareaand196streamsamplesintheNuecesRiverbasin.Forwellandstreamsampleswithdissolvedorganicnitrogenmeasurements,nitrogenasNO3 wascalculatedandcategorized.Streamandwellsamplesarecomparedfor(C)285 EdwardsAquiferwellsamplesintheeastareaand4,647streamsamplesintheGuadalupeandSanAntonioriverbasinsand(D)128Edwards Aquiferwellsamplesinthewestareaand434NuecesRiverbasinstreamsamples.

CAFOsoverthecontributingandrechargezonesare limitedcomparedtotheartesianzoneoftheEdwards Aquifer,wheretheaquiferisshieldedabovebyimpermeableformations(Figure12).Withtheexceptionofan increaseinUvaldeCountybetween1969and1982,agriculturalactivityasmeasuredbytheamountofcropland harvestedwasrelativelyconstantordecliningforall countiesduringtheperiodwhennitratelevelswererisingintheEdwardsAquifer(Figures9and11).Forthe studyarea,thetotalsforcattleandcalveswerefairly constantfromthe1950to1992censusesbutdeclinedby 48percentfrom1992to2022.Together,Figures11and

Figure11.Censusdataoncroplandharvested,reportedbycounty,as ameasureofagriculturaltrends.

12conﬁrmtheopinionsofpreviousauthorsthatagricultureisunlikelytobethecauseofrisingnitratelevels.

UsingtheSWATmodel,SullivanandGao(2016) concludedthatatmosphericdepositionofnitratewasthe mostimportantinputofnitratetowaterchargingthe EdwardsAquifer.OnemeasureofsuccessoftheSWAT modelwasthatthecalibratedshallowaquifernitrate concentrationwasconsistentwiththatreportedforthe EdwardsAquiferbyMusgroveetal.(2010).ThecontrastinobservedvaluesbetweentheEdwardsandthe Trinityaquifers(Figures6and7)wasnotaddressed. TheMODFLOWmodel(Sullivanetal.,2019)used averagenitrateconcentrationsfromTWDBdatafor areasbeyondtheSWATmodelstudyforboththe EdwardsandtheTrinityaquifers.Inparticular,nitratein thecrossﬂowfromtheTrinityAquiferwasmodeledas 9.34mg/L(2.11mg/LnitrateasN).Anaverageis stronglyinﬂuencedbyextremevaluesandisnotrepresentativeofnitrateintheTrinityAquifer(Figures6B and14).NeitherstudyaddressedelevationofbackgroundlevelsofnitrateintheEdwardsAquifercomparedtotherelativelyundisturbedlevelsmeasuredin the1940sand1950s(Figure9).

Bothregionaldistributionandtimingofatmospheric nitrogendepositionindicateitisnottheprincipalcause ofelevatednitratelevelsintheEdwardsAquifer.

Reactivenitrogenoxide(NOx)islargelyproducedby anthropogenicactivityandintheUnitedStateshas declinedsince1980.Thesharpestdeclinehasbeensince 1995asaresultofthe1990CleanAirActAmendments. Atmosphericammoniadepositionhasbeenessentially ﬂatintheUnitedStatessince1990(NationalAtmosphericDepositionProgram,2016).Nationalmapsof atmosphericdepositionofammonium,nitrate,andtotal nitrogenfrom1986to2016shownoconsistenttrend (NationalAtmosphericDepositionProgram,2020)that couldberelatedtotheriseinnitratelevelsinthe EdwardsAquiferforthesameperiod(Figure9).The largestsourcesofNOx aremotorvehiclesandpower generation.Consequently,depositionofNO3 ishighest inurbanareas(NationalAtmosphericDepositionProgram,2016).Theeffectisillustratedforthisstudyarea byanitrate–Nmapfor2012,whichshowsa50percent declineinatmosphericnitratedepositionfromBexar CountytoUvaldeCounty(ReedyandScanlon,2017, theirFigure16).Consequently,atmosphericdeposition doesnotexplainthesimilarityofEdwardsAquifer nitratemeasurementsintheeastandwestpartsofthe studyarea(Figure6).

AsnotedbyReedyandScanlon(2017),theEdwards TrinityAquifer(Figure1)haselevatednitratelevels, whicharehighestinthenorthlargelyduetoagricultural activity.AssignmentofGWDBmeasurementstonitrate categoriesillustratethis(Figure13A)inshowinga north-to-southdeclineinmeasurednitrate.However, countiesimmediatelynorthofthestudyareahave

observednitratelevelssince1990thatarecomparableto thoseintheEdwardsAquifer(Figure13B).Thisraises thepossibilitythattheEdwardsTrinityAquiferiscontributingnitratetotheEdwardsAquifer.Acrossmostof thestudyarea,anynitratefromtheEdwardsTrinity AquiferwouldhavetoreachtheEdwardsAquifervia spring flowcontributionstocontributingzonestreamsor bycrossflowintotheTrinityAquifer.Neitherstream measurementsnorTrinityAquifermeasurementssupportasignificantnitratecontributioninthoseareas.In westernUvaldeCounty,samplesdesignatedasEdwards TrinityhaveelevatednitratelevelsclosetotheEdwards Aquiferwherecrossflowoccurs.However,oneindicationthatcross fl owfromthewestfromtheEdwards TrinityAquiferisnottheprimarysourceofnitratein theEdwardsAquiferisthatEdwardsAquifernitrate concentrationsarefairlyconstantacrossthestudyarea (Figures6and7).Awesternsourcewouldshowwestto-eastdilution.Inaddition,Lindgrenetal.(2004)concludedinfl owtotheEdwardsAquifer,includingboth thenorthernandthenorthwesternboundaries,wasonly 6.5percentoftotalrecharge.Furthermore,thetiming ofnitrateconcentrationsintheEdwardsTrinityAquifer differsfromthatoftheEdwardsAquiferinthatitrose fromthe1940sto1990sbutdeclinedafterward.

Musgroveetal.(2016)primarilyuseddatafrom BexarCounty,togetherwithdatafromComalSprings, toattributeelevatednitratelevelsintheSanAntonio segmentoftheEdwardsAquifertoanthropogenic sourcesinurbanizedareasoftherechargezone.Thetwo

Figure12.2019distributionofcultivatedcropsandhayorpasture(MRLC,2019).

Figure13.ForEdwardsTrinityAquifersamplesfromtheGWDB,thecategoriescorrespondingtonitratemeasurementsweredetermined.(A) Foreachwell,thecategorycorrespondingtothemaximumnitratemeasurementisdisplayedinmapview.(B)Thepercentageofcategoriesfor 420wellsincountiesinorclosetothestudyarea(Bandera.Blanco,Crockett,Edwards,Gillespie,Hays,Kerr,Kimble,Kinney,Real,Sutton,and ValVerde)wasdeterminedforsamplestakenstartingin1990.(C)Themedianvalueofnitratewasdeterminedbydecadeforwellsinthecounties inorclosetothestudyarea.

areasdistinguishedonthebasisofpopulationchange canbeusedtotestwhetherurbanizationissufﬁcienton itsowntoexplainthenitrateobservations.Medinaand Uvaldecountieshaverelativelysmallpopulationsand lowpopulationgrowth.Developmentovertherecharge zonethereremainslow(Figure15).Withrespectto ﬂow intheEdwardsAquifer,MedinaandUvaldecounties areupgradientoftheurbanizationassociatedwiththe increaseinpopulationofBexar,Comal,andHays countiessince1990(Figure5).Ratherthanlownitrate valuesintheEdwardsAquiferinMedinaandUvalde counties,nitratemeasurementsareelevated(Figures6 and7).

Opsahletal.(2018)evaluatedwaterqualityinthe EdwardsAquiferusingrainfalldata,surfacewatersamples,groundwaterdatafromsixwellsinBexarCounty, andasinglewellinMedinaCounty.Isotopicsamples

analyzedbyOpsahletal.(2018)andMusgroveetal. (2016)foundbothnaturalandanthropogenicsourcesfor nitrate,andbothstudiesindicatethathighervaluesof deltanitrogen-15ofnitrate(d15N-nitrate)markgreater inputofhumanorpetwaste.Thesamplesfromthe MedinaCountywellhadthelowestmeasured d15N-nitrate, whichtheauthorsconsideredtobenatural.However,the isotopicsignatureofsoilnitrogenandammoniumfrom ammoniumnitrateoverlap(Opsahletal.,2018).Bothstudiesconsideredlowerdissolvedoxygeningroundwaterversussurfacewatertobeanindicationofnitriﬁcationtaking placeintheEdwardsAquifer,andMusgroveetal.(2016) interpreteditasevidenceofurbanizationontherecharge zone.TheGWDBhad68measurementsofdissolvedoxygenfrom1994to2022fromwellsinBexar,Comal,and Hayscounties,withamedianvalueof5.9mg/L.For 14,085streamsamplesfromtheGuadalupeandSan

Figure14.Nitratemeasurementssince 1990aregroupedintoone ofsevencategoriesforGWDBsamplesfromtheGlenRose FormationoftheTrinityAquifer.Barchartsshowthepercentage ofeachcategoryfortheeasternareafor401samplesandthewesternareafor198samples.Alsoshownarethecategoriescorrespondingintheeasttothesampleaverage(4.7mg/L)andmedian (1.3mg/L)andinthewesttothesampleaverage(2.4mg/L)and median(0.38mg/L).

Antonioriverbasinsoverthecontributingandrecharge zones,themedianvaluefordissolvedoxygenis8mg/L (TCEQ,2024).Thedropindissolvedoxygenfromsurface samplestogroundwatersamplesintheeasternareaisconsistentwiththepreviousstudies.However,themedian valuefor48wellsamplesfromtheEdwardsAquiferfrom 1996to2022inMedinaandUvaldecountiesis6mg/L, andfor808streamsamplesfromtheNuecesRiverbasin overthecontributingandrechargezones,themedianvalue fordissolvedoxygenis8.6mg/L.Combinedwithlower totalnitrogeninstreamthaningroundwatersamplesacross thestudyarea(Figure10CandD),theconsistentdropin dissolvedoxygenmaybeanindicatorofnitriﬁcationof ammoniumfromANFO.

IfelevatednitratelevelsintheEdwardsAquiferare theresultofurbanizationovertherechargezone,one wouldexpectrisingnitratelevelstocoincideintime withincreaseddevelopmentofthatarea.Landusemaps conﬁrmtheobservationbyBushetal.(2000)thatdevelopmentovertherechargeandtransitionzoneswas mostlyafter1992(Figure15).FortheSanAntoniosegmentoftheEdwardsAquifer,Musgroveetal.(2016) didnotusehistoricaldataofwater-qualitymeasurementsotherthanreportsfromComalSpringsfromthe 1930sand1940s.BasedonTWDBwelldata,median EdwardsAquifernitratelevelsinthe1940sand1950s werenearbackgroundlevels(Figure9).Bythe1960s, nitratelevelsweredistinctlyelevated.FortheSanAntoniosegmentoftheEdwardsAquifer,theplotsbydecade showthatelevationofnitratelevelsprecededintensive developmentovertherechargezone,soanadditional explanationisrequired.

Datesofoperationforallquarriesidenti ﬁ edonFigure1arenotavailable.Quarriesareknowntohave beenpresentintheareafromthelatenineteenth

century,e.g.,Cameron(1994)andHaas(1995).A2010 WPAPapplication(Forster,2010)notedthattherehad beensigniﬁ cantquarryoperationsontheEdwards rechargezoneformorethan80years.Therisein nitrateconcentrationintheEdwardsAquifer(Figure9) coincideswiththeentryofANFOonalargescalein thecommercialexplosivemarketinthemid-1950s,and ANFOdominatedthemarketbythe1960s(Moreira, 2012).

Elevatednitratevaluesarefoundwellintotheartesian zoneoftheEdwardsAquifer(Figure7).Long-distance transportfromsourcesalongtherechargezoneisconsistentwithnitratebeingnearlyinertingroundwater (StarksandChampion,2001).Tracertestsdocument thatsolutionfeaturesallowrapidpassageofwaterin karsticaquifers(Johnsonetal.,2010).InastudyofsamplesfromaPWSwell ﬁeldinBexarCountylocated approximately10kmsoutheastoftherechargezone, Jaguckietal.(2011)showedthatwaterisrelatively young(,1–41years),andwellmixedoverthe150-m (500ft)producinginterval.Frommodeling,theyconcludedthatasaresultofrapidtransportofwaterthrough karstfeatures,morethan40percentofproducedwateris lessthan2yearsold.Contaminantsintroducedintothe rechargezonewouldbedetectableinthePWSwells withinayearandreach90percentoftheintroducedcontaminantlevelin10years.Reductionofacontaminant sourcewouldbedetectableinonetoseveralyears.Nitrate intheEdwardsAquiferhasshownatleasta60-yearrise inconcentration(Figure9),whichindicatesanincreasing amountofnitratecontaminationrelativetothepotential fordilutionbyrecharge.Jaguckietal.(2011)notesome variabilityintheproportionofoldwaterinregionalwells thattheyrelatetotraveltimesandthatwouldimplysaturationofanycontaminantswouldnotbeuniforminthe EdwardsAquifer.

Makinganorder-of-magnitudeestimateofresidual nitratethatmayhaveresultedfromminingallmapped quarriesrequiresestimatingtheacreageminedsince themid-1950sandusingthattoscaleupthepotential nitrateresidualfromquarrying8.09 3 105 m 2 (200 acres)inComalCounty(Table2).Ingeneral,limestone aggregateproductionroughlyincreaseswiththe increaseinTexas ’spopulation(KyleandElliot,2019). Theincreaseinpopulationfrom1955through2022is 88percentoftheincreasesince1930(Macrotrends, 2024).Thatpercentageleadstoanestimateofthe quarryareaminedsince1955of3.5 3 107 m 2 (8,700 acres)andanitrateresidualestimateof1.7 3 108 kg, oranaverageof2.4 3 10 6 kgannuallyover67years. MedianannualrechargetotheEdwardsAquiferisestimatedtobe6.8 3 10 8 m 3 from1934through2021 (EAA,2021).Relativetomedianannualrecharge,the

Figure15.Urbandevelopmentinthestudyareain(A)1992(Sohletal.,2018)and(B)2019(MRLC,2019).

averagepotentialannualnitratecontributiontothe aquiferisontheorderof3.6mg/L. FlowintheEdwardsAquiferisdominatedbyconduitswithcomparativelylittlecontributionfromthe largermatrixvolume(Hauwert,2011).Lindgrenetal.

(2004)citeanestimateofwater-ﬁlledporespaceforthe SanAntoniosegmentoftheEdwardsAquiferof2.13 3 1011 m 3 (1.73 3 107 acreft).Forkarsticaquiferslike theEdwardsAquifer,conduitvolumeasapercentageof totalporosityisestimatedtobeasmuchas1percent

(TaylorandGreene,2008).Relativetothatestimateof conduitvolume,thepotentialannualnitrateresidualis ontheorderof1.1mg/L.

Thetimingofnitrateincreaseintheeastandwest areas(Figure9)resultingfromtheintroductionof ANFOtoaggregateoperationswouldbeexpectedtobe similar.However,nitrateconcentrationsobservedinthe westarehigherthanmightbeexpectedcomparedto easterncountiesbasedonthenumberandsizeofquarries(Figures1and6),particularlyinUvaldeCounty. QuarriesinUvaldeCountyareclusteredinarelatively smallareaandareseparatedfromtheremainderofthe SanAntoniosegmentbya flowrestriction,theKnippa Gap(Figure1),whichwasoriginallydefinedfroma steepeningofpotentiometriccontoursintheeasternpart ofthatcounty(Lindgrenetal.,2004).Thoseauthors describethe flowpatternincentralUvaldeCountyas complexandpoorlydefined.Clarketal.(2013)determinedthattheKnippaGapreflectsachangeinorientationofrelayrampsfromsouthwestdippinginMedina CountytonortheastdippinginUvaldeCounty.The EdwardsAquiferlikelyhasadifferentdilutionhistoryin UvaldeCountycomparedtoareastotheeast,whichmay influencetheobservednitratelevels.Inaddition,theelevatednitratelevelsingroundwateroftheEdwardsTrinity PlateauAquiferinnorthwesternUvaldeCountymaycontributenitratetotheEdwardsAquiferbycrossflow.

FourSanAntonioWaterSystemwell ﬁeldsproduce waterexclusivelyfromtheEdwardsAquifer:Castle Hills,Geronimo,MountainLaurel,andTexasResearch Park.Forthesewell ﬁelds,nitratemeasurementsmade in2022rangedfrom5.39to9.86mg/L(SAWS,2023). ThesevaluescorrespondtogroupsinFigures6and7, whichshowelevationofnitratelevels.ForwellwaterqualityanalysesfromtheTWDBdatabase,nitratevalues abovetheMCLremainrarefor2020through2022,but 74percentofTWDBnitratemeasurementswere8mg/L orgreater,whichisthecategoryharmfultosomefreshwateraquaticorganisms.

CONCLUSIONS

MeasurementsofnitrateintheEdwardsAquiferare elevatedrelativetowhatmightbeexpectedfromunpollutedsourcesandhaveclearlyincreasedovertimetoa pointwheretheypotentiallythreatenaquaticorganisms withintheEdwardsAquiferkarstenvironment.Neithersurfacewaterfromthecontributingzonenorcross-fault ﬂow fromtheTrinityAquiferaccountforelevatednitratemeasurementsintheEdwardsAquifer.Inaddition,itisclear frombothgeographicandtemporalperspectivesthatneitherurbandevelopmentovertherechargezonenoratmosphericdepositionofnitrogenaccountfortheobserved nitratelevelsintheEdwardsAquifer.

UseofANFOinquarryingoperationshasnotbeen previouslypublishedasasourceofnitratepollutionin theEdwardsAquifer.ThehighsolubilityofANFO,the largevolumesofANFOusedinquarryoperations,and thehighpercentageofexplosiveresiduals,togetherwith thefactthatquarryoperationsareaknownsourceof nitratepollutionofgroundwaterelsewhere,makeitclear thataggregatequarriesarelargecontributorsofnitrate totheaquifer.Quarrydistributionalignswellwiththe geographicdistributionofelevatednitrateconcentrationsintheEdwardsAquifer.That,togetherwiththe coincidenceintimeofincreasingnitratelevelswiththe large-scaleintroductionofANFOtoaggregatemining, indicatesthatquarriesareasigniﬁcant,ifnotthemain, sourceofnitratepollutionintheaquiferandmakethe EdwardsAquiferausefulexampleoftheimpactof aggregateminingonkarsticaquifers.

ACKNOWLEDGMENTS

GeorgeVeniprovidedextensiveinputtothecontent andorganizationofthispaper.DougWiermanandBrian Smithprovidedhelpfulreviews.

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DeterminingGroundwaterTableinSlopesWith HorizontalDrainsandDesignFrameworkfor LengthandSpacing

MIHIRALAKRUWAN*,AKIYOSHIKAMURA, AND MOTOKIKAZAMA

DepartmentofCivilandEnvironmentalEngineering,TohokuUniversity,6-6-06, AobaAramaki,Aoba-ku,Sendai,Miyagi,Japan980-8579

KeyTerms: SlopeStabilization,Landslides,Drainage, HorizontalDrains,WaterTableProfile,PoreWater Pressure,SlipSurface

ABSTRACT

Horizontaldrains(HDs)arecommonlyimplemented inslopestabilizationtoreducetheporewaterpressure (PWP);however,theyalsocausecomplexthree-dimensional(3D)variationsinthegroundwatertable(GWT), whichrequireintricate3D flowmodels.Toaddressthis challenge,weproposeanovelsemi-empiricalmethod basedonaseriesofnumericalsimulationsvalidated throughnumericaland fieldstudiestodeterminethe GWTofaslopewithHDs.Subsequently,theproposed methodwasextendedtocalculatetheaveragePWP(U) actingonapredefinedslipsurfacevialinearintegration. Thedecreaseof U (DU)resultingfromHDscanbeused toevaluatetheperformanceofHDs.Tocreatedesign chartsthatconsidertheimpactoflengthandspacingof HDson DU foraspecificslope,wedevelopedaPythonbasedcomputerprogram.Twocasestudieswereconducted,whichshowedthat DU increaseswithlonger HDsandshorterspacing.Theresultsalsoindicated thatextendingtheHDsbeyondaparticularlengthdoes notsignificantlyaffect DU;itishighlysensitivetothe spacinginshortHDsandnotsensitiveinlongHDs. Furthermore,wefoundthatthetotallengthofHDs requiredtoachieveatarget DU islessinwidespacings thaninshortspacings.Inconclusion,longHDswith widespacingsaremoreeffectiveandeconomical.Owing totheuniquenatureofeachslopestabilityproblem,this studyoffersapracticaltoolforanalyzingtheeffectivenessofHDsinsteadofprovidingageneralguide.

INTRODUCTION

DesignofHorizontalDrainsinLandslideMitigation Projects

Horizontaldrains(HDs)wereinitiallyintroducedfor groundwatermanagementinagriculturalland(Stuytetal.,

*Correspondingauthoremail: samarasekara.okandapala.arachchige.don. m.b8@tohoku.ac.jp

2005).Today,theyarewidelyusedasaneffectivetechniquefordrainagemanagementinavarietyofgeotechnicalapplications,includingslopeandembankmentstability improvement(Nonveiller,1981;LauandKenney,1984; Martinetal.,1995;Caietal.,1998;Forrester,2001; Rahardjoetal.,2003,2011;Tsaoetal.,2005;Ghiassian andGhareh,2008;Shresthaetal.,2008;andHindsetal., 2021),drainagemanagementbehindsoilnailingand retainingwalls(ChenandChen,2013;GeotechnicalEngineeringOfﬁce,2017;LakruwanandKulathilaka,2020, 2021),enhancementoftheliquefactionresistanceofsoil (Fasanoetal.,2019,2021),andaccelerationofconsolidation(Fengetal.,2020).Thisstudyfocusedonevaluating theeffectivenessofHDsinenhancingslopestability.

TheeffectivenessofHDsinenhancingslopestabilityis primarilybasedonseveraldesignparameterssuchaslocation,spacing,length,anddraintype(Rahardjoetal.,2011; Lakruwanetal.,2021;andAbedetal.,2024).Rahardjo etal.(2003)showedthatinstallationofHDsatthebottom ofaslopewasmoreeffectivethaninstallationofHDsat higherelevations,thusconfirmingtheresultsofLauand Kenney(1984)andMartinetal.(1995).Additionally,some studieshavetriedtoidentifyamaximumlimitforthe lengthofHDs(Royster,1980;LauandKenney,1984;Cai etal.,1998;andCooketal.,2008),andseveralattempts havebeenmadetodevelopdesigncharts(Kennyetal., 1977;Stanic,1984),buttheyaregenerallyonlyvalidfor thespecificslopeconditions(ResnickandZnidarcic,1990). Additionally,thegeneralguidelinesprovidedbyCooketal. (2012)forHDspacingbasedonthesoiltypevarysignificantlydependingonthesoilandrechargeconditions.

PreviousstudieshaveproventhatthedesignofHDsis highlysite-speciﬁcandthatprovidinggeneralizedguides isnotfeasible.Hence,insteadofageneralizedguide,a novelmethodthatisapplicabletoawiderangeofslope conditionsisproposedinthisstudytouniquelyanalyze theeffectsofthelengthandspacingofHDsonthepore waterforcereductiononaslipsurfaceand,subsequently, thestabilityenhancementofaslope.

GroundwaterTableProfilebetweenParallelandLong HDs

Amongtheseveraltheoreticalmethodsdevelopedto obtainthegroundwatertable(GWT)proﬁlebetween

Lakruwan,Kamura,andKazama

HDsinagriculturallands(Hooghoudt,1940;Ernst,1954; andKirkham,1958),theHooghoudt(1940)equation, whichisbasedonDarcy’slawandtheDupuit-Forchheimer theoryofhorizontal ﬂow,iswidelyused.Luthin(1966)presentedtheworkofHooghoudt(1940),whichwasoriginally presentedintheNetherlands,as:

PoreWaterPressureonaSlipSurface

Theaccumulationofporewaterpressure(PWP)on theslipsurfacecanbeidentiﬁedasthemajortriggering factorforrain-inducedslopefailures.Severalstudies haveevaluatedslopestabilitybasedontheporewater forceontheslipsurface( U)(Tsaoetal.,2005;Conte andTroncone,2012,2018).

NovelMethodtoObtainGWTProfileandDesign

ProcedureforHDs

Here, hm denotesthemaximumGWTheightatthe mid-pointbetweentheparallelandinfinitelylongHDs (m), S representsthedrainspacing(m), R corresponds totheconstantorsteady-staterechargefromtheground surface(orinfiltration)(m/s), ks denotesthesaturated hydraulicconductivityofthesoil(m/s),and de reflects theequivalentdepthtotheimpermeablelayer(m), whichaccountsfortheradial flowinthevicinityofthe pipethatdoesnotsatisfytheDupuit-Forchheimer assumption.Gregetal.(2013)presentedthefollowing relationshipsfordetermining de:

Currentlyavailableanalyticalandempiricalmethods donotadequatelyaddresstheeffectof3Dvariationinthe GWTinshortHDsonslopes.Additionally,two-dimensional(2D) flowsimulationmodelsneglectthe3Deffect oftheGWTprofile,potentiallyresultinginainadequate design(Lakruwanetal.,2022;Abedetal.,2024).Consequently,toaccuratelycapturethecomprehensiveeffectof the3DvariationoftheGWT,complex3D flowsimulationmodelsarenecessary.However,settingupanaccurate3Dmodeltorepresentanactualslopeischallenging, andthetimeandcomputationalpowerrequiredtoiterate suchcomplexmodelstoobtaintheoptimaldesignparametersforHDssignificantlyincreasethecomplexityofthe process.

where D isthedepthoftheimpermeablelayer(m), rd is thepiperadius(m),and

Althoughafewstudies(Gregetal.,2013;Conteand Troncone,2018)haveusedtheHooghoudt(1940) methodforslopestabilityapplications,itsapplicability toslopesislimitedbecausetheassumptionofsemi-infinitelylongandparallelHDsisinvalidformostslope stabilizationapplications.AlthoughtheHooghoudt (1940)methodassumesauniformvariationintheGWT profilealongtheHDs,thelimitedlengthsresultinnonuniformGWTvariationsalongtheHDsonslopes,leadingtoacomplexdistributionoftheGWTprofilein three-dimensional(3D)space.

CrenshawandSanti(2004)werethe firstresearchers toaddressthisissueandproposedanempiricalmethod fordeterminingthe3DvariationintheGWTalongshort HDs.However,therequirementofaGWTheightof approximately20ft(6m)behindthedrain fieldandthe inabilitytopredicttheGWTprofilebeyondtheHD field aredrawbacksofCrenshawandSanti’s(2004)method.

Incontrast,thisstudyproposesanovelsemi-empirical methodforpredictingthesteady-stateGWTprofileof HDswithlimitedlengthsbasedonaseriesofnumerical simulations.Thismethodwasvalidatedthroughaseries ofnumericalcasesand fieldstudies.Theproposed methodhasfewerlimitations,requiresminimalcomputationalpower,canbeconvenientlyimplemented,and notonlycanpredicttheGWTprofileintheHD field, butalsotheslopebehindtheHD.

Furthermore,weextendtheproposedGWTprediction methodtoobtainthe U andreductionof U (DU)byHDs. Wethendescribetheprocedurefordevelopingadesign charttoselecttheoptimallengthandspacingoftheHDs foragivenslope.Todemonstratetheeffectivenessofour proposedmethodanddesignprocedure,weperformed twocasestudies,andweprovideseveralguidelinesfor selectingtheoptimallengthandspacingofHDs.

TheproposedGWTpredictionmethodanddesign procedureallowdesignerstoefﬁcientlyperformHD designwhileconsideringthe3Dnatureoftheproblem.

OBJECTIVES

Theobjectivesofthisstudyaresummarizedas follows:

1.Todevelopanovelsemi-empiricalmethodtopredict theGWTproﬁleofaslopewithHDsbasedonthe observationofnumericalsimulationsandtovalidate

theproposedmethodthroughacomprehensiveseries ofnumericalcasestudiesandarigorous ﬁeldstudy, ensuringthereliabilityandapplicabilityoftheproposedmethod;

2.ToestablishanaverageGWTproﬁlethatadequately captures3Dvariations,whichmaybeutilizedin2D numericalsimulations,providingamoreaccurateand efﬁcientapproachforanalyzingslopestability;

3.Toquantifythe U and DU valuesinthepresenceofaslip surfacebyintegratingacrosstheaverageGWT,andto thoroughlyanalyzetheeffectoftheHDlengthandspacing on DU,highlightingthesigniﬁcanceofselectingsuitable valuestoachieveeffectiveandeconomicaldesign;and

4.Toproduceadesignchartforspeciﬁcslopeconditions, enablingdesignerstoconﬁdentlyselectoptimalHD lengthsandspacingbasedonthedesignrequirements, andtoprovideinsightsandrecommendationsforthe selectionofHDlengthsandspacingtoachieveanoptimaldesign.

Thestructureofthisarticle(Figure1)issummarized asfollows:

1.ProposedMethod:Thenovelsemi-empiricalmethod developedtodetermineorpredicttheGWTonaslope

withHDsbasedonaseriesof3D ﬁniteelement(FE) analysesconductedonslopeIispresentedandshown inFigure2.TheaverageGWTobtainedfromthe resultsissubsequentlyusedtocalculatethe U and DU

2.Validation:TheproposedmethodforpredictingGWT, U,and DU iscomprehensivelyvalidatedthrougha numericalcasestudyonslopesIandIIinFigure2and a ﬁeldstudy.

3.ResultsandDiscussion:TheeffectoftheHDlength andspacingonthe DU (andfactorofsafety[FS] enhancement, DFS)valueisrigorouslyevaluatedusing slopesIandIIinFigure2.

PROPOSEDMETHOD

NovelSemi-EmpiricalMethodtoPredictGWTProfile ModelSetupandBoundaryConditions

SeriesofFEsteady-state ﬂowanalyseswereperformedona30-m-wide(inthe Y direction)sectionof slopeIusingPLAXIS3D(CONNECTEditionV20) (PLAXIS,2020).Ten-nodetetrahedralelementswere usedforFEdiscretization.The3DFEmodelofslopeI isshowninFigure3.Table1presentsthedetailsofthe

Figure1.Methodologyforthestudy.

ﬁlesusedinthestudy:(a)slopeI,and(b)slopeII.

numericalmodelanalysis,andtheparametersareshown inFigures3and4.

Establishingtheupstreamhydraulicboundaryinslopes ischallengingduetoitscomplexbehaviorinﬂuencedby hydrologicalconditionsoutsidetheslopedomain.Followingpreviousstudies(Dasetal.,2022;Lerniaetal.,2022), aconstant-headDirichletboundarywithatotalheadof Hi (D þ hi,where hi istheheightoftheGWTattheboundary relativetotheHDlevel)wasused.Althoughthisupstream

headexhibitsseasonalvariation(Lerniaetal.,2022),using thehighheadvalueoftherainyseasonisaconservative approachinsteady-stateanalysis.Theconstraintsforthe locationoftheupstreamboundaryareoutlinedinthesectionon “ConstraintsoftheProposedMethod.” ANeumann boundaryconditionwasassumedatthetopsurfacetosimulatetheinﬁltrationduringrain.Althoughinﬁltrationhas temporalvariability,weusedtheaveragevalueofthepeak rainfallasaconstantrecharge(R)toprovideaconservative

Figure3.The30-m-wide3DFEmodelofslopeIusedtodevelopthepr oposedsemi-empiricalmethodtoobtaintheGWTpro ﬁ le( S ¼ 5m).

Figure2.Slopepro

Lakruwan,Kamura,andKazama

Table1. DetailsoftheFEmodelsusedtodevelopempiricalrelationships(withparametersasillustratedinFigures3and4).

DetailsofPLAXISModels

D (m) B (m) L (m) hi (m) S (m) n 105015,20,25,30,352,5,810,50,0.05,0.1,0.2 9020,30,40,50,60,701050,0.1 205015,20,25,30,35550,0.1

DefiningZonesintheGWTProfile

estimateinsteady-stateconditions.Thisrechargerepresentsthein ﬁ ltrationaftersurfacerunoff.Additionally,a0.1mcutoffvaluewasusedtopreventexcessive pondingonthesurface.Linedrainswithzerowater pressurewereemployedastheHDs,neglectingthe effectofentranceheadlossandpressureheadinside fully ﬂ owingpipes.

Weobservedthreedistinctregionsofgroundwater fl owonslopeswithHDs,whichwerecharacterizedby fl owdirection(Figure5).IntheregionbehindtheHDs, the X -directionalcomponentofthegroundwater fl ow ( qx )dominatesthe Y-directionalcomponent(q y),resultingina2D fl owinthe XZ plane.Conversely,when HDswerepresent, qy becamedominantover q x,leading to2D fl owinthe YZ plane.Thetransitionfromthe XZ planetothe YZ planeoccurredthroughanarrowregion attheendoftheHDs.Becausecleardistinctions betweenthethreeregionscouldnotbeestablished basedsolelyongroundwater fl ow,weexaminedthe GWTpro fileatthemid-sectionbetweentheHDs(on the XZ plane).

ThegradualincreaseintheGWTintheHD ﬁeldwas idealizedintotwolinearproﬁleswithdifferentgradients

Table2. Summaryofvalidation,showingMAEand R2 valuesbetweentheGWTsfrom3DFEmodelsandproposedmethod.

TotalLength (Zones1,2,and3)

MAE(m) R2

OnHD (Zones1and2)OnZone3

R2 MAE(m) R2

Figure10(a)

n ¼ 0.1/S ¼ 2.5m/hi ¼ 22m

L ¼ 10m0.2150.9980.3050.7860.1950.997

L ¼ 20m0.1560.9990.1970.8690.1400.999

L ¼ 30m0.1920.9990.2380.8100.1590.998

L ¼ 40m0.2380.9990.2950.7390.1750.998

L ¼ 50m0.3130.9980.3810.6380.1880.998

n ¼ 0.1/S ¼ 5m/hi ¼ 22m

L ¼ 10m0.1930.9980.3630.7890.1560.998

L ¼ 20m0.2140.9980.3430.8230.1600.998

L ¼ 30m0.2690.9980.3860.8140.1870.997

L ¼ 40m0.3260.9970.4530.8020.1830.997

L ¼ 50m0.4050.9960.5360.7920.1650.998

n ¼ 0.1/S ¼ 10m/hi ¼ 22m

L ¼ 10m0.2100.9980.5020.8140.1480.998

L ¼ 20m0.3020.9960.5070.8140.2170.995

L ¼ 30m0.3610.9960.5090.8440.2580.993

L ¼ 40m0.4120.9950.5430.8580.2640.992

L ¼ 50m0.4790.9930.6210.8740.2200.994

Figure10(b)

S ¼ 10m/L ¼ 30m/hi ¼ 12m

n ¼ 0.050.1800.9970.2770.9010.1120.997

n ¼ 0.10.2000.9960.2960.9130.1320.995

n ¼ 0.30.6870.9720.4470.8940.8550.607

Figure10(c)

n ¼ 0.17/S ¼ 10m/L ¼ 30m

hi ¼ 17m0.2580.9970.4130.8370.1500.998

hi ¼ 12m0.1780.9970.3120.8350.0850.998

hi ¼ 7m0.1720.9930.2520.8310.1150.992

hi ¼ 2m0.1580.9510.2400.6400.1000.917

Figure10(d)

n ¼ 0.1/S ¼ 5m/L ¼ 30m/hi ¼ 22m

h ¼ 2.5°0.3390.9960.5970.7290.1590.999

h ¼ 5° 0.2890.9970.5040.8860.1380.999

h ¼ 10° 0.4980.9940.4980.9410.4980.981

tosimplifytheproposedmethod.Zone1correspondsto theregionwith flowinthe YZ planeandexhibitsa gradualincreaseinGWTwithasmallgradient.Zone2 waslocatednearthedrainend,whereasuddenincrease intheGWToccurred,correspondingtothetransition region.Thelengthofthisregionisapproximatelyequal tothedrainspacing( S)andhasasteepergradientthan thatofzone1.Finally,zone3representsthecurved GWTpro filebehindtheHD field,wherethe fl owisin the XZ plane.

Insummary,wedefinedthreedistinctzonesintheGWT profilebasedonourobservationsof flowdirectionand changesintheGWTgradientalongtheprofile.Thesezones arelabeledaszones1,2,and3,asillustratedinFigure4.

EstablishingtheGWTProfile

Whenthesaturatedhydraulicconductivityofthesoil wasisotropicandhomogeneous,wedistinguishedthe GWToftheslopeintotwopartsasformedasaresultof hi andtherelativerechargeintensity( n)fromthetop surface(orinﬁltration)(Figure4).

n ¼ Rechargefromthetop ðRÞðm=sÞ Hydraulicconductivity of thesoil ðks Þðm=sÞ (5)

TheheightsoftheGWTbetweenzones1and2and betweenzones2and3aredenotedas hc and he (m), respectively.Then, hc and he aredividedintotwoparts obtainedfrom hi and n,suchthat

Figure5.The X and Y directional ﬂowvelocities(qx and qy,respectively)ofslopeIinthesectionatthequarterpointbetweenHDs(S ¼ 10m, L ¼ 25m).

Figure4.Typicalgroundwatertableproﬁleofaslopeatthemid-sectionbetweenHDs.

he0,(b) hc0,(c) her,and(d) hcr.

hc ¼ hc0 þ hcr ; (6) and

he ¼ he0 þ her ; (7)

where hc0 (m)and hcr (m)denotethecomponentsof hc causedby hi and n,respectively;and he0 (m)and her (m) denotethecomponentsof he causedby hi and n,respectively.Basedonthenumericalmodelanalysisresults, wederivedthefollowingempiricallinearrelationships for he0, hc0, her,and hcr,asshowninFigure6:

he0 Sp ¼ 2:103 hi B L þ 0:034; (8)

hc0 Sp ¼ 0 930 hi B L þ S þ 0 019; (9)

her hm np ¼ 0 090 B L ðÞ 0 072; (10) and hcr hm np ¼ 0 032 B L þ S ðÞþ 0 368; (11)

where L isthelengthoftheHDinthe X direction (m), B isthelengthof h i attheupstreamboundaryof theslopefromtheHDstartingpoint,and h m indicates themaximumGWTheightobtainedusingtheHooghoudt(1940)method(Eq.1)forparallelandin finitelylongdrains,where h i isthemeasuredheightof theGWT,locatedsuf fi cientlyawayfromtheHD fi eld.

TheGWTproﬁleinthemid-sectionbetweenHDscan bepredictedasfollows.

Figure6.Empiricalrelationshipsfor:(a)

• Zone1:

Zw ¼ hc L S X þ D; (12)

where Zw istheGWTproﬁle(m),and D denotesthe depthtotheimpermeablelayer(m).

• Zone2:

Zw ¼ he hc S ðX L þ S Þþ hc þ D; (13) and

• Zone3:Zone3maybecharacterizedbytwodifferent methods,inlocalcoordinatesandglobalcoordinates.

LocalCoordinates(xz Space) Figure5showsthatthe groundwater ﬂowinzone3mainlyoccursinthe X direction, allowingtheuseoftheDupuit-Forchheimerassumptionfor horizontal ﬂow.UsingDarcy’slawandtheDupuit-Forchheimerassumption,thedischarge( q ,inm 2 /s)per unitslopewidthacrossasectionatpointP,asshown inFigure4,canbeobtainedas:

q ¼ ks z Dz Dx (14)

Theincrementofthewater ﬂow, Dq,isequaltothe rechargefromthetop:

Dq ¼ DxR: (15)

Byintegrationandapplyingboundaryconditions,

q ¼ qi þ B L x ðÞR; (16)

where qi isthewater ﬂowfrombehindtheslope(m2/s).

SubstitutingEq.16intoEq.14,weobtain

qi ks þ B L x ðÞn dx ¼ zdz: (17)

Byintegrationandapplyingboundaryconditions,

z 2 ¼ H 2 e þ H 2 i H 2 e B L þ B L ðÞn x nx 2 ; (18)

where He ¼ he þ D (m),and Hi istheconstantheadat theupstreamboundaryofthemodel hi þ D (m).

GlobalCoordinates(XZ Space) The z isconvertedto Zw inglobalcoordinatesystembyreplacingxwith(X L)

Z 2 w ¼ H 2 e þ H 2 i H 2 e B L þ B L ðÞn X L ðÞ

nX L ðÞ2 : (19)

CorrectionforInclinedHDs

HDsaretypicallyinclinedtofacilitategravitydrainage, resultinginahigherGWTproﬁlethanhorizontallyinstalled HDs.Therefore,weintroducedacorrectiontotheproposed methodtoaccountfortheinclinationsoftheHDs.

Thecorrectioninvolvesadjustingthevaluesof hc and he.First,thevalueof hi iscorrectedwithreferenceto theelevationsoftheHDattheboundaryofzones1and 2andthedrainendasfollows(Figure4):

hich ¼ hi L S ðÞtan h ðÞ; (20) and hieh ¼ hi Ltan h ðÞ; (21) where h istheinclinationangleoftheHDstothehorizontal,and hich (m)and hieh (m)arethecorrected hi valuesforcalculating hc0 and he0,respectively.Thevaluesof L and S aremeasuredinthehorizontaldirection.Note thatthevaluesof hcr and her remainunchangedbecause theyaresolelybasedontherechargefromthetop. Finally,thecorrected hc and he valuescanbeobtainedas:

hc ¼ hc0 þ hcr þ L S ðÞtan h ðÞ; (22) and he ¼ he0 þ her þ Ltan h ðÞ: (23)

InthisGWTpredictionmethod,zones1and2provide empiricalsolutions,whereaszone3representsatheoretical GWTproﬁlewithanempiricalboundary(he).

Theaccuracyoftheproposedmethodforpredictingthe GWTatthemid-sectionbetweentheHDswasvalidated throughacomprehensiveseriesofnumericalcasestudies usingthe3DFEmodelofslopeIIanda ﬁeldstudy.

AverageGWTProfileforSlopeswithHDs

TheGWTprofilevariationbetweentheHDsonthe YZ planeforslopeIhavingHDswith10mspacingand30m lengthisshowninFigure7.TheGWTprofilebetweenthe drainsconformedtothehalf-ellipticalpatternproposedby Hooghoudt(1940)alongthelengthofthedrains.However,a uniformGWTprofilewasobservedbehindtheHD field. Hence,theaveragevalueoftheGWTprofilealongthelength oftheHDs(zones1and2),denotedas(Zw(avg)),asshownin Figure8,canbeusedtocharacterizetheGWTprofileforfurthercalculationsof U, DU,andfactorofsafety( FS)enhancement( DFS),aswellasincorporationinto2Dnumerical simulations(seeSupplementalMaterialAppendixA).

PWPonSlipSurface

TheobjectiveofHDsistoenhancetheslopestabilityby reducingthePWPactingontheslipsurface.Once Zw(avg) (m)isdeterminedfromtheproposedmethod,theaverage

¼ 10m, hi ¼ 6m,and n ¼

porewaterforce( U [kN/m])actingontheunitwidth ofaprede ﬁ nedslipsurface( Z s [m])canbecalculated vialinearintegrationbetween Z w(avg) and Z s (Figure 9).Followingthemethodofslices,theslipsurfaceis dividedintoa ﬁ nitenumberofsliceswithlinearbottoms.Theporewaterforceonaslice( u i [kN/m])can beobtainedas

where qw isthedensityofwater(kg/m3), g isthegravitationalacceleration(m/s2),and ai istheangleofthe bottomoftheslicerelativetothehorizontalline.Then, thetotalaveragePWPontheslipis(kN/m2):

Next,thereductionofaveragePWPontheslipsurface(DU [kN/m2])canbeobtainedas:

U0 istheinitialaveragePWPontheslipsurface.

ConstraintsoftheProposedMethod

TheconstraintsoftheproposedGWTprediction methodandconcomitantdesignchartareasfollows:

• Theslopeisassumedtoconsistofisotropicand homogeneoussoil.

• Ahorizontalimpermeablelayerispresentatthebottomoftheslope.

• Aknowngroundwaterlevelexistssufﬁcientlyaway fromtheslopethatcanbeusedastheupstreamconstantheadboundary.Itisrecommendedtolocatethis boundaryatleast15mawayfromtheHD ﬁeld,which istheminimumdistanceconsideredinthisstudy.

• Anequivalentconstantrechargeratemustbedeﬁned forinﬁltration.

• Thereisauniformdistributionofwaterlevelinthe Y direction.Inthecaseofanon-uniformdistribution, particularly hi,itisrecommendedtodividetheslope intosectionswithuniformwaterlevelandapplythe methodtoeachsectionseparately.

ﬁlebetweenHDsinzones1and2forcalculating U, DU,and DFS.Thesevalueswereusedin2Danalyses.

Figure7.GroundwatertableproﬁlebetweenHDsonthe YZ planeatdifferentlocationsonthe X axis(L ¼ 30m, S

0.1inslopeI).

Figure8.AverageGWTpro

DesignProcedure

Theproposedmethodinvolvesmultiplerigorousiterative calculationstepstodeterminetheoptimallengthandspacingofHDs.Toovercomethetime-consumingnatureofthe designprocess,aPython-based(Python3.6.2;Rossumand Drake,2009)computerprogramwasdevelopedtoautomate thecalculations.Thestep-by-stepdesignprocedureforHDs, basedontheframeworkproposedinthisstudy,isoutlined inFigureB1(SupplementalMaterialAppendixB).

VALIDATIONOFTHEPROPOSEDMETHOD

Thissectionpresentstheveriﬁcationoftheproposed GWTpredictionmethodusingaseriesofnumerical casestudiesanda ﬁeldstudy.Also,theaccuracyofthe calculationmethodfor U and DU isdiscussed.

ValidationoftheProposedGWTPredictionMethod throughNumericalCaseStudies

TheproposedGWTpredictionmethodwascomprehensivelyverifiedunderawiderangeofconditionswithin theaforementionedlimitations.VerificationwasperformedonslopeII,consideringvariationsintheHD length,spacing,andinstallationangle,rainfallconditions, andinitialGWT.ThepredictedGWTprofileobtained usingtheproposedmethodwascomparedwiththeGWT profileatthemid-sectionofthe3DFEmodel.Toprevent waterponding,acorrectionwasintroducedtotheproposedmethod,particularlyforthetoeregion.Inaddition toavisualcomparison,aquantitativeevaluationwasconductedusingthemeanabsoluteerror(MAE)betweenthe predictedand3DFEmodelGWTs.TheMAEwascalculatedasfollows:

where Zw(Predict) istheGWTprofilepredictedusingthe proposedmethod, Zw(3DFEModel) istheGWTatthemidpointbetweentheHDsobtainedfromthe3DFEmodel, and m isthenumberofdatapoints.Comparisonsofthe GWTprofilesweremadeat5mintervalsinthe X direction.TheMAEvalueswerecalculatedfortheentire lengthoftheslopeandseparatelyfortheHD field (zones1and2)andbehindtheHDs(zone3).TheMAE valuesaresummarizedinTable2.

Figure10Ashowsacomparisonofthepredictedand 3DFEmodelGWTprofilesforhorizontallyinstalledHDs withdifferentlengthsandspacings.ThesmallMAEvalues indicategoodagreementbetweentheGWTprofiles.Generally,zones1and2showedlessagreementbetweenthe GWTprofilesthanzone3,whichcanbeattributedtothe empiricallinearidealizationoftheGWTinzones1and2. Incontrast,thetheoreticalpredictionoftheGWTprofile withanempiricalboundaryinzone3resultedinlower MAEvalues.Furthermore,theagreementbetweenthe GWTprofilesdeterioratedwithanincreaseinthespacing andlengthoftheHDs.WhentheHDlengthincreased,the distancetotheupstreamboundaryoftheslope(distanceto hi)decreased,leadingtopoorpredictions.Therefore,itis crucialtoestablishthe hi atasufficientdistanceawayfrom theexpectedlengthoftheHDstoensureaccurateresults.

Figure10Bcomparesthepredictedand3DFEmodel GWTproﬁlesforhorizontallyinstalledHDsatdifferentrainfallintensities(n).Higherrainfallintensitiesresultedina loweragreementbetweentheGWTproﬁles,andaslight overestimationinzone2wasalsoobserved.Thelowerconformityindicatedbythelower R2 value(Figure11)forthe

Figure9.AverageGWT( Zw(avg)),slipsurface(Zs),andcalculationof FS ofaslopebymethodofslices.

Figure10.ValidationoftheproposedGWTpredictionmethodby3DFEmodelfor:(a)differentlengthsandspacings( n ¼ 0.1)(slopeII);(b)differentrainfallintensities( S ¼ 5m)(slopeII);(c)different hi values( n ¼ 0.17)(slopeII);and(d)inclinedHDs(S ¼ 5mand n ¼ 0.1)(slopeII).

predictionof hcr canbeidentifiedasthereasonforthis behavior.However,itdidnotsignificantlyaffecttheaccuracy oftheGWTprediction.Figure10canddconfirmstheaccuracyoftheproposedmethodinpredictingtheGWTunder differentinitialGWTconditionsandHDangles,respectively.

Figure11comparesthe hc and he valuespredictedby theproposedmethodwiththevaluesobtainedfromthe 3DFEmodels.ThelowMAEandhigh R2 valuessuggestthattheproposedmethodprovidesagoodpredictionof hc and hc

ValidationofProposedGWTPredictionMethodby FieldStudy

TheGinigathhenalandslide,locatedatbridgeno.48/2 attheAvissawella-Hatton-NuwaraeliyaRoadinSri Lanka,wasselectedfor ﬁeldvalidationoftheproposed method.Thedetailsofthelandslidesweredocumented byLakruwan(2019)andLakruwanandKulathilaka (2020,2021).Thelandslidemassconsistedofthick colluviumandaresidualsoillayerunderlainbyhighly

crystallinemetamorphicrock.Theinitialslopefailure occurredonJune22,2014,followedbyexcavationat thetoeforroadwideningandpropagationtotheupper slopeduringtherainyseason.Thelandslidewasmitigatedbyimprovementsindrainageandsoilnailing.

Theaveragehomogeneousandisotropichydraulic conductivityofthesoiloverburdenwasfoundtobe8 3 10 6 m/s,andthebedrockwasconsideredtobeimpermeable(Lakruwan,2019).Thewaterlevelsofthetwo observationwells,W1andW2,located1.7mand42.38 mawayfromHD ﬁeld,respectively,weremonitoredafter the20-m-longHDswith5mspacingwereinstalled(Figure12).Becausethehorizontalimpermeablebottomlayer onlyexistedinthatsection,onlythetopslopesection

demarcatedbythedottedlinerectangleinFigure12 wasconsideredforthevalidationoftheproposed method.Twocaseswereselectedforvalidation:Case 1,norainfallfromFebruary27toMarch21,2018,and Case2,450mmcumulativerainfallfromMay18to23, 2018(n ¼ 0.11).SincethewaterlevelofW2wasrather constantwithtime,itwasselectedasthe Hi (B ¼ 62.38m and Hi ¼ 15.00m[Case1]/15.58m[Case2]).The observedandpredictedwaterlevelsofW1inCase1(zero elevationatthetopoftheimpermeablelayer)were2.67 and2.77m,respectively.Theobservedandpredictedwater levelsofW1inCase2were3.61and4.06m,respectively. Therefore,despiteaslightoverestimation,theproposed methodcanaccuratelypredictthewaterlevel.

Figure11.Comparisonofpredicted(a) hc and(b) he valuestothevaluesobtainedfromthe3DFEmodel.

Figure12.FieldvalidationoftheproposedGWTpredictionmethodfortheGinigathhenalandslide.

Lakruwan,Kamura,andKazama

ValidationoftheProposedMethodforObtaining U and DU

SlopeIwasusedforthevalidationoftheproposed methodforobtaining U and DU.Theaverage U (kN/m) fromthe3DFEmodelwasobtainedbyintegratingthe PWPvaluesofthe51stresspointslocatedontheFEmesh oftheslipsurface(FigureB2inSupplementalMaterial AppendixB).Threepoint-rowswerelocated,oneonasectionontheHDandoneeachonsectionsatthemid-and quarter-pointsbetweentheHDs.Eachrowconsistedof19 pointsat2.5mspacingsinthe X direction.

Thegoodagreementbetweenthe U valuesobtained fromthe3DFEmodelsandtheproposedmethod(Eq. 24andEq.25)forslopeI(hi ¼ 6mand n ¼ 0.1),as showninFigure13a,conﬁrmsitsvalidity.Also,the plotsof DU inFigure13bconﬁrmtheaccuracyofthe proposedmethodincalculating DU.

Thiscomprehensivevalidationconfirmedtheaccuracy oftheproposedGWTpredictionmethodforslopeswith HDsunderawiderangeofconditionssubjectedtothe aforementionedlimitations.Moreover,itvalidatesthe predictionof U and DU inthepresenceofpredefinedslip surface.Therefore,theproposedmethodcanbeusedas anefficienttoolfordesigningHDswhileovercomingthe difficultiesofcomplex3Dnumericalsimulations.

RESULTSANDDISCUSSION

Thissectionpresentsexamplesofapplicationsofthe proposedmethodforthedesignofHDsandprovides

severalconclusionsonselectingappropriatelengthsand spacingforHDs.

EffectofHDonPWPandEffectiveStressonSlip Surface

A3DFEstress-deformationanalysiswasperformedon slopeIusingthestrengthreductionmethodtoobtainthe PWPontheslipsurfaceand FS.Figure14showsthePWP distributionontheslipsurfaceobtainedfromthe3DFE modelofslopeIwithadrainspacingof10m.ThePWP decreasedwithHDlength,andnofurtherreductionoccurred beyondnormalizeddrainlength(L/Ls,where Ls isthe lengthtotheslipsurface)of2.0.Theeffectivestress increased,asshowninFigure15,whilecomplyingwiththe decreaseinPWP,whicheventuallyresultedinsimilar behaviorinthe FS enhancement.

Figures13providesfurtherinsightsintotheeffects oftheHDlength.InFigure13,both U and DU achieveuniformvaluesforlongHDs,andafurther increaseinthedrainlengthbecomesinsigni ﬁ cant, whichcon ﬁ rmstheresultsofCaietal.(1998),Cook etal.(2008),LauandKenney(1984),andRoyster (1980).ThisphenomenonisexplainedinFigureB3 (SupplementalMaterialAppendixB).InshortHDs, thefailurewedgeextendsintoallthreezonesofthe GWT,and U becomeshighlysensitivetothelengthof theHDsbecausethehighGWTinzone3gradually movesawayfromthefailurewedge.Withanincrease inHDlength,zones3and2graduallymoveaway fromthefailurewedge,eventuallylimitingthefailure

Figure13.Validationoftheproposedmethodtoobtain(a) U and(b) DU ( n ¼ 0.1)(slopeI).

wedgetozone1.Inzone1, U showsnegligiblesensitivitytodrainlengthincrementsbecausethegradient oftheGWTisverysmall.

Effectof n on U

Figure16illustratestheeffectof n,rangingfrom0.01 to0.2,on U fordifferentdrainlengthsandspacing.The U valuesat n ¼ 0.01and n ¼ 0.2aredenotedas U0.01 and U0.2,respectively.HighGWTsinwidelyspaced HDsarehighlysensitivetosurfacerecharge,making n signiﬁcantlyaffect U atlarge S values.Thiseffect reducesastheGWTlowersincloselyspacedHDs.Furthermore,theimpactof n on U issigniﬁcantinshort HDsanddecreaseswithincreasingdrainlength,dueto theshiftintheslipsurfacelocationfromzone3tozone 1,asdiscussedintheprevioussection.

ProducingDesignCharts

Adesignchartcanbeusedtoef ﬁ cientlyselectthe lengthandspacingofHDstoachievethedesired DU or DFS valuesforaparticularslopefailure.Theprocedurefor calculating DFS isoutlinedinSupplementalMaterial AppendixC.Giventheheterogeneityofthefailures,the designchartsbyKennyetal.(1977)andStanic(1984)were developedforuniqueslopeconditions.Therefore,insteadof producinggeneralizedcharts,weproposecreatingunique designchartsforagivenslopetoselectthedrainlengthand spacing.Thelimitationsonthechartusedareoutlinedinthe section “ConstraintsoftheProposedMethod.” Figure17showsthedesigncharts,andvariationin DU and DFS withdrainspacingfordifferent L/Ls,produced forslopesIandII.Thenon-compatibilityofthedesign chartsforslopesIandIIshowsthattheyareuniquefora

Figure14.PWPdistributionontheslipsurfacefordifferent L valuesobtainedfromthe3DFEmodelanalysis(S ¼ 10m, Ls ¼ 10m, D ¼ 10m, B ¼ 50m, hi ¼ 5m,and n ¼ 0.1).

Figure15.Distributionoftheeffectivestressontheslipsurfacefordifferent L valuesobtainedfromthe3DFEmodelanalysis(S ¼ 10m, Ls ¼ 10m, D ¼ 10m, B ¼ 50m, hi ¼ 5m,and n ¼ 0.1).

givenslope,andageneralizeddesignchartcannotbe adopted.

Areductioninthesensitivityofthedrainlengthforlong HDsisalsoreﬂectedinthedesigncharts,wherethedesign

chartsindicatethattheeffectofdrainspacingismoresigniﬁcantinshortHDs,whereasitisnotsigniﬁcantinlong HDs.InlongHDs(i.e.,whenthefailurewedgeislocated inzone1),increasingthespacingcausesonlyaslight

Figure16.Effectof n on U fordifferentdrainlengthsinslopeI(S ¼ 5m, Ls ¼ 10m, D ¼ 10m, B ¼ 50m,and hi ¼ 6m).

increaseintheGWTproﬁleinsidethefailurewedge. Therefore,onlyasmallincrementin U isobservedwhen increasingthespacinginlongHDs,whichisalsoshown byAbedetal.(2024).

Atargetvalueof DFS or DU canbeachievedwitha smallerHDlengthperunitslopewidthwhenlargedrain spacingsareused(Figure18).Forexample,toachieve DU of9.9kN/m2 ( DFS 0.6)inslopeI,5.3and1.7mpipe lengthspermeterwidthofslopearerequiredwhenthe spacingis1.0and5.0m,respectively.Therefore,theuse

ofalargerdrainspacingismoreeconomical.However, twomainissuesarisewithawidedrainspacing.Oneissue istheriskofnon-functioningHDsforvariousreasons, suchascloggingofthe ﬁlterscreen,mineralprecipitation, androotgrowthinsidethepipe(Martinetal.,1995;Lee, 2013).Inthecaseofanon-functioningdrainpipe,the drainspacingwillbedoubled,and DFS willbereducedto 0.54and0.35fortheinitialdrainspacingof1.0and5.0m, respectively.Therefore,asmallspacingismoreconservativeinsuchsituations.Thisissuecanbeeliminatedusing

Figure18.TotalHDlengthperslopewidth(m/m)toachievetarget DU for:(a)slopeIand(b)slopeII.

Figure17.Designchartfortheselectionofdrainspacingandlengthsfor:(a)slopeIand(b)slopeII.

longHDsbecausetheeffectofspacingisnotsignificant. ThenextissueassociatedwithwidelyspacedHDsisthe requirementforapipewithahigherdischargecapacity. Therefore,atrade-offexistsbetweenthepipe flowcapacityandtotalpipelength,andthe flowcapacityofthe drainpipeshouldalsobeconsideredintheselectionof drainspacing.

CONCLUSIONS

Thefollowingconclusionshavebeendrawnfromthis study.

1.Anovelsemi-empiricalmethodthatcanbeusedto obtaintheGWTpro ﬁ leofaslopewithHDssubjectedtoconstantrechargefromthesurfaceisproposedinthisstudy,alongwithadesignframework forHDlengthandspacing.Theaccuracyoftheproposedmethodwascomprehensivelyvalidatedthrough aseriesofnumericalcasestudiesforawiderangeof conceivableconditionsandmonitoringdataofan actualslope.

2.Theproposedmethodwasextendedtodeterminethe PWPforceonapredeﬁnedslipsurfacevialinearintegrationbetween Zw(avg) and Zs,whichwassubsequentlyveriﬁedthroughanumericalcasestudy.The Zw(avg) canbeusedin2Dnumericalmodelstoreplicatethe3DeffectsoftheGWT.

3.APython-basedcomputerprogramwasdevelopedto automatemultiplerigorousiterativecalculationsteps fordeterminingtheoptimallengthandspacingofthe HDs.Auniquedesignchartwasproducedforaspeciﬁcslopeusingthedevelopedcomputerprogram.

4.DesignchartsofslopesIandIIwereusedtoevaluatethe effectsoflengthandspacingoftheHDsonthePWP reductionand FS incrementofaslope.TheHDsmust onlybeextendedtoalengthequaltothedrainspacing beyondtheslipsurface.Thelengthandspacingofthe HDsrequiredtoachievethetarget FS increasecanbe obtainedfromthedesignchartproducedforaparticular slopecondition.AlthoughlengtheningtheHDsismore effectivethanreducingthedrainspacing,theriskof drainblockageanddischargecapacityofthedrainpipe mustbeconsideredwhenselectingthedrainspacing.

SUPPLEMENTALMATERIAL

SupplementalMaterialassociatedwiththisarticlecanbe foundonlineat https://www.aegweb.org/e-eg-supplements.

ACKNOWLEDGMENTS

ThisresearchwassupportedbytheJapanSocietyforthe PromotionofScience(JSPS)Grants-in-AidforScientiﬁc Research(KAKENHI)grant(no.JP24K00974).

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UsingtheTotalApparentSpacingApproachto OptimizeFoundationDrainOrientations

WILLIAMM.ROMAN1* ChiefGeologist,GannettFleming,Inc.,CampHillPA

JOHNN.BEREZNIAK2

2277SagamoreDrive,StateCollege,PA16803

KeyTerms: ConcreteDams,FoundationDrains, Discontinuities,OptimumDrillingDirection,Uplift,Design

ABSTRACT

Designoffoundationdrainholesinbedrockwhere fracturescontrolthepermeabilitymaybeoptimized basedonthediscontinuitypattern,speci ﬁ callythe spacingandorientationofthepredominantdiscontinuitysets.Whensuitablerepresentativeoutcropsare available,eithernaturallyorbymeansofexcavation, jointspacingandorientationcanbemeasureddirectly bytraditionalmethodsusingameasuringtapeand compassorpockettransitornewermethodsusing computersoftwaretomeasurevirtualjointsurfaces withinadigitalterrainmodeldevelopedfromdigital photogrammetryorlightdetectionandranging(LiDAR) surveys.Whensuitableoutcropsarenotavailable, jointspacingandorientationcanbemeasuredusing orientedcoredrillingtechniquesordownholeimaging tools(e.g.,opticalandacousticsondes)thatprovide digitalimagesoftheboreholewall.Withanunderstandingofthebedrockjointingpatternobtained fromoneormoreoftheabovemethods,thetotal apparentspacing(TAS)approachcanbeusedto optimizetheorientationofdrainholes.TheTAS approachemploysthevectordotproductformula tominimizethetotalapparentspacingbetween discontinuitiesandmaximizethenumberofdiscontinuitiesinterceptedbythedrainhole.TheTAS approachwasusedtooptimizetheorientationof foundationdrainholesduringrehab ilitationofthe GilboaDaminSchoharieCounty,NY.TheTAS approachhasotherapplications,e.g.,optimizing theorientationofexploratorycoreholestocollect discontinuitydataortheorientationofgroutholes forfoundationgrouting.

1Retired;presentaddress:1080StateParkRd,Newport,PA17074

2Previouslyat:U.S.ArmyCorpsofEngineersHeadquarters,Washington, DC,SeniorCivilEngineer(retired)

*Correspondingauthoremail: bill.roman.bretz@gmail.com

INTRODUCTION

Upliftpressureiscriticaltoboththeslidingandoverturningstabilityofconcretedamsf oundeddirectlyonrock. Theupliftpressurebeneathsuchadamisafunctionofthe hydraulicgradient,whichisassumedtovarylinearlyfrom headwaterattheheelofthedamtozeroortailwateratthe toeofthedam(Figure1a),assumingahomogeneousand isotropicconditionwherehydraulicconductivitydoesnot changesigniﬁcantlyoverthedamsite,whichisnotoften thecaseintherealworld.Inthecasewherehydraulicconductivitydoeschange,therelationshipbetweenupliftpressureandheadwaterandtailwaterpressureisnon-linear. StudiesdonebyGrenobleetal.(1995)showedthatrising reservoirlevelscandifferentiallydeformdiscontinuities inthefoundationandcausethehydraulicconductivityto increaseattheheelofthedamanddecreaseatthetoe (taperedjoint).Anon-linearresponsemayproducean upliftpressuredistributionthatis greater thantheconventionallinearassumptionforuplift(EbelingandPace, 1996a).Non-linearresponsesofupliftpressurestoreservoirlevelhavebeenanalyticallymodeled(Ebelingand Pace,1996b)andphysicallyobservedinseveraldams (StoneandWebsterEngineeringCorporation,1992).The expansionandcontractionofdownstreamfacesdueto seasonalambienttemperaturechangesmaycausegreater changesinupliftpressurethanchangesinheadwaterlevels (StoneandWebsterEngineeringCorporation,1992).

Installationofdrainsbeneaththedamcanalterthe hydraulicgradientandhelptoreducetheupliftpressure (Figure1b).Drainsareconsideredtobethemostreliable andcost-effectivewayofreducingfoundationupliftpressures,especiallyforstructuresfoundedonrock(USACE, 2005).Theeffectivenessofthedrainagesystemdepends onthedepth,size,andspacingofthedrains;thecharacter ofthefoundation;andthefacilitywithwhichthedrainscan bemaintained.

Thehydraulicconductivityofajointedrockmassiscommonlycontrolledbyitssecondarypermeability,soinmany civilengineeringapplications,bedrockdiscontinuitiesdominatethedraindesignprocedures.Assumingotherdiscontinuitycharacteristicssuchasaperture,length,and ﬁllingare relativelyuniform,foundationdrainholesdrilledatthe

Figure1.Schematicsshowingtheoreticalupliftdistributionbeneathaconcretedam(A)havingnodrainsand(B)withfoundationdrains.Modiﬁed fromU.S.ArmyCorpsofEngineers(2005,FigureC-4).

optimumorientationandinclinationwithrespecttothe orientationandspacingofthepredominantdiscontinuities shouldrelieveupliftpressuremostefﬁciently.However,it shouldbenotedthattheseverityofupliftpressuremaybe independentofdiscontinuityorientation(Grenobleand Amadei,1990)asdiscontinuityapertureand ﬁllingmay controlupliftpressuresmorethandiscontinuityorientation (Murphyetal.,2002).

Whenotherdiscontinuityparametersarenotrelatively uniformandresultinananisotropicdistributionoffoundationhydrostaticpressures,optimizationofdrainhole orientationsbasedsolelyontheorientationandspacing ofthepredominantdiscontinuitiesisnotappropriate. Anisotropichydrostaticpressuresmayoccurinfoundations wheremultipleepisodesofstructuraldeformationdueto plateconvergenceorriftinghaveproduceddiscontinuity setshavingsignificantlyvaryingdiscontinuityparameters. Variationsinstressregimesmayaffectdiscontinuityaperture andlength,andpreferential flowof fluidsduringstructural deformationmayresultinprecipitationofmineralsthat completely fillcertaindiscontinuitysets.Drillingfoundationdrainholesattheoptimumorientationandinclination withrespecttotheorientationandspacingofthepredominantdiscontinuitiesisappropriateonlywhendiscontinuity parametersarerelativelyuniform,acasemorelikelyto existwithinbedrockfoundationsthathaveexperiencedonly minorstructuraldeformation.Sitereconnaissanceandsubsurfaceinvestigationshouldseektoidentifyandcharacterize thosediscontinuitiesthataffectthehydraulicpressuresin thefoundation,whichcouldincludelessprevalentdiscontinuitiesand,insomecases,evenonesinglediscontinuity.

Severalauthorshaveaddressedaspectsofdrainhole design.Cooketal.(2007)discussedthestateoftheart andpresentedsuggestionsforimprovementinthedesign

oflandslidehorizontaldrains.Twoapproachesforoptimizing boreholeorientationhavebeenpresentedin Environmental& EngineeringGeoscience.ZhouandMaerz(2002)developed anapproachto ﬁndtheoptimumdrillingdirectionformaximizingaborehole’sinterceptionofdiscontinuitysets.The basicconceptinvolves ﬁndingadrillingdirectionthatminimizesthelinearsamplingbiasindex(LSBI),whichisa functionoftherelativeanglebetweentheorientationofthe boreholeandthemeanorientationofthepolesofeachof thediscontinuitysets.Theoptimumdrillingdirectionisthe directionalongwhichtheLSBIisminimized.Haneberg (2009)observedthatwhiletheLSBIminimizationapproach producescorrectresults,itsuffersfrommultipleshortcomingsthatinhibititsutility.Haneberg(2009)presentedan approachthatbuildsupontheapproachtakenbyZhouand Maerz(2002)byincorporatingchangestomakeitpotentiallymoreusefulandgeometricallymeaningful.Haneberg (2009)describedthisapproachtooptimizeaborehole’s orientationbyminimizingthelinearsamplingangular deviationandprovidedexamplesproducedusingaseries offunctionswrittenforthecomputerprogramMathematica (Haneberg,2004).

Thispaperdescribeshowthetotalapparentspacing (TAS)approachoptimizesdrainholeorientationusinga minimizationprocessbasedondiscontinuityspacingand orientation.TheTASapproachseekstominimizethetotal apparentspacingofdiscontinuitiesalongthedrainhole andtomaximizethenumberofdiscontinuitiesencountered bythedrainhole.Beyondthefoundationdrainholeorientationexampleprovidedherein,useoftheTASapproach tostrategicallyorientadrillholesoastointersectthemaximumnumberofdiscontinuitiesalongthedrillholefora givendrillinglengthmaybeusefulforotherpurposeswithin

therealmsofgeotechnicalexploration,rockengineering,and foundationtreatment.

BEDROCKJOINTING

OriginsandPatterns

Fracturesoccurinresponsetostressesthatexceedthe strengthoftherocktoresisttheresultingforce.Fractureproducingstressesmaybetensile,compressive,shear,ora combinationofstresstypes.Stressesmayvaryinstrength andorientationovertimeandspace,sofracturesmayoccur inmultiplesetshavingvaryingorientationsandcharacteristicswithinaregion.Onceformed,fracturescanbemodiﬁedasaresultoftectonicandthermalstressandstrain, changesin ﬂuidpressure,andphysical,chemical,andbiologicalprocessessuchaserosionandvalleystressrelief, dissolutionorprecipitationofminerals,anddegradationby roots(NASEM,2020).

Planarbedrockdiscontinuities,whichmaynotnecessarily be fl at,oftenoccurinsetsofdiscontinuitieshaving semi-parallelorientationsorfallingwithinarangeof similarorientations.Horizontaltogentlydippingsedimentaryrockswithlimitedtectonicstressesoftenfeaturetwo nearlyorthogonaljointsetsthatarebothroughlyperpendiculartoasetofbeddingjointsdevelopedalongbedding planes.Onejointsetiscomposedof “strikejoints,” which generallystrikeinaboutthesamedirectionasbedding strike,whiletheothersetiscomposedof “ dipjoints, ” whichgenerallystrikeroughlyparalleltothebeddingdip direction.Additionaldiscontinuitysetsareoftenpresentin sedimentaryrockssubjectedtohighertectonicorother externalstresses.Asimilarblockyjointingpatternmay occurinigneousrockshaving flowbanding. “Flat-lying joints” areapproximatelyhorizontalandstrikeparallel tothe flowlinesindicatedbythepreferredorientationofthe longaxesoftherockcrystals.Asecondjointsetiscomposedof “Qjoints” or “crossjoints” thatstrikenearlyperpendiculartothe flowlines.Athirdsetiscomposedof “S joints” or “longitudinaljoints” thatdipsteeplyandstrike paralleltothe flowlines(KrynineandJudd,1957).Inigneousrockmasseslacking flowbanding,localandregional stressandstraindistributionstendtodefinethefracture pattern. “Tensilejoints” formgenerallyparalleltothemaximumstresstrajectory,and “shearjoints” formatanangle ofroughly20to35degreestothemaximumstresstrajectory (NASEM,2020).

SurveyMethods

Thepatternofdiscontinuitieswithinarockmass,specificallythetypicalorientationandspacingofdiscontinuities, mustbedeterminedinordertousetheTASapproach.The orientationofrelativelyplanardiscontinuitiescanbe expressedbyanazimuth(eitherstrikeordipdirection)and

adipangle.Discontinuityspacingandorientationdatacan becollectedusingoneormoreofseveralavailablesurvey methods.Properlyconductedsurveyscanfurnishdatawitha highprobabilityofaccuratelyrepresentingthetypicalorientationandspacingofthediscontinuitieswithinarockmass. Whensuitablerepresentativeoutcropsareavailable, eithernaturallyorbymeansofexcavation,discontinuity spacingandorientationcanbemeasureddirectlybytraditionalmethods(e.g.,linearscanlineorwindowsampling) usingoneormoretoolssuchasameasuringtapeanda compass,pockettransit,smartphone,ortablet.Corrections forobservationbiashavebeendevelopedforscan-line sampling(Terzaghi,1965)andforboreholesampling, whichmaybeconsideredasasubsurfacescanline(Mauldon andMauldon,1997).Observationalbiasmayberemoved byorientingscanlinesperpendiculartooneanother andbyrecordingthetracelength,orientation,andpoint ofintersectionofeverydiscontinuityalongthescanlines (LaPointeandHudson,1985).Forwindowsampling,the windowshouldbeaslargeaspossibletominimizesampling bias.Ideally,atleasttwowindowsofsimilarsizeshouldbe establishedonadjacentrockexposureshavingdifferent (preferablyorthogonal)orientations(Priest,1993).Window samplingreducestheobservationbiasduetothediscontinuityorientationandlengthassociatedwithscan-linesampling; however,biasmayremainasaresultofcurtailmentofthe discontinuitiesduetooutcropcover.

Newermethodsusecomputersoftwaretomeasurevirtual discontinuitysurfaceswithinadigitalterrainmodelofthe rockoutcropdevelopedbymeansofdigitalphotogrammetryorlightdetectionandranging(LiDAR)surveys (Hanebergetal.,2006;TononandKottenstette,2007; Haneberg,2008;KemenyandTurner,2008;KottenstetteandShaffner,2008;Shaffneretal.,2009;Kemeny etal.,2011;andWatts,2011).UsingbothdigitalphotogrammetryandLiDARimagingmaybeusefulinsome circumstances(Otooetal.,2011).Althoughusefulfor measuringlocationsandorientationsofdiscontinuities, photogrammetrymaynotprovidecriticalgeologicalinformationthatcanbegleanedonlybyhumanexaminationof thediscontinuities(GatesandHaneberg,2012;Battulwar etal.,2021).Traditional ﬁeldcharacterizationandinput fromexperiencedprofessionalsmayenhancethereliability oflaserscanning(Farny,2017).

Whensuitableoutcropsarenotavailable,discontinuity spacingandorientationcanbemeasuredusingoriented coredrillingtechniquesordownholeimagingtools(e.g., opticalandacousticsondes)thatprovidedigitalimagesof theboreholewall.Providedboreholeinstabilitydoesnot precludetheuseandretrievaloftheimagingsonde,boreholeimagingservesasapowerfultoolformeasuringfractureorientationsandspacings,bettercharacterizingzonesof brokencore,andunravelingthemysteriesofcorelosses.

Thesurveymethodsusedshouldprovideanunderstandingofthevariabilityinorientationsanddipangles ofthediscontinuities.Thisunderstandingisveryimportant inultimatelyselectingthetypicaldiscontinuityorientation anddipangle.Largervariabilityinthedatawillmakethe selectionofthetypicaldiscontinuityparametersevenmore important.Moreover,discontinuitydatashouldbecollected asclosetothesiteaspossible.Discontinuitydatafroman off-sitelocationshouldbecomparedtoavailableinformationfromthesitetodetermineitsapplicability.

THEORY

Assumptions

TheTASapproachforidentifyingtheoptimumfoundationdrainholeorientationisbasedonthefollowing generalassumptions:

1.Jointingistheprimarypathwayofwater ﬂowandpressurethroughtherockmassformingthedamfoundation.

2.Surveysareproperlyconductedandaccuratelyrepresent thetypicaldiscontinuityspacingandorientationofthe predominantdiscontinuitysetswithinthedamfoundation.

3.Thespacingbetweenindividualdiscontinuitieswithin asetisgenerallyuniformthroughoutthedamfoundation; consequently,theaveragediscontinuityspacingofeach setcanbeused.

4.Theorientationsofdiscontinuitieswithinasetare generallyuniformthroughoutthedamfoundation;consequently,theaveragediscontinuityorientationofeach setcanbeused.

5.Thediscontinuitysetsarepersistentthroughoutthedam foundation.

6.Adrainholedirectiontrendingparalleltotheplane representingtheaverageorientationofadiscontinuity sethasaninﬁniteapparentspacingandisexcludedas anoptimumfoundationdrainholedirection.

7.Thediscontinuitysetsarerelativelyuniforminaperture, length,and ﬁlling.TheTASapproachmaybemodiﬁed byaddingweightingfactorswhenthisisnotthecase.

Ifdiscontinuityspacing,orientation,andpersistence aresigniﬁcantlynon-uniform,thenconsiderationshould begiventosubdividingthefoundationintoregionsor areasofsimilardiscontinuitypropertiesandperforming separateevaluations.

Concept

Foradrainholetodrainwatermosteffectivelyfroma jointedrockmass,thedrainholemustbeorientedsothat itinterceptsthediscontinuitynetworkmostfrequently; i.e.,thehighestnumberofdiscontinuitiesisintercepted perunitlengthofdrillhole.Sincethediscontinuitynetwork

consistsofacertainnumberofpredominantdiscontinuity sets,adrainholewillintercepteachpredominantdiscontinuitysetofthediscontinuitynetworkatacertainaverage distancealongthelengthofthedrainhole.Optimizingthe drainholeorientationthusrequiresaminimizationofthe averagedistancesbetweenjointsetsinterceptedbythe drainhole.Thedotproductformulaprovidesameansto accomplishthisminimization.

Thedesignatedpredominantdiscontinuitysetsshouldbe thosethatsigniﬁcantlycontributetopressureinthefoundationofthestructure.Whenthecharacterizationoffoundationconditionsdeterminesthatmultiplediscontinuitysets contributetopressureinthefoundation,butthattheircontributionsvarysigniﬁcantly,considerationshouldbegiven toassigningweightingfactorstothepredominantdiscontinuitysetsortosubdividingthefoundationintoregions orareasofsimilardiscontinuitypropertiesandperforming separateevaluations.

Formulas

Althoughdiscussedintermsof “joints” and “jointsets,” theformulasbelowareapplicabletothebroadercategory ofstructuralbreaksencompassedbytheterms “discontinuity” and “discontinuitysets.”

Thedotproduct(orscalarproduct)oftwothreedimensionalCartesianvectors

a ¼ðax ; ay ; az Þ and b ¼ðbx ; by ; bz Þ isdeﬁnedas:

a

Itcanbeproven(Stewart,2003,p.844)thatEq.1 isequivalentto

where ja j a and jb * j b denotethemagnitudesofvectors a and b * ,and h istheanglebetween a and b * .

Let n * bethenormalvector(pole)ofaplane P thatrepresentstheaveragejointorientationinajointset.Letthe magnitudeof n * (denoted jn * j n)representtheorthogonal (true)averagespacingofthejointswithintheset.Let r beanarbitraryvectorrepresentingtheproposeddrain holeorientationstartingattheoriginandconnectingthe origintoanarbitrarypointonplane P.Themagnitudeof r * (denoted jr j r )representstheapparentaveragespacing ofthejointswithinthesetalongthedrainhole(Figure2).

Let r 0 beavectorwithinplane P connectingthetip of n withthetipof r asshowninFigure2.Vector r 0 is thusequaltothedifferenceof r * and n * :

Figure2.Schematicdrawingillustratingthevectorrepresentingthe poletothejointset(n ),thevectorrepresentingtheproposeddrain holeorientation( r !),andthevectorconnectingthem(r 0 )withinthe planerepresentingtheaveragejointorientation(P).

r 0 ¼ r n ; (3) or,equivalently, r isequaltothesumof n and r 0 :

r ¼ n þ r 0 (4)

Replacing a and b ! with r 0 and n inEq.2showsthat thedotproductofthesevectorsiszerosince r * 0 ? n * , andthereforetheincludedangleisarightangle:

r 0 n ¼jr 0 jjn jcosh

¼ r0 ncos90

¼ 0: (5)

Takingthedotproductof r * and n * andsubstitutinginthe equalitiesofEq.3andEq.4,usingthedistributiveproperty ofthedotproduct,andknowingthat r 0 n ¼ 0,yields:

r n ¼ðn þ r 0 Þ n

¼ðn þðr n ÞÞ n

¼ n n þðr n Þ n

¼ n 2 þ r 0 n

¼ n 2 þ 0

¼ n 2 (6)

Usingtheconventionalunitvectornotationof

^ a ¼ a =ja j¼ a=a,thedotproductof r * and n * canalso bewrittenas:

r n ¼ðr ^ r Þðn ^ n Þ

¼ rn ^ r ^ n (7)

Settingthetwosolvedequalitiesforthedotproduct of r and n equaltoeachotheryields:

rn ^ r ^ n ¼ n 2 (8)

Becausethedotproductoftwovectorsisascalar,Eq. 8canbedividedby n ^ r ^ n :

r ¼ n= ^ r ^ n ðÞ (9)

Let ^ n 1 representtheaveragepoletoJointSet1and ^ r representtheproposeddrainholeorientation.Insphericalcoordinates,theunitvectorsare ^ n 1 ¼ð1; h1 ; u1 Þ and ^ r ¼ð1; h; uÞ.ConvertingtoCartesiancoordinatesproduces ^ n 1 ¼ðsinu1 cosh1 ;

and

UsingEq.1totakethedotproductofvectors ^ n 1 and ^ r yields:

Substitutingtheproductofvectors ^ n 1 and ^ r intoEq. 9anddeﬁningthisquantityas d1 yields:

Hence, d1 istheapparentjointspacingforJointSet1 whendrillingwithazimuth h andplunge u.

Equation11approximatestheapparentspacingofdiscontinuitiesbelongingtoJointSet1alongatheoreticaldrain hole.Similarequationscanbewrittentosolvefortheapparentspacingofotherrecognizedjointsetsalongatheoretical drainhole.Thetotalapparentspacingapproachminimizes thesumoftheapparentspacingsofallselectedjointsets alongatheoreticaldrainhole,whichisgivenby:

Xwhere k ¼ numberofselectedjointsets, ni ¼ truejointspacingofthe ithjointset, ui ¼ jointdipangleofthe ithjointset, hi ¼ jointdipdirectionofthe ithjointset, u ¼ drainholeinclination,and h ¼ drainholeazimuth.

Itshouldbenotedthattheangularmeasurements inEq.12referencesphericalcoordinatesasusedin

mathematics,so h and hi refertotheangularmeasurementfromthe x axisinacounterclockwisedirection withinthe X- Y plane(witheast ¼ 0degrees),and u and ui refertotheangularmeasurementfromthe z axis (withzerodegreesrepresentingverticallyupwardand 180degreesrepresentingverticallydownward).Ifjoint dipdirectionanddrainholeazimutharemeasuredincompassbearings(0to360degreeswithnorth ¼ 0degrees), thenthecompassbearingshouldbesubtractedfrom90 degreestoconvertto h and hi .Ifjointdipangleanddrain holeinclinationaremeasuredfromthehorizontalplane (with90degreesrepresentingverticallyupwardand 90 degreesrepresentingverticallydownward),thentheangle shouldbesubtractedfrom90degreestoconvertto u and ui .

Application

Equation12providesaformulatominimizethesum oftheapparentspacingsofallselectedjointsetsalongatheoreticaldrainhole,whichisthepurposeoftheTAS approach.ApplicationoftheTASapproachwilltypically involvethefollowingactivities:

1.Performtheresearchandsiteinvestigationsneededto characterizethediscontinuitieswithintherockmass formingthedamfoundationandtogroupthediscontinuitiesintosets.

2.Identifythediscontinuitiesorsetsofdiscontinuities thatsigniﬁcantlyinﬂuenceuplift.

3.AssesstheTASapproachassumptionstodetermine whethertheTASapproachisapplicable.

4.IfthevariationintheparametersoftheselecteddiscontinuitysetsindicatestheTASapproachisnotapplicable, thenconsidersubdividingthefoundationintozonesfor separateevaluationsand/orassigningweightingfactors totheselecteddiscontinuitysets.

5.Determinethebest-ﬁtorientationsoftheselecteddiscontinuitysets.

6.Determinetherangeoffavorabledrainholeorientations.

7.Foreachpossiblefavorabledrainholeorientation,use Eq.11tocalculatetheapparentspacingofeachjoint setandsumtheapparentjointspacingsalongthe drainholeperEq.12.Acomputerspreadsheetwill facilitatethesecalculations.

8.Usethecalculatedresultstoidentifytheoptimumdrain holeorientation.

ThelevelofeffortdevotedtoapplyingtheTASapproach shouldbecommensuratewiththerelativeimportanceofthe draindesignintheoveralldamdesignorstabilityanalysis. Forexample,ifreductioninupliftpressurebyfoundation drainsisnotbeingfactoredintothestabilityanalysis (aconservativeapproach),thenalessrigorousapplication oftheTASapproachmaybewarranted.Rehabilitationof GilboaDamprovidesanexampleoftheapplicationofthe TASapproach.

Figure3.TopographicmapshowinglocationofGilboaDaminSchoharie County,NY.

GILBOADAMEXAMPLE

DuringtherehabilitationofGilboaDaminNewYork State,theTASapproachwasusedtooptimizedrainhole orientation.

GilboaDam

GilboaDamislocatedintheCatskillMountainsin southernSchoharieCounty,NY(Figure3).Thedamis situatedapproximately125mi(200km)north-northwest ofManhattanand37mi(60km)southwestofAlbany, NY.GilboaDamimpoundsSchoharieReservoir,which isoperatedbytheNewYorkCityDepartmentofEnvironmentalProtection,andwhichisakeycomponentof thecity’swatersupplysystem.SchoharieReservoirhas anormalstoragevolumeofapproximately20.8billion gallons(7.88 3 107 m3)andcovers1,159acres(469hectares)atnormalpool.Thereservoirhasawatershedarea ofapproximately314mi2 (813km2).

TheeasternportionofGilboaDamconsistsofa160-fthigh(48.8m),1,324-ft-long(404m)concretespillway foundedonrockandhavingacrestelevationof1,130ft (344.4m).Duetothepresenceofadeep,pre-glacialchannel

(CS),earthembankment(EE),anddownstreamretainingwall(RW).

inthebedrock,thewesternportionofGilboaDamconsistsof a180-ft-high(54.9m),700-ft-long(213m)earthembankmentwithacrestelevationof1,150ft(351.5m).Aretaining wallextendsdownstreamfromthejunctionoftheconcrete spillwayandearthembankment(Figure4).

ConstructionofGilboaDambeganin1919andwas completedin1927.AmajorrehabilitationofGilboaDam beganin2006andwascompletedin2014.Therehabilitation includedinstallationof80post-tensionedrockanchorsinthe spillway,placementof57,000yd3 (43,580m3)ofconcrete buttressonthedownstreamsideoftheconcretedam,a newreinforcedconcretelininginthespillwaychanneland plungepool,installationofanew9-ft-diameter(2.7m) low-leveloutlet,andadditionofagallerywithfoundation drains.In2015,theprojectwasnamedtheAssociationof StateDamSafetyOfﬁcialsNationalDamRehabilitation ProjectoftheYear.

SiteGeology

BedrockinthevicinityofGilboaDamispartofthe Catskillclasticwedgeandconsistsgenerallyofthicksandstonebedsandthinnerbedsofsiltstone,shale,andmudstone,whicharelocallymappedastheMoscowFormation oftheHamiltonGroup(Fisheretal.,1961).Thesediments formingtheseMiddleDevonianrocksappeartohavebeen depositedwithinamigrating,deltaicshorezonebetween marinesedimentstothewestandnon-marine(ﬂuvial)sedimentstotheeast(BridgeandWillis,1994;BridgeandJarvis,1998;andBridge,2000).Gilboaisthesiteofoneof theworld’soldestfossilforests,andduringtheoriginal constructionofGilboaDam,treestumpfossilsweresent toseveralmuseums(BartholomewandBrett,2003;Hernick,2003).Excavationofaformerquarrydownstreamof thedamduringthedamrehabilitationprovidedan opportunityforresearcherstoobserveandmapaportionofthe

fossilforest(Steinetal.,2012).Theareawasglaciated, andGlacialLakeGrandGorgeandGlacialLakeSchoharie occupiedtheSchoharieValleyduringtheWisconsinanstage (Rich,1935;Titus,2003).

CoreboringsatGilboaDampenetratedasandstonedominatedportionoftheMoscowFormation.Bridgeand Willis(1994,pp.1441–1443)describedtheGilboaDam sectionastwo35-m-thicksandstone-dominatedsequences separatedbya5-mr-thickmudstone-richintervalwithsedimentsofthetwosandstonebodiesshowingmainlywave andtidalcurrentinfluencelowerdownandoffshore-directed currentinfluencehigherup,whichreflectsdepositionin waterbecomingincreasinglyshallowandclosertoland. Inadditiontosandstone,damrehabilitationcoreborings encounteredminorbedsofcalcareoussiltstone,shale,and mudstone,someofwhicharevisibleinoutcropsbelowthe dam(Figures5,6,and7).Analysesofseveral finer-grained markerbedsusingthethree-pointmethodindicatedthe overallbedrockstrikeisN80°W,andtheoveralldipis 1.5degreestothesouth-southwest.

JointingCharacterization

Scan-LineSurvey

RockoutcropsalongSchoharieCreekbelowGilboa Damwereexaminedaspartofthereconnaissanceeffort forthedamrehabilitation.Duringadetailedlinesurvey consistingof fivetraverses,theorientationsof45discontinuitiesweremeasured,andnotesweretakenontheirproperties(e.g.,length,continuity,roughness,infilling,thickness, waviness,andmoisture).Discontinuityorientationdata aresummarizedinTables1,2,and3.Littlevariationwas observedindiscontinuityproperties,asmightbeexpected inrelatively flat-lyingbedshavingonlyminordeformation duetoplateconvergenceduringtheAlleghenianorogeny,

Figure4.AerialviewofGilboaDamshowingtheconcretespillway

Figure5.Sandstonebedsundercutbylessresistantbedsoneastside ofSchoharieCreekgorgedownstreamofGilboaDam.

sodiscontinuitysetswerenotweighted,andthedamsite wasnotsubdividedintozonesduringthefoundationdrain designprocess.

JointOrientation

Mostmeasureddiscontinuitysurfaceswerevertical, andallbutonehadadipof75degreesormore.Thegreat circlesandpolescorrespondingtothe44steeplydipping measurementsareplottedinastereonetinFigure8.RocscienceDipssoftwarewasusedtodeterminetheglobal best-ﬁtorientationsofthethreeidentiﬁedjointsets,which

Table1. JointSet1measurements.* TraverseJoint

ID Trend (°) Distance (ft)No. Strike Direction(°) Dip

Dip Direction(°) A2255105290 2259206990 20519504290 19022604590 19028805090 18535904790 B20051105086140 190201503688306 190201604590 190251805090 190331904787137 190412001790 190512104275312 C19032403290 16762502490 167102703290 167182803090 D15353004990 153123204790 153193403690 153233504390 153263603786307 E19783804690 197174105488324 197314304590 197364505590

Mean04389

Median04590

Best-ﬁtplane04389313

Best-ﬁtpole(plunge/trend) 1133

*Trendsanddirectionsareexpressedincompasscoordinates. 5ft ¼ 1.5m.

expressedintermsofstrikeanddipareN43°E/89°NW, N75°W/88°NE,andN5°W/89°SW.Thethreejointsets wereobservedonboththeeastandwestsidesoftheSchoharieCreekgorgebelowthedam(Figures9and10).The threeidentiﬁedjointsetsappeartobeofregionalsigniﬁcance,astheybearsimilaritiestojointsystemsdepictedin theGilboaandSchoharie15-minutequadrangles(Isachsen andMcKendree,1977a)andtobrittlestructuresdelineated intheGilboa15-minutequadrangle(IsachsenandMcKendree,1977b).Thenortherlystrikeofthethirdjointsetis paralleltotheoveralltrendofSchoharieCreekthroughthe gorgecontainingtheSchoharieReservoirandGilboaBlenheimReservoir,apowersupplyreservoirlocateddownstreamofGilboaDam.

JointSpacing

Measurementstakenduringthedetailedlinesurvey providedanapparentspacingofjointsalongeachtraverse. Thetruespacingbetweenjointscanbecalculatedby rewritingEq.11asfollows:

Figure7.Fossilmudcracksona boulderwithinSchoharieCreek gorge.

Figure6.Steeplydipping,solutionedjointsinacalcareousbedon theeastsideofSchoharieCreekgorge.

Table2. JointSet2measurements.*

TraverseJoint

ID Trend (°) Distance (ft)No. Strike Direction(°) Dip Angle(°)

B20001029790

190161229085020

190161327987009

190171428383013

190221727584185

190472229187201

C19002328684016 16782628686016

D15322928390 153113128584015 153153328482014

E197103928285192 197164028788017 197354428790

Mean28586

Median28686

Best-ﬁtplane28588015

Best-ﬁtpole(plunge/trend) 2195

*Trendsanddirectionsareexpressedincompasscoordinates. 5ft ¼ 1.5m.

nn ¼ dn ðsinun coshn sinu cosh þ sinun sinhn sinu sinh þ cosun cosuÞ; (13)

wherenn ¼ thetruejointspacingofjointset n, dn ¼ theapparentjointspacingofjointset n, un ¼ theplungeofthepoletojointset n, hn ¼ thetrendofthepoletojointset n, u ¼ theplungeofthetraverse,and h ¼ thetrendofthetraverse.

First,anaverageapparentjointspacingwascalculated foreachjointsetforeachtraversehavingatleasttwomeasurementsforthejointset(Table4).Thetraversetrend wasdeterminedfrom ﬁeldmeasurementwithaBrunton compass.Incaseswherethetraversetrendwasnotconstant, aweightedaveragetrendwascalculatedbasedonthe

Table3. JointSet3measurements.*

TraverseJoint

Figure8.Stereonetplotoftheplanesandpolescorrespondingtothe jointmeasurementsandthebest-ﬁtorientationforeachset.

lengthsofeachtraversesegment.Next,theaveragetrue jointspacingwascalculated,andthenaweightedaverage wascalculatedforeachjointsetbasedonthelengthsof thetraverses.Theweightedcalculatedaveragetruejoint spacingforJointSets1,2,and3was3.7ft(1.1m),9.4ft (2.9m),and6.8ft(2.1m),respectively.

DrainHoleDesign

Orientation

Basedontheprincipalcomponentandaveragejoint spacingofeachjointset,thederivedequationswereused tocalculatetheapparentspacingbetweenjointsofeachset

*Trendsanddirectionsareexpressedincompasscoordinates. 5ft ¼ 1.5m.

gorge.

Figure9.JointSets1,2,and3ontheeastsideofSchoharieCreek

fordrainholestrendinginanupstreamdirectionand plungingfrom45to70degreesfromhorizontal.AnExcel spreadsheetwasusedtoperformthecalculationsin 1-degreeincrementsofazimuthandplunge,so4,706 possibledrainholeorientationswereevaluated.Apartial plotoftheresultsindicatesthattheshallowestplunge (45degrees)isthemostfavorableoftheholeinclinations considered(Figure11).

Themostfavorableazimuthsareclusteredaround142, 249,71,and203degrees,inthatorder.Holeshavingthese azimuthsandplunging45degreesareranked1st,15th,77th, and203rdwhentheresultsaresortedbytotalapparent spacing,i.e.,thecombinedapparentspacingofeachjoint setwithintheproposeddrainhole.Thecalculationsindicateadrainholetrending142degreeshastheoptimum totalapparentjointspacingandwillinterceptjointsbelongingtoJointSets1,2,and3every5.2ft(1.6m),20.9ft (6.4m),and17.1ft(5.2m),respectively,onaverage (Figure12).

Aspartofapeerreviewoftheproposedfoundation draindesign,includingthefavorabledrainorientations identiﬁedusingtheTASapproach,analternateanalysis wasperformedusingtheLSBIapproachofZhouandMaerz (2002).UsingtheLSBIapproach,themostfavorabledrain holeazimuthswerefoundtobeclusteredaround140,74, 251,and199degrees,inthatorder(Figure13).Table5 providesacomparisonoftheresultsforfavorabledrain holeazimuthsusingthetwoapproaches.

Tooptimizeboreholeinclination,theLSBIapproach calculatestheLSBIbasedontheanglebetweentheboreholeinclinationandtheaveragedipsofthejointsetsprojectedontoaverticaleast-westplane(0to180degrees). Drainholesinclined45to90degreesfallintherangeof inclinationsfrom45to135degrees.SimilartotheTAS approach,theLSBIapproachindicatesthatshallowerinclinationsaremorefavorable(Figure14),whichintuitively makessensegiventhenear-verticalaverageorientationsof thethreejointsets.BoththeTASandLSBIapproachesare thoughttobecomputationallysimilarintheapplicationof therespectiveapproachandproducecomparableresults. TheTASapproachusesaformulathatfactorsinbothboreholeazimuthandinclinationtocalculatethetotalapparent spacingalongtheborehole,whiletheLSBIapproachcalculatesaseparateLSBIfortheboreholeazimuthandfor theboreholeinclination.

DrainHoleLength

Foragivendrainholeorientation,drainholelength influencesthereductionoftheupliftpressuresthemost, followedbydrainholespacinganddrainholediameter,in thatorder(daSilva,2005).Drainholelengthhasalarge influenceonthevaluesoftheupliftpressuresupto approximatelyhalfthevalueoftheupstreamreservoir depth.Fromthispointonwards,theinfluenceofthedrain holelengthdecreasesuptoavalueequaltothereservoir

B258.0904677.74.0 C279.9901545.04.0 D297.0902155.35.0 E253.0902849.34.1 225592B257.0904769.49.49.4 C268.690828.07.8 D297.0901336.54.8 E253.09025312.512.5 3591A260.090929.02.36.8 E253.09023223.08.6

¼ 1.5m.

Figure10.JointSets1,2,and3onthewestsideofSchoharieCreek gorge.

Table4. Summaryofcalculatedaveragejointspacing.*

Figure11.PlotoftotalapparentspacingofJointSets1,2,and3fordrainholestrendingfrom071to251degreesandinclinedfrom45to70 degrees.

depth.Afterthat,itsinﬂuenceisverysmall(daSilva,2005). Currentdesignstandardssuggestthatfoundationdrains shouldbeinstalledtoaverticaldepthoftwo-thirdstothreequartersofthedepthoftheexistinggroutcurtain(USACE, 1995).Thedepthofthefoundationdrainsshouldnever exceedthedepthofthegroutcurtain(ASCEandUSCOLD, 1967).Geologicsetting,engineeringparameters,andoperationalandperformanceobjectivesalsoneedtobeconsidered inevaluatingthedesigndepthofthedrainholes.

AtGilboaDam,the ﬁnaldrainholedesignconsisted oftworows(AandB)ofdrainholes.RowAdrainholes weredrilledS38°Eatanangleof45degreesfromverticaland27ft(8.2m)intorock.RowBdrainholeswere drilledvertically10ft(3m)intorock.Giventhecalculatedapparentjointspacings,eachangledRowAdrain holewouldbeexpectedtointerceptatleast ﬁve,and possiblysix,jointsbelongingtoJointSet1andatleast one,andpossiblytwo,jointsbelongingtoJointSets2 and3(Figure12).TheverticalRowBdrainholeswould beexpectedtointerceptmainlybeddingjointsdipping 1.5degreestothesouth-southwest.

DrainHoleSpacing

Thedrainholespacingwillbecontrolledbythe apparentspacingofjointsetsalongthedrainhole,which hasalreadybeendetermined,andtheapparentspacingof jointsetsalongthegallerywherethedrainholesareto

bedrilled.Inordertoavoidgapsbetweendrainholes,the drainholespacingshouldnotexceedtheapparentjoint spacingalongthegalleryofeachjointsetmultipliedby thenumberofjointsbelongingtothejointsettheoreticallypenetratedbythedrainhole.

Theapparentjointspacingofeachjointsetalongthe gallery(dn)canbecalculatedbymodifyingEq.11to considertheazimuthandplungeofthegalleryinsteadof theazimuthandplungeofthedrainhole:

where nn ¼ truejointspacingofjointset n, un ¼ jointdipangleofjointset n, hn ¼ jointdipdirectionofjointset n, u ¼ galleryinclination,and h ¼ galleryazimuth.

Thenumberofjointstheoreticallypenetratedbyeach drainhole(Nj)isgivenby:

where L ¼ lengthofthedrainhole,and dn ¼ apparentspacingofjointset n alongthedrainhole. Thus,themaximumspacingofdrainholesalongthe gallery(Dmax)foraparticularjointsetisgivenby:

Figure12.Schematicplanviewsalongdrainholepathshowingcalculatedaverageapparentspacingofdiscontinuitiesof(a)JointSet1, (b)JointSet2,and(c)JointSet3.

calculationsassumingadrainholelengthof27ft.For JointSets1,2,and3,themaximumdrainholespacings are38.5,22.9,and11.0ft,respectively.JointSet3controlsthemaximumdrainholespacingsincethestrikeof JointSet3hasthesmallestangulardeviationfromthe drainholeorientation.

Drainspacingmustaccountfordeteriorationoffoundation drainsovertimeduetoclogging,incrustation,mineralization, etc.,aswellasconsiderationofnon-linearupliftdistribution. Majorhydraulicstructuresmustbedesignedwithbothresiliencyandredundancy,andareliableandrobustfoundation drainagesystemisoneofthemostimportantaspectsofmaintainingastabledam.Adrainspacingof10ft(3m)was selectedfortheRowAdrainholes.Theabovecalculation doesnotapplytoRowBdrainholessincetheyweredrilled vertically.Aspacingof10ft(3.0m)wasalsoselectedfor theRowBdrainholes.

DrainHoleConstruction

Nominal3-in.-diameter(7.6cm)drainholesweretypically drilledwithpercussiverotarymethodsusingwater ﬂushata spacingof10ft(3m)(Figure15).Whereexploratoryholes wererequired,NQwirelinedouble-tuberockcoringwas speciﬁed.Ateachdrillinglocation,oneholewasdrilledS38° Eatanangleof45degreesfromverticaland27ft(8.2m) intorock,andasecondholewasdrilledvertically10ft(3m) intorock.Intotal,248drainholesweredrilledat124drilling locations.Thedrainholesweredrilledpriortothecreationof thedamgalleryduringconstructionofthedownstreamconcretebuttress(Figures16and17).

Upliftwasconsideredinthedesignofthesitestructures atGilboaDam.Thespillway,spillwaychannel,plunge poolconcretepanels,andretaining/trainingwallswere designedassumingfulluplift.Upliftoftheconcreteslabs inthespillwaychannelandplungepoolwasaddressed usingpassiveanchorbarsgroutedintobedrock.Although thegalleryfoundationdrainsinthespillwayandthe underdrainsystemwithinthespillwaychannelandplunge poolwereincludedinthedesign,thedrainswerenot accountedforasanupliftreductionmethodsincethegalleryandspillwaychannelweredesignedtobebelowtailwaterelevationduring ﬂoodconditions.

SUMMARY

AtGilboa,thegallerytrendsapproximately253degrees andplungesabout3degreesfromhorizontal.Table6 providesasummaryofthemaximumdrainholespacing

Fracturesdevelopinarockmassinresponsetostresses thatexceedthestrengthoftherocktoresisttheresulting forces.Discontinuitiestypicallyexhibitpreferredorientationsandoftenoccurassets,witheachsetconsistingof regularjointssub-paralleltoeachother.Ineachset,discontinuitiesusuallyhavethesamegeneralorientationand exhibitrelativelycommonphysicalcharacteristics.Rock massesmaycontainmultiplediscontinuitysets,andthree tofoursetsaretypicalofmanyrockmasses.

Figure13.PlotofLSBIforJointSets1,2,and3fordrainholeswithazimuthsfrom071to251degrees.

Upliftisoneoftheprimaryloadsthataffectthestabilityof aconcretedamfoundedonrock.Thehydraulicconductivity ofajointedrockmassiscommonlycontrolledbyitssecondarypermeability,andinmostcivilengineeringapplications involvingrockfoundations,thesecondarypermeabilitycontrolsthedraindesignprocedures.Drainsareusedtoreduce dangerousupliftpressurebeneathhydraulicstructuresand mustbecorrectlyplacedinthree-dimensionalspacetointersecthydraulicallysigniﬁcantdiscontinuitiesthatarecapable offracture ﬂow.

Numerousmethods,suchasscan-lineoutcropsurveys andcoreholeimaging,canbeusedtosystematicallycollect thedatarequiredtocharacterizethejointingpatternofa rockmass.Properlyconductedsurveyscanprovidedata ontheorientation,spacing,andphysicalconditionofthe discontinuitysetswithinarockfoundationorrockslope. Thesedatacanprovidethe apriori knowledgeaboutthe

discontinuitystructureoftherockfoundationthatisneeded toeffectivelyutilizetheanalyticalapproachesdescribed herein.TheTASapproachcanbeusedtodeﬁneanoptimum drillingdirection,thatis,thecorrectorientationandinclinationalongwhichaboreholecanbedrilledtointersectas manydiscontinuitiesaspossibleforagivendrillinglength. Thisanalyticaltoolhasadirectapplicationinthedesignof systematicdrainsinstalledinajointedrockfoundation.

LIMITATIONS

AzimuthTAS(ft)AzimuthLSBI 14243.21404.49 24944.02514.95 7145.9744.90 20348.81995.92

LSBI ¼ linearsamplingbiasindex;TAS ¼ totalapparentspacing. *Drainholestrendingdownstreamnotconsidered.5ft ¼ 1.5m.

Primaryandsecondarypermeabilitymayoverlapin somerockfoundations.Inthesecircumstances,rockfoundationsmayexhibitprimarypermeabilitythatisgenerallysimilartosecondarypermeability.Theapproachespresented hereindonotexplicitlyconsidertheeffectofprimarypermeabilityonseepageand,hence,cannotbeusedtoproperly designafoundationdrainagesystemthatissignificantly affectedbyprimarypermeability.Flownets,orotheranalyticalornumericalmethods,arerequiredtodesignafoundation drainagesystemlargelyinfluencedbyprimarypermeability. TheformulausedintheTASapproachwillprovide anin fi nitetotalapparentspacingincertainscenarios. Forexample,usingtheformulatooptimizeascenario includingadiscontinuitysetorientedexactlyparalleltothe proposeddrainholewillhaveasolutionofinfinity,since thedrainholewilltheoreticallyneverinterceptadiscontinuitysetorientedexactlyparalleltothedrainhole.Inthis

Table5. Nodesofoptimumdrainholeazimuths.*

Figure14.PlotofLSBIforJointSets1,2,and3fordrainholeswithinclinationsfrom45to135degrees.

case,iftheproposeddrainholeorientationisdeemedtobe necessaryforsomereason,thenapreferablesolutionmight betooptimizetheorientationofonesetofdrainholes basedonthenon-paralleldiscontinuitysetsandaddasecondsetofdrainholeshavinganorientationoptimizedto intercepttheparalleldiscontinuityset.

Likewise,whenmultiplesetsofdiscontinuitiesexist andinﬂuenceuplift,optimizingthedrainholeorientation basedonananalysisfactoringinallthediscontinuitysets mayresultinacompromisesolutionthatmaybelesseffectivethanadesignincorporatingavarietyofdrainhole orientations,eachoptimizedtotargetasinglesetorgroup ofdiscontinuitysets,e.g.,onedrainholeorientationoptimizedtointersectnear-verticaldiscontinuitysetsandone drainholeorientationoptimizedtointersectnear-horizontal beddingjoints.

Regardlessoftheresultingoptimizeddrillingorientation developedbytheTASoralternatemethods,theactualoptimumorientationresultsmaybelimitedbyfactorsoutside

thenumericalanalysis.Thephysicalcharacteristicsofthe siteandthelimitationsofequipmentandaccessmustbe takenintoaccountpriortodesignatinganorientationfor boreholesinadrillingworkplan,scopeofwork,orcontract.Theanalysismayindicatethatdrainholeshavinga shallowplungefromhorizontalaremostfavorable,butthe mechanicsofexecutingsuchanorientationmaypreclude thefeasibilityofdrillingadrainholehavingthatorientation.Theaccessandlogisticsofasitemayalsopreclude usingequipmentthatmightotherwisebeabletoachieve theoptimizedorientation.Undertheseconditions,alternativescenariosmustbeconsideredtodeterminethebest practicalsolutionduringtheanalysisanddesignphase,rather thanafterequipmentandpersonnelhavebeenmobilized. Drillingtoleranceandthepotentialfordeviationshould alsobeconsideredwhenputtingtheresultsofthesemethods intoaspeciﬁcapplication.Atgreaterinclinationsfromvertical,theboreholeismorelikelytodeviatefromthedesired orientation,allotherfactorsbeingequal.

Table6. Summaryofcalculatedmaximumdrainholespacing.

5ft ¼ 1.5m.

Despitearigorouslyexecutedsiteinvestigationprogram, considerablegeologicuncertaintiesmaystillexist.Any siteinvestigationprogrammustbethoroughlyevaluated forsamplingbiasandotherdeficiencies.Siteinvestigation deficienciesandinadequaciesmustbethoroughlyconsideredwhen finalizinganyfoundationdraindesign.

Conclusionsbaseduponlimitedsamplingdatamay introducesigniﬁcantepistemicuncertaintyintothegeotechnicaldesignprocess.Thisuncertaintyshouldbefullyrecognizedandconsidered,andsoundengineeringprinciplesand judgementarestillrequiredwhenapplyingtheanalytical approachesdescribedherein.

CONCLUSIONS

TheTASapproachcanbeusedtodeﬁneanoptimum drillingdirection,thatis,thedirectionalongwhichaborehole

canintersectasmanydiscontinuitiesaspossibleforagiven drillinglength.Given apriori knowledgeofthejointingpatternwithinarockfoundation,thisapproachprovidesameans torationallydesigntheoptimalorientation,depth,andspacingofrockfoundationdrains.Drainsproperlyinstalledatthe optimalorientation,depth,andspacingwillmoreeffectively andefﬁcientlydiminishthehydraulicpressurebeneathaconcretedamfoundeddirectlyonjointedrock.Whenproperly applied,theTASapproachcansupersedethelong-standing rules-of-thumbthathavehistoricallybeenusedtodesignthe spacing,depth,andorientationoffoundationdrains.

TheTASapproachhasnumerousgeotechnicalexplorationandrockengineeringapplications.Specificapplications includetheorientationofdrillholesusedfor:geotechnical siteinvestigations,permeationgrouting,permeationgroutingverification,pressure(packer)testing,installationof geotechnicalinstrumentation,groundwatersamplingand productionwells,rockslopeinvestigationanddesign,installationofrocksupportanchors,andcollectionofrockcore samplesforengineeringpropertytesting,etc.TheTAS approachmaybeappliedstrategicallytointerceptasmany discontinuitiesaspossibleortofocusoninterceptingthose discontinuitiesofgreatestsignificance,orthosethatpose thegreatestrisksduetostructuralorseepageconcerns.

ACKNOWLEDGMENTS

TheauthorsthankDr.AbdulShakoor,formerco-editor of EnvironmentalandEngineeringGeoscience,forhis invitationtosubmitthismanuscripttothejournalaftera presentationontheTASapproachatanAssociationof EnvironmentalandEngineeringGeologistsAnnualMeeting. TheauthorsalsothanktheNewYorkCityDepartmentof EnvironmentalProtectionforpermissiontousetheGilboa Damrehabilitationprojectasaworkedexample;Jake Romanforhiskindassistanceonthemathematicalaspects ofthemanuscript;andthepeerreviewersfortheirmany

Figure15.Drilling3-in.-diameter(7.6cm)drainholeatthedownstreamtoeofthedam.

Figure16.Constructionofdamgallery(nearcenterofphoto)during constructionofasteppedconcretebuttressonthedownstreamface oftheexistingdamduringdamandspillwayrehabilitation.

Figure17.Completedfoundationdrainwithinthenewdamgallery.

insightfulcommentsandthoughtfulsuggestionsfor improvingthemanuscript.

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Developmentofa3DGeologicModelUsedinthe SeismicHazardandLiquefactionHazardAnalysis

ofMadisonCounty,Tennessee

VALARIEJOYNERHARRISON*

DepartmentofEarthSciences,UniversityofMemphis,109JohnsonHall,Memphis, TN38152-3430andTennesseeDepartmentofEnvironmentandConservation, MemphisEnvironmentalFieldOfﬁce,8383WolfLakeDrive, Bartlett,TN38133-4119

HAMEDTOHIDI

DepartmentofCivilEngineering,UniversityofMemphis,104EngineeringScience Building,Memphis,TN38152andMIGIZIGroup,Inc.,201160thStreetSouth, Suite401,Seattle,WA,98448

ROYVANARSDALE

DepartmentofEarthSciences,UniversityofMemphis,109JohnsonHall,Memphis, TN38152-3430andDepartmentofEnvironmentalScience,RhodesCollege,2000 NorthPkwy,Memphis,TN38112

CHRISCRAMER

CenterforEarthquakeResearchandInformation,UniversityofMemphis,3890 CentralAvenue,Memphis,TN38152-3050

DAVIDARELLANO

DepartmentofCivilEngineering,UniversityofMemphis,104EngineeringScience Building,Memphis,TN38152

KeyTerms: 3DGeologicModeling,MadisonCounty, Tennessee,NewMadridSeismicZone,Mississippi Embayment,Quaternary,Tertiary,Paleogene,Cretaceous

ABSTRACT

MadisonCountyofwesternTennesseeisapproximately100kmsoutheastoftheNewMadridseismic zoneand,thus,issubjecttopotentiallydangerous seismicshakingandliquefactionevents.Holocene ﬂoodplainalluviumandassociatedPliocene/Pleistocene terracesoftheForkedDeerandHatchieRiverscover alargepartofMadisonCounty.Underlyingthese unconsolidatedsedimentsareapproximately ∼650m ofpoorlyconsolidatedPaleogeneandCretaceousterrestrialandmarinesediments,whichareunderlainby Paleozoiclimestonebedrock.Uniformregionalgeology usedinthe2014UnitedStatesGeologicalSurveyseismichazardmapsassumesahomogenousseismicvelocitystructureabovebedrock.Three-dimensional geologicmappingofthe ∼700mofQuaternary,

*Correspondingauthoremail: Valarie.harrison@tn.gov

Neogene,Paleogene,andCretaceoussedimentsconductedinthisstudyprovidesamoredetailedvelocity structure,resultinginamoreaccurateanddetailed mapoftheexpectedgroundmotionsinMadison Countyduringfutureearthquakes.Inaddition,the surfacegeologicmaprevealsregionsthataresusceptibletoliquefaction.Thispaperdiscussesthemethods usedtoconstructthesurfacegeologymap,subsurface structurecontourmaps,isopachmaps,and3Dgeologicmodelusedintheseismichazardandliquefaction hazardanalysesofMadisonCounty,Tennessee,and summariesoftheresultsoftheseanalysesare presented.

INTRODUCTION

Locatedapproximately100kmsoutheastoftheNew Madridseismiczone,MadisonCounty,Tennessee,and itsprimarycityofJackson(population68,114)(Figure 1)maybeaffectedbybothearthquakegroundshaking andliquefactionintheeventoffuturelargeNewMadrid earthquakes(Chiuetal.,1992;Coxetal.,2001,2006, 2013;Tuttleetal.,2002,2019;Cramer,2006;Csontos

Figure1.(A)LocationofMadisonCounty,Tennessee.(B)Geologicallymapped7.5-minutequadranglesofMadisonCounty.TheMercer,Spring Creek,andCedarGrovequadranglesarehighlightedinyellowandhavenotbeengeologicallymappedandpublished.Thebasemapisa1-m DEMofMadisonCountywiththeblacktowhitecolorrampshowingelevationsfromlow(93m)tohigh(204m),respectively. Harrison,Tohidi,VanArsdale,Cramer,andArellano

3DGeologicMappingofMadisonCounty,Tennessee

andVanArsdale,2008;CramerandBoyd,2014;Cramer etal.,2018a,2023).The2014regionalUnitedStates GeologicalSurvey(USGS)probabilisticseismicground motionmapspredictpeakgroundacceleration(PGA) from0.2to0.7g,wheregisgravitationalacceleration, forMadisonCounty(Figure9A)intheeventoffuture M7.3–7.7NewMadridseismiczoneearthquakes.However,USGSseismicgroundmotionmapsarebasedon uniformstandardgeologyanddonotincludetheeffects ofnear-surfacegeology(Petersenetal.,2014),which canbesigniﬁcantintheeventofalargeproximalearthquake(Crameretal.,2023).Inthispaper,wedetailthe processofconstructingthesurfacegeologicmap,structurecontourandisopachmaps,andthe3Dgeologic modelofMadisonCountythatwasusedintheseismologichazardmodelingthatisdetailedinCrameretal. (2023).Thisresearchdocumentsnear-surfacegeology andproducesamoreaccurateanddetailedseismichazardanalysisaswellasliquefactionhazardanalysis.This studyalsoillustratestheusefulnessofcontinued3DgeologicandseismologicmappingperipheraltotheNew Madridseismiczone,whichincludeslargeportionsof westernTennessee,westernKentucky(Zhuetal.,2021), southwesternIllinois,southeasternMissouri,andnortheasternArkansas.

GeneralGeologyofMadisonCounty

MadisonCountyconsistsoflowland ﬂoodplainsof theSouthForkandMiddleForkoftheForkedDeerrivers,HatchieRiver,HatchieRivertributaries,terracesof theserivers,andareasofPaleogeneandLateCretaceous strata(Figure2)(Brown,1968a,b;Parks,1968a,b,c,d, e,f,g;Saucier,1987;Jewel,2006;Antonacciand Hoyal,2011,2012a,b,2014).AthindepositofPleistoceneloessoverliestheterracesandbothPaleogeneand Cretaceoussediments.Beneaththe ﬂoodplainandterracedepositsinmostofMadisonCountyareEocene (»34Ma)sedimentsthatoverlieolderPaleogene,Late Cretaceous,andPaleozoicstrata(Figure2andTable1). NofaultshavebeenmappedinMadisonCounty,andthe countyisseismicallyinactive(Figure2A).

Approximately3.6Ma(Odumetal.,2020),theancestralMississippiRiversystem ﬂowedthroughthecentral UnitedStatesanddeposited ﬂoodplainsedimentsnow discontinuouslypreservedasahigh-levelterracecalled theUplandComplex(VanArsdaleetal.,2007;Cox etal.,2014;CupplesandVanArsdale,2014;Lumsden etal.,2016).Remnantsofthe3.6Materracehavebeen mappedinObion,Dyer,Lauderdale,Tipton,andShelby countiesinwesternTennessee(Figure1A)(VanArsdale etal.,2007).Remnantsofterraceshavealsobeen mappedinMadisonCounty(Saucier,1987).Incision throughtheUplandComplexthroughoutthelowerMississippi/OhioRiversystembeganintheearly

Figure2.(A)7.5-minutequadranglesofMadisonCountymappedby AntonacciandHoyal(2011;2012a,b;2014),Brown(1968a,b),Jewel (2006),andParks(1968a,b,c,d,e,f,g).QuadrangleslabeledinFigure 1.LineA-A’ inJacksonNorthquadrangleindicateslocationofcross sectioninB.(B)West–eastgeologiccrosssectionoftheJacksonNorth 7.5-minutegeologicquadranglemapwith4Xverticalscale(modiﬁed fromJewel,2006).(C)GeologiccolumnofJacksonNorth7.5-minute geologicquadranglemap(modiﬁedfromJewel,2006).

Table1. Stratigraphyofthesurfaceandsubsurfacegeologyofthe JacksonNorth7.5-minutequadranglemapinnorthcentralMadison County(Figure1B)(modiﬁedfromJewel,2006).Thisstratigraphyis appropriateforallMadisonCountyandwasinterpretedbasedonoil andgasandhydrologictestwells.

PeriodGroupandFormationSymbol QuaternaryAlluviumQal Tertiary(?)andQuat.FluvialDepositsQTf TertiaryWilcoxGroupPortersCreek ClayClaytonFm. Tw Tp Tcl CretaceousOwlCreekFm.McNairySand DemopolisFm.CoffeeSand Ko Km Kd Kc

PaleozoicsUndifferentiatedLimestonePal Harrison,Tohidi,VanArsdale,Cramer,andArellano

Pleistocenewithgrowthofcontinentalicesheetsand resultantlowersealevels(Saucier,1994;VanArsdale etal.,2007).DuringthePleistocene,theHumboldt, Hatchie,andFinleyterracesformedalongtheObion, ForkedDeer,andHatchierivers(Saucier,1987;Rodbell, 1996)anduptofourloesslayersweredepositedinwesternTennessee(Markewichetal.,1998).InDyerCounty (Figure1A),Rodbell(1996)identifiedPeorialoess (,20ka)andunderlyingRoxannasilt(.65kaloess)on topofHumboldtandHatchieRiverterracealluvium (Reichenbacheretal.,2022).Theresearchersbelieve thattheancestralMississippi/Ohioriversystemandits tributariesentrenched .65kainwesternTennessee becausetheHumboldtterracealluviumisoverlainby the .65kaloessbasedontheDyerandLauderdale counties’ geologichistories.Subsequently,theMississippi/Ohioriversystemanditstributariesfurther entrenchedtoformtheHatchie(also .65ka)andthen Finley(»22ka)terracelevels(Harrison,2022).The finalentrenchmentoftheMississippianditstributary riversinwesternTennesseeoccurredwithinthelast 22,000years,resultinginmostofthemodern floodplain alluviumbeing ,12ka.

METHODS

ThesurfacegeologyofMadisonCounty exceptfor theMercer,CedarGrove,andSpringCreek7.5-minute topographicquadrangles hasbeenmappedatascaleof 1:24,000(Figure2A)(Brown,1968a,b;Parks,1968a, b,c,d,e,f,g;Jewel,2006;AntonacciandHoyal,2011, 2012a,b,2014).Forthisgeologicandearthquakehazard investigation,MadisonCountywasdividedintothree surfacegeologicunits:Holoceneriver ﬂoodplainalluvium,Plioceneand/orPleistoceneriverterracealluvium, andsemi-consolidatedsedimentsofPaleogeneandCretaceousage.BedrockbeneathMadisonCountyis

Paleozoiclimestoneanddolomite(Figure2andTable1) (Jewel,2006).TocompletethesurfacegeologyofMadisonCounty,theMercer,CedarGrove,andSpringCreek 7.5-minutequadrangles(Figure1B)wereinterpretedby using1-mlidardigitalelevationmodels(DEM)andprojectingthesurfacegeologyofadjacent7.5-minutepublishedgeologicquadranglemaps(Figure2A).Mostof theMercerquadrangleinMadisonCountywasmapped bySaucier(1987)asHatchieRiverterrace,andSaucier ’s mapformsthebasisforourmappingofthisquadrangle. TheremainingareawithintheMercerQuadranglewas mappedbyextendingalluviumfromstreamsinthesurroundingquadranglesthroughtheHatchieRiverterrace. TocompletethemappingofMadisonCounty,theSpring CreekandCedarGrovequadranglesweresubdividedinto 27polygons.Thepolygonsadjacenttothepublishedsurfacegeologyweresplitintothethreeclassificationsof Holocene floodplainalluvium,Pliocene/Pleistoceneterrace alluvium,andPaleogeneandCretaceoussedimentsbased onsurfaceelevation.Thesepolygonswerethenreclassified untilthepatternattheperimeteroftheDEMmatchedthe patternoftheknownadjacentpublishedsurfacegeology (Figure2A).Thisprocesswasrepeatedforeachpolygon. TheresultingrastersweremergedintoamosaictorepresenttheinterpretedsurfacegeologyofthethreequadranglesandmergedwiththegeologyclassificationofTable1 toproduceasimplifiedsurfacegeologicmapofMadison County(Figure3).

Subsurfacelithologicboringlogswereacquiredfrom petroleumexplorationwells,TennesseeDepartmentof EnvironmentalControl,andtheStateofTennesseeDepartmentofEnvironmentandConservationDivisionofGeology.Theseboringscontainlithologicdescriptionsofthe Neogene,Paleogene,Cretaceous,andPaleozoicstratathat reachadepthof »410mbelowtheJacksonNorthgeologicquadranglenearthemiddleofMadisonCounty(Figures1Band2B).

ThesubsurfacegeologyofMadisonCountyisillustratedinthecross-sectionthataccompaniesthe7.5minuteJacksonNorthGeologicQuadranglemap(Figure 2B)(Jewel,2006).ThebasalportionofthePaleocene ClaytonFormationisdifﬁculttodifferentiatefromthetop oftheunderlyingCretaceoussediment(theOwlCreekFormation)insomepublishedquadrangles(e.g.,Figure2B)as theyarebothdescribedasconsistingofglauconiticclay andsand.Therefore,theseunitsarecombinedinthose mapsbasedontheirsimilarlithologies.Becausethe lithologiesareindistinguishable,theyareexpectedto havethesameseismicvelocities.Withinthisstudy,the term “topofCretaceous” isusedevenwhereathinsectionofPaleoceneClaytonispresent,andthus,theuncertaintyofthetopoftheCretaceousstrataisconsideredin the ﬁnalground-shakinghazardmaps.Theelevationsof thetopsoftheWilcoxGroup,PortersCreekClay,and

3DGeologicMappingofMadisonCounty,Tennessee

Figure3.SimplifiedgeologicmapofMadisonCountyforthisseismicgroundmotionandliquefactioninvestigation.LiquefactionprobabilitycurvesinFigure9AandBwerecalculatedforthe floodplain strata(Holocene floodplainalluvium)andnon-floodplainstrata (Pliocene/Pleistoceneterracealluvium,Paleogenestrata,andCretaceous strata).EarthquakeliquefactionpotentialishighfortheHolocene floodplainareasofMadisonCounty.

Cretaceousweredeterminedbysurfacecontactelevations frompublished7.5-minutegeologicquadranglemapsin Madisonandadjacentcounties(Figure2A)andfrom welldata.AlltopofPaleozoicelevationpicksinMadison Countyarefromwelldata.

StructurecontourmapsweremadeshowingtheelevationsofthetopsoftheClaiborneGroup,Cretaceous strata,andthetopofthePaleozoicstrata(Table1). ThesemapsweremadeusingtheNaturalNeighborcontouringalgorithm.Planartrendsurfacesofthetopsof theWilcoxGroupandPortersCreekClaywerealso made.Theplanartrendsurfaceswereusedtoextrapolate thetopsoftheWilcoxGroupandPortersCreekClay undermostofMadisonCounty.Controlforthetopof theCretaceouswasobtainedfromoutcropdataineasternMadisonCountyandadjacentcountiesandfrom petroleumexplorationwells.ThetopofPaleozoicstructurecontourmapwasconstructedfromoilwellexplorationdata.Strikesanddipsofthemappedunitswere determinedfrom ﬁrstordertrendmapsoftheunits’ elevationdata.

Near-surfacegeologicunitthicknessesarenecessary foracomprehensiveevaluationoflocalgroundmotion andliquefactionpotential(Crameretal.,2023).Therefore, isopachmapswereconstructedoftheLowlandHolocene

alluvium,Pliocene/Pleistoceneterracealluvium,Claiborne Group,WilcoxGroup,PortersCreekClay,andCretaceous sections.LowlandQuaternaryalluviumthicknesseswere locallydeterminedfrompublished7.5-minutegeologic quadranglemapsthathadcross-sectionsacrossriver floodplains.Inaddition,roadbridgecrossingsofthemiddleandsouthforksoftheForkedDeerRiverinMadison Countywerevisited,andtheheightfromthebridgedeck tothebottomofthedeepestpartoftheriver flowing beneaththebridgewasmeasuredbydroppingaweighted measuringtapetotheriverbed.Immediatelyadjacentto thebridgecrossings,1-mlidarDEMdatawereusedto maketopographicprofilesacrossthe floodplain.Bridge deckelevation, floodplainsurfaceelevationderivedfrom the1-mlidardata,andtheheightofthebridgedecktothe bottomofthechannelwereusedtoestimatethe floodplain thicknessatthebridgecrossingsasfollows:Thebridge deckheightabovechannelbottomminusbridgedeck heightabove floodplainequalschanneldepth,which equalsapproximate floodplainalluviumthickness.

Thecalculatedelevationsofthebaseofthe floodplainswereusedtoapproximateplanartrendsurfacesof thebaseofthealluviumbeneaththesouthandcentral ForkedDeerRiver floodplains.Anisopachmapofthe southandcentralForkedDeerRiver floodplainswas madebysubtractingtheelevationsofthe floodplainbase planartrendsurfacefromthe floodplainsurfaceelevations.TheisopachmapofthePliocene/Pleistoceneterrace alluviumwasdeterminedbysubtractingtheelevationsof thebaseoftheterracealluviumfromthegroundsurface elevations;thus,thethinsurfaceloesswasincludedin thisthickness.TheClaiborneGroupisopachmapwas derivedbysubtractingtheelevationofthetopoftheWilcoxFormationfromthesurfaceelevationwheretheClaiborneisexposedandfromtheelevationofthetopofthe Claibornewhereit’scoveredbyHolocene floodplain alluviumorPliocene/Pleistoceneterracealluvium.IsopachmapsoftheWilcoxGroup,PortersCreekClay,and Cretaceoussectionweremadebysubtractingtheelevationsfromthebasalstructurecontourmapfromtheoverlyingstructurecontourmapofeachmappingunit(e.g., PortersCreekClaytopelevation – Cretaceoustopelevation ¼ thicknessofPortersCreekClay).Fordatausedin thisstudy,seeAppendixA.

ThesurfacegeologyofMadisonCounty,structure contourmaps,andisopachmapswerecompiledintoa 3DmodelandanimationofthegeologyofMadison County(AppendixB).S-wavevelocitiesweredeterminedforthemappedsurfaceandsubsurfacegeologic units,andinconjunctionwiththeunits’ respectivethicknesses,averticalseismicvelocityproﬁlewascalculated forevery0.5km2 ofMadisonCountyanda10-kmsurroundingarea.Adetaileddiscussionoftheseismic methodologyispresentedinCrameretal.(2023).

GeotechnicalEngineeringAnalysis

GeotechnicalanalysisofMadisonCountywasconductedtodevelopliquefactionprobabilitycurves (LPCs)basedongeotechnicalsoilborings,groundwater levels(GWL),andseismicgroundmotionestimates. TheLPCsweredevelopedusingthethree-stepprocedureutilizedbyRixandRomero-Hudock(2006), Crameretal.(2018a,b;2019;2020a,b;2021;2022; 2023)andsummarizedinTohidietal.(2021).However, anoverviewofeachstep,includingspeciﬁcmodiﬁcationsincorporatedforMadisonCounty,isprovidedas follows.

Step1involvescalculatingthefactorofsafetyagainst liquefactionateach0.6-mdepthincrementtoadepthof 20mofeachsoilboringusingthesimplifiedprocedure ofSeedandIdriss(1971)andtheYoudandIdriss(2001) relationshipfordeterminingthecyclicresistanceratio basedonstandardpenetrationtest(SPT)data.Atotalof 233soilboringlogswerecollectedfromvariousorganizations.However,only69ofthesesoilboringlogsmet thefollowingscreeningcriteriaforliquefactionanalysis: (1)Boringsmustbeatleast20m(66ft)deep,(2)boringsmustincludeN-valuesandUnifiedSoilClassificationSystem(USCS)soilclassificationstoadepthof15 m(50ft),and(3)theboringlocationsmusthavelatitude andlongitudecoordinates.

AftersummarizingtheboringlogsforN-values, USCSsoilclassiﬁcation,and ﬁnescontentusingadepth incrementof0.6mtoatotaldepthof20m,theGWLat eachboringlocationwasestimatedfromtheGWLcontourmapofSchrader(2008)andcorroboratedwithdata from23wellsprovidedbytheJacksonEnergyAuthority (JEA),whichindicatedastablewaterlevelovera15yearperiod(from2006to2020).Themeasurements fromthese15yearsindicatethatthechangeinwater levelisminor.Additionally,JEAhasnotencountered anysuddenriseinwaterlevelsnearrivers.TheJEA GWLsareingeneralagreementwiththelevelsindicated bythecontourmapofSchrader(2008).Therefore,the SchraderGWLcontourmapwasutilizedtoobtainthe GWLateachsoil-boringlocationtoperformliquefactionanalysis.

Step2consistsofcalculatingtheliquefactionpotentialindex(LPI)ateachsoil-boringlocationbasedonthe Iwasakietal.(1978,1982)method.Tohidi(2022)providesanoverviewoftheLPImethodaswellasthe equationsusedtoobtainLPI.LPIvaluesaredetermined ateachsoil-boringlocationforPGAvaluesof0.1,0.2, 0.3,0.4,0.5,0.6,0.7,0.8,0.9,and1.0gandearthquake magnitudes(Mw)of5,5.5,6,6.5,7,7.5,and8.Thus, LPIvaluesaredeterminedforeachof70possiblecombinationsofPGAandMw,i.e.,70earthquakescenarios.

Step3consistsofdevelopinganLPCfortheprobability ofexceedingatargetLPIvalueof5and15foreachprimary

surficialgeologicunit.However,duetotheverylimited numberofsoilboringsavailableinsomenear-surfacegeologicunits,surfacegeologywasseparatedinto floodplain alluviumandnon-floodplainstrataasmappedinFigure3. Thenon-floodplainstrataconsistofriverterracealluvium andTertiary-Cretaceousstrata.However,fornon-floodplainstrata,allboringsarewithintheterracealluvium, andtherearenoboringswithintheTertiary-Cretaceous strata.TheTertiary-Cretaceousstratahavethehighestelevation;consequently,theyhavethelowestGWLandthe lowestprobabilityofliquefaction.Insummary,LPCs weredevelopedbasedonSPTdataof25soilborings withinthe floodplainalluviumand44soilboringsfrom theterracealluvium.

LPCsdeterminedforthisstudyarebasedonthe probabilityofexceedinganLPIvalueof fi ve,whichis moderateliquefactionpotentialbasedontheresultsof Iwasakietal.(1978,1982)andToprakandHolzer (2003).TheprobabilityofexceedinganLPIof fi veis basedonthenumberofsoilboringswithLPIexceeding fi ve(cumulativefrequencydistribution).AnexampleofcomputingtheprobabilityofLPI . 5isgivenin Tohidietal.(2021)andTohidi(2022).Similarly,LPCs arealsodevelopedbasedontheprobabi lityofexceedinganLPIvalueof15,whichis “ severe ” liquefaction potential.

RESULTS

SurfaceGeologicMap

ThesurfacegeologyofMadisonCounty(Figure 2A)wasgeologicallydifferentiatedintothreemappingunitsforseismicandliquefactionhazardanalyses(Figure3):Holoceneriver ﬂ oodplainalluvium, Pliocene/Pleistoceneriverterracealluvium,andsemiconsolidatedsedimentsofPaleogeneandCretaceous age.TheFigure2Bcross-sectionfromtheJackson North7.5-minutegeologicquadrangleisrepresentativeofthegeologythatunderliesmostofMadison County.

StructureContourMaps

ThetopofthePaleogeneandCretaceousstrataare exposedineasternmostMadisonCounty.Overmuchof thecounty,1–25mofPliocene/Pleistoceneterracealluviumand1–9mofHoloceneriveralluviumoverliePaleogeneandCretaceousstrata(Figure3).

Structurecontourmapsweremadefromsurfacedata ofthecontacttopsoftheClaiborneGroup,WilcoxGroup, PortersCreekClay,andCretaceousstratamappedin thepublished7.5-minutegeologicquadranglemapsof Figure2(Figure4AthroughF)(Brown,1968a,b;Parks, 1968a,b,c,d,e,f,g;Jewel,2006;AntonacciandHoyal,

3DGeologicMappingofMadisonCounty,Tennessee

Figure4.(A)Surfaceoutcropdata(bluedots)usedtomakestructurecontourmapin “B” ofthetopoftheClaiborneGroupwithbluelinesshowingpublishedquadranglecross-sectionlines.PlanartrendsurfacestructurecontourmapsoftheWilcoxGroup(C),thePortersCreekClay(D), Cretaceousstrata(E),andthePaleozoicstrata(F)withstratigraphiccontactandboreholedatapointsusedtoconstrainthetopsoftheseunits. Elevationisinmeterswithrespecttosealevel.

2011,2012a,b,2014)andfromdrillholedata.Planartrend surfaceswere ﬁttotheelevationsofthetopsoftheWilcox GroupandPortersCreekClaytomakeplanartrendsurface structurecontourmaps(Figure4CandD).Fittingplanes totheseformationtopsisjustiﬁedbecausenofaultsor foldshavebeenmappedinMadisonCounty(Figure2A). Strikeanddipofthegeologicformationsweredeterminedfromthecontourlinesintheirplanartrendsurfaces(Figure4CandD).Theseunittopsslopewesterly atlessthan1°:TheWilcoxGroupisN28°E,0.21°NW, andPortersCreekClayisN27°E,0.27°NW.Thestrikes anddipsofthetopsoftheCretaceous(N28°E,0.48°NW) andPaleozoic(N21°E,0.44°NW)inMadisonCounty weredeterminedfromthecontourlinesontheirstructurecontourmaps(Figure4EandF).Thestructural analysisinthisstudyshowsthat,inMadisonCounty, theentirestratigraphicsectionfromthetopofthe PaleogenetothetopofthePaleozoicdipswesterly at ,1°.

IsopachMaps

Holoceneriver floodplainalluviumthicknessmeasurementsalongtheForkedDeerriversandHatchie Rivertributariesrangefrom1to9m(Figures5and 6A).The floodplainalluviumthicknessinthisstudy,as determinedfromthebridgemeasurements(Figures5and 6A),isanestimatebecause floodplainalluviumcould underliethechannelbaseorthechannelbasecouldbe incisedintoPaleogenestrata.Basedonavailableborehole dataandlithologicdescriptionsintheavailablequadrangles,the floodplaininthisstudyisinterpretedtocontaina surfacesiltandclayoverbanklayer,whichoverliespoint barsandthatmaycontaingravel.

Pliocene/Pleistoceneterracealluviumthicknessisvariablewithvaluesrangingfrom1to25m(Figure6B).In general,terracealluviumisthickestalongdrainage dividesandthinstowardsmodernstreams.ThePliocene/ Pleistoceneriverterracesaretopographicallyhigherthan theHolocene ﬂoodplainsandconsistofsurfacesiltand claythatoverliessandandgravel(Figure2C).Athin layerofPleistoceneloessoverliestheriverterracealluvium.Thereareatleasttwoterracelevelsapparentin1-m lidarDEMdata,butthedifferentterracelevelsarenot hereindifferentiated.

PaleogeneandCretaceousstrataareatgroundsurface ineasternmostMadisonCountyandunderliewestern MadisonCounty(Figures2and4andTable1).Insome areas,thePaleogenestrataarecoveredwithathin veneerofPleistoceneloess.TheClaiborneGroupthickensnorthwesterly(Figure6C),butthicknessisirregular becauseitsuppersurfacehasbeenerodedtovariouselevationsbytheentrenchmentofPliocene/PleistoceneriversandtheongoingentrenchmentofHolocenerivers acrossMadisonCounty(Figure2A).TheWilcoxGroup

(Figure6D)andPortersCreekClay(Figure6E)thickento thenorthwest,andtheCretaceoussectionthickenssoutherly(Figure6F).TheClaiborneGroupaverages33mthick, WilcoxGroup37m,PortersCreekClay107m,andthe Cretaceousstrataaverages237mthickinMadisonCounty.

SeismicGroundMotionMaps

USGSseismicgroundmotionmapsarebasedona uniformstandardgeologyanddonotincludetheeffects ofnear-surfacegeology(Petersenetal.,2014).Seismic groundmotionmapsforMadisonCountyweredeveloped(Crameretal.,2023)usingthegeologicalmodel describedinthispaper.Theshearwavevelocities(Vs) usedintheUSGShazardmodelcalculations(Petersen etal.,2014)areuniformlyhigherthantheaverageshear wavevelocityovertheuppermost30m(Vs30)usedto developourgroundmotionmaps.TheactualshallowVs valuesareaverageswithuncertaintywithinageological formationfrommultichannelanalysisofsurfacewaves/ refractionmicrotremor(MASW/ReMi)measurementsat representativesitesinMadisonCounty.ThedeeperVs values,whichhaveamuchsmallerimpactonseismic hazard,arefromthemeasurementsinsimilargeological stratainShelbyCounty,adjustedfordifferencesindepth ofburial.VsvaluesforhazardcalculationsinMadison Countyareassignedtotheformationsinthe3Dgeology modelusingformationalaveragesanduncertaintyinthe mannerusedinGombergetal.(2003).Twoofthemost importantearthquakeprobabilisticmapsfromCramer etal.(2023)areillustratedhere(Figure8)corresponding totheresponseofstructureslessthantwostorieshigh (PGA)andstructures 10storieshigh(1.0seconds). Theunitsshownonthesemapsareaccelerationing, whichistheaccelerationattheearth’ssurfaceof980.655 cm/s/s.Atshortperiods(PGA),soilresponseinwestern MadisonCountyde-amplifiesgroundmotions(Figure 8C)relativetotheUSGSmaps(Figure8A)duetononlineareffectsathighergroundmotionsclosertotheearthquakesource.Butnonlineareffectsarereducedineastern MadisonCountyduetoincreaseddistancefromtheearthquakesourceandincreasedsoilstiffness(consolidation) ofthenear-surfacegeology,andhence,shortperiod groundmotionsaresignificantlyamplified.Atlongperiods(1.0seconds),soilresponseislessaffectedbynonlinearsoileffects,andhence,groundmotionsareamplified (Figure8D)relativetotheUSGSmaps(Figure8B), whichdependsonthedistancefromthesourceandsoil stiffness(Crameretal.,2023).

GeotechnicalEngineeringResults

WhereastheLPCsforP[LPI.5]wereintroducedin Crameretal.(2023),theLPCsforP[LPI.15]arenewly presentedinthispaper.Figure9AandBshowtheLPCs

3DGeologicMappingofMadisonCounty,Tennessee

Figure5.Holocenelowland floodplainalluviumthicknessmeasurements.Themaponthetoprightshowsnumberedlocationsof floodplaintopographic1-mLidarprofilesandcorrespondingbridgemeasurementswiththewhitelineshowingtheborderofMadisonCounty.The1–8 floodplainsurfaceprofilesand floodplainthicknessmeasurementsarelocatedin “A.” Reddotisbottomofchannelmeasuredatbridgelocations.

Harrison,Tohidi,VanArsdale,Cramer,andArellano

Figure6.IsopachmapsoftheLowlandHolocene ﬂoodplainalluvium(A),Pliocene/Pleistoceneterracealluvium(B),ClaiborneGroup(C), WilcoxGroup(D),PortersCreekClay(E),andCretaceousstrata(F).Contoursareinmeterswithrespecttosealevel.

Figure7.3DmodelofthesurfaceandsubsurfacegeologyofMadisonCounty.ColorscorrespondtocolorsusedinFigure2A.Modelis50Xverticalscale.3DanimationisinAppendixB. 3DGeologicMappingofMadisonCounty,Tennessee

for ﬂoodplainalluviumandnon-ﬂoodplainstrataforthe probabilityofexceedingLPIof5and15denotedasP [LPI.5]andP[LPI.15]versustheratioofPGAover magnitudescalingfactor(MSF).MSFcorrectsforearthquakemagnitudesotherthan7.5andiscomputedusing therevisedrelationshipbyIdriss(1999).

Aswasexpected,theForkedDeerRiversandHatchie Rivertributary floodplainalluviumLPCsaresignificantlyhigherthanthenon-floodplainstrataLPCsfor bothP[LPI.5]andP[LPI.15]duetoashallowgroundwatertableandthepresenceofmoresandysoils. Crameretal.(2023)evaluatedearthquakeliquefaction potentialsforMadisonCountyoverarangeofNew Madridearthquakemagnitudesandepicenterlocations. OnesuchevaluationwasmadeassumingaM7.3Axial faultearthquake.Suchanearthquakewouldresultinan LPI . 5onthe floodplainsinMadisonCounty(Figure 3),thusresultinginmoderateliquefactionpotential.

CONCLUSIONS

Holocenealluvium,withestimatedthicknessesrangingfrom1to9m,underliestheMiddleForkandSouth ForkofForkedDeerRiverandHatchieRivertributary

floodplainsinMadisonCounty.Thesurfacegeologyof MadisonCountyalsoconsistsofextensivePliocene/ Pleistoceneriverterracesthathavebeenincisedby Holocenerivers.WespeculatethattheterracesofMadisonCountyareremnantheadwater floodplainsofthe ancestralMississippi/Ohioriver.TherearenoradiometricdatesfortheMadisonCountyterraces,butitis believedthattheterracesareequivalenttothosemapped inwesternmostTennessee(Saucier,1987;VanArsdale etal.,2007;Lumsdenetal.,2016;Odumetal.,2020; Harrison,2022;Reichenbacheretal.,2022).Specifically, thehighestMadisonCountyterracemaybeequivalent tothe3.6MaUplandComplexandsequentiallylower terracesequivalenttotheHenderson,Humboldt,and Hatchieterracespreservedinthedownstreamreachesof theForkedDeerRiverinDyerandLauderdalecounties andtheHatchieRiverinLauderdaleandTiptoncounties (Figure1A)(Harrison,2022;Reichenbacheretal., 2022).Thisterraceinterpretationisconsistentwiththe regionalterracemappingbySaucier(1987)whereinhe mappedHumboldtandHatchieterracesalongthe HatchieRiverinsouthwesternMadisonCounty. Surfaceandsubsurfacegeologicmappinghasprovidedthe3Dgeology(Figure7andAppendixB)and

Figure8.Twopercent–in–50yearhazardmapsshowingthe2014U.S.GeologicalSurvey’snationalseismichazardmapforsiteclassBCconditionsfor(A)peakgroundacceleration(PGA)and(B)1.0secondspectralacceleration.(C)and(D)equivalenthazardmapsaccountingforthe effectsoflocalgeologyforMadisonCounty(fromCrameretal.,2023). Harrison,Tohidi,VanArsdale,Cramer,andArellano

thePliocene-QuaternarygeologichistoryofMadison County.Thisresearchisofvaluetothosewhoseeksubsurfaceinformation,suchaspotentialsandandgravel resourcesintheriverterracesofMadisonCounty.As showninFigure8,thelocalnear-surfacegeologysignificantlyinﬂuencestheexpectedgroundmotionsduring futureearthquakesand,thus,providesamoreaccurate anddetailedseismichazardassessmentthantheuniform regionalgeologyusedinthe2014USGSseismichazard maps(Petersenetal.,2014).Whencomparingthe groundmotionmapinthisstudywiththeUSGSseismic hazardmapsatshortperiods(PGA),thesurfacegeology

de-amplifiesthegroundmotioneffectsinwesternMadisonCountyandamplifiestheseeffectsineasternMadisonCounty(Figure8).Thisisduetoanincreasein consolidationintheCretaceousstrataandcloserproximitytotheNewMadridearthquakesourceinthewest. Whencomparing1.0-secondhazardmaps,thereisan overallincreaseinground-shakinghazardinMadison Countythatdependsonsoilstiffnessanddistancefrom theearthquakesource.TheLPCsandtheassociatedliquefactionhazardmapsdevelopedforMadisonCounty indicatethatliquefactionpotentialishigherintheForked DeerRiverandHatchieRivertributary floodplainsdueto

3DGeologicMappingofMadisonCounty,Tennessee

Figure9.MadisonCountyliquefactionpotentialindex(LPI)–basedliquefactionprobabilitycurves(LPCs)of ﬂoodplains(A)andnon-ﬂoodplains (B)fromstandardpenetrationtestdata.ThegraphsshowtheprobabilityofexceedingLPIof5and15versustheratioofpeekgroundacceleration overmagnitudescalingfactor.TheLPCsforP[LPI.5]wereintroducedinCrameretal.(2023),buttheLPCsforP[LPI.15]arenewtothis paper.

shallowgroundwatertableandthepresencesandysoils. ThisstudyofMadisonCounty,Tennessee,furtherillustratestheimportanceofknowingthelocalnear-surface stratigraphyandstratathicknessintheappraisalofpotentialseismicgroundshakingandliquefaction.

ACKNOWLEDGMENTS

WethankLukeEwing,WilliamMann,andElaine FoustoftheTennesseeDepartmentofEnvironmentand Conservation(TDEC),andtheU.S.ArmyCorpsof Engineers,theTennesseeDepartmentofTransportation, andConstructionMaterialsLaboratory,Inc.,forproviding dataneededinthisproject.Wealsoacknowledgethe importanceofthequadranglesmappedbyVinceAntonacci ofTDEC,whichwereintegralinmakingthisgeological model,andtheJacksonEnergyAuthorityandDr.John Smithforprovidingwellgroundwaterdata.

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APPENDICES

AppendixAcontainsspreadsheetsofthedatapoints usedtocreatethestructurecontourmaps.AppendixB containsavideoanimationofthe3Dgeologicalmodel ofMadisonCounty,Tennessee.

TheAppendicescanbefoundat: https://www.aegweb. org/e-eg-supplements 3DGeologicMappingofMadisonCounty,Tennessee

UsingLandslidestoLocatetheEpicenterfor theHistoric1882EarthquakeinColorado

JUSTINMARCHLEWICZ*

512CavettStreet,VanWert,OH45891

PAULSANTI

1516IllinoisStreet,Golden,CO80401

KeyTerms: Landslides,Seismic,Colorado

ABSTRACT

OnNovember7,1882,thelargestearthquakein Colorado’srecordedhistoryimpactedanareaestimated tobe470,000km2 (181,000mi2).Whiledifferingaccounts ofintensitywererecordedbyvariouscommunitiesat thetime,thedistributionofreporteddamagesindicated alikelymagnitudebetween6and7.Thisstudyaimed torefinethisestimateusinglandslideswithintheregion thatwerelikelytobeco-seismic.Previouslyproposed methodologywasusedtocalculatesurfaceroughness valuesforlandslidesmappedatvariousscalesdigitized andcompiledbytheColoradoGeologicalSurvey.A searchradiusof54kmsurroundingtheepicentral estimatefortheearthquakewaschosentoevaluate therelativeagesoflandslideswithinthedefinedregion. Whiletheproposedmethodologyrequiresacalibration usingage-datedlandslideswithinthedataset,theuse ofsurfaceroughnessasarelativeagescaleprovedto besufficientlyusefultoconstrainanareawithinthe studythatshowedanomalouslyhighsurfaceroughnessvalues.Basedonthisearthquakebeingoneof theonlylargeearthquakestohaveoccurredinrecorded historyinColorado,aswellasalackofwidedistributionintheanomalousregionexpectedfromastorm event,thisanomalousareacontaininghighsurface roughnesslandslidesacrossvaryingbaselithologieswasconcludedtohaveahighprobabilityof beingco-seismicallyinduced.Utilizingthisinformation, anewinitiationellipsewasgeneratedfromthesedata.

INTRODUCTION

OnNovember7,1882,thelargestearthquakerecorded inColorado ’shistoryaffectedanareaestimatedtobe 470,000km 2 (181,000mi 2 ).ThereportedMercalli intensitiesatthetimerangedfromIItoVII,andbyutilizingregressionequationsthatcomparedintensitytoarea

*Correspondingauthoremail: jmarchlewicz@mines.edu

felt,KirkhamandRogers(1986)assignedamagnitudeof ML6.2 6 0.3.Whilemanyattemptshavebeenmadeby priorresearcherstoconstraintheepicenterofthishistoric earthquake,itisstilluncertainonpreciselywhichfault theslipoccurred.WhileColoradohasnothadadditional earthquakesclosetothismagnitudesince1882orin recordedhistory,itisimportanttoattempttodecipher wherethisearthquakeoccurredsothatadditionalstudiesof thefaultcanbeconductedtoseeifanotherearthquakelike theonefeltin1882ispossibletooccuragain.

Thepurposeofthisstudywastoconstrainanewepicenterestimateforthe1882earthquakebyutilizingco-seismic landslidesresultingfromthisevent,adistinctlynew approach.Ifitispossibletodifferentiatethoselandslides thatlikelyoccurredaroundthesametimeasthisearthquake,thentheselandslidescanbeusedtoconﬁ rmthe currentepicenterestimateoridentifyanewlocation.

BACKGROUND

Severalpreviousstudieshavebeenundertakentobetter understandthisearthquake.Theprimarystudyonwhich thisresearchisbasedcomesfromKirkhamandRogers (1986),whocompiledreportedintensitiesfollowingthe 1882earthquake.Asstatedwithintheirstudy,basedon theworkofDamesandMoore(1981),itwasfoundthat fewerthan200,000peoplelivedinColoradoatthetime oftheearthquake,withmanylivinginruralareas.Additionally,severalotherfactorscomplicatedtheaccuracy ofhowtheearthquakewasreportedatthetime,withthe earthquakeoccurringonamajorelectiondayaspolls closed,causingsomenewspaperstoentirelyneglect reportingthetremor.Othersourcestendedtoexaggerate theimpactsoftheearthquake’seffectsortosimplynot reportitatall,astheirreadersonlywantedtoreaduplifting stories.Finally,hearsayreportsemergedincertainnewspapers,whereearthquakeeffectsintownswithdamagewere reportedbytownswherenomajordamageoccurred.By attemptingtominimizetheseeffectswhenevaluatingpublishedreportsofdamagefromthisearthquake,Kirkham andRogers(1986)assembledanisoseismalmapfromthis dataset,notingtheparticulardifﬁcultythatemergeddueto

Figure1.KirkhamandRogers(1986)isoseismalmapconstructedutilizingreportedintensitiesfollowingthe1882earthquake.Colorizationprovided byColoradoGeologicalSurvey.

conﬂictingreportsandasomewhatnonsensicalgeographic distributionofreportedintensities.Afterareevaluationof thereports,andbyassigningdifferinglevelsofcredibility tothepublishedarticlespertainingtodamagereported, theyconstructedanisoseismalmapofthefeltarea,choosingapredictedepicenterthattheystatedintheirreport maybeoffbyuptohalfadegreeoflatitudeorlongitude. Anupdatedandcolorizedversionofthismap(Kirkham andRogers,1986)publishedbytheColoradoGeological Survey(CGS)isshowninFigure1.

PriorreportsfromDamesandMoore(1981)had suggestedanepicenteralongtheDudleyGulchgraben (seelocationonFigure2),butfromradiocarbondating ofunfaulteddepositsabovethefault,KirkhamandRogers (1986)concludedthatnosurfacerupturesalongthatfault hademergedforatleast1,230 6 60years.Additionally, Eckert(1982)thoroughlystudiedtheDudleyGulchgraben andfoundnoevidenceofeitherhistoricorrecentmovementalongthefault.BeforetheDamesandMoorereport, anotherestimatewasmadebyHadsell(1968)utilizing asimilartechniquetoKirkhamandRogers(1986)but

relyingonadifferentweightingofreportedintensities (Figure2).FortheisoseismalmapthatKirkhamand Rogers(1986)generated,thereportedintensitiesfrom ColoradoSpringswereassignedalesscredibleweight thangivenbyHadsell,leadingtoanepicentralestimate farthernorththanHadsellhadpredicted(Figure2).Additionally,theKirkhamandRogers(1986)estimatewaslater givenfurthercredibilitybySpenceetal.(1996),whoanalyzedthe1984LaramieMountainsearthquakeandcomparedthegeneratedisoseismalmapforthateventwith theonecreatedforthe1882earthquake.Fromtheirstudy, itwasproposedthatwhilethefeltareaforthe1882earthquakeshouldlikelyhavebeenlarger,theKirkhamand Rogers(1986)epicenterestimatewasroughlycentralto thisnewproposedfeltarea,consistentwiththedatafound fromthe1984LaramieMountainsisoseismalmap.

METHODOLOGY

Toconductthisstudy,theepicenterestimatemadeby KirkhamandRogers(1986)wasusedasastartingpoint

Figure2.Locationsofpredictedepicentersforthe 1882earthquakeproposedbyKirkhamandRogers(1986),DamesandMoore(1981),andHadsell (1968)withinColorado.ESRIbasemapisincludedforreference.

asalikelyzoneforlandslidesinitiatedbytheearthquake. TheKirkhamandRogers(1986)epicenterwasused duetoadditionals upportfortheepicentralareafrom Spenceetal.(1996).Ut ilizingestimatesfromKeefer (1984),amaximumlikelyinitiationradiuswouldbe approximately36kmaroundtheepicentrallocation, witha54kmradiusincludedduetouncertaintyinthe estimatedepicentrallocation.Whileattemptingtoverifythevalidityofthisinitiationradius,archivednewspaperaccountsoftheeventweresearchedinasimilar mannertothatdonebyKirkhamandRogers(1986)to determineifrockfallsorlandslidesbeinginitiated fromthisearthquakewererecorded.From ThePueblo DailyChieftain (1882)publishedonNovember11,1882, anarticletitled “MotherEarth” wasfounddescribing rockfallsandalandslideasrecountedbyranchmenliving intheRockyMountainsnearPueblo,CO,atthetime. Whiletheseaccountsfallwelloutsidetheinitiationradius

predictedbyKeefer(1984),itislikelythatthelowattenuationofseismicenergythroughtheigneous/metamorphic bedrockoftheRockyMountainshelpstoaccountforthese observations.Duetouncertaintiesinthetrueextentofpossibleco-seismiclandslidesinitiatedbythisevent,a54km radiuswaschosenasthelargestsearchradiustorepresent 150%thesizeoftheprimarysearchradius.Anadditional, smallersearchradiusof18kmwasincludedas50%of theprimarysearchradiustoobservevaryingspatialtrends betweentheseregions.Arepresentationofthesearch radiiawayfromtheKirkhamandRogers(1986)epicenter estimateisshowninFigure3,withaninlayofprior mappedlandslidesdigitizedbytheCGSusedtoconduct thisstudy.OncethisdatasetwasloadedintoArcGISPro, thelithologywithinthestudyareawasderivedfrom Tweto(1979),asshowninFigure4.Additionally,light detectionandranging(LiDAR)and1mdigitalelevation model(DEM)dataforthestudyregionwereloaded

Figure3.AreaofinterestwithlandslidesidentiﬁedbyColoradoGeologicalSurveycoloredbysourceandradiioverlainwithmaximumlandslide initiationradiusdeﬁnedbyKeefer(1984)andFrontRangeepicenterpredictedbyKirkhamandRogers(1986).

throughtheU.S.GeologicalSurvey(USGS)3DElevation Program(3DEP)LidarExplorerapplication(U.S.Geological Survey,2024),andaslopeanglemapwasgenerated,as showninFigure5.

Withinthestudyarea,signiﬁcantnumbersofCGSidentiﬁedlandslidesoverlieregionsdesignatedasglacial driftorunconsolidatedmaterial.TheremainderofdesignatedlandslideslieontopofMesozoic-agedsedimentaryclasticrock,theInglesideandFountainFormations, Proterozoicmetamorphicrocks,andgraniticrocksofthe 1700Maagegroup.Fromastudyofthisinformation,the landslidespresentwithinthestudyareagenerallyfellinto twocategories:highslopeangles( .40°)underlainby whatwasconsideredtobecompetentbedrock(igneous andmetamorphosedigneousrock),andshallowerslope angles(,40°)underlainbywhatwasconsideredtobeless competentbedrockorsediments(sedimentaryformations andglacialdrift).

Inadditiontoananalysisofslope,therangeofslope aspectswithinthestudyareawasalsocalculated,using theaspecttoolwithinArcGISPro(Figure6).Following this,asummationofrastercellswithinthedatasetidentifiedby45°aspectincrementswasperformed,andthedata wereexportedtovisualizethequantityofcellsidentified acrosstheentirestudyregionwithinthe54kmsearch radiusaswellasonlyonCGS-identi fi edlandslides. Whiletheseattemptstounderstandandcharacterize thestudyareawereuseful,inordertoidentifylandslides thatwerelikelytobeco-seismic,itwasnecessarytoestimateanageforlandslidesintheregion.Followingthe methodologydefinedbyLaHusenetal.(2016),acalculationofsurfaceroughnesswasperformedforthe611 de fi nedlandslideswithinthedataset.Toconductthis analysis, first,theslopemapgeneratedusingtheUSGS 1mDEMwasclippedtotheextentoflandslidesidentified bytheCGSwithinthestudyareaacrossseveralmapped

Figure4.GeneralizedlithologyforthestudyareafortheU.S.GeologicalSurvey.ThecentralportionoftheFrontRangeisprimarilydeﬁnedby igneousandmetamorphiccomponents,withsedimentaryandunconsolidatedsedimentslyingtotheeastandwest.Mappingwasconductedby Tweto(1979).

scales(24,000,48,000–100,000,250,000,andBoulder CountyLiDARdataset).Fromthede ﬁ nedmethodology,surfaceroughnessisde ﬁ nedastheaveragestandarddeviationofslopewithinalandslidedeposit.To achievethis,thefocalstatisticstoolwithinArcGISPro wasusedtocalculatethestandarddeviationwithin landslidedepositsfora3m 3 3mwindowpercell. Next,histogramsofrecordedstandarddeviations weregeneratedforeachlandslide,andameanstandarddeviationvaluewasrecordedforeachlandslide. Whiletheproposedmethodologyadvisedtheuseof thezonalstatisticstooltocalculatethemeanofthestandard deviationvaluesassignedtoallpixelswithineachlandslide polygon,ArcGISProwasunabletosuccessfullycomplete thiscalculationforanunknownreason,someanstandard deviationsdeterminedfromhistogramswereusedinstead. Overall,thereisnodifferenceinthemeanvaluesthatwere

calculated,withtheonlydifferencebeingtheamount oftimethatprocessingrequired.Themethodologyfrom LaHusenetal.(2016)alsocallsforacalibrationofthisrelativeage-datingmethodwithabsolutedatingoflandslide depositswithinthedataset,butthiswasbeyondthescope andfundingofthisstudy.Assuch,thisstudyprimarilyfocusedontheuseoftheserelativeageswiththe assumptionthathighercalculatedsurfaceroughness valuesindicatedyoungerlandslidedeposits,andlower valuesindicatedolderlandslidedeposits,duetoyounger landslidedepositsurfaceshavinghadlesstimetoweather tolessvariabletopography.

RESULTS

Followingthesummationofaspectsperrastercell withinboththe54kmstudyareaandonlywithin

Figure5.SlopeanglescalculatedfromU.S.GeologicalSurvey1mdigitalelevationmodelfromlightdetectionandranging(LiDAR)datautilizingthe slopetoolwithinArcGISPro.

CGS-identi fi edlandslides,rosediagramsweregeneratedfromthisprocesssplitbyidentifiedcardinaldirectionsforaspect,illustratedinFigure7.Overall,therose diagramfortheentirestudyareashowsnoclearpattern,as expectedforanindiscriminateanalysisofalargearea. However,fortherosediagramgeneratedonlyforcells identifiedbyCGSaslandslides,aclearpreferenceforE-W orientationsemerges,aswellasadistinctlackofN-Sorientations.Whileitisnotimmediatelyclearwhythistrend exists,itispossiblethatpreferentialweatheringmayoccur onN-Sslopeswithinthestudyarea,whichmadeidenti fi cationoflandslidesmorechallenging,orsimply thatfewerN-S –orientedslopesarepresentwithinthe studyarea.

Followingthecalculationofsurfaceroughnessvalues perlandslide,ananalysisofthesevalueswasconducted toevaluatethedistributionwithineachofthefourscales atwhichlandslidesweremapped,aswellasacombined

setwithallrecordedlandslidesincluded.Figure8shows thedistributionofsurfaceroughnessvaluescalculated withintheentiredataset.Whileafewnotableoutliersexist withinthedata,overallanormaldistributionofroughness valuesappearstoemerge.Additionally,areviewoflandslidesgivenparticularlyhighorlowaveragestandarddeviationvalueswasconductedtoevaluatethevalidityofthe scoring.Notopographicirregularitiesorclearerrorsindata wereseenfortheseoutliers.

Whilethedataappeartofollowaroughlynormaldistributionoverall,thenextstepwastoevaluatewhethera spatialdistributionexistedwithinthedataset.Ifanarea ofanomalouslyhighsurfaceroughnessvaluescanbefound overaregion,thenitispossiblethata “recent” triggering eventcausedtheselandslidestoform.Whilethisstudyused aqualitativeformofagedating,thisdatasetcouldbecalibratedfromquantitativeagedatingoflandslidesincluded withinthedatasetforfurthervalidation.

Tobeginthisanalysis,centroidsforthelandslides presentwithinthestudyareawerecalculatedtobetter visualizetheclusteringofdatapresentwithinthedata set,aswellastopreparefordatacontouring.Figure9 isamapofthestudyareawithlandslidecentroidscoloredbysurfaceroughnessvalues.Togaugeifthere wasaspatialtrendbetweentheKirkhamandRogers (1986)epicenterandthesurfaceroughnessvaluesassigned tothelandslidecentroids,theneartoolwithinArcGISPro wasusedtocalculatedistancesbetweenlandslidecentroids andtheKirkhamandRogers(1986)epicenter.Figure10is achartcomparingsurfaceroughnessvaluesperlandslide centroidcomparedagainstthedistanceofthecentroidtothe KirkhamandRogers(1986)epicenter.Asshown,there isnotastrongtrendbetweendistancestotheKirkham andRogers(1986)epicenterandsurfaceroughnessvalues, withageneralnegativetrendemerging(containingsome datascatter)around35km.

BasedonthecolorpatternsinFigure9,aspatialtrend appearedtoexistwithinthedata,withagroupingof rougher(younger)valuesinthemid-lowerleftquadrant. Toinvestigatethispossibility,akrigedrasterwasgenerated tocontourthesurfaceroughnessvaluesofallpointswithin thedataset.Thisrougherregionisevenmoreapparentin Figure11.Overlainonthis ﬁgureisadatasetofmapped faultswithinColoradofromtheUSGS(Tweto,1979)to identifyfaultsthatintersectedthisregion.Wechoseanew epicenteralongthefaultlyingclosesttothecenterof theregion,indicatedonthemapasapoint.Additionally, oncethisnewepicenterwaschosen,surfaceroughness valueswereplottedagainstthedistancesofthelandslide centroidstothisnewepicenter,andtheresultsareshown inFigure12.Fromthisgraph,thedatascatterseenwhen conductingthesameprocessasthatconductedforthe KirkhamandRogers(1986)epicenterinFigure10demonstratesanincreaseinthecorrelationwithanegativelinear

Figure6.AspectmapofthestudyareageneratedutilizingtheaspecttoolwithinArcGISProwithU.S.GeologicalSurvey1mdigitalelevation model.

Figure7.Rosediagramoftabulatedaspectpixelswithintheentirestudyarea(left)andoverthepixelswithindigitizedlandslideregionsidentiﬁedbytheColoradoGeologicalSurvey(right).Colorscalerepresentscalculatedangleofpixel,andradiirepresentthenumberofpixelsidentiﬁed foreachaspect.

trend,morethandoublingthe R2 valueassociatedwiththe ﬁt.Whilenoteverylandslidecontainedwithinthisdataset islikelytohaveaco-seismicorigin,forsuchastrongtrend toemergefromthisanalysis,itislogicaltoconcludethata largeeventinrecenthistoryoccurredinthisregion,causing severallandslidesatasimilartime.

DISCUSSION

KirkhamandRogers(1986)concludedthateventhough theirmethodologyrequiredare-analysisofreported intensities,theywerestillabletoidentifyarelatively constrainedregioninwhichtheearthquakeepicenter

Figure8.Distributionofsurfaceroughnessvaluesforthe611landslideswithinthestudyarea.Withtheexceptionofafewoutliers,agenerally normaldistributionisformedwithinthedata.

Figure9.Centroidsoflandslideswithinthestudyareacoloredbycalculatedsurfaceroughnessvalues.

waslikelytofall.Theynotedthattheirestimateof theexactepicenterwaslikelytobeoffbyasmuchas ahalfdegreeoflatitudeorlongitude.Fromouruseof themethodologyproposedbyLaHusenetal.(2016),

evenwithoutanabsoluteagecalibration,therelative agesestimatedbycalculatingthesurfaceroughness valuesoflandslidesidenti ﬁ edaregionofanomalouslyhighroughnessvalues.Thisidenti ﬁ edregion

Figure10.SurfaceroughnessvaluesforeachlandslidecomparedtothedistanceoftheircentroidfromtheepicenterindicatedbyKirkhamand Rogers(1986).

Figure11.Krigedsurfaceroughnessmapgeneratedusinglandslidecentroids,overlainbyColoradofaultsmappedbyU.S.GeologicalSurvey(from Tweto,1979).Analternateepicentermarkedbya ﬁlledredcircleislocatedapproximately35kmsouthwestofKirkhamandRogers(1986)epicenter.

fallswithintheestimationoferrorcitedbyKirkham andRogers(1986),withthenewpredictedepicenter being35kmaway.Whilethenewepicenterestimate waschosenbasedonproximitytothecenterportion

oftheidenti ﬁ edregionofhighroughness,itismore appropriatetodesignateanellipsebasedontheinformationobtainedtofurtherconstrainthelikelyarea wheretheepicentermayhavebeen.Figure13shows

Figure12.Surfaceroughnessvaluesforeachlandslidecomparedagainsttheircentroid’sdistancetothenewpredictedepicenter.

anewellipsede fi ningazoneoflikelyinitiationforthe 1882earthquake,centeredaroundournewestimated epicenter.Thisellipsewasde fi nedtobestcontainthe regionwithsurfaceroughnessvaluesabove2.707, representingtheanomalouszonethisstudyidenti fi ed. Tobetterillustratethesizeofthisellipsecomparedto previousstudies,thepossibleearthquakeinitiation ellipseproposedbyKirkhamandRogers(1986)is includedwithinthe fi gure.Basedonourresults,we demonstrateasigni ficantreductioninthesizeofthe likelyinitiationarea.

Whiletheoriginalaimofthisstudywastoidentify landslideswithinthestudyareathathadthepotentialto beco-seismicforastatisticaldeterminationofanepicenter,thestrongspatialtrendobservedwithinthesurfaceroughnessvaluesfortheregionidentiﬁedalikely productofthe1882earthquake.Whileitispossiblethat alocalizedstormcouldhaveproducedalargenumberof temporallyconsistentlandslideswithinthisrelatively

smallarea,wepresumeitisunlikelythatsuchalocalizedandpowerfulstormoccurredinthepast.Tofurthersupportthisreasoning,datacollectedduringand followingtheexceptionallylarge2013FrontRange rainfalleventwereincorporatedintothestudyutilizing datacompiledbyColoradoStateUniversity(CSU,2013) forpeakcumulativerainfalltotalsover12hoursduringtheevent(Figure14),andthisdatasetwasthen comparedwithUSGS-compileddebris fl ows/landslidesrecordedfollowingthisevent(Figure15).As shown,regionswithhigher12hourrainfalltotalsappear tobecorrelatedwiththenumberofrecordeddebris fl ows/landslides,butthecorrelationisnotperfect.The zoneofdebris fl owsandlandslidesfrom2013covers anareaover50kminwidthandover150kmlong, encompassingthemainconcentrationoftheinventory inFigure15.ThisissimilartotheareaofhighroughnesslandslidesfromtheinventoryweusedinFigure11,usingaroughnessof1.77(approximatelythe

Figure13.NewproposedinitiationellipsecomparedtotheoriginalinitiationellipsedeﬁnedbyKirkhamandRogers(1986).

Figure14.Cumulativerainfallmeasurementsover12hoursduringthe2013FrontRangerainfallevent(right)pairedwithlocationsexceeding 50,100,500,and1,000yearrecurrenceintervalsfor12hourprecipitationtotals(left).DataarefromCSU(2013).

medianvalueinFigure8)asthethreshold these arethethreecolorsinFigure11indicatingtheareas ofhighestroughness.Inthisbriefcomparison,then, astormaslargeasthe2013stormwouldbeneeded toaffectanareathislarge.Giventhatthe2013storm surpassedthe1,000yearrecurrenceinterval,it wouldberaretohaveexperiencedastormofthis magnituderecentlyenoughthatthelandslideswould stillhavethehighroughnessvaluesmeasured,and evenmorerareforsuchaneventtohaveoccurred butremainunrecordedincontemporaneousnewspaperaccounts.

CONCLUSION

The1882earthquakethatoccurredinColorado causedwidespreaddamagetoexistingstructures acrosslargedistancesbasedonreporteddamagesand eyewitnessaccounts.WhileDamesandMoore(1981) indicatedthatsomeofthereportswereexaggerated orunder-representativeofthetrueimpactsofthis earthquakeanditsaftershocks,KirkhamandRogers (1986)effectivelydecipheredthedataavailableto thematthetimetodevelopageographicalellipsethat theywerecon fi dentcontainedtheepicenterforthis earthquake.Theirspeci fi cassignmentofanepicenter hadlesscon fi dencebutwaschosenasthebest fi tfor theavailabledata.Whiletheirmethodologywas basedondatathathadvariablecredibi lity,ourwork forthisstudycorroboratestheirestimations,including thelikelyerrorinthedistancebetweentheirchosen

epicenterandthelocationwesuggestforanewinitiationellipse.Byutilizingdataavailabletoday,suchasa USGS1mDEMforthestudyareaandadigitized repositoryoflandslidesfrompriorFrontRangemapping effortsatvariousmapscalescompiledbytheCGS,a morepreciseconstraintontheepicenterlocationwas identi ﬁ edusingthemethodologyproposedbyLaHusenetal.(2016)thatreliedontopographicsurface roughnessasaproxyforage.Bydeterminingaregion withinthestudyareawithaconcentrationofhigher thanaveragesurfaceroughnessvaluesforlandslides, wesuggestthatthisellipseprovidesabetterconstraint onthelocationofthe1882earthquakeepicenter.Furthermore,wesuggestthattheregionwasunlikelyto haveexperiencedalargeandfocusedprecipitation eventwithintherecenttimeframeneededtogenerate suchaconcentrationofyounglandslideswithhigh roughnessvalues.

Additionalworktocon ﬁ rmthenewepicenterlocationwouldincludeagedatingoflandslidesandof unfaultedsedimentsabovesomeofthefaultslocated withinthisregion.Ontheotherhand,thepossibility existsthatthefaultresponsibleforthisearthquakeis blind,withKirkhamandRogers(1986)estimating thehypocentraldepthtoberoughly20kmbeneath thesurface.Whileitmayprovechallengingto ﬁ nd thefaultdirectlyresponsibleforthisearthquake,itis ofimportancetoknowmoreinformationaboutthe causativestructureduetothepotentialhazardsthat anadditionalearthquakeofthismagnitudecouldpose totheFrontRangeandsurroundingregionsshouldit occuragain.

Figure15.Debris ﬂows/landslidesidentiﬁedbyU.S.GeologicalSurveyfollowingthe2013FrontRangerainfalleventoverlyingColoradoState Universitycumulativerainfallmeasurements(CSU,2013).

ACKNOWLEDGMENTS

IwouldliketothankDr.JimMcCalpinforbringing thisprojecttomyattention,asthisresearchhaslarge implicationsforthefutureestimationofthepossibility forlargeearthquakeswithintheFrontRangeandassuch wasworthyofadditionalconsiderationbeyondthe constraintsthatpriorconductedresearchwasableto placeuponthesuspectedepicenterofColorado’s1882 earthquake.Additionally,IwouldliketothankDr.Paul Santiforhiscontinuedsupportthroughoutthisprojectin providingideasfornextstepsandbestpracticesfor processingandviewingthedatathatwereabletobe obtained.Finally,IwouldliketothankmywifeTessfor hersupportduringmytimeatColoradoSchoolof Mines,asnoneofthiswouldhavebeenpossiblewithout heratmyside.

REFERENCES

CSU(ColoradoStateUniversity),2013, 12HourPrecipitationTotalsand RecurrenceIntervalsfor9/12/2013 [created9/13/2013]:ColoradoState UniversityPrecipitationSystemsResearchGroup,FortCollins,CO. DAMESAND MOORE,1981,GeologicandSeismologicalInvestigations forRockyFlatsPlant:ReportpreparedforU.S.Departmentof EnergyunderContractNumberDE-AC04-80AL10890.

E CKERT,A.D.,1982, TheGeologyandSeismologyoftheDudley GulchGrabenandRelatedFaults,PiceanceCreekBasin, NorthwesternColorado :M.S.Thesis,UniversityofColorado atBoulder,CO,139p.

HADSELL,F.A.,1968,HistoryofearthquakeactivityinColorado: Colorado SchoolofMinesQuarterly,Vol.63,No.1,pp.57–72.

KEEFER,D.K.,1984,Landslidescausedbyearthquakes: GeologicalSociety ofAmericaBulletin,Vol.95,No.4,pp.406–421.

KIRKHAM,R.M. AND ROGERS,W.P.,1986,Aninterpretationofthe November7,1882,Coloradoearthquake.InRogers,W.P.and Kirkham,R.M.(Editors), ContributionstoColoradoSeismicityand Tectonics A1986Update:SpecialPublication28,Colorado

GeologicalSurvey,DepartmentofNaturalResources,Denver,CO. Electronicdocument,availableat https://coloradogeologicalsurvey. org/publications/colorado-seismicity-tectonics-1986

LAHUSEN,S.R.;DUVALL,A.R.;BOOTH,A.M.; AND MONTGOMERY, D.R.,2016,Surfaceroughnessdatingoflong-runoutlandslidesnear Oso,Washington(USA),revealspersistentpostglacialhillslope instability: Geology,Vol.44,No.2,pp.111 –114.

SPENCE,W.;LANGER,C.J.; AND CHOY,G.L.,1996,Rare,largeearthquakesattheLaramideDeformationFront Colorado(1882)and Wyoming(1984): BulletinoftheSeismologicalSocietyofAmerica, Vol.86,No.6,pp.1804–1819.

ThePuebloDailyChieftain,1882,MotherEarth,Page4:Electronic document,availableat https://www.coloradohistoricnewspapers.org/? a¼d&d¼CFT18821111-01.2.17&srpos¼2&e¼ en-20-CFT-1–img-txIN%7ctxCO%7ctxTA-%22Mother þEarth%22 1882 2

TWETO,O.,1979, GeologicMapofColorado:U.S.GeologicalSurvey Open-FileReportOFR92-0507. https://pubs.usgs.gov/of/1992/ofr92-0507/

U.S.GeologicalSurvey, 2024,3DEPLiDARExplorer:Electronic application,availableat https://apps.nationalmap.gov/lidarexplorer/#/ .Accessed7September2023.

BookReview

IntroductiontoWasteManagement:ATextbook

(SYEDE.HASAN)

Reviewby:ThomasOommen

DepartmentofGeologyandGeologicalEngineering,UniversityofMississippi, University,MS-38677

INTRODUCTION

TheUnitedNationsHumanSettlementsProgram notes, “Managingsolidwastewellandaffordablyisone ofthekeychallengesofthe21stcentury,andoneofthe keyresponsibilitiesofacitygovernment” (UN-Habitat 2010,p.xix).Improperwastemanagementcanleadto environmentaldegradation,soilandwaterpollution,and wildlifeharm.Mismanagementofhazardouswastecan posesigniﬁcanthealthriskstocommunitiesandthe environment.Wastemanagementhelpsconservenatural resourcesbyreducingwastesenttolandﬁllsandrecoveringmaterialsthatcanberecycledorreused.

Landfillsareasignificantsourceofmethaneemissions,apotentgreenhousegascontributingtoclimate change.Landfillspaceis finiteandexpensive,andasthe amountofwastegeneratedcontinuestoincrease, finding newlandfillsitesbecomesmoredifficult.Therefore, properwastemanagementpracticesandpoliciesare essentialtoprotectpublichealthandtheenvironmentand conserveresources.In IntroductiontoWasteManagement, Hasan(2022),anexperiencededucator,hascomeforth withacollegetextbookinwastemanagement,distilling informationfroma35-yearteachingcareer.Thebookis writtenforstudentsmajoringinenvironmentalscience, environmentalengineering,environmentalstudies,environmentalgeology,resourcemanagement,andsustainabilityprograms.Itcoversallmajortypesofwaste:municipal solidwaste(MSW),hazardouswaste,medical/healthcare waste(MW),nuclear/radioactivewaste,andelectronic wasteinadditiontoachapteronwasteminimizationand pharmaceuticalsandpersonalcareproducts(PPCPs).

SCOPEANDORGANIZATIONOFTHETEXT

ThisbookbyProfessorHasanprovidesanoverview ofthewastemanagementindustry,includingitshistory, currentpractices,andprospects.Itcoverskeytopics, suchaswastegenerationandcharacterization,collection andtransportation,treatmentanddisposal,andenvironmentalimpactsfromitsmismanagement.Theauthor presentsthematerialclearlyandconcisely,makingthe bookunderstandabletoreaderswithlittletonoprior

knowledgeofthesubject.Thebookiswell-organized witheachchapterbuildingonthepreviousonetoprovideacomprehensiveunderstandingofthetopicat hand.Oneofthebook’sstrengthsisitsuseofreal-world examplesandcasestudiestoillustratetheconceptsdiscussed.Thecontentisillustratedwith85 ﬁgures nearly allincolor and87tables,whichmakethematerialeasiertounderstand.Thebookincludesdiscussionofcontemporarytopics,suchasCOVID-19waste,itsproper management,andhowtodealwithMWfromfuture pandemics/epidemicsthatareprojectedtoincreasein frequencyandintensityinthewakeofclimatechange.

Thebookisorganizedinto10chaptersthatcanbeutilizedasthecontentforaone-semestercoursetointroducestudentstowastemanagement.The10chapters providestudentswithafundamentalunderstandingof wastemanagement.Eachchapterbeginswiththechapter ’slearningobjectivesandconcludeswithchaptersummary,studyquestions,relatedsupplementaryreadings, webresources,anddetailedreferences.Solvednumerical problemsoncalculationoflandfilllifeandarea,waste quantities,groundwater flow,etc.,enhancethevalueof thebook.Atextbook flavorpermeatesthroughoutthe book,andthisstylemakesiteasyforthefacultytoadopt itforanintroductorycourseinwastemanagement.

Chapter1discussestheevolutionofwastemanagementpracticesandthedevelopmentofmodernwaste managementsystems.Thechapteralsodefineswaste anditsclassificationintodifferentcategories.Overall, thischapterprovidesasolidintroductiontothesubject, providingacomprehensiveoverviewofthe fieldof wastemanagementanditshistory.Chapter2discusses theessentialsofgeology,itsrelevancetowastemanagement,thevariousgeotechnicalaspectsofearthmaterials, andtheirsignificanceintheselectionofwastedisposal facilitiesandremediationofcontaminatedsites.The chapteralsoexplainstheprinciplesoftoxicologyand relatesthemtohumanandecologicalhealthissuescaused bywastemismanagement.Chapter3describesthehistory andevolutionofenvironmentallawsintheUnitedStates andexplainslawsdealingwithwastemanagementand pollutioncontrol.ThechapteralsodiscussestheEuropean Union’senvironmentallawsalongwithUnitedNations

guidelinesandrecommendations.Chapter4reviewsthe historicalevolutionofMSWmanagementintheUnited Statesandprovidesdetaileddiscussionsofcommon wastedisposalmethods,suchaslandfillsandincinerators, andenvironmentalandhealthimpactsassociatedwith them.Chapter5beginswithdifferentiationbetweenhazardoussubstancesandhazardouswasteandexplains healthproblemscausedbyhazardouswastemismanagement.Thechapteralsocoverssourcesandgeneratorsof hazardouswasteandmethodsforitsstorage,transportation,treatment,anddisposal.Chapter6describesthehistoryandneedforregulatingMW,anddescribesthetypes, sources,andhazardsassociatedwithMWanditssafe management.Thechapterdiscusseswastetypesgeneratedinhospitals,bestmanagementpractices,andthebenefitsofsegregatingcommonwastefromregulated medicalwaste(RMW);summarizesmethodsoftreatment anddisposalofRMW;andreviewsSARS-Cov-2and otheremergingdiseasesandtheirimpactsonMWmanagement.Chapter7describesthehistoryofradioactivity, nuclear fission,atomicphysics,andnucleartechnology. Thechapteralsoprovidesasoundknowledgeof nuclearwastemanagementandtheglobalstatusof high-levelwastedisposal.Chapter8describesthe natureandsourceofelectronicwasteandthelaws governinge-wastemanagement.Thechapteralsosummarizestheproblemofincreasinge-wastequantity andthehealthandenvironmentalimpactsassociated withitsinformalrecycling,highlightingtheurgent needfordealingwithitsmanagementinasustainable

way.Chapter9discussesthehistoricaldevelopmentof wasteminimizationapproachesandcomparesandcontrastsintegratedwastemanagementandsourcereduction.Thechapteralsoexplainsthezero-wasteconcept andU.S.lawsrelatedtowasteminimizationandrecyclinganddescribesinnovativewasteminimization trends.Chapter10de ﬁ nesPPCPs;explainstheirnature andthreattohumanandecologicalhealth;andidentiﬁ esthesource,occurrence,andhazardsofPPCPs.

SUMMARY

Introductiontowastemanagement:Atextbook isthe onlybookavailableinthemarketthatoffersacomprehensivecoverageofall ﬁvemajortypesofwaste:solid, hazardous,medical,nuclear,andelectronic.Overall,this bookisanexcellentresourceforstudents,professionals, andanyoneinterestedinlearningaboutwastemanagement.Theauthor ’sexpertiseinthe ﬁeldisevident,and thebookprovidesathoroughandup-to-dateoverview ofthesubject.Ifyouarelookingforacomprehensive guidetowastemanagement,thisbookisamust-read.

REFERENCES

HASAN,S.E.,2022, IntroductiontoWasteManagement:ATextbook: JohnWiley&Sons,Hoboken,NJ.436p. UN-Habitat,2010, SolidWasteManagementintheWorld’sCities: WaterandSanitationinWorld’sCities2010:Earthscan,London. 257p.

Environmental&EngineeringGeoscienceJournal (ISSN10787275),Copyright © 2024bytheAssociationofEnvironmental& EngineeringGeologists(AEG)andtheGeologicalSocietyof America(GSA)ispublishedquarterly.EditorialOffices:The UniversityofMississippi,DeptofGeologyandGeological Engineering,University,MS38677.AccountingandCirculation Offices:AEG,2840WestBayDrive#141,BelleairBluffs,FL 33770-2620.Call(727)940-2658x2004tosubscribe.Periodicals postageispaidatAEG,2840WestBayDrive#141,Belleair Bluffs,FL33770-2620.

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Allrightsreserved.Nopartofthispublicationmaybereproducedor transmittedinanyformorbyanymeans,electronicormechanical, includingphotocopying,recording,orbyanyinformationstorageand retrievalsystem,withoutpermissioninwritingfromAEG.

THISPUBLICATIONISPRINTEDONACID-FREEPAPER

EDITORS

THOMAS OOMMEN

DepartmentofGeologyand GeologicalEngineering TheUniversityofMississippi University,MS38677 toommen@olemiss.edu

ERIC PETERSON

DepartmentofGeography, Geology,andtheEnvironment IllinoisStateUniversity Normal,IL61790 309-438-5669 ewpeter@ilstu.edu

K AREN E.SMITH ,EditorialAssistant,kesmith6@kent.edu

ADVISORYBOARD

SANTI,PAUL ColoradoSchoolofMines DALAL,VISTY MarylandDepartmentof theEnvironment HASAN,SYED UniversityofMissouri, KansasCity

NANDI,ARPITA EastTennesseeStateUniversity SASOWSKY,IRA D. UniversityofAkron

ASSOCIATEEDITORS

ACKERMAN,FRANCES RambollAmericasEngineering Solutions,Inc. BASTOLA,HRIDAYA LehighUniversity

BEGLUND,JAMES MontanaBureauofMinesand Geology

BRUCKNO,BRIAN VirginiaDepartmentof Transportation CLAGUE,JOHN SimonFraserUniversity,Canada DEE,SETH UniversityofNevada,Reno FRYAR,ALAN UniversityofKentucky GARDNER,GEORGE MassachusettsDepartmentof EnvironmentalProtection GHOSH,SUMAN CaliforniaDepartmentof Conservation

HAUSER,ERNEST WrightStateUniversity KEATON,JEFF WSPUSA MAY,DAVID USACE-ERDC-CHL POPE,ISAAC BookReviewEditor SANTI,PAUL ColoradoSchoolofMines SCHUSTER,BOB SHLEMON,ROY R.J.Shlemon&Associates,Inc.

STOCK,GREG NationalParkService

SWANSON,SUSAN (SUE) BeloitCollege

ULUSAY,RESAT HacettepeUniversity,Turkey

WEST,TERRY PurdueUniversity

SubmittingaManuscript

Environmental&EngineeringGeoscience (E&EG),isaquarterlyjournaldevotedtothepublicationoforiginalpapersthatareofpotentialinteresttohydrogeologists,environmentalandengineeringgeologists,and geologicalengineersworkinginsiteselection,feasibilitystudies,investigations,designorconstructionofcivilengineeringprojectsorinwaste management,groundwater,andrelatedenvironmentalfields.Allpapers arepeerreviewed.

Theeditorsinvitecontributionsconcerningallaspectsofenvironmental andengineeringgeologyandrelateddisciplines.Recentabstractscanbe viewedunder “Archive” atthewebsite, “http://eeg.geoscienceworld.org” Articlesthatreportonresearch,casehistoriesandnewmethods,andbook reviewsarewelcome.Discussionpapers,whicharecritiquesofprinted articlesandaretechnicalinnature,maybepublishedwithrepliesfrom theoriginalauthor(s).Discussionpapersandrepliesshouldbeconcise.

Tosubmitamanuscriptgotohttps://www.editorialmanager.com/EEG/ default.aspx.Ifyouhavenotusedthesystembefore,followthelink atthebottomofthepagethatsaysNewusersshouldregisterforan account.Chooseyourownloginandpassword.Furtherinstructions willbeavailableuponloggingintothesystem.Uponsubmission,manuscriptsmustmeet,exactly,thecriteriaspecifiedintherevisedStyle Guidefoundathttps://www.aegweb.org/e-eg-supplements. Manuscriptsoffewerthan10pagesmaybepublishedasTechnical Notes.ThenewoptionalfeatureofOpenAccessisavailableuponrequestfor$750perarticle.

Forfurtherinformation,youmaycontactDr.ThomasOommenatthe editorialoffice.

Coverphoto

LimestonequarryintheEdwardsAquiferRechargeZone,wherenitratesand fueloilusedinquarryexcavationmayentertheaquifer.Photographcourtesyof GeorgeVeni.Seearticleonpage173.

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