F EBRUARY 2023 VOLUME XXIX, NUMBER 1
Special Issue on Dams and Levees
Guest Editors - Kevin S. Richards, Gary D. Rogers, and Cassandra Wagner
THE JOINT PUBLICATION OF THE ASSOCIATION OF ENVIRONMENTAL AND ENGINEERING GEOLOGISTS AND THE GEOLOGICAL SOCIETY OF AMERICA
SERVING PROFESSIONALS IN ENGINEERING GEOLOGY, ENVIRONMENTAL GEOLOGY, AND HYDROGEOLOGY
Volume29,Number1,February2023
1IntroductiontoSpecialIssueonDamsandLevees
KevinS.Richards,GaryD.Rogers,andCassandraWagner
3PotentialforHydraulicFracturewhenDrillinginDamsandLevees
JeffreyA.Schaefer
17BestPracticesforPlanningandImplementingSiteInvestigationsatFederallyOwnedorRegulated DamsandLevees
KathleenBensko,BryanK.Simpson,ThomasA.Terry,andScottR.Walker
41DamandLeveeSafetyRiskduringRemedialConstruction
JeffreyA.Schaefer
53RoughRiverDamSafetyModificationProjectEvolution
StevenShifflettandWillAilstock
67SonicDrillingonEmbankmentDamsandLevees
MarkS.ElsonandStevenD.Widincamp
KEVINS.RICHARDS*
1530WillowStreet,LakeForest,IL60045
GARYD.ROGERS
SchnabelEngineering,11AOakBranchDrive,Greensboro,NC27407
CASSANDRAWAGNER
DamSafetyProductionCenter,USACE,12596WBayaudAvenue,Suite400, Lakewood,CO80228-2019
InSeptember2019,thechairsoftheFoundations CommitteeattheU.S.SocietyonDams(USSD)and theDamsandLeveesTechnicalWorkingGroupatthe AssociationofEnvironmentalandEngineeringGeologists(AEG)begandiscussingthepossibilityoforganizingajointworkshop.Thisworkshopwasthefirst formalcollaborationbetweenthesetwocommittees. Thecollaborationwasrelativelyeasybecausemany membersofAEGandUSSDhaddualmemberships andhadbeencoordinatinginformallyduringtheyears beforethiseffort.
Earlyon,aworkshopplanningcommitteewas formedanditwasdeterminedthatthebestformat forthisworkshopwouldbetoinviteavarietyof speakers,withspecializedexpertise,whocouldshare theircurrentpracticesformanagingriskduringinvestigationsandconstructionathigh-hazarddamsor levees.TheU.S.ArmyCorpsofEngineers(USACE) recentlyissuedanEngineeringandConstruction Bulletin(ECB),“InterimApproachforRisk-Informed DesignsforDamsandLeveeProjects”(USACE ECB2019,ECBNo.2019-15,https://www.wbdg. org/FFC/ARMYCOE/COEECB/ecb_2019_15.pdf), thatcalledforinclusionofriskinallphasesofdesign andconstructionandhadarelativelynewengineering regulation(ER),“DrillinginEarthEmbankment DamsandLevees”(31Dec2014,ER1110-1-1807, https://www.publications.usace.army.mil/USACEPublications/Engineer-Regulations/).Theworkshop providedavenuetosharehowvariousfederalagencies wereimplementingriskmanagementpracticesandto hearfromconsultantsandcontractorsaboutcurrent challengesconcerningriskmanagementrequirements duringinvestigationsandconstruction.Oneoftheprimarygoalsofthisworkshopwastoemphasizethese issuesand,asaresult,tohelpadvancethestateof
*Correspondingauthoremail:kevin.richards@comcast.net
thepracticetoimproveriskmanagementthroughall stagesofdesignandconstruction.Onetakeawayfrom theworkshopwastheimportanceofcoordinatingrisk managementexpectationsduringthepre-bidphase. Identifyingspecificriskmanagementproceduresthat maybenecessaryforthecontractdocumentsallowsforeasyinclusionofthemintheworkandthe contractor’sbid.
WeareindebtedtoAEG,USSD,SchnabelEngineering,9800JebStuartParkway,Suite200,Glen Allen,VA23059,andPrimeResins,2291Plunkett Road,Conyers,GA30012forsponsoringthisworkshop.Wewouldliketothankourprogramchair, GaryRogers,SchnabelEngineering,forhishelpwith selectingandorganizingtheinvitedspeakers.The workshopwasheldinFortLauderdale,Florida,from December6–8,2021,andincludedafieldtripto HerbertHooverDiketoobservevariouscutoffwall installationmethodsthatwereinprogress.Kudosto theU.S.ArmyCorpsofEngineers,theU.S.Bureau ofReclamation,theFederalEnergyRegulatoryCommission,theTennesseeValleyAuthority,andallthe consultantsandcontractorsfortheirparticipation intheworkshop.Approximately100professionals attendedandthelivelyaudiencediscussionsduring theworkshopwerequiteinformativeandverymuch appreciated.Notallinvitedspeakerswereableto providetechnicalpapersforthispublication,but theworkshoppresentationsarepostedathttps:// www.aegweb.org/dams-levees.Weencourageyouto checkthemout.
Asguesteditors,wehopethatyouenjoythisspecial editionof EnvironmentalandEngineeringGeoscience andjoinusinourquesttoimproveriskmanagement practicesinyourfutureplanningforconstructionand investigationsathigh-hazarddamsandlevees.
SchaeferGeotechnicalConsultingLLC,12102RehlRoad,Louisville,KY40299
KeyTerms: HydraulicFracture,EmbankmentDams, Levees,Drilling,Grouting,ConfiningStress,Undrained Strength,PressureMeasurement
Hydraulicfracturingofembankmentdamsorlevees andtheirsoilfoundationsisarealconcernwheninvasive techniquesareusedforinvestigationandremediation. Methodsusingwater,drillingmud,compressedair,and grouthaveresultedinfracturingoftheembankmentsor foundationsasevidencedbycracking,lossofcirculation,connectionstootherborings,andblowoutsonembankmentslopes.Numerousincidentshavebeendocumentedintheliterature,butmanymoreundocumented casesarelikelytohaveoccurred.Topredicttheoccurrenceofhydraulicfracture,apracticalmodelrequired byU.S.ArmyCorpsofEngineersregulationswascomparedtolaboratoryandfielddata.Themodelrequires estimationofthetotalminorprincipalstressandthe undrainedshearstrengthofthesoil.Methodsforestimatingtheseparametersarediscussed.Pressuremeasurementandcomparisontothecorrectlimitingpressures,intermsoftotalstress,arecriticalfactorsinthe evaluationofhydraulicfracturepotentialandthepotentialtodamageundergroundstructures.Decisionstodrill intodamsandleveesshouldbemadeonlyafterproper evaluationoftherisk.Drillingtechniquesthatdonotrequirefluidsshouldbeusedwhenpossible.Iffluidsare required,thenpressuresshouldbelimitedtogravity.If higherpressuresmustbeused,thenthepressuresmust bemeasured,andeffortsmustbemadetokeepthembelowtheestimatedthresholdforhydraulicfracture.
Drillingorotherinvasivetechniquesthatusedrilling fluids,water,grout,air,orslurrycanpotentiallycause hydraulicfractureofembankmentdamsorlevees andtheirfoundations.Beforeconductinginvasiveprocedures,itisthedesigner’sresponsibilitytounderstandthepotentialriskassociatedwiththatprocedure. Hydraulicfracturingcanincreasetheriskofdamor
*Correspondingauthoremail:jschaefer.geo@gmail.com
leveefailurebydamagingtheimpervioussoilsections oftheembankments.Itcancauseasignificantincrease inthepotentialforconcentratedleakerosion,displacementoftheembankment,reductioninshearstrength, andadverseenvironmentaldischarges.Hydraulicfracturingcausedbydrillingcanalsoleadtotheimproper interpretationofinvestigationresults.Forexample,if allthedrillingfluidislostinaboringintheimpervioussectionofthedamcore,whatarethepossible interpretations?Isthereaverypervioussoillayerora preexistingdefect,ordidthedrillingcausethesoilto crackduetohydraulicfracturing?
Numerousincidentshaveoccurredondamsand leveeswheredrilling,grouting,andotheractivities havecausedhydraulicfracturing.Sherard(1973)documentedmanycasesofcrackinganddrillfluidloss,includingincidentsatseveralU.S.ArmyCorpsofEngineers(USACE)projects.TheincidentlistedinTable1 atWisterDamoccurredduringattemptedremediationoftheembankmentaftera1949eventwhenthe damsufferedmajorconcentratedleakerosion.Mud groutingwithonlygravitypressurealongthecenter lineofthedamwasattemptedtoaddresspotentialembankmentdefectsatunknownlocations.Afterplacing32,140cubicfeet(910m3 )ofgrout,thegrouting operationwassuspendedduetothelargenumberof cracksbeinggeneratedinthecrestofthedam(Erwin andGlenn,1992).
AspartoftheforensicinvestigationoftheTeton Damfailure(IndependentPaneltoReviewCauseof TetonDamFailure,1976),theindependentpanelperformedafieldhydraulicfracturingtestontheintact remnantembankmentsection.Thetestsweredoneto determineifhydraulicfracturewasapotentialfailure modethatcontributedtothefailure.Ironically,while drillingtheboringstodothetests,theysuddenlylost drillwaterandfracturedtheembankment.Additional attemptsweremadetodrillwithair,butreturnblockagescausedadditionalfracturing.
Numerousadditionalhydraulicfractureincidentswerefoundbytheauthorthroughvarious communicationswithUSACEDistricts.TheseincidentsaresummarizedinTable2.
Table1. SummaryofpotentialhydraulicfracturingincidentsfromSherard(1973).
DjatiluhurDamIndonesiaSuddenwaterlossincrestboringinthecore. ElIsiroDamVenezuelaCorecrackedfromfoundationrockgrouting.
GarrisonDamNorthDakotaAhighpercentageoftheholesdrilledinapartiallycompletedembankmentexhibitedamajorloss ofdrillwaterwithintheembankment.
GraminhaDamBrazilCompletedrillfluidlossduringpiezometerinstallations.
HartfordDamGeorgiaAcrestboringlocatedoveranabutmentlostallthedrillwateratadepthof20ftand(6m)then continuouslybetween20and55ft(6and17m).
IlhaSoteiraDamBrazilAboringwasdrilledtoinstallapiezometerintotheclayembankment,andwaterlossoccurredat adepthof36ft(11m).
LaVillitaDamMexicoAboringaccomplishedwithrotarydrillingintoheavybentoniteabruptlylostfluidat40ft(12m). Atthesametime,aninstrument100ft(30m)awayalongthecrestreactedtothefluidloss.
LivingstonDamTexasWaterlossesduringdrillingforpiezometerinstallationexperiencedinmorethanhalftheborings.
LovewellDamNebraskaTwenty-sixboringslostdrillfluidinimperviousembankment.
MatahinaDamNewZealandExcavationofembankmentrevealedgroutfractures.
ShekPikDamHongKongNumerousboringsexperiencedfluidlossduringthedrilling. WisterDamOklahomaCrestcracksoccurredfromgrouting.
YardsCreekUpper ReservoirDam
NewJerseyDrillwaterlostinboringsinthelowerhalfofthecore.
WhenreviewingrecordsfromtheNationalArchives forariskassessment,noteswerefoundfrom1966 recordingadiscussionwithaseniorUSACEengineer. Theengineerstatedthatineveryinstancewherethe coreofexistingdamsinTulsaandFt.WorthDistrictshadbeendrilled,drillingfluidhadbeenlost. ThisincludedSanAngelo,Toronto,CouncilGrove, andCanyonDams.
Cracking,groutleaks,excavatedgrout-filledfractures,andslurry-filledfracturesareallexamples
ofphysicalevidenceofhydraulicfracturethatoccurredasaresultofdrilling,grouting,orslurrywall construction.
Figure1showsagroutleakthatoccurredduring groutingofarockstageover200ft(61m)below theworkplatformatWolfCreekDam.
Figure2showsgroutleaksfromthesheetpileinterlocksontheworkplatformintheupper-left cornerphoto.Thevibratingwirepiezometerspike indicatedonthegraphwiththeredarrowcorrespondstotheleak.Thepiezometermeasureda
Table2. AdditionalUSACEhydraulicfracturingincidents.
DamLocationIncidentDescription
AddicksDamTexasGroutleaksfromleveeembankment/foundation.
Grout-filledhydraulicfracturesfoundinexcavationofembankment.
AmericanRiverLeveeCaliforniaHydraulicfracturefromslurrywallconstruction.
BarkerDamTexasGroutleaksfromleveeembankment/foundation.
Grout-filledhydraulicfracturesfoundinexcavationofembankment.
CenterHillDamTennesseeGroutleaksandworkplatformdeformation.
EastBranchDamPennsylvaniaGroutlossduringpiezometerinstallationinimperviousdamembankment. Completewaterlossduringdrillinginimperviousdamembankment.
GreenhavenPocketLeveeCaliforniaHydraulicfracturefromslurrywallconstruction. HartwellDamHartwellDrillfluidlossadjacenttoconcretesection. KentuckyLockKentuckyGrouthydraulicfracturesfoundinembankment.
LittlevilleDamMassachusettsWaterlossandpiezometercommunicationduringdrillingadjacenttoconduit. MosulDamIraqNumeroushydraulicfracturesignaturesingroutdata.
MudMountainDamWashingtonSlurryfromcut-offwallconstructioncausednumerousembankmentcracksandhydraulic fractures.
PatokaSaddleDamIndianaGrout-filledhydraulicfracturesfoundinembankment. RedRockDamIowaGrouthydraulicfractureduringremedialgrouting.
ThomastonDamConnecticutDrillfluidlossadjacenttoconduit.
WhittierNarrowsDamCaliforniaCompletewaterlossduringdrillinginimpervioussection. WolfCreekDamKentuckyGroutleaksthroughworkplatform,crestcracking.
pressurespikethatexceededtheelevationofthe workplatformbymorethan100ft(30m)ofhead. WhengroutingvoidsbelowtheoutletworksonAddicksDam,groutleaksexiteddownstreamandright oftheconduit.Figure3showstheleakthatoccurred duetoahydraulicfracture.
Asimilargroutingprogramwasperformedtofill voidsundertheconduitatBarkerDam.Later,duringconstructionofadownstreamconduitfilter, grout-filledcracks(Figure4)causedbyhydraulic fracturewereuncoveredintheexcavation.
Figure1.Groutleaksemergingthroughcracksintheworkplatform slabonWolfCreekDam.
Figure3.GroutleakthatoccurredatAddicksDamwhengrouting voidsundertheoutletworksconduitlocatedofftherightedgeof thephoto.
AnexcavationinanembankmentontheKentucky Lockprojectrevealedexamplesofbothhorizontal andverticalhydraulicfracturesinthesoil(Figure5a andb).Theseweretheresultofapreviousgrouting projectwherehighpressureswereusedinkarstrock groutstages.
Duringtheconstructionofadeepcut-offwallin MudMountainDam,WA,thedamembankment wasfracturedfromtheslurryusedtosupportthe excavation.Therewerenumerouslargeslurrylosses intheembankmentalongwithvisiblecrackingof thedam.Thedamwasconstructedinaverysteep narrowrockvalley,whichresultedintheembankmentsoilshavinglowconfiningstress(Figure6a). Thismadethedamsusceptibletocracking.Figure 6bshowsanexampleofalongitudinalcrackinthe workplatformatthecrestofthedam,andFigure 6cshowsacrackinthesideofacut-offwallpanel excavation.SeeDavidsonetal.(1992)formore details.
WhenconstructingaslurrywallinapoorlycompactedembankmentontheGreenPocketLevee, ahydraulicfractureoccurred,resultinginalarge crackandasignificantslurryloss.Thecrackinitiatedattheexcavationandpassedunderandbehind theexcavatoralongthecrest,anditthenturnedand exitedatthetoeofthelevee(Figure7a).Figure7b showsanexamplephotooftheslurry-filledcrack foundinanexcavationoftheembankment.
AsimilarincidentoccurredontheAmericanRiver Leveeduringconstructionofaslurrywall.The poorlycompactedleveecouldnotsupportthe slurry,andahydraulicfractureoccurred.Figure8a isasurfaceexpressionofthefracture,andFigure8b
showstheslurry-filledfracturefoundinanexcavationoftheembankment.
Aspartofaresearchproject,USACE(1998)WaterwaysExperimentStationevaluateddrillfluidpressuresandthepotentialforhorizontaldirectional
Figure6.(continued)
drilling(HDD)tocausehydraulicfracture.The groutleaksandfractureswereeasilyidentifiedwith thered-dye-coloreddrillfluid(Figure9).
Duetothesignificantnumberofknownincidents, wecanconcludethatitisnotrareforhydraulic fracturestooccurfromintroductionoffluidsinto embankmentsandtheirfoundationsduringdrilling, grouting,orunbalancedslurryconstruction.Intheauthor’sopinion,itisalsolikelythattherearemanymore casesthathavenotbeenreportedduetoamisinterpretationofdrillfluidlossorexcessivegrouttakes.
Severalmechanismsforhydraulicfractureinsoils havebeenproposedbydifferentresearchers.Aliteraturereviewrevealedthattherearemanydifferentconceptsandmodelswithwhichtoestimatedamaging fracturepressuresinsoils,including tensilestress, undrainedshearstrength, unconfinedcompressionstrength,
Figure7.(a)Sketchshowingthelocationofahydraulicfracture thatoccurredontheGreenPocketLevee.(b)Photographshowing aslurry-filledhydraulicfracturefoundinanexcavationoftheembankment.
elastictheory, linearelasticfracturemechanics, empiricalformulas,and cavityexpansion.
Thesemodelsrangefrompracticaltoverytheoretical.Inthisstudy,itwasdesiredtodetermineifthere wasapracticalmodelthatwouldgivereasonableresults.Sherard(1973,p.273),statedthat,“Fromapracticalstandpoint,acrackmaybecausedtoopenona givenplaneiftheeffectivestressactingontheplane goestozero;thatis,ifthetotalstressontheplaneis equaltoorlessthanthewaterpressure(providedthe soilcannotwithstandtensilestress).”
Documentationintheliteraturerelatedtofieldhydraulicfracturetesting(Bozozuk,1974;Calcagno, 1983;ChenandZhang,1989;Hamoucheetal., 1995;andSherard,1973)revealedthatthepressurerequiredtoinitiatehydraulicfracturewasalwayssomewhatgreaterthantheminorprincipal stress.Thefieldtestresultsshowedthatthefracturepressurewasapproximately300to3,000psf
(14to144kPa)greaterthantheestimatedminorprincipalstress,whichsupportstheideathat thesoilstrengthcontributestothefracturepressure resistance.
TheapproachtakenbyAndersenetal.(1994)was basedonthegeneralprinciplethathydraulicfracturingcanoccurifthegroutpressureexceedstheminorprincipalstressplusthetensilestrengthofthe soil.ThiswassupportedbyastudybyAlfaroand Wong(2001),whereinthevariousconceptswereinvestigatedandcomparedtolaboratorytesting,and thetensilestrengthapproachbestapproximatedlaboratorytestsforhydraulicfracture.AdditionalbackgroundonhydraulicfracturingcanbefoundinSherard(1970),Sherardetal.(1972),Sherard(1986),and Bjerrumetal.(1972).
Becausetensilestrengthisnotcommonlydeterminedforsoils,itisproposedthattheundrained strengthofthesoilcanbeusedtoreasonablyapproximatethetensilecapacityofthesoil.Hydraulicfracture ofclaysoccursinanundrainedloadingconditionbecausethereisaquickloadactingonanimpervious soil.Severalresearchershavepublisheddatafromlaboratoryhydraulicfractureteststhatalsoincludedinformationontheundrainedstrengthofthesoil.The author’sevaluationofthemeasureddatafoundinthe literaturereviewrevealedthattheboreholefracture pressurecancloselyapproximatethemeasuredfracturepressureinthelaboratoryusingtheminimum principalstressplustheundrainedstrengthofthesoil intermsoftotalstress. P
which,fornormallyconsolidatedsoils,wouldreduce to:
Ifenoughdatawereprovidedinthepapers,theresultswerereplottedandoverlainwiththeproposed predictionfromEq.1.Figures10and11showthatthe predictionfromEq.1agreeswellwiththelaboratory
hydraulicfracturetestsandhighlightstheimportance oftheundrainedshearstrengthofthesoil.
TheHDDindustrycommonlyusestheDelftCavity ExpansionEquation.Thecavityexpansiontheorywas firstdevelopedbyVesic(1972)andthenfurtherdevelopedforuseinHDDprojectsbyresearchersatDelft Geotechnics,LugerandHergarden(1988).Thetheory isthatastheannularfluidpressureincreases,theboreholeradiuswillexpand.Initially,thedeformationwill beelastic,butasthepressureincreases,thedeformationwillbecomeplastic.Asthezoneofplasticdeformationincreasestothegroundsurface,blowoutwill occur,anddrillingfluidwillflowtothesurface,creatinga“fracout”or“inadvertentreturn.”USACEguidancedocumentEM1110-2-2902—Conduits,Pipes, andCulvertsAssociatedwithDamsandLeveeSystems(USACE,2020),describesseveralissueswiththis equationandapproach.Theprimaryissuesare(1)the equationisusedtolimitpressuretopreventdrilling fluiddischargeonthesurfaceratherthanpreventing theinitiationofhydraulicfractureand(2)thethe-
oryisbasedonisotropicstressconditions,whichalmostneverexistinrealsoils.Fordamandleveesafety, weareconcernedwithpreventingdamage,notjust fluidreleaseonthesurface.However,forclays,ifwe simplifytheequationbyassumingundrainednormally consolidatedsoilconditionsandtheminorprincipal stressisusedinsteadoftheisotropicstress,theDelft equationreducesdowntobeequivalenttoEq.2given above.
TheapproachusedbyUSACEinER1110-1-1807— DrillinginEarthEmbankmentDamsandLevees(USACE,2014)shouldbeimplementedwithintheprofessionalpracticeandfollowedwheneverdrillingorother invasivetechniquesareusedondamorlevees.Ifpossible,theuseofdrillingfluidsshouldbeavoidedin orneardamorleveeembankments.Ifdrillingfluids mustbeusedduetothedrillingobjectiveorthesubsurfaceconditions,theDrillingProgramPlan(DPP) mustcontainananalysiswithcalculationsforthepotentialtocausedamageandthemeasuresthatwillbe usedtominimizerisk.ForUSACEdams/levees,all DPPsthatproposetheuseoffluidsmustbereviewed bytheStandingCommitteeonDrillingandInstrumentation,ledbytheUSACEGeotechnical,Geology, andMaterialsCommunityofPracticeLeadandapprovedbytheDamSafetyOfficerortheLeveeSafety Officerattherelevantdistrict.Thetotalminorprincipalstressplustheundrainedstrengthofthesoilshould beusedtoestimatethehydraulicfracturepressure. TheUSACEregulationdoesnotrecommendafactorofsafety.Designersshouldselectanappropriate factorofsafetybasedontheimportanceofthestructureandlevelofuncertaintyintheanalysis.Itwould bereasonabletouseafactorofsafetyinthe1.3to 1.5range.
ThefirstparameterinEq.1andEq.2isthetotalminorprincipalstress.Todeterminethisforflatground conditionswithnormallyconsolidatedsoils,thesteps are:
1.Determinethesoilunitweights.
2.Estimatetheporepressureusingseepagemodeling ormeasureddatafrompiezometers.
3.Calculatetheverticaleffectivestress.
4.Estimate k0 byusingtheequationinJaky(1944), whichgivestherelationshipbetweenthemajorand minoreffectiveprincipalstressesas
However,therealconditionsfordamsandmanyleveesaremuchmorecomplicated.Theconfiningstresses cannotbeaccuratelyestimatedbysimplecalculations duetothefollowingissues:
Embankmentsalwayshaveslopedgroundconditions.
Majorandminorstressesarenotalwaysverticaland horizontal.
Theupperzonesoftheembankmentmaybeoverconsolidatedduetolocked-incompactionstresses, whilethelowerzoneswillbenormallyconsolidated becausethesoilweightisgreaterthanthecompactionstresses.
Thephreaticsurfacecanvarysignificantlyacrossthe coreofadam.
Irregularrocksurfaces,pinnaclesandslots,steep abutments,overhangs,conduits,concretedamcontacts,anddifferentialsettlementwillallcauselocalizedlowconfiningstressconditions.
SeeSherard(1986)foragooddiscussiononthespecialconsiderationsforlow-stresszonesthatcontribute tohydraulicfracturing.
Withmodernnumericalanalysistoolssuchas SIGMA/W,FLAC,andPLAXIS,atwo-dimensional modelcanbedevelopedtoestimatethestaticstresses inanembankmentdamatthelocationoftheproposed drilling.Inspecialcaseswithcomplexgeometry,threedimensionalmodelsmaybeappropriate.Thiswilltypicallyrequireevaluatingdifferentsections,including bothlongitudinalandtransversesections.Thegeometryofthemodelshouldbebasedonconstruction recordsandanyavailableboringdata,sothefoundationgeometryandembeddedstructuralfeatures,such asconduits,areproperlyincluded.Multiplepoolelevationsandpiezometricconditionsmayneedtobe consideredbasedonthepotentialforpoolfluctuations.Alinearelasticsoilstrengthmodelcanbeused. Theunitweightofthematerials,anestimateofPoisson’sratio,andcharacterizationofgroundwaterconditionsaretheprimaryvariablesrequired.Poisson’sratio isusedinthemodeltodeterminetheratioofhorizontaltoverticalstress(k0 ).
Oncethemodelisconstructed,itisloadedwithselfweight(gravity),andstressesarecalculated.Contour plotsofdepthversusstressprofilesofthetotalminimumstressescanbedeveloped.Thisanalysiswillprovideanapproximateestimateofthestressesinthedam
andfoundation.Moredetailedevaluationcanbedone usingmultistagesequencemodelingandmorerigorous materialmodels.SeeMcCookandGrotrian(2010)for proceduresformorerigorousmodelingforhydraulic fracture.Multiplesimplifiedrunscanbeperformed parametricallybyvaryingtheinputparameterstoestimatetherangeofstresses.
Itispossibletomeasure insitu stressusingtools suchastheself-boringpressuremeter(Benoit,1995).
Theself-boringpressuremeterisconsideredtobethe mostreliablemethodformeasurementoflateralstress insoils.Thepressuremeterisplacedinthebottomof aboringandadvancedusingarecessedcuttingbit.
Afteradvancementtothedesireddepth,testingisdelayedtoallowforthedissipationofporepressure.
Themembraneisthenpressurized,andtotalpressure
versuscavitystrainismeasured.Thetotalhorizontal stressisdeterminedateachstrainarmbyinspectionof thetotalpressureversuscavitystrainplots.Thepressureatwhichthemembraneliftsoff(beginsdeforming thesoil)isequaltothe insitu totalhorizontalstress. Schmertmann(1985)providedadditionalinformation on insitu lateralstressandmethodsformeasurement. Measurementof insitu stresscanreallyonlybeused asaconfirmationoftheestimatedstresses,sincea boringusingdrillfluidmustbeperformedtodothe measurement.
Cautionshouldbeusedbeforedrillinganyboringsnexttoconduitsorsteeprockabutments.Numerousincidentsofdrillfluidlossadjacenttoconduits
orabutmentshaveoccurred.Stressesaroundconduits canbeverylow.Itispossibletohavetensilestresses adjacenttoconduitsduetoarchingeffects,asdemonstratedinaresearchreportbyCasagrandeandCovarrubias(1970)fortheUSACEWaterwaysExperiment Station.Iftensilezonesarepresent,thentheuseofany drillingfluidwilllikelycausehydraulicfracture.Figure 12showsacommonconditionwhereoutletworksconduitswereconstructednearasteeprockabutment.In theareamarkedbytheredarrowinFigure12,arching ofthesoilswilloccur,andtheconfiningstressesinthe areabetweentherockabutmentandtheconduitwill beverylow.
ForthesecondpartofEq.1andEq.2,the undrainedstrengthofthesoilisrequired.Thiscanbe determinedfrompreviouslaboratoryorfieldtestingor correlationstoindexproperties,oritcanbeestimated usingcriticalsoilmechanics.Aconservativeestimate canbeobtainedbyassumingtheembankmentdamor leveeisnormallyconsolidated,andEq.2canbeused.
Thisassumptionisvalidinmostcases,exceptin theuppersoilzones,wherethelocked-incompaction stressesaregreaterthantheoverburdenstress,andthe undrainedstrengthwillbemorethanestimated.Ifno previoustestingforundrainedstrengthisavailable,for normallyconsolidatedclays,theundrainedstrength ratiototheverticaleffectivestresscantypicallybeestimatedas ½ sin φ orintherangeof0.22to0.25. Equation5canthenbeusedtoestimatetheundrained strength.
Figure13showsdamagetoabottomoutlettunnel linercausedbyexcessivegroutpressureswhenrefusal pressureadjacenttothetunneldidnotconsiderthetotalpressure.
Whencomparingestimatedfracturepressuresto pressuresinthefield,therelevantvaluetouseisthe totalpressure.Thetotalfluidpressureistheinitial porepressureplustheadditionalpressureappliedby thecolumnoffluidorgrout.Pressuremeasureddown theholeinthefieldwillbethetotalpressure.Groutingprojectstypicallyuseeffectivepressurestomonitor groutingprograms.Thisisappropriatefordeterminingtheeffectivenessofthegrouting.Inorderforthe grouttobepushedintosubmergedrockjoints,theinjectionpressuremustfirstovercomethe insitu groundwaterpressure.Theadditionalpressureistheeffectivepressure.However,totalpressureshouldbeused toevaluatehydraulicfracture,andeffectivepressure shouldbeusedtoevaluategroutingeffectiveness.Also, whendeterminingtheloadappliedtounderground structures,thetotalpressureshouldbeevaluated.
Onewaytoimproveourabilitytomonitorpressuresiswiththeuseofinstrumentedpackers,which candirectlymeasurethetotalgroutorwaterpressure directlyatthetopofthegroutorwaterpressuretest stage.Detailsofaninstrumentedpackerdevelopedby ACTforuseontheRoughRiverDamprojectcanbe foundinIvanovetal.(2017).Itisalsocommonfor HDDdrillingequipmenttohavepressuresensorsat thecuttingheadtodirectlymeasurethedrillfluidpressure.Forstandardgeotechnicaldrilling,directmeasurementoffluidpressuresinnotcommonlydone.If pressurizedfluidsareused,amethodtomeasurethe appliedpressureatthedrillrigshouldbeused,which willneedtobeaddedtothecalculationofthestatic headofthedrillfluid.Oncethethresholdpressures
Figure14.Simplifiedexampleofhydraulicfracturepressurecalculation.
Column1:DepthofCalculation.
Column2:MajorTotalStress = Column1 × γsoil .
Column3:PorePressure = (Column1 DepthtoWater) × γwater ifpositive,else0.
Column4:MajorEffectiveStress = Column2 Column3.
Column5:MinorEffectiveStress = Column4 × k0 .
Column6:MinorTotalStress = Column5 + Column3.
Column7:UndrainedStrength = Column4 × 0.5 × sin φ
Column8:FracturePressure = Column6 + Column7.
Column9:GroutFluidPressure = Column1 × γgrout .
Column10:FactorofSafety = Column8/Column9.
arecalculated,apressurereliefvalvecouldbeusedto protectagainstpressurespikes,whichcouldhappenif cloggingoccursinthedrillfluidreturnpath.
Figure14showsasimplifiedexampleoffracture pressurecalculationsforgravitygroutingonaflat groundsite.Thisisforillustrativepurposesonlyand doesnotapplytoconditionsthatarefoundinand underdamsandlevees.Theboringisplannedtobe drilledinasingleclaysoillayertoadepthof100ft (30m).Calculationswillbedonefor5ft(1.5m)in-
crementsofdepth.Thesoilunitweightis120pcf(18.9 kN/m3 ).Thegroutunitweightis80pcf(12.5kN/m3 ). Theinitialwatertableisatadepthof50ft(15m).The drainedstrengthisassumedtobe φ = 30degrees.The k0 valueiscalculatedusingEq.3.
Notethatthelowestfactorofsafetyisnotatthebottomoftheboring.Groundwaterhasasignificantinfluenceonthefracturepressures.Practitionersshould considersettingupthecalculationsforsmallincrementsofdepthandincludeallsoillayerspresenton thesite.Fordamsandleveeswherenumericalmodelingisusedtoestimatethestresses,valuesoftotalminorprincipalstressversusdepthcanbeoutputfrom
HydraulicFracturePotentialduringDrillinginDams
themodelingsoftwareandusedinasimilarspreadsheettocalculateprofilesforfracturepressure,drill fluidorslurrypressure,andfactorofsafetyversus depthforeachproposedboringorslurry-supported excavation.
1.Numeroushydraulicfractureincidentshaveoccurredondamsandlevees.
2.Holesshouldnotbedrilledindamsorleveesunless thereisawell-reasonedcasethatjustifiestherisk.
3.Aplanshouldbedraftedtomanagetheriskinthe bestwaypossibleto“donoharm.”
4.Riskcanbeminimizedthroughcarefulplanningof boringlocation/samplesites.
5.Theuseofsonicdrillingorhollowstemaugers shouldbeconsideredtoavoidtheriskofdrilling withfluids.
6.Iffluidsareused,onlygravitypressureshouldbe used,ifpossible.
7.Ifpressurizedfluidsmustbeused,thenahydraulic fractureanalysismustbeperformedtodetermine theestimatedfracturepressure,estimatedfluid pressurefromdrillingactivity,andestimatedfactor ofsafety.
8.Pressuresshouldbemonitoredandkeptbelowthresholdsusinganappropriatefactorof safety.
9.Ifpossible,apressurereliefvalveshouldbeused toprotectagainstspikesinpressurecausedbydrill fluidreturnclogging.
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Erwin,E.D.andGlenn,J.M.,1992, ASTMSTP1129—Plastic ConcreteSlurryWallforWisterDam,SlurryWalls:Design ConstructionandQualityControl:AmericanSocietyforTestingandMaterials,Philadelphia,PA.
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Ivanov,D.;Horvath,D.;andHeenan,D.,2017,Advancesin measuringgroutingpressuresusinganinstrumentedpacker. InByle,J.,Johnson,L.,Bruce,D.,ElMohtar,C.,Gazzarrini, P.,andRichards,T.(Editors), Grouting2017:AmericanSocietyofCivilEngineers,Honolulu,HI,pp.313–322.
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Seed,H.B.andDuncan,J.M.,1981,TheTetonDam—Aretrospectivereview.In ProceedingsoftheTenthInternationalConferenceonSoilMechanicsandFoundationEngineering,Vol.3: Stockholm,Sweden,pp.219–238.
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KATHLEENBENSKO
FederalEnergyRegulatoryCommission,OfficeofEnergyProjects,DivisionofDam SafetyandInspections,888FirstStreet,N.E.,Washington,DC20426
BRYANK.SIMPSON
U.S.BureauofReclamation,GeotechnicalServicesDivision,EngineeringGeologyand Geophysics,DenverFederalCenter,POBox25007(86-68320),Denver,CO 80225-0007
U.S.ArmyCorpsofEngineers,RiskManagementCenter,12596W.BayaudAvenue, Suite400,Lakewood,CO80228
SCOTTR.WALKER*
TennesseeValleyAuthority,DamSafetyGovernance&Oversight,1101MarketStreet, LP1F-C,Chattanooga,TN37402
KeyTerms: DamSafety,LeveeSafety,Geotechnical SiteInvestigations,DrillingProgramPlans,Drilling Guidance,FederalAgency
Management,regulation,andreviewoftheplanning andexecutionofintrusivesiteinvestigationsvarywithin eachoffourfederalorganizationsinvolvedwithdam andleveesafetybaseduponvariousrolesandresponsibilities.TheU.S.BureauofReclamation(USBR),the TennesseeValleyAuthority,andtheU.S.ArmyCorps ofEngineers(USACE)areowner-operatorsofdams thatservemultiplepurposes,includingfloodcontrol,hydropower,watersupply,andnavigation.TheFederalEnergyRegulatoryCommission(FERC)regulatesandinspectsnon-federalhydropowerdams,anddevelopsand implementspolicies,programs,andstandardstoensure thesafetyoftheselicensedprojects.Eachorganization (exceptFERC)isalsoresponsibleforoversight,rehabilitation,andrepairofstructuresincludingdamaged levees,canals,anddams,includingsome(inthecaseof USBRandUSACE)thatareoperatedandmaintained byothers.Thispaperprovidesabriefoverviewofthe variousregulationsandguidancerelatedtotheplanning andexecutionofintrusivesiteinvestigationsconducted atstructureswithineachorganization’sportfolio.Italso
*Correspondingauthoremail:srwalker3@tva.gov
providesaprogrammaticoverviewoftheprocessesutilizedbythedifferentfederalorganizationsforplanning, reviewing,andimplementingintrusiveinvestigations.It coversspecificrequirements(guidanceandregulations), timeframesthatareinvolvedwiththereviewprocess, anddiscussesupdatesandchangesthatareplannedor beingconsidered.
Thepurposeofthepaperistoexplainhowthefour majorfederalorganizationswithdamandleveesafety programs—theU.S.BureauofReclamation(USBR), U.S.ArmyCorpsofEngineers(USACE),Tennessee ValleyAuthority(TVA),andFederalEnergyRegulatoryCommission(FERC)—planandimplement intrusivesiteinvestigationsforhydraulicstructures. Eachorganizationhasdifferentprimarypurposesand rolesrelatedtodamsandleveeswhichaffectwhyand howsiteinvestigationsforeachareperformedand implemented.
Topicsdiscussedincludethefollowing:
Asummaryofeachorganization’srolesandresponsibilitiesrelatedtodamandleveesafety,including thepurposesoftheirdamsandlevees,regulatory position,emergencyandpost-emergencyworkfor others,andworkforotheroutsideentitiesrelatedto damandleveesafety
Adescriptionofthebasicsiteinvestigationprocessestheorganizationsallfollowandhowriskassessmentsareutilizedininvestigationplanning Abriefdescriptionofhoweachorganizationplans andexecutesintrusivesiteinvestigations,including adiscussionofthevariousregulationsandguidance documentseachorganizationfollows Contracting,communications,levelofinvolvement, andoversightoftheintrusivesiteinvestigation process
DiscussiongenerallyfollowstheorderofUSBR, USACE,TVA,andFERC.Thisreflectsaprogression fromfewertomoreentitiesinvolved,andfromanalmostfullyinternalprogram(USBR)toamixedinternal/externalprogram(USACE)toowneroversightof theprocess(TVA)toregulatoroftheprocess(FERC).
Notalldamandleveesafety-relatedaspectsthatare importanttositeinvestigationprogramsarediscussed. Additionalrequirementscouldincludepreparedsubmittalssuchashealthandsafetyplans(HASPs), qualityassuranceandqualitycontrol(QA/QC)processes,TemporaryConstructionEmergencyAction Plans(TCEAPs),siteaccess,realestateissues,utility clearances,tribalconsultations,NationalEnvironmentalPolicyAct(NEPA)and/orStateHistoricPreservationOffice(SHPO)reviewsorrestrictions,andother archeological,biological,orhistoricalevaluations.
USBR
Establishedin1902,theUSBR(alsoreferredtoas Reclamation)isbestknownforthedams,powerplants, andcanalsitconstructedinthe17westernstates(see Figure1).Thesewaterprojectsledtohomesteading andpromotedtheeconomicdevelopmentoftheWest. Reclamationhasconstructedmorethan600damsand reservoirsincludingHooverDamontheColorado RiverandGrandCouleeDamontheColumbiaRiver.
Reclamationisalsothesecondlargestproducerof hydroelectricpowerintheUnitedStates.USBR’s53 powerplantsannuallyprovidemorethan40billion kilowatthours,generatenearly$1billioninpowerrevenue,andproduceenoughelectricitytoserve3.5millionhomes.Today,theUSBRisacontemporarywatermanagementagencywithastrategicplanoutlining numerousprograms,initiatives,andactivitiestohelp thewesternstates,NativeAmericantribes,andothers balanceamultitudeofcompetingusesforwater.The missionistoassistinmeetingtheincreasingwaterdemandsofthesestatesandentitieswhileprotectingthe environmentandthepublic’sinvestmentinReclamation’sinfrastructure.
Reclamationplacesgreatemphasisonfulfillingwaterdeliveryobligations,waterconservation,waterrecyclingandreuse,anddevelopingpartnershipswith customers,states,andNativeAmericantribes,and findingwaystobringtogetherthevarietyofintereststoaddresscompetingneedsforlimitedwater resources.
In1853,USACEbeganworkontheWashington Aqueduct,whichsupplieswatertotheDistrictof Columbia(WarDept,1939)andisstilloperatedby USACE.Subsequently,USACEbecameinvolvedin floodcontrolandnavigationalongtheMississippi Riverin1879withthecreationoftheMississippi RiverCommission(StatutesatLarge,1881).In1885 USACEcompletedthreereservoirsinMinnesotanear theheadwatersoftheMississippiRiverforflood controlandnavigationpurposes(USACE,1885and 2020a).TheRansdell–HumphreysFloodControlAct of1917(StatutesatLarge,1917)wasthefirstfederal FloodControlActpassedbyCongress,andthenthe FloodControlActof1936(StatutesatLarge,1936a) madefloodcontrolafederalpolicyandofficiallyrecognizedUSACEasthemajorfederalfloodcontrol agencywithofficesandprojectsacrossthecountryand overseasterritories(Figure2).
USACEself-regulatesitsdamsandleveesandinspectsandpartiallyregulatesothersunderaprogram authorizedbyPublicLaw(PL)84-99(Statutesat Large,1955andU.S.C.,2020c).USACEisthelargest producerofhydropowerintheU.S.asmeasuredby hydroelectricgenerationcapacityinmegawatts.Additionally,USACEprovidesfloodfightingassistance andaidstheFederalEmergencyManagementAgency (FEMA)indisasterrecoveryefforts.USACEmanagestheNationalInventoryofDamsandtheNational LeveeDatabaseandisworkingwithFEMAonthe NationalLeveeSafetyProgram.
TVAisacorporateagencyandinstrumentalityof theUnitedStates,createdonMay18,1933,when CongresspassedtheTennesseeValleyAuthorityAct of1933(TVAAct;StatutesatLarge,1934andU.S.C., 2020b)inresponsetoPresidentFranklinDelanoRoosevelt’svisionfor,“acorporationclothedwiththe powerofgovernmentbutpossessedoftheflexibilityandinitiativeofaprivateenterprise”(Roosevelt, 1938).Incontrasttoauthorizationsforotherfederal agencies,theTVAActreferstoTVAas“theCorporation”anddescribesitspurposesasimprovingnavigabilityandprovidingfloodcontrolontheTennessee
River,initiatingenvironmentalimprovements(reforestationandimprovingfarmingpractices),stimulatingeconomicdevelopmentwithintheTennesseeValley,andprovidingfornationaldefense.
Asthenation’slargestpublicpowerproducer,TVA utilizesmultiplegenerationsources,includingnuclear, naturalgas,coal,andrenewables.Hydroelectricrepresentsabout10percentofTVA’soverallgenerationmix, andtwenty-nineofTVA’sriverdamprojects(along withtheRaccoonMountainPumped-Storagefacility) generateelectricity.TVAceasedreceivingfederalappropriationsin1999andmadeitsfinalscheduledrepaymentonCongress’original$1billioninvestmentin theTVApowersystemin2014.TodayTVAisfullyselffunded,withnearlyallrevenuederivedfrompower sales.WhilethisfiscalmodeldifferentiatesTVAfrom USBRandUSACE,akeysimilarityisthatTVA’sDam SafetyProgram(DSP),likethoseofotherfederaldam owners,isself-regulated.
TVAoperateswithinapowerserviceareathat coverspartsofsevenstates,asshowninFigure3. AllbutoneofTVA’sriverdamsarelocatedwithin thedrainagebasinoftheTennesseeRiver,whichis alsodepictedinFigure3.TVA’scurrentriverdam
inventoryincludes49projectscomprisedof88individualstructures(includingsaddledams,dikes,etc.). TVAconstructed42riverdamprojects(80totalstructures),purchasedsixotherprojects(sevenstructures), andacquiredoneprojectfromUSACEaspartofthe TVAAct.
Inadditiontotheriverdams,TVA’sDSPoversees ninewaterretentionstructuresconstructedtosupport ongoingfossilandnucleargeneration,sixleveesoriginallyconstructedforvectorcontrolandwhichnow providerecreationandwildlifehabitatbenefits,and onehistoricgristmilldamsituatedonlandTVAacquiredaspartofaflowageeasement.Allnon-river damsareclassifiedaslow-hazardpotentialstructures. AseparateregulatoryprogramatTVAprovidesgovernanceandoversightofcoalashandcoalwashfinesimpoundmentsatactiveanddecommissionedfossilgenerationsites.
FERC’soriginbeganwiththepassageoftheFederalWaterPowerActof1920(FWPA;Statutesat Large,1921)whichprovidedthefederalgovernment
withmorecomprehensivecontrolofthenation’snavigablewaterresources.In1935theFWPAbecame PartIoftheFederalPowerAct(U.S.C.,2020a),underwhichtheFederalPowerCommissionevolvedinto anindependentregulatoryagency(McKernan,1950). In1977,asaresultoftheDepartmentofEnergyOrganizationAct(StatutesatLarge,1980),theFederal PowerCommissionwasre-designatedastheFederal EnergyRegulatoryCommission.Sincetheenactment oftheFederalPowerAct,CongresshasassignedexpandedresponsibilitiestoFERCthroughvariouslaws toincludeproposalreviewofinterstatenaturalgas pipelines,storageprojects,liquifiednaturalgasterminals,responsibilitiesrelatingtobulk-powersystemreliabilityandcybersecurity,andlicensesfornon-federal hydropowerprojects.
FERC’soversightofnon-federalhydropowerdevelopmentishandledbythreeofsixdivisionswithin theOfficeofEnergyProjects.TheDivisionofDam
SafetyandInspections(D2SI)executestheCommission’sDamSafetyandPublicSafetyProgramsbyimplementingpolicies,programs,andstandardsfordam safetytoensurethatjurisdictionalprojectsareinspectedandevaluatedintheirdesign,construction, operation,maintenance,andsecurityphasestoprotectlife,health,property,andtheenvironment.Project compliancetothetermsandconditionsofthelicensesandexemptionsregardingdamsafetyarealso assessed.D2SI’soversightroleisexplainedintheCode ofFederalRegulations(CFR;CFR,2020).D2SIis comprisedoffiveregionalofficesasshowninFigure4, andaHeadquartersofficeinWashington,D.C.,totalingabout130personnel.
FERCregulatesover2,500hydropower-producing projectswithintheUnitedStates.Federallyowned hydropower-producingprojectsdonotfallwithin FERC’sjurisdiction;however,FERCdoesregulate privatelydevelopedhydropowerfacilitieslocatedat
USACE-andUSBR-ownedprojects.(Therearecurrentlynoprivatelydevelopedhydropowerprojectsat TVAfacilities.)TheprojectswithinFERC’spurview varygreatlyintype,size,design,age,purpose,andoperation.Thenumberoflicensedprojectsandlicensees resultsinaratherdynamicportfoliobecauseitfrequentlychangesasnewprojectsarelicensedandother licensesaresurrendered.Figure5showsthedistributionofbothFERC-regulatedandfederalhydropower projects.
Figures6through8summarize(asapplicable),the inventory,structurepurposes,roles,anddegreeto whichdamorleveesafetyriskisappliedtoeach organization’soveralldamsafetyprogramandintrusivesiteinvestigations.Thenumberofregulated structuresforUSBRandUSACEreferstoadditionalfacilitiestheyregulate.Emptycellsindicate not applicable.Collectively,thesefiguresprovideasummaryofthesimilaritiesanddifferencesbetweeneach organization.
Asummaryoftheorganizationalrolesshownin Figure8isasfollows:
Self-governing–organizationregulatesitsownactionsrelatedtodamorleveesafety.
Regulator–regulatesallaspectsofdamorlevee safety.TheFERCregulatoryroleislimitedtothose structuresthatarepartofnon-federalhydropowerproducingfacilities(however,thefacilitiesmaybe locatedatfederalprojects).Pertinentstructuresrelatingtothesefacilitiesareincluded,suchassaddledams,etc.USACE’sregulatoryroleislimitedto leveesthatwerebuiltbyUSACEandturnedover toalocalsponsortooperateandmaintain,structuresforwhichanentityhaselectedtoparticipate inthePL84-99program(whichprovidesreimbursementforspecificdamagestoleveesthatresultfrom high-waterevents),andreviewofrequestsbynonUSACEentitiesforpermitstomodify,alter,oroccupyanyexistingUSACE-constructedpublicworks projectorassociatedlandsthatwouldotherwise beprohibitedby33U.S.C.§408(U.S.C.,2020d). (ThisstatuteiscommonlyreferredtoasSection 408,andtheassociatedauthorizationsareknownas 408permits.)
Emergencyassistance–aidsduringfloodemergenciestolocalsponsorsuponrequest.
Repairandrehabilitation–forstructuresthatare partofthePL84-99programsrelatedtoleveesand limiteddamsthatwerebuiltbyUSACEandarenow operatedandmaintainedbyalocalsponsor.USACEalsoactsasFEMA’sengineeringandcontractingbranchwhenrespondingtonationallydeclared disasterssuchashurricanesandfloods.
USBR,USACE,TVA,andFERCaretransitioningtowardfullyrisk-informedmanagementoftheir respectivedamandleveesafetyprograms.However, internaldesignandguidancestandards,externalregulations,treatyobligations,andcontractsdocontinue tocontrolsomeaspectsoftheprograms(tovarying degrees)andsiteinvestigationsisoneareawhereriskinformedprocesseshavebeenslowertoimplement.
USBRisthefurthestalongintheprocess,withthe majorityofitsdecision-making,design,andsiteinvestigationsinformedbyrisk.TVAandFERCaretransitioningtorisk-informedprocessesformanagementof theirfacilities,includingsiteinvestigations,butboth alsohavestandards-basedcriteriaandrequirements thatmustbefollowed.
USACEisusingrisktomanagedamandleveeportfoliosandtoinformdesigns;however,forsiteinvestigations,integrationofriskvariesbetweenthedam andleveeprograms.USACEtypicallyavoidsdrilling inorneardams(particularlyearthembankments)exceptinsupportofdesigns,aspartofissueevaluation studies(IESs)followingriskassessments,ortorepair orreplaceinstrumentationorreliefwells.Leveesfully ownedandoperatedbyUSACEaregenerallytreated similarlytothedamswhenitcomestointernalinitiationofsubsurfaceinvestigationprograms.However, unlikedamsthatoccupycomparativelysmallfootprints,leveesarelongstructuresthatoftenparallelmajorriversformanymiles.Thisgeometryandgeography commonlyplacetheminconflictwithtransportation andutilitycorridorsandmakethemattractiveaslocationsforparksorsimilaropenspace.Entitiesexternal toUSACEthatwishtoplacestructuresin,on,orunderUSACE-ownedleveesutilizetheSection408permitprocess(whichdoesnothaveariskcomponent)to requestpermissiontoconductsubsurfaceinvestigation insupportoftheirneeds(e.g.,designofbridges,directionalboresforpipelines,utilitytowers,picnicsheltersandoutbuildings).TheSection408processalso appliestodrillinginleveesthatareownedandoper-
USBRandUSACEjointlyoperateseveralfacilities, withtheUSBRtypicallymakingday-to-dayoperationaldecisionsrelatedtowaterandhydropower,while USACEtypicallymakesdecisionsrelatedtofloodcontrol.Theentitythatcontrolsthemainoperationisconsideredtheleadorganizationanddetermineswhich ofthetwowillleadindamsafetypractices.ExamplesoftheseincludetheFolsomDamjointuseproject andtheGarrisonDamproject’sSnakeCreekEmbankment,whichseparatesLakeAudubon(whichis controlledbyUSBRforirrigationwatersupply)from LakeSakakawea(whichiscontrolledbyUSACEprimarilyforfloodcontrol).
TVA’sportfolioincludes10damswithnavigation facilities—nineontheTennesseeRiverandoneon theClinchRiver.Ateachfacility,TVAownstheinfrastructureandisresponsibleformaintenanceofthe dam,whileUSACEisresponsibleforoperationand maintenanceofthenavigationlocks.TVAmanages reservoirlevelsanddischargesacrossthesystemto supportnavigation.USACEdesignedandisnowcontractingforandmanagingconstructionofnewnavigationlocksattheKentuckyandChickamaugaprojects becauseUSACEcanreceivethefederalappropriations necessarytofundtheprojects.Additionally,thenorthernendoftheTennessee–TombigbeeWaterway(which isoperatedandmaintainedbyUSACE)is8.5miles upstreamfromTVA’sPickwickLandingDam.WaterfromPickwickLakeisreleasedwhenvesselslock throughJamieWhittenLockandDaminMississippi.
TVAalsocoordinatescloselywiththeUSACEdistrictofficesinNashvilleandCincinnatiforroutineand floodoperationsontheTennesseeandCumberland
Rivers.Thelowermostreservoirsoneachriver(KentuckyLakeandLakeBarkley,respectively)arehydraulicallyconnectedbyanavigationcanal.TVA runsthehydraulicmodelforthecombinedsystem andschedulesnormaloperationdischargesfromboth dams.TVAalsoscheduleshydroelectricgeneration fromtheotherUSACEdamsupstreamontheCumberlandandprovidesUSACEwithdischargesfrom
GreatFallsDam(whichistheonlyTVAriverdam intheCumberlandwatershed).Duringfloodevents ontheOhioRiver,USACEschedulesdischargefrom KentuckyandBarkleyDamstoreduceimpactstothe lowerOhioandMississippiRivers.Inaddition,TVA utilizesUSACEcadrestoassistwithroutinesemiquantitativeriskassessments(SQRAs)forselected dams.
Figure6.Summaryofstructureinventories.
Althoughtherearenocurrentjointoperations,TVA andUSBRcollaboratedcloselyintheyearsimmediatelyfollowingpassageoftheTVAAct.Becauseit tooktimetocreateadesignbranch,TVAhiredUSBR todevelopthedesignsforNorrisandWheelerDams (whichiswhyNorrisistheonlyTVAdamwithdrum gates).TVAiscurrentlyworkingwithUSBRonrehabilitationofringsealgatesatHiwasseeDam,thedesignforwhichisvirtuallyidenticaltothoseatGrand CouleeDam.
TherearenumerousprojectswhereUSACEor USBRisthedamownerandoperatorwitha third-partyhydropowerproviderthatisregulated byFERC.ForUSACEstructures,thehydropower providergenerallycontrolstheday-to-dayoutflow operationsincoordinationwithUSACE,whileUSACEcontrolsthefloodpoolandfloodcontrol operations.Iftheprojectsalsohavenavigation, thenUSACEcontrolsthenavigationthroughthe locksandthenavigation-relatedpools;examples
oftheseincludemanydamsalongtheColumbia River.
ForReclamationassets,thirdpartiesarepermittedunderaleaseofpowerprivilege(LOPP)contract.LOPPprojectsmustnotimpairtheefficiency ofReclamation-generatedpowerorwaterdeliveries, jeopardizepublicsafety,ornegativelyaffectanyother Reclamationprojectpurpose.
TVAcoordinateswithFERClicenseesthatoperate damswithintheTennesseeRiverwatershed(mostnotablyontheLittleTennesseeRiverdownstreamfrom FontanaDam),buttherearenoformaljointoperationsbetweenFERC,theirlicensees,andTVA.
ThepartiesinvolvedandthereasonsforconductingintrusiveinvestigationsatstructuresownedorregulatedbyUSBR,USACE,TVA,orFERCvary.For structuresownedbyoneoftheorganizations,theprocessmaybeself-initiated(suchastocollectinformationtobetterquantifyanidentifiedpotentialfailure mode(PFM),ortosupportdesignofamodification), orinitiatedbyanoutsideorganization(e.g.,transportationdepartments,pipelineowners,orthird-party hydropowerdevelopers).Forstructuresregulatedby USBRorUSACE,theprocessistypicallyinitiatedby anoutsideorganization;inadditiontotheexamples above,thesemayalsoincludealocalfloodcontrolor irrigationdistrict.ForFERC-regulatedstructures,the processisalwaysinitiatedbythelicensee.
Eachfederalorganizationutilizesregulationsand guidelinesthatdescribepermissibleorpreferredinvestigationtechniques,minimumqualifications,andsubmittalrequirements.Whilethespecificsofthevarious documentsdiffer,theunderlyingfundamentalprincipleis DONOHARM tothestructure.Theprimary governingdocumentsforeachorganizationareas follows:
USBR: GuidelinesforDrillingandSamplinginEmbankmentDams (USBRDrillingGuidelines;USBR, 2014)
USACE: DrillinginEarthEmbankmentsandLevees (ER1110-1-1807;USACE,2014a)
FERC: GuidelinesforDrillinginandNearEmbankmentDamsandtheirFoundations,Version3.1 (FERCDrillingGuidelines;FERC,2016)
TVA: EvaluationsandDesignofDams (TVAStandardProgramsandProcesses(SPP)-27.001; TVA,2022a),whichreferencestheFERCDrilling Guidelines
Supportingdocumentsthatmaybeusedtoassist withtheplanningofintrusiveinvestigations,including
internalandexternalguidancefordrillingandabandonmentofborings,andmethodologies,arelisted inthe References section.TheseincludeAustralian DrillingIndustryAssociation[ADIA],2020;ASTM, 2014,2016,2017,2018a–e;Dustmanetal.,1992;Farrar,1999;FEMA,2005;Luteneggeretal.,1995;Ruda andBosschler,2005;TVA,2013;USBR,1990,1998a, 1998b,2001;USBRandUSACE,2019;andUSACE, 2020b.
Whiletheintentofthisarticleisnottoreiterateinformationpublishedinthesedocuments,thefollowing areconsiderationsthatconstitutegoodpracticeand aregenerallyapplicabletointrusiveinvestigationplanningforprojectsatanydamorlevee(whetherforor regulatedbyafederalorganizationornot).
ResearchingAvailableBackgroundInformation
Researchistheleastinvasiveandsafestwayto gatherinformation.Thistaskincludesgathering,organizing,andreviewingallavailableinformationabout thestructureandsubsurfaceconditions.Potential sourceswillvaryfromprojecttoprojectbutshould includeallrecordsheldbytheownerandpriorownersattheiroffices(local,regional,andnational),at thesite,historicarchives(newspaper,county,state, andfederal–includingtheNationalArchivesand RecordsAdministration),stateandfederalagencies (e.g.,UnitedStatesGeologicalSurvey;stategeologicalsurveys;UnitedStatesDepartmentofAgriculture, NaturalResourcesConservationServicesoilsurveys), andotherresourcesasneeded.
Itisessentialtounderstandthefollowinginformationataminimumaspartofthepreparationforarisk assessmentandanysiteinvestigation:
Originaldesignplansandspecificationsforthe structure
As-builtdocumentation,andhistoryofmodifications
Recordphotosfrompreconstruction,duringconstruction,andpost-construction
Relevantanalyses(particularlystabilityandseepage)
Bedrockgeology,includinginformationaboutgeologicstructure,seismotectonicfeatures,joints,or discontinuities
Surficialgeologyincludingthegeomorphologyof thesiteandsurroundingarea
Hydrogeologicconditions
Inspectionrecords,includinginstrumentationreadings,recordsandperformanceevaluations
Muchofthisinformationistypicallycontained inthesupportingsummarydocumentationforeach structure;however,forvariousreasonsthisisnot
Table1. Supportingsummarydocumentnomenclatureby organization.
Organization
SupportingSummaryDocumentName andAcronym
USBRComprehensiveReview(CR)
USACEPeriodicAssessment(PA)orRiskAssessment (RA)
TVASupportingTechnicalInformation(STI)
FERCSupportingTechnicalInformationDocument (STID)
USBR = U.S.BureauofReclamation;USACE = U.S.ArmyCorps ofEngineers;TVA = TennesseeValleyAuthority;FERC = Federal EnergyRegulatoryCommission.
alwaysthecaseinpractice.Eachorganizationusesdifferenttermsforthesupportingsummarydocuments; seeTable1fortheorganization-specificnomenclature.
Intrusiveinvestigationprogramsshouldbedesigned bygeotechnicalandgeologicalprofessionalswithexperienceindamandleveesafety.Thefirststepisto definethequestionstobeansweredaswellasthegoals andtargetsoftheprogram,whichwillinformthelocationsanddepthsfortheexplorations,thesamples andotherinformationtocollect,andtheinstrumentationtoinstall.Inmanycases,intrusiveinvestigationprogramsareinitiatedfollowingriskassessments. ForPFMswithadispositionof“InsufficientInformation,”drillingorexcavationmaybenecessaryto collecttheinformationrequiredtoproperlyevaluate thePFM.ForSQRAsorquantitativeriskassessments (QRAs),informationfromtheinvestigationsmaybe neededtoreduceuncertaintyrelatedtorisk-driving failuremodesidentifiedbytheSQRAortoinformor refinenodalestimatesinaQRA.
Theneedfortheinformation,however,mustbebalancedagainsttheriskassociatedwithcollectingit.To thatend,theplanningteamshouldidentifywhatinformationcanbeobtainedbynon-invasiveandminimally invasivemethods.Additionally,forsomeinvestigation programs,phasingmaybeappropriate,withtheprogramtoendwhensufficientinformationisavailable toanswerthequestionsposed,thegoalshavebeen achieved,ortheriskofcontinuingtheprogrambecomesunacceptable.
USBR,USACE,TVA,andFERCallrequire detailedplansforfieldinvestigationswithsimilarcontent,althoughnomenclatureandspecificsof howinformationispresenteddifferbyorganization.
FollowingFERC’smethodfororganizinginformation, intrusiveinvestigationplansshouldgenerallyinclude:
(1)Purpose,objective,andjustificationforthework, includinganswerstothefollowing:
• Whydoyouwanttodrillordiginthedamor levee(i.e.,putadefectinthestructure)?
• Didyouconsiderotheralternatives?
• Howwilltheinformationhelpyoumakeadecision?
• Whatifyoudidnothavetheinformation? Wouldthedecisionchange?
(2)Existinginformationcollectedduringtheresearchphaseshouldbesummarizedandkeyinformationpresented,includingbutnotlimitedto thefollowing:
• Geologic,hydrogeologic,andgeotechnicalinformation
• Boringlogsandothersubsurfaceexplorations
• As-builtdrawingsandspecifications;record photographsoforiginalconstructionandmodifications
• Relevantdesignandconstructionreports
• Relevantanalyses
• Inspectionandassessmentreports
• Instrumentationplans,installationrecords, data,andperformancereports
(3)Essentialgeologicandengineeringdrawings shouldshowthefollowingitems:
• Embankmentzones
• Detailsofsubsurfacematerialclassification, geologiccontacts,andcontinuityinterpretationssupportedbyallnearbydrillingandsamplingdetails
• Depthtothetopofrockandallotherzonesof importance
• Piezometerlocationsshowingscreenedinfluencezonesandrecordedpiezometriclevelstied tothereservoirwaterlevel
• Otherinstrumentationsuchasinclinometers, movementmonuments,etc.,showninthecontextofthefoundationgeologycontactsandinterpretations
• StandardPenetrationTest(SPT)blowcounts orother insitu andlaboratorytestresultsdefiningengineeringproperties
• Geophysicaldata,whereuseful(e.g.,crosshole shearwavevelocityprofiles)
• Estimatedextentofanyzonesofinterest,both naturalandmade-made(e.g.,groutholes)
• Seepageareastiedtoembankmentzonesand geologicunits,wherepossible
• Locationsofallinternalandexternalstructures,includingseepagecontrolfeatures,conduits,etc.
• Locationandtypesofanydistressfeatures (seepage,wetspots,sandboils,sinkholes,etc.)
(4)Drillingscopeandmethodology:
• Numberandlocationofproposedborings
• Utilities,surfaceandundergroundobstacles,and accessibility
• Materialsexpectedtobedrilled,sampled,and tested
• Depth,diameter,bearing,andinclinationofborings
• Drilling,sampling(e.g.,SPT,shelbytube,HQ3 wirelinerockcore),and insitu testingmethods (e.g.,vaneshear,pressuremeter,waterpressure [packer]tests,etc.)
• Detailsoftheproposeddrillingequipmentand tooling
• Instrumentationandboreholecompletionrequirements(influencezone,seals,etc.)
• Personnelresponsibleforloggingmaterialsand assuringgeologicdrawingsareupdatedregularly duringthedrillingprogram
• Fieldandlaboratorytestingprogram
• Verificationthat insitu testingpressureswillnot causehydraulicfracturing
(5)Boreholecompletionshouldanswerthefollowing questions:
• Howdoyouplantouseorrepairthedefect?
• Haveyoucheckedthetremiegroutingpressures (hydrostatichead)tomakesuretheywillnot hydraulicallyfracturethestructure?
• Doesgroutingneedtobestagedtocontrolthe pressures?
• Howwillyoubackfillacrossadrainagefeature?
• Istheinstrumentationplanned?
(6)Instrumentationinformation:
• Plannedconstructiondetailsandconstruction methods
• Verificationthatwelldevelopmentandrehabilitationwillnotcausehydraulicfracturingor damagethecasings
• Isinstrumentationtemporaryorpermanent? Willitbeautomated?
(7)Documentationandcoordinationshouldanswer thefollowingquestions:
• Whataretheloggingproceduresorspecifications(e.g.,UnifiedSoilClassificationSystem,InternationalSocietyforRockMechanics, organization-specific)?
• Whatformswillbeused?
• Whatinformationneedstobecollectedinthe fieldduringthedrilling?
• Whoisresponsibleforupdatingtheinformationonthedrawings?
• Howfrequentlywilltheboringlogsbesentin fromthefieldtoupdatethedrawings?
• Whodecidesonchangestotheplanduringthe investigation,andhowarechangesapproved?
(8)Personnelexperienceshouldincludethefollowing:
• Explorationteam:Listmembersoftheexplorationteamthatdevelopedthesiteinvestigationplan.Includename,organization,title, registration,andyearsofexperience
• Drillrigoperators:Includenameandyearsof experience.Noteexperiencedrillinginornear damsorlevees
• Fieldsupervisionpersonnel:Includename,organization,title,registrations,andyearsofexperiencewithdamandleveeinvestigations
(9)Evaluationofpotentialrisks:includeanevaluationoftheriskofhydraulicfracturing,erosion, contaminationofdrainagefeatures,heave,orany otherpotentialdamage.Thisshouldincludethe following:
• Adetaileddescriptionofanydrillingfluiduse includingdetailsonthecirculationsystem,locationswherefluidwillcontactsoil,andcirculationpressuresthatwillbeused
• Monitoringneedsduringdrillingandacontingencyplaniflossofdrillingfluidorothercomplicationsareobservedduringdrilling
• Measurestominimizetheriskofdamagetothe dam,levee,orfoundation
• Measurestopreventthepossibilityofcrosscontaminationandleakagefromconfinedand separategroundwateraquifers
• Measurestopreventdrillcontactwithstructuralfeatures,suchasconduits
• Nearbyinstrumentswhosebehaviorwillbe monitoredduringtheinvestigationandtheexpectedresponse,includingthresholdandlimit values,andcontingencyplansforanunexpectedresponse
• Anemergencyactionplanincludingalistof emergencyequipmentandsuppliestohaveonsite(phoneorradio,filtermaterials,groutmaterials,etc.)
• Potentialremediationproceduresforrisks identified
(10)Other:
• Siteaccessmethods
• Environmentalprotectionmeasures
• NEPA,SHPO,orotherrequirementsandrestrictions
Afinalconsiderationisthatinallcases,damand leveesafetyshouldbeprioritizedabovescope.Owners, consultants,anddrillingcontractorshavebeenguilty
ofpushingtocompleteascopeofworkwhilelosingfocusontheoverallsafetyofthestructure.Ifthe subsurfaceconditionswereknown,therewouldbeno needfortheexplorationprogram.Thus,iftheconditionsencountereddifferunfavorablyfromwhatwasexpected,itbecomescriticaltostopandre-evaluatethe planandtheassociatedrisk.Thisis(inpart)whyminimumexperiencerequirementsexistforpersonnelexecutingtheprograminthefield.
Thissectionprovidesadditionaldetailsregarding planningand/orreviewofintrusiveinvestigationprograms,specifictoeachorganization.
Oncetheneedforanintrusiveinvestigationprogram isjustified,amultidisciplinaryteamdeterminesexplorationcomponentsrequiredtoadequatelyaddressthe dataneeds.Ideally,theexplorationteamincludesthe principalengineer,principalgeologist,geophysicists, andexpertsinlaboratoryanalysis, insitu testing,and otherdisciplinesasneeded.Dataneedsandinvestigationplansarediscussedtoensurecompatibility.Items todetermineincludethefollowing:
Purposeoftheinvestigation
Costoftheexploration
Requiredsampletypeandsize(disturbedorundisturbed)
Acceptabledrillingandinvestigativemethods
Depth,diameter,andinclinationofdrillingrequired Materialstobedrilledandsampled
Utilities,surfaceandundergroundobstacles,andaccessibility
Developmentoffieldexplorationrequests(FERs) occursinternallywithintheUSBRforavarietyof clients,withthelargestpercentagefromtheUSBR DamSafetyoffice.OtherclientsforFERsincludethe BureauofIndianAffairs,BureauofLandManagement,NationalParkService,stateagencies,andother waterdistrictauthorities.Typically,FERsaredevelopedfortheDamSafetyOfficeattheTechnicalServiceCenter,alongwithotheragencies.FERsforother agenciesoccasionallymaybedevelopedwithinthevariousUSBRregions.
AmajorityofFERsarewrittentobettercharacterizesubsurfaceconditionsinsupportofexisting damsafetyissueevaluations,correctiveactionstudies,orfinaldesignsfordammodifications.Other casesforFERdevelopmentincludeassessmentor feasibility-levelsubsurfacecharacterizationsofnew damsites,retrofittingexistingdamsforhydropower, linearprojectssuchascanalsorpipelines,andmany otherprojectapplications.
FERsarewrittenfollowingtheUSBRDrilling Guidelines(USBR,2014),whichsupersedeUSBR’s 1989policydocument.TheUSBRDrillingGuidelinesprovideagencypolicyfortheinvestigationofembankmentdams,includinginvestigationplanning,site preparation,boreholeadvancement,subsurfacetesting,andboreholecompletion.
Specifically,theUSBRDrillingGuidelinesrequire considerationofwhethertheneedforcollecteddata justifiesthecostandpotentialrisktothestructurecreatedbythedatacollectionprocess.Thisincludesthe determinationofpotentialconsequencesifnoaction istakencomparedwithsubsequentinvestigationcost estimates.Consequencesincludeboththeriskandlikelihoodofworseningconditions,whichcoulddriveup futurecostsofremediationifrequired.
Damfoundationgeometryandhazardoffracturing Instrumentationandcompletionrequirements
Typicaldamsafetyinvestigationsincludesampling andtestingmethodstodeterminethepotentialforliquefaction(dynamicstability),seepageandpiping(includingfiltercompatibility),staticstability,collapse offoundationsoils,andcracking.Emergencyrepairs anddamremediationactivitiestypicallyrequiresome drillingcomponenttocollectneededdata.
Specificconsiderationsrelatedtointrusiveinvestigationsincludethefollowing:
Hydraulicfracturingduringthedrillingprocess and/orboreholecompletioncanopenseepagepaths andcreateconditionsconducivetointernalerosion, piping,andultimatelydamfailure.Locationsorsituationsthatrequireparticularcare(orpreferably avoidance)areasfollows:
◦ Inornearcutofftrenchesorcutoffwallswhere stressconcentrationsand/orhighgradientsare morelikelytoexist
◦ Nearstructuresorconduitswithinembankments
◦ ImperviousZone1corewithaninternalslope ratiosteeperthan0.5horizontalto1vertical (0.5H:1V)
◦ ThinZone1core
◦ UpstreaminclinedZone1core
◦ Nearabutmentswhereabruptchangesinslopes (shoulders)occur
◦ Inareaswheretheembankmentissubjectto transversedifferentialsettlementduetolarge changesinthicknessofcompressiblefoundation soils
◦ Nearabutmentssteeperthan0.5H:1V
◦ Imperviouszonesconsistingofsiltandmixtures offinesandandsilt(e.g.,lowplasticitysoilsthat aremoreeasilyfractured)
◦ Drillinginorthroughgroutcurtains
◦ Nearareassoftenedorweakenedbyseepage
◦ Inlocationsofknowndefectsincompactedfill Damagetointernaldrainagefeatures,eitherdueto drillingorcontaminationwithgroutduringboreholecompletionorabandonment
Drillingorexcavatingatthetoeofadamwherea criticalhydraulicgradientorartesianconditionsexist,includingtheneedtoconstructadrillingberm
Drillingandsamplinglooseorunconsolidatedmaterialsandbalancingdrillingtechniques(e.g.,mud rotary)againstthepotentialforhydrofractureifthe holeisadvancedthroughtheembankment
Presenceofloosecollapsiblesoilswherewetting couldleadtocollapse,cracking,andultimatelyfailureofthestructure.Collapsiblesoilsarepresentin variouslocationsinthearidwesternstatesandunderliesomeearthdamsandsaddledikes.
Qualificationsandexperienceoftheleaddriller whenuseofReclamationcrewsisnotanoption
Preferred,restricted,andprohibiteddrillingtechniques
Instrumentationinstallationsandboreholecompletionorabandonment.Whenusingneatcement grout,specifyingtypeKcement,oraddinguptoone percentgypsumoraluminumpowderwillgivethe cementexpansiveproperties,whichmaybeadvantageousinembankmentdamswhereinternalseepage isanissue.
InadditiontointrusiveinvestigationsconductedundertheFERprocess,drillingassociatedwithremediationcarriesrisksthatmustbeconsidered.Examples includetheuseofcompressedairasadrillingfluidfor advancinggroutholesandjetgroutingwhichinvolves drillingholeswithveryhighairandfluidpressuresat rapidrates.
USACEsiteinvestigationsfallundermultipleUSACEregulatoryandguidancedocuments(USACE 2001,2014a,2014b,2018,2019,2020,2021)foundin the References section.
TheDrillingProgramPlan(DPP)processdescribed inER1110-1-1807(USACE,2014a)appliestositeinvestigationsatUSACE-ownedandUSACE-regulated damsandlevees,includingsiteinvestigationsconductedunderSection408.Thisincludesanyintrusive workinto,through,orbelowUSACEdamsandlevees withafederalinterest.
ER1110-1-1807includesabackgroundsectionon thepotentialproblemsrelatedtodrillingindamsand leveesandspecificallyidentifiesbasicrisksandpotentialbenefitsofdrillingwithfluidsandtheneedtobalancetheneedforusingfluidswiththepotentialrisks tothestructures.Thedocumentlaysoutthepolicyrequirementsasfollows:
DPPsneedtodiscussoraddressthefollowing topics:
◦ Justificationfortheplannedexploration(s)
◦ Existingdatareview
◦ Restrictionontheuseoffluidsfordrillingindams andlevees
◦ Riskidentificationandmitigation
◦ Boreholecompletion
◦ Drillingpersonnelrequirementsforthedrillerand on-siteoversightbyageologistorengineer
◦ Approval
◦ Exemptionrequirements
AppendixBofER1110-1-1807containsfurther guidanceandageneraloutlineofwhatshouldbeincludedintheDPP.Thesecontentsaresimilartothose describedpreviouslyinthedocumentationsectionand alsoincludethefollowing:
Riskevaluation:thissectionshouldincludeallthe potentialproblemstheproposedinvestigationplan couldcausetothestructureandhowtheyhavebeen addressedbytheplan,includingemergencymeasuresandemergencycontacts
DistrictDamorLeveeSafetyOfficercertification thattheplanfollowsER1110-1-1807
Additionalrequirementsbeyondwhatislistedabove includeaminimumoffiveyearsofexperiencedrilling indamsandleveesforthedrillerandfieldpersonnel loggingtheboringsandtheneedforeitheralicensed engineerorgeologisttobeonsiteandinresponsiblechargeofthedrillingwork.Thispersonneedsto beknowledgeableenoughabouttheplanneddrilling tobeabletorespondtoanysituationsoremergenciesthatmayarise.Lessexperiencedpersonnelare notexcludedfromloggingborings,providedafully experiencedsupervisoryprofessionalispresentonsiteduringdrillingoperations.ER1110-1-1807coversalldrillingindamsorleveeswithUSACEinterestnomatterthepurpose,includingbutnotlimited toinvestigations,instrumentationreplacement,relief wells,andconstruction-relateddrillingincludingcutoffwalls,grouting,andinstallationofutilitiesthrough orbelowthestructures.
USACEreviewoftheDPPs,whethergeneratedinhouseorbyconsultants,forUSACEinvestigations orinvestigationsbyothersunder408permits,allfollowthesamebasicprocess.Requirementsarelisted
inER1110-1-1807,withadditionalguidanceprovided byEM1110-2-2902(USACE,2020).Theplansare firstreviewedbythelocaldistrictstoensuretechnicalcontentisappropriateandforquality-controlconsiderations.TheplansarethensubmittedforAgency TechnicalReview(ATR)bythedrillingandinstrumentationcommitteeoftheGeotechnical,Geology, andMaterialsCommunityofPractice(GG&MCoP). ThereviewprocessismanagedfortheGG&MCoP bytheRiskManagementCenter.TheATRbythe GG&MCoPisnormallydonebytwotothreemembersfromacrossUSACEwhoaregeologistsand/or geotechnicalengineers.Reviewcommentsareprovidedbacktothedistrictthatsubmitstheplanfor review.
Goalsandconsiderationsofthisprogramareas follows:(1)Donoharmtothestructures.(2)Fluids usedwhenneededduringthedrillingandabandonment(grouting)oftheboringshavealowriskofcausinghydraulicfracture.(3)Thedrillinghasalowriskof causingdamagetothestructure.(4)Theinvestigation plannershavethoughtthroughanddescribedpotentialproblemsalongwithappropriateresponseplans andmaterialstohaveavailabletoaddresspotentialissues.(5)Bereadytoreactappropriatelytoanyproblemsthatmayarise.
USACE’sreviewprocessforinternallydeveloped DPPsandassociatedworkdevelopedtosupportan IESisslightlydifferentfromthereviewprocessfor DPPspreparedbyentitiesexternaltoUSACE.The teamsthatdeveloptheplanspresentthemtoagroup ofseniorgeologistsandgeotechnicalengineerswith significantexperienceinplanningandexecutingfield workondamsandlevees.Thesepresentationsbythe riskassessmentteamaretypicallyvirtualmeetings. Duringtheseinteractivemeetings,thereviewersask questionsandprovidecomments,suggestions,andrecommendationsbacktotheteamwithfollow-upcommentswrittenbytwoorthreeofthereviewersprimarilyfocusedontheDPPportionsoftheplan.These plansaregenerallyphasedwithpredeterminedtriggers forthenextphaseofinvestigationandevaluationas needed.Whensiteinvestigationissuesarise,thereview teamsubjectmatterexperts(SMEs)arere-engagedto assisttheriskassessmentteaminfindingsolutions. ThisactiveinvolvementofSMEswiththeriskassessmentteaminupdatingandmodifyingtheplanshas reducedUSACE’stimeandcostrelatedtositeinvestigationsandriskassessments.
Inallcases,thereviewmanagerandreviewersare availabletodiscusstheircommentsandresolution ofthemwiththesiteinvestigationdesignteamto helpfinalizetheplansandwhenneededdiscussmodificationsduringthesiteinvestigationsorintrusive activities.
AsdepictedinFigure9,theheadofTVA’sDSPis theAgencyDamSafetyOfficial(ADSO),aposition currentlydelegatedbyTVA’schiefexecutiveofficerto thechiefoperatingofficer.FromtheADSO,thereare twobranches:governanceandexecution.Governance includestheDamSafetyOfficer(DSO),andtheDam SafetyGovernance&Oversight(DSG&O)organizationthatprovidesday-to-daygovernanceandoversightoftheDSPonbehalfoftheDSO.Ontheexecutionside,AssetOwners(AOs)aretheexecutivesfor thevariousbusinessunitsthatownresponsibilityfor assetsthatclassifyasdams.AOsdesignateaResponsibleEngineeringManager(REM)foreachstructure intheirrespectiveinventories,andtheREMisresponsibleforensuringthatDSPrequirementsaremetfor theirassignedstructures.
DamSafetyServices(DSS)providesin-housetechnicalsupport.Forintrusiveinvestigations,theAOwill generallyengageaconsultanttohelpdeveloptheexplorationprogram,andtheconsultantwillinturnhire theexplorationcontractor.
TherequirementsoftheTVADSParedescribedin aseriesofinternaldocumentsreferredtoas“Procedures.”Thetop-tierdocumentdefinestheframework oftheprogramitself;eighttier-downsectionsdescribewhatisrequiredconcerningevaluationand design,construction,operationandmaintenance,inspections,instrumentation,emergencypreparedness, governance,andriskassessment;andtheneachAO developsaparallelsetof“Procedures”thatdefine howtherequirementsoftheTVA-levelprocedureswill beimplementedwithintheirparticularbusinessunit. Overall,TVA’sDSPismodeledafterboththeFERC frameworkandFEMA’sFederalGuidelinesforDam Safety(FEMA,2004).Tothatend,theFERCDrilling Guidelines(FERC,2016)areincorporatedintoTVA’s DamSafetyProceduresbyreference.
SeparateorganizationswithinTVAactasowners andregulatorswhenitcomestointrusiveinvestigations.Foratypicalriverdaminvestigationproject, theREMservesastheprojectmanager;ageotechnicalengineer,geologist,orstructuralengineerfrom DSSservesasthetechnicalmanager;andresponsibilityfordevelopmentandexecutionoftheexploration scopeiscommonlyoutsourcedtoanexternalconsultant.AOsarerequiredbyprocedure(TVA,2022aand 2022b)tosubmitinformationaboutplannedintrusive inspectionsforreview,andDSG&OreviewstheAO’s submittalonbehalfoftheDSO.
ThetypicalprocessforplanninganintrusiveinvestigationataTVAdamstartswhenanAOidentifiesthe needforsubsurfaceinvestigation(tobetterquantify risk,supportthedesignofamodification,etc.).TVA
developsthegeneralscopeandoverallprojectobjectivesandcontractswithaconsultanttoplantheprogram.Theconsultantthendevelopsaninvestigation planandothersupportingdocuments,asrequired. Next,theAOinformsTVA’sDSOoftheplannedwork andsubmitssupportingdocumentationtoDSG&O forreviewandcomment.
Intrusiveinvestigationandsubmittalsareintended tobescalablecommensuratewiththescope,complexity,andriskoftheproject.DSG&Oexpectsto receiveaworkplanthatgenerallyincludesthefollowinginformation(asapplicable):(1)reason(s)forthe investigation,includingjustificationthattheneedfor subsurfacedataorinstrumentationneedsoutweighs therisksassociatedwiththeintrusiveinvestigation(s);
(2)location(s),depth(s),inclination(fromvertical)and bearing(asapplicable)ofproposedinvestigationholes;
(3)dimensions(length,width,depth,andsideslope angles)oftrenchesortestpits;(4)anticipatedsubsurfaceconditions;proposedinvestigationanddrilling techniques,includingchangeoversbetweendrilling methodsandtype(s)ofplannedsamplingand insitu testing;(5)lengthoftimethatboringsand/orexcavationsareexpectedtoremainopen(notbackfilled);(6) plannedholecompletion(i.e.,instrumentationinstallationorabandonment),includingproposedproducts andmaterials,mixingproportions,andinstallation andplacementtechniques);(7)documentationthat boththeresponsibleon-siterepresentativeandlead drillrigoperatorandequipmentoperatormeetexperiencerequirements;(8)plannedsiterestoration andrepair,includingpatchingofreinforcedormass concrete,asphalt,orturf;and(9)anoverallproject schedule,includingthedurationofdrillingactivities. Fornon-drilledexcavations(testpits,testtrenches, etc.),adescriptionofthebackfillplan,including materialspecifications,placementtechniques,and compactionrequirements,shouldbeincluded,andif instrumentationisplanned,notewhichPFM(s)the instrumentsareintendedtomonitorordetect.When applicable,theriskofcausinghydraulicfractureof theembankmentand/orunderlyingformationdueto theintroductionoffluid(whetherduringdrilling, in situ testing,orbackfillingthehole)alsoneedstobe consideredandevaluated,andmethodsforcalculating andmonitoringallowablehydrostaticand/orapplied pressuresinthefieldneedtobedescribed.
Acontingencyplantoaddressand/ormitigatethe risksassociatedwithdrillingchallenges,geologichazards,and/ordamsafetyissuesthatcouldbeencounteredortriggeredbytheintrusiveinvestigationactivitiesneedstobeprovided.Thisrequirementoriginatedduetotheextensivepotentialforkarstacrossthe TennesseeValleyandhasexpandedbasedonindustrylessonslearned.Potentialdrillingchallengesand
hazardstoaddressinclude(butarenotlimited to)voidsandroddrops,lostcirculation,heaving sands,artesianconditions,pipingandinternalerosion, blowout,contaminationofdrainagematerials,inducementofslopeinstability,presenceofhazardousand explosivegasses,andthepresenceofburiedorembeddedconduitsandotherappurtenances.Anexampleof acontingencymeasureisstockpilingmaterialsnecessarytoconstructareverse-gradedfilterandcoordinatingwithamaintenancebasetostationabackhoeor excavatoratornearthedamwhendrillingnearthetoe ofanembankmentinknownkarstterrain.
TheotherrequireddocumentisaQualityManagementPlan(QMP).TheQMPisintendedtoensureintrusiveinvestigationactivitiesmeetTVA’sdamsafety andcontractrequirementsbyensuringQAandQC activitiesareperformedcompetentlyandconsistently andinamannerthatwillidentifyandaddressdam safetyissues.TheQMPcommonlyincludesanaccountabilitymatrixthatdescribeswhoholdstheultimatedecision-makingauthorityforvariousactivities throughoutexecutionoftheinvestigationprogram, andwhoisauthorizedtostopthework.
AttheAO’sdiscretion(orifrequiredbyTVA’s DSO),twoadditionaldocumentsthatmayberequired (typicallyformorecomplexand/orhigherriskinvestigationprograms)areatemporarysupplementtothe EmergencyActionPlan(TEAP)andTemporaryInstrumentationMonitoringPlan(TIMP).TheTEAP addressesdetectionofandresponsetoeventsduring intrusiveinvestigationsthatcouldthreatenthesafety ofthestructureanddescribestheresponsibilitiesand communicationproceduresshouldanemergencyoccur.ATIMPidentifiestheinstrumentstoberead, thefrequencyofreadings,andresponsibilitiesfordata evaluationduringtheinvestigationprogram,aswell asalertandactionthresholds,communicationprocedures,andresponseprotocolsifathresholdisexceeded.WhenaTIMPisnotrequired,theworkplan shoulddescribeanymonitoringofexistinginstrumentsand/orinspectionsplannedtobeconducted duringtheinvestigationprogram.
DSG&Oreviewsthesubmittalforcompliancewith TVAproceduresandgeneralindustrybestpractices (includingtherequirementsoftheFERCDrilling Guidelines)andprovidescommentstotheAO. DSG&OalsodeterminesiftheprojectrequiresformalDSOconcurrence.(Concurrenceisgenerallyreservedforprogramsthatareunusuallycomplexor whichcarryelevateddamsafetyrisk;forexample,the programscopeinvolvesdrillingthroughthecoreofan embankmentdamorinjectingfluidsunderpressure.) Oncecommentsareresolved,DSG&OeitherrecommendsthattheDSOconcurwiththeAO’splanned program,or(morecommonly)notifiestheAOthat
concurrenceisnotrequired,andthatDSG&Otakes noexceptiontotheexecutionoftheproject.Inthe eventofsignificantmodificationstotheplannedactivities,AOsarerequiredtoinformtheDSOandallowtimeforsupplementalreviewandcommentby DSG&Oasnecessary.
Separatefromthesubmittalreviewdescribedabove, AOsalsoneedtoobtainadrillingandchippingpermitand/oranexcavationpermitinaccordancewith TVATechnicalInstructionTI-DS-0002(TVA,2022c). ThisisaninternalTVAprocessthatevaluatespotentialconflictswithundergroundorembeddedinfrastructure.Drillingandchippingpermitsapplytopenetrationsgreaterthan2in.(5cm)inconcreteandto geotechnicalboringslessthan12in.(30cm)indiameterinconcrete,rock,earthenembankments,orareas adjacenttothedam.Excavationpermitsapplytotest pitsorotherexcavationsopenedbynon-rotarymethodsandboringsgreaterthan12in.(30cm)indiameter. Forinvestigationsofnavigationstructures,theprocess alsoincludescoordinationwiththeUSACELockmaster.Additionally,TVArequiresaHASPbedeveloped (althoughthisrequirementoriginatesoutsideofDam SafetyandDSG&OdoesnotformallyreviewHASPs).
DSG&OalsoreviewsconstructionandmodificationprojectsonbehalfoftheDSOandmakesrecommendationstotheDSOaboutprovidingformal concurrence.DSG&Oconductsanoveralldesignreviewandforconstructionprojectsthatincludedrilling (e.g.,secantpilewalls,shaftfoundations,confirmation borings,etc.)aspartoftheproject.DSG&Oreviews thedrillingscopeforconsistencywithtypicalindustry standardsandTVAprocedures,andAOsareobligated toresolveDSG&O’scommentstothesatisfactionof theDSObeforeconcurrenceisprovided.
Oncetheintrusiveinvestigationprogram(orconstructionandmodificationproject)iscomplete,theresultsneedtobeincorporatedintothenextregularupdatetotheSTIdocumentforthedam.
D2SIisresponsiblefordevelopingandimplementingFERC’sDamSafetyProgramtoevaluatethe integrityofwaterretainingstructures,theadequacy oftheemergencyactionplansdevelopedtoachieve publicsafety,andthesecuritymeasuresdevelopedto includethethreatofcyberattacks.Thisincludesoversightofthedesign,construction,operation,andmaintenanceofthedamsandwaterconveyancestructures withinFERC’sjurisdiction.Theregulatoryauthority vestedtoFERCisdescribedin18CFRPart12(CFR, 2020)whichalsooutlinestheresponsibilitiesofthe projectlicensees.D2SIhasonecriticalmission:the safetyofdamsunderFERC’sHydropowerProgram.
TherolesandresponsibilitiesthatFERCplaysduringdesignandconstructiondiffergreatlyfrommost agencies.FERCisnottheowneroftheprojectsand doesnotdesignorprepareplans,specifications,or contractdocuments,nordoesitgetinvolvedwithany contractorprocurement.Astheregulatorybody,for theconstructionofanewprojectorremediationof anexistingprojectaddressingPFMs,FERCservesto reviewandapprovetheplanandscheduledeveloped bythelicenseetoperformtheworkanddefinethe pathmovingforwardtoensurethesafetyoftheproject (Figure10).
Damsclassifiedashavingahighhazardpotential, havingaheightgreaterthan32.8ft(10m)abovethe streambed,orthatimpoundmorethan2,000acre-feet (2.5millionm3 )ofwater,fallundertherequirements ofPart12DoftheCommission’sregulations.InApril 2022,arevisiontotheregulationsbecameeffective thatchangesthewayPart12Dinspectionswillbeexecuted.AsdescribedinChapter16oftheEngineering GuidelinesfortheEvaluationofHydropowerProjects (FERCEngineeringGuidelines;FERC,2021a),the inspectionswillalternateeveryfiveyearsbetweena PeriodicInspection(PI)conductedbyanIndependentConsultant(IC)andaComprehensiveAssessment(CA)performedbyanICandateamofSMEs. APIisprojectperformancefocused,whileaCAisa morein-depthandcomprehensiveassessmentofthe project.ThescopeoftheCAincludesaPotentialFailureModesAnalysis(PFMA)alongwithaLevel2 RiskAnalysis(L2RA)asdefinedinChapter18ofthe FERCEngineeringGuidelines(FERC,2021c).The PFMAprocessisdescribedinChapter17oftheFERC EngineeringGuidelines(FERC,2021b)andincludes identificationandscreeningofPFMs.TheL2RAthen estimatestheriskassociatedwithcrediblePFMstoinformprioritizationoffutureriskreductionactivities.
BoththePFMAandL2RAprocessesmayidentifytheneedforsite-specificinvestigations.Duringthe PFMA,theinformationavailablemaybeinsufficient tofullyevaluatesomepostulatedPFMsandaccurately determinetheircredibility.DuringtheL2RA,limited informationmayintroduceahighdegreeofuncertaintywhenestimatingtheriskassociatedwithacrediblePFM.Subsurfaceinvestigationtocollectadditionalgeologicandgeotechnicalinformationthenbecomesnecessarytoclosedatagaps,supportadditional evaluationsandanalyses,orrefineriskestimates.Informationobtainedfromsiteinvestigations,associated testing,andinstrumentationcanalsobeusedtoinformdesignofbothremediationandnewconstruction projects,andtoaidindetermininganyRiskReduction Measuresthatmaybenecessary.
Afteritisdeterminedthatasiteinvestigationisrequired,theDPPispreparedandsubmittedbyali-
censeetoFERC.TheDPPisrequiredbyFERCfor anydrillingprojectandmustfollowtheFERCDrilling Guidelinesthatprovideinformationonthehazardsof drillingatanembankmentdamandguidanceforplanningandpreparingaDPP.Theguidanceprovidedin theFERCDrillingGuidelinesalignsverycloselywith thoseofUSACEandUSBR.Thedifferencebetween theseorganizationsisbasicallyhowtheDPPsrequired foranydrillingprojectarehandled,processed,and implemented.
AConstructionPFMAmayberequiredbythe RegionalEngineerbeforeconstructionbutafterthe drawings,specifications,procurementofthecontractor,andproposedmeansandmethodshavebeendeveloped.Thisassessmentisdesignedtoidentifyany PFMsthatmayresultfromconstructionprocedures, sequences,orprojectloadingvariations.Itservesto providetheopportunitytomakemodificationstothe workorincorporateInterimRiskReductionMeasures tobuydownconstructionrisks.Afterprojectcompletion,aPost-ConstructionPFMAmayalsobeperformedtoevaluateandaccountforthenewactual conditionsimpactingtheproject.Thiscouldinclude changesinloading,unknownfoundationcharacteristicsrevealed,ornecessarydesignchangesmadeduring construction(FERC,2021b).
TheadministrationofFERC’sinvolvementwiththe projectsistypicallyperformedbytheregionaloffices, headedbytheRegionalEngineer.OncetheDPPis
received,theregionalofficeentersitintoatrackingsystemwhereitproceedsthroughaverycarefully orchestratedandmethodicprocess,asillustratedin Figure11.
SomeofthecommondeficienciesnotedinDPPs receivedbyFERCareinadequateinformationon theessentialgeologicandengineeringdrawings,and incompletedetailsdescribingthedrillingscopeand methodology,emergencyprocedures,boreholecompletionmaterialsandmethods,personnelexperience, andtheevaluationofthepotentialrisksandcorrective remedialmeasures.Dependingonthespecificcircumstancesandcomplexnatureofthescopeofwork,some submittalreviewsinvolvethecollaborationofboth theregionalofficeandSMEsstaffedatHeadquarters. Allreviewcommentsarediscussedandcompiledand providedtothelicenseebyletter.Onceallcomments havebeenaddressedtothesatisfactionofFERC,authorizationtoproceedtothenextstepisprovidedby theRegionalEngineer.
TheUSBRhasfullin-housedrillingcapability. Site-specificsurface-subsurfaceexplorationsareperformedfollowingUSBRFERsandareexecutedpri-
marilybyin-houseUSBRpersonnel.Therearedrill crewsandequipmentlocatedwithinvariousUSBR regions.Thesepersonnelarespeciallytrainedfor drillingintoandneardamstomaximizedatacollectionandminimizedamsafetyriskrelatedtosubsurfaceexploration.USBRgeologistsarealsoonsitefull timetodirectdrillingoperationsandcollectandlog samples.
Dependingondemand,outsidedrillingservicesare occasionallyprocuredandused,buttypicallyonlyfor non–dam-specificexplorations(pipelines,foundations, etc.).
SiteinvestigationworkbyandforUSACEvaries bydistrictandproject.Somedistrictsutilizein-house personnelandequipmentasmuchaspossible,while otherdistrictsprefertousecontractors.Forcontracted work,theoversightandlevelofUSACEinvolvement on-sitewillfollowthecontractdocuments.Difficultaccessandspecialtydrillingarealmostalwaysdoneby contract.
Beforedrillingcommences,thedrillersandUSACE shouldmakesurethereisanapprovedDPPandthat theyhaverevieweditandarefollowingitsrequirements.ThelevelofUSACEinvolvementintheplanningandon-siteactivitieswillvarybycontract.Inall casesthedrillingplanshouldbedevelopedandapprovedfollowingER1110-1-1807(USACE,2014a), andwherefluidsareused,followingtheguidancein
EM1110-2-2902(USACE,2020b)relatedtohydraulic fracturingcalculations.Iffluidsarebeingusedduringthedrillingorbackfillingprocess,thefluidpressurewillneedtobemonitoredusingafloatingneedlepressurevalvetorecordthemaximumpressure spikesthatcanoccurinstantaneouslyandareoften unnoticed.
Drillingdonebynon-USACEentitiesforwhatever reasonalsoneedstofollowER1110-1-1807(USACE, 2014a),andwherefluidsareused,followtheguidanceinEM1110-2-2902(USACE,2020b)relatedto hydraulicfracturingcalculations.Forpipesandconduitsthatarebeinginstalledusingdirectionaldrilling orsimilarmethods,theyalsoneedtofollowtheguidanceinEM1110-2-2902,particularlysection5.6on hydraulicfracturingcalculations.
TVAdoesnothavein-housedrillingcapability,and in-houseresourcesfortestpitsarelimited.In-houseresourcesforfieldoversightofinvestigationprojectsare alsolimited,soTVAtypicallyoutsourcesintrusiveinvestigationprojectstoprivateconsultants,whointurn subcontractfordrillingorexcavationservicesfollowingTVArequirements.Theconsultantsprovideexperiencedfieldstaffwhooverseetheinvestigations,log samples,anddocumentthework.TVAstaffarecom-
monlyonsiteforthekickoffandtoobserveprojectcriticalactivities.
OncetheDPPisacceptedandthenoticetoproceedisgrantedforthesiteinvestigationworkto start,FERC’srolecontinuesbutnotinthecapacityoffull-time,on-sitepresence.Theconstruction progressiscarefullymonitoredtypicallythroughregularlyscheduledmeetingsand,ataminimum,monthly inspectionsbytheregionaloffice.PIsmayalsoinvolvestafffromHeadquarters.Boringlogsarerequiredtobesubmittedwithin24hoursofholecompletionandadraftofthefieldboringlogsand dailyworklogsmayberequestedandsubmittedto theFERCregionalofficedaily.ChangesordeviationsfromtheDPParerequiredtobecoordinated withFERCalongwithnotificationofanycomplicationsthatarisesuchasvaryingsiteconditions,the needfordesignmodifications,orunexpecteddevelopments.Anyconditionsthatpresentthemselvesas damsafetyissuesareexpectedtotriggerastop-work response,implementationofrisk-reductionmeasures, and,ifnecessary,activationoftheTCEAP.These incidentsarerequiredtobereportedimmediately toFERC.
Theprimaryreasonsforperformingsiteinvestigationsatexistingdamsandleveesaretosupportdam andleveesafetydecisions,toreviewmodificationsthat addressdeficiencies,orwhenaddinghydropowertofacilities.Potentialfuturemodificationsincludeincreasingwaterstoragethroughdamraisesandmodificationsinstorageoperations.
Theuseof3Dsoftwareforplanningandimplementingsiteinvestigationswithgeoreferenced3Dhydrogeomodelstoimproveunderstandingofthestructures andfoundationsisincreasingindustry-wide.Examplesincludeamarkedincreaseintheuseofinstrumentedequipmentforhorizontaldirectionaldrilling, grouting,andcutoffwallinstallation,andweanticipatethesetechnologiesmovingtoreal-timemonitoringofsite-investigationdrillingequipment.The sensorandrecordingequipmentalreadyexist;itis justamatterofaddingthemtositeinvestigation equipmenttostartobtainingadditionalinformation onthedrillingandgroutingprocessesusedforthese investigations.
Updatestovariousguidanceandsupportdocumentsareinprogressorunderconsideration,asdiscussedinthefollowingsections.
USBR
Updatestothefollowingdocumentsareexpectedin thefuture:
EngineeringGeologyFieldManual,VolumesIandII GuidelinesforDrillingandSamplinginEmbankment Dams
USACEisnearingtheendoftheprocesstoupdate ER1110-1-1807,andthemostsignificantchangeisto includealldams,levees,andappurtenantstructures includingconcretedamsandfloodwalls,notjustembankments.Theupdatealsoincludesspecificreference toEM1110-2-2902forhorizontaldirectionaldrilling andothertrenchlessinstallationmethods,andabandonmentofsimilarstructuresandpressuregrouting.It alsoincludesclarificationthatitcoversconstructionrelateddrillingandreliefwells,andtheneedtoaddress existingon-siteinfrastructure,includingbutnotlimitedto,reliefwells,conduits,instrumentation,andcutoffwalls.OtherrelateddocumentsthatarebeingupdatedincludeER1110-2-1156(USACE,2014b),and EngineeringandConstructionBulletin(ECB)2019-15 (USACE,2019).
AnupdatetotheTVA-levelProceduresbecame effectiveApril1,2022.TVA-SPP-27.001nowmore specificallyreferencesintrusiveinvestigations,althoughnosignificantchangestotheprocessorapproachwereimplemented.Revisedbusinessunit-level procedurestoalignwiththeTVA-levelupdateswillbe developedinthecomingmonths.
TheTVA-levelproceduresaretypicallyupdatedon a3-yearcadence,andthenextupdateisanticipated tobemoreextensive.Additionally,developmentofan engineeringguidelineortechnicalinstructionspecific tointrusiveinvestigationsisunderconsideration.
Anupdatetothe FERCGuidelinesforDrillingin andNearEmbankmentDamsandtheirFoundations is inprogress.Thisupdatewillincludethefollowing:
ArevisiontotheEmbankmentDamssection Expansiontoincludedrillinginconcretedams,appurtenantstructures,andothertypesofdams
Additionally,theDrillingProgramPlanrequirementsarebeingrefinedtoaddressspecificrisksasapplicabletothestructuretype.Thenewoutlineisbeing
designedtobeuser-friendlyandapplicable(asneeded) toalltypesofdamstructures.
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JEFFREYA.SCHAEFER*
SchaeferGeotechnicalConsultingLLC,12102RehlRoad,Louisville,KY40299
KeyTerms: DamSafety,LeveeSafety,RemedialConstruction,ConstructionRisk,ConstructabilityEvaluation,RiskMitigation,Cofferdams,Risk-Informed Decisions
Structuralmodificationstodamsandleveescancreatelifesafetyrisksduringtheconstructionperiod.It isimportantforallpartiesinvolvedtounderstandthe potentialrisk,evaluatemethodstomitigaterisk,and considertheriskwhenmakingdecisions.Mostimportantly,itiscriticaltoalwaysstriveto“donoharm”and preventactivitiesthatcouldpotentiallyincreaserisk. Thiscasehistorywillprovideexamplesofconditions thatcanleadtoincreasedriskduringconstructionincludingexcavationsthatlowerthecrestofadamor levee,excavationsatthetoeofanembankment,rapid embankmentconstruction,blockedbypasstunnels,spillwaymodifications,excavationsforoutletculverts,and damagefromgrouting.Akeycomponentofmanyconstructionprojectsisatemporarycofferdamtoprotect theworkareafromflooding.Cofferdamshaveahighpotentialtocreateriskduringconstruction.Animportant steptohelppreventincreasingriskduringconstruction istohavemultipleconstructabilityreviewsduringthedevelopmentoftheprojectdesign.Ifthepotentialforconstructionriskisproperlyidentified,theriskcanoften bemitigatedorreduced.Decision-makerinvolvementis necessarytoassurerisksareunderstoodandproperdecisionscanbemade.Severalcasehistoriesarepresented todemonstratehowrisksduringremedialconstruction areevaluatedandincorporatedintodecision-making.
Whenadecisionismadetoreducerisksatadamor levee,astructuralmodificationmaybeimplemented. Sometimesastructuralmodificationcanpotentially exposethepublictoevengreaterrisksduringthe timethatittakestoconstructthemodifications.Itis criticaltoalwaysstriveto“donoharm”andprevent activitiesthatcouldpotentiallyincreaserisk.Some
*Correspondingauthoremail:jschaefer.geo@gmail.com
examples(notall-inclusive)areprovidedforthereader tounderstandtheneedtoevaluatethepotentialfor proposedremedialmeasuresthatincreaseriskduring construction.Theterm“constructionrisk”isusedto describethisriskduringconstruction.Construction riskcanhavedifferentmeanings,suchascostriskor scheduleriskduringconstruction.However,forthis document,constructionriskreferstothelifesafety oreconomicriskthatexistsduringtheperiodof constructionwhenadamorleveeisbeingmodified. Lifesafetyriskistherisktotheworkersandpublic downstreamofadamorlevee.Itistheproductofthe probabilityoftheloading,theprobabilityoffailure, andtheexpectedliveslostwithfailure.Economic riskissimilarbutisbasedontheexpectedcostsof damagestostructuresandfacilitiesdownstream.
Whenselectinganddesigningdamorleveealternatives,itisimperativetoevaluateandunderstand theconditionsthatcanleadtoincreasedriskduringconstruction.Thefollowingareafewexamples ofcommonconditionsassociatedwithdamandlevee remediation.
Excavationsthatlowerthecrestofthedamorlevee canincreaseitssusceptibilitytofloodovertopping.An examplewherealeveeprotectingSacramento,CA,is temporarilydegradedfortheconstructionofacutoffwallisshowninFigure1.Itistypicaltodegrade theleveeuptoone-halftheleveeheighttoprovidea workplatformforthecutoffwallconstructionequipmentandtoreducetheslurryhead,loweringtherisk ofhydraulicfracturefromtheslurry.
Excavationsatthetoeofadamcanincreaseitssusceptibilitytoslidinginstabilitybyremovingmassand allowingpotentialslidingsurfacestodaylightinthe excavation.Figure2showsanexamplewhereexcavationofthematerialatthetoeofadamwouldcausea stabilityproblem.
Excavationsthatremoveaportionofthedownstreamslopeorfoundationofanembankmentcan leadtoashortenedseepagepathandincreasedsusceptibilitytointernalerosion.Figure3showsanex-
amplewhereaditchwasexcavatedalongthetoeofa levee(LeveeUnit8ontheWhiteRiver,Washington, IN).Thisledtotheinitiationofinternalerosioninto theditchwhichunderminedthelandsideslope,causinginstability.Thiseventuallyresultedinbreachofthe levee.
Rapidconstructionofembankmentscanincrease thelikelihoodofslopeinstability.Therearenumerous casehistorieswhereembankmentswereconstructed ataratethatwastoofastfortheporepressuresin theembankmentand/orthefoundationtodissipate, andaslopefailurewasinitiated.Figure4showsFort PeckDam(Redlingeretal.,2018)andWacoDam (Stromanetal.,1984)whichbothhadmajorslopefailuresduringconstruction.Thecauseoftheslopefailuresonthesetwodamswasacombinationofapoor understandingofthegeologiccharacteristicsoftheir foundationsandtherapidconstructionthatcaused
veryhighconstructionporepressures.Althoughthese examplesareofslopefailuresthatoccurredinthe originaldamconstruction,thisisalsoaconcernif embankmentfillsforremediationareconstructedtoo quickly.
Ifbypasstunnelsareusedtodivertwaterduring constructionofadam,thereisapotentialthatthetunnelswillcollapseorbecomecloggedduringconstruction,leavingtheprojectwithnowaytopassinflows whichcouldleadtoovertopping.AtItuangoDam (Colombia,SouthAmerica),anauxiliarybypasstunnelcollapsedandalandslideblockedtheentrancesto theprimarybypasstunnels,leavingtheprojectwithno reliablemechanismtocontrolthepool.
Spillwayremediationthattakesthespillwayoutof servicetemporarilycanincreasethechanceofuncontrolledreleasesorovertopping.Fullorpartial replacementofstructuralfeatures,suchasspillways, couldpotentiallyresultinareleaseofwaterifa bulkheadorcofferdamweretofail.Figure5shows
anexamplewhereaU.S.ArmyCorpsofEngineers (USACE)spillwaygateistakenoutofserviceandrepairsarebeingmadebehindtemporarybulkheads.
Repairworkongatescanalsopotentiallycause gatestoinadvertentlyopenandresultinanunplanned waterrelease.ArecentaccidentonClevelandDamin Vancouver,Canada,occurredwhenrepairswerebeing madeonaspillwaygateanditsuddenlyopened,resultinginamajorreleasethatkilledfishermeninthe riverdownstream(SchmunkandMigdal,2020).
Temporaryremovaloffloodprotection(leveeor floodwall)canexposetheprotectedareatoincreased floodrisk.Figure6showsareplacementofalevee
tainconstruction.Failureofthelinercouldincreasetheriskofinternalerosion.Overtoppingriskisalsoincreasedifthebottomoutletwasnotabletobeusedforrequireddischarges(U.S.ArmyCorps ofEngineers,MosulDamTaskForce).
drainageculvertwithnocofferdamorothertemporary floodprotectioninplace.Theleveeprotectspartofthe cityofClarksville,IN.
Constructingremedialelementslikegroutcurtains andcutoffwallsadjacenttoexistingstructurescan leadtodamageandpotentialinitiationofinternalerosion.Areasnexttoexistingstructuresaremorelikely tohavelowconfiningstressandembankmentmaterialsthatarenotaswellcompactedastherestoftheembankment,makingthemmoresusceptibletohydraulic fracturing.Figure7showsanexamplewherehigh groutingpressuresdamagedaconcreteandsteelliner inadam’sbottomoutlettunnel(MosulDam,Iraq). Pressuresfromtheadjacentgroutcurtainconstructionpenetratedthe5-ft(1.5-m)-thickconcreteliner andbuckledthe1-in.(2.5-cm)-thicksteelliner.Thedeformationwasapproximately3ft(1m)intothetunnel and40ft(12m)long.Fortunately,thesteellinerdid notfail,andthetunnellinerwasabletoberepaired. Hadthesteellinerburst,therewasapotentialto initiateinternalerosionintotheconduit.
Pressuresinducedfromdrillingfluidsorgrouting canbelargeenoughtoproducedamageeitherbycausingthegroundtofractureordisplace,orboth,and increasetheriskofinternalerosionfailuremodes. Figure8showsanembankmentdam(AddicksDam, TX)excavationthatwasfracturedbygrouting,and Figure9isacrackinthecrestofWolfCreekDam (Jamestown,KY),adamcrestthatwasassociatedwith grouting-induceddisplacements.
Wheredamsandleveeshavedrainagefeaturesthat areimportanttotheperformanceofthestructure,uncontrolleddrillingfluidsorgroutingactivitieshavethe
potentialtoclogdrainagesystemsandincreasetherisk ofinternalerosionfailuremodesorincreasingpore pressures,leadingtoslopefailure.Improperlyconstructedfiltersordrainscanpotentiallyprovideanunfilteredexitforseepage.Thisincludesfilterlayersthat wereplacedtoothin,aremissingorinadvertentlyremoved,ordrainpipesthatarenotproperlyconnected orbrokenduringinstallation.
Akeycomponentofmanyconstructionprojectsis acofferdamtoprotecttheworkareafromflooding. Designofcofferdamsrequiresthesameriskconsiderationsasanyotherwater-retentionstructure.Con-
sequencesneedtobeconsidered.Willfailureofthe cofferdamresultinuncontrolledreleaseofimpounded waterthatfloodspopulationdownstreamorwithinthe leveedareaorjustthefloodingoftheworkarea?The trade-offsbetweencostandriskreductionneedtobe weighed.Itisoftenadvisabletoinvolvethedecisionmakersintheselectionofdesignfloodsforcofferdams. Ananalysisneedstobedonetodeterminetherisk ofovertoppingduringconstructionandotherfailure modes.
Tomitigatetheriskofbreachingduetoovertopping,manycofferdamsaredesignedwithcomponents toallowcontrolledfloodingoftheexcavationarea. Thecofferdam(fortheOlmstedLocks,Olmsted,IL) inFigure10wasfloodedbeforebeingovertoppedby 2ft(0.6m)for2weeks.Iftheexcavationhadnot beenfloodedbeforeovertopping,thecellularcofferdamsurelywouldhavefailed.Headdifferencewas over100ft(30.5m)fromthesandfoundationexcavationfloortothecofferdamcrest.Aftertheflood,other thantheextensivecleanupofthedepositedmud,only minorrepairofsomefillerosioninafewsheet-pilecell topswasrequired.
Thiscasehistorydemonstratestheimportanceof consideringconstructionriskwhendeterminingthe heightofacofferdam.Thisisespeciallytruewhen thecofferdamreplacesthereservoirdamastheprimarydammingsurfaceduringtheconstructionperiod.Itwasdeterminedthatoutletworksontwoadjacentdryreservoirs(AddicksandBarkerDamsin Houston,TX)requiredreplacementtomitigateaninternalerosionalongtheconduitfailuremode.Numerousvoidshadbeendetectedundertheconduitsand stillingbasins.Atthattime,thesereservoirsweredeterminedtohavethehighestriskintheUSACEportfolio ofdamsduetotheirhighprobabilityoffailureandthe veryhighestimatedlossoflifedownstreamifthedams weretofail.Figure11showsoneoftheoutletworks beforetheconstructionofthereplacement.
Toreplacetheoutletworks,aportionoftheexisting damhadtoberemoved.Tocreateaprotectedwork areaforthedamremovalandoutletworksreplacement,largeearthcofferdamswererequired.Thesecofferdamsnotonlyprovidedaprotectedareaforconstructionbutalsoservedastheprimarydamthat heldbackthereservoirduringconstruction.Figure12 showsadrawingofoneofthecofferdamsrequiredto buildthenewoutletworks.
Aconstructabilityreviewwasperformedaspartof thestudyphaseandtheteamrecommendedthatthe cofferdamsbebuilttomatchthefullheightofthe
dams.Avalueengineering(VE)studywasperformed withamajorrecommendationtoreducetheheightof thecofferdams.TheVEteambelievedthatthecofferdamdidnotneedtobeequaltotheheightoftheexistingdamandthatareductioninheightwouldprovide costsavingswithlittleincreasedrisk.Estimatedsav-
ingswere$1.3millionforeachdam.TheVEteamapparentlydidnotunderstandorappreciatetheconcept ofconstructionrisk.Theproposalwasrejectedbased ontheincreasedliferiskto1.2millionpeopleandthe potential$60billionineconomiclossesintheevent ofamajorfloodevent.Thecofferdamwouldserveas thedamembankmentthroughoutconstruction.Accordingly,theestimatedsavingspaledincomparison tothepotentiallossoflifeandeconomicconsequences ifamajorfloodeventhadoccurredandthecofferdam wasovertoppedorbreached.Thetopelevationofthe cofferdamwasdeterminedbasedonariskassessment duringthestudyphaseandmaintainingthecofferdam elevationthesameastheexistingdamwaswarranted asaresultoftheproject’shighconsequences.
Duringconstruction,thelargestfloodinthe project’shistoryoccurredduringHurricaneHarvey. Between35and40in.(89and102cm)ofrainfell inthearea.Bothreservoirsreachednewrecordpools approximatelyjust10ft(3m)belowthecrest.At oneofthedams,theVEstudy’sproposedcofferdam crestwasbelowthenewrecordpool.Ifthecofferdamsweredesignedwithalowercrestelevation,significantdamageswouldhaveoccurredfromovertoppingand/orincreaseddischargestopreventtheover-
toppingofthecofferdams.Figure13showsoneofthe cofferdamsduringthenewrecordflood.Thiscasehistoryisagoodexampleofthe“donoharm”philosophythatmustbefollowedwhenperformingremedial constructiontoensurerisksarenotincreasedduring construction.
Oneofthekeytaskstohelppreventincreasedrisk duringconstructionistohavemultipleconstructabilityreviewsduringdevelopmentoftheprojectdesign.Ataminimum,constructabilityreviewsshould beperformedduringthestudyphase,aswellasduringthe65percentplansandspecificationdevelopment.Constructabilityreviewteamsshouldinclude personnelwithexperienceinthedesignandconstructionofprojectssimilartotheproposedalternative.Thereshouldalsobegeologistsandgeotechnicalengineersexperiencedinthegeologicconditions attheprojectsite.Duringthestudyphase,alternativerisk-managementplansareformulated.Constructionriskcanbeamajorfactorinselectingtheriskmanagementplan.Ifconstructionisnotconsidered duringthestudyphase,aplanmaybeselectedwhich isnotfeasiblebecauseofintolerableconstructionrisk, leadingtounexpecteddelaysandcostincreases.Duringdesign,specialelementsandspecificationrequirementscanbeincorporatedtomitigateandreduce risk.
Allconstructionrisksmustbeidentifiedandproperlymitigatedtoreducethemtotolerablelevels.Constructionriskcanbeevaluatedusingengineeringjudgmenttoqualitativelycharacterizetheriskorafull eventtreeanalysistoquantitativelyestimatetherisk,
dependingonthecomplexityandlevelofriskinvolved. Methodsforestimatingriskcanbefoundin“Best PracticesinDamandLeveeSafetyRiskAnalysis” (U.S.BureauofReclamation[USBR]–USACE,2019).
Ifthepotentialforconstructionriskisproperly identified,theriskcanoftenbemitigatedorreduced. Afewexamplesarepresentedbelow.
1.ExcavationSequencing.Aslopeexcavationcanbe sequencedtoalwaysbeunloadingthetopofthe slopeinsteadofthebottomwhichmaydestabilize theslope.
2.ExcavationSizeLimits.Thelengthofopentrench excavationscanbelimitedtothatwhichcanbe backfilledthesamedaytominimizeprolonged exposureconditions.USBRdevelopedasolution toimprovetheseismicstabilityofadamby excavatinglow-strengthalluvialmaterialsatthe toeofthedamandreplacingthemwithahigherstrengthconcretezone(Harrisetal.,2013).Figure14showshowthedesignmitigatedtherisk ofslopeinstabilityduringconstructionbybuilding shortsectionsofbracedexcavations(MoormanIslandAuxiliaryDam,Folsom,CA).
3.EncasementWall.Specialtechniquescanberequiredonportionsofaprojecttomitigaterisk. Forexample,ontheEastBranchDam(Wilcox, PA)cutoffwallproject,secantpilesusingthecased method(Figure15)wereusedtoconstructaprotectivebarrieraroundthealignmentforanewdeep cutoffwallinanareaoftheembankmentthatwas damagedbyapreviousinternalerosionincident. Thenewdeepcutoffwallusedthehydromillpanel method.Thesecantpilesprovidedabarriertoeliminatetheriskoftheslurryusedwiththehydromill initiatingerosionintheembankment.
4.ProtectiveGroutLines.Anotherexampleisthe MississinewaLakeDam(Peru,IN)cutoffwall projectwheresignificantslurrylossesoccurred whileexcavatingkarstrockwithahydromillinthe demonstrationsection.Thecontractwasmodified togroutthekarstbedrockbeforeexcavatingcutoffwallpanels.Figure16showsthegroutlinesjust upstreamanddownstreamofthecutoffwallalignment.Thisprocedureisnowcommonlyusedwhen installingcutoffwallpanelsinkarstorhighlyfracturedrocktomitigatetheriskofexcavationcollapse,erosion,andunwantedslurry.
5.WorkSchedule.Often,thescheduleofworkcanbe optimizedtoreducerisk.Thereservoirwatersurfaceelevationmaydrivetheriskduringconstructionandthelikelihoodofreachingvariouseleva-
tionsmayvaryduringtheyear.Thus,onewayto minimizethetemporaryincreaseinriskduringconstructionistoadjusttheconstructionscheduleso thathigh-riskactivitiesoccurwhenthereservoiris likelyatthelowestlevel.Thiscanbeaccomplished byacceleratingtheconstructionscheduletoreduce thewindowoftimetheprojectisatrisk.
6.ReservoirRestrictions.Reservoirrestrictionscanbe implementedtolowerthepooltoreducethefrequencyanddurationthatthereservoirexceedstargetelevationsduringtheperiodofconstruction. Poolrestrictionscaninfluenceallthreeaspectsof theriskequation:
• restrictionscanreducethefrequencyofthe load,
• canreducethemagnitudeoftheloadandlower probabilityoffailure,and
• canreducethepotentialconsequencesdueto lesswatervolumeavailableinabreach.
7.DemonstrationTestSections.Anothermethodof mitigatingriskduringconstructionistorequire
demonstrationsectionstobeconstructedaway fromthepermanentworkorinlesscriticalareas beforegoingintofullproduction.Demonstration testsectionscanbeaproof-of-conceptexerciseand provideearlyinformationtoevaluatethesuitability ofequipment,excavationprocedures,construction techniques,andmaterials.Demonstrationtestsectionsoftenrevealproblemswithaproposedmethod ofconstructionthatcanthenberemediedbefore initiatingfullproduction.Testsectionshavebeen constructedformanyoftheUSACEcutoffwall projects.
8.ReliableCofferdams.Thefacilityownershouldtake responsibilitytoprovidethedesignfloodfortemporarycofferdamswhenportionsofadamorleveeare temporarilyremoved.Thisresponsibilityhasbeen frequentlyassignedtocontractors,sometimeswith negativeoutcomes.Thecofferdamheightshouldbe arisk-informeddecision.Figure17(FolsomDam AuxiliarySpillway,Folsom,CA)isanexampleof spillwayconstructionriskmitigatedbyconstructing afull-heightcofferdamwithreliabilityequivalentto themaindam.
Perhapsoneofthemostimportantaspectsregardingconstructionriskisdecision-makerinvolvement.Decision-makersforeveryorganizationare different.FortheUSACE,theywouldinclude: DamandLeveeSafetyofficersattheDistrictand Division,DamandLeveeSafetycommunityofprac-
ticeleadsatHeadquarters,directorsoftheDamSafety ModificationCenter,andtheRiskManagementCenter.Otherdecision-makerscouldincluderegulators liketheFederalEnergyRegulatoryCommission,local stakeholders,orotherorganizationsforjointmultipurposeprojects.Designersofmodificationsneedtobe awareofdesignandconstructionsituationsandtiming thatcouldresultinanincreaseinrisktothestructure
andthepopulationdownstreamorwithintheleveed area.Attimes,itisnecessarytoacceptahigherlevel ofrisktemporarilytoachievethelong-termbenefits ofriskreduction.However,thedesigneraloneshould notbethejudgeofwhatleveloftemporarilyincreased riskisacceptable,howlongthatriskwouldbepresent, andhowmuchmoneyshouldorshouldnotbespent tomitigatethoserisks.Arisk-informeddecisionon constructionriskcanbemadeifalloftheinformation ismadeavailabletothedecision-makers.Thereare manywaystodealwithincreasedriskduringconstruction;someinvolveadditionalfundingtoconstructengineeredmitigationmeasurestooffsetrisks,andothers involvetheuseofscheduleadjustmentsandconstructiontiming.Alloftheoptionsshouldbeevaluatedso thataninformeddecisioncanbemade.
1.Manyconditionscanleadtoincreasedriskduring construction.
2.Allstructuralmodificationsshouldbedesignedto minimizethepotentialtoincreaserisk.
3.Constructabilityreviewsshouldbeperformedto evaluatethepotentialriskassociatedwiththeremedialmeasure.
4.Qualitativeorquantitativemethodscanbeusedto evaluaterisk.
5.Waystomitigatetheriskshouldbeexplored.
6.Constructiontimingandscheduleadjustments shouldbeevaluatedforeachalternative.
7.Arisk-informeddecisiononconstructionofremedialmeasurescanbemadeifallthekeyinformation isconsideredandmadeavailabletothedecisionmakers.
8.Donoharm;donotincreaseriskorcausedamage whiledoingtheremediation.
MostofthecontentinthiscasehistoryisalsocontainedinarecentUSACEdraftupdatetoChapterH3ConstructionRisk,BestPracticesinDamandLevee SafetyRiskAnalysisthatwaspreparedbytheauthor priortoretiringfromtheUSACE.
Harris,M.J.;Lindquist,E.S.;Jameson,R.;andPorter,T., 2013,Acollaborativesuccess–ConstructionoftheMormon Islandauxiliarydamkey-blockforseismicrehabilitation. 2013 AssociationofStateDamSafetyOfficials(ASDSO)Annual ConferenceProceedings:AssociationofStateDamSafetyOfficials,Providence,RI.
Redlinger,C.G.;Ferguson,K.A.;andBerre,L.M.,2018,80th anniversaryoftheFortPeckDamconstructionslide. 2018AssociationofStateDamSafetyOfficials(ASDSO)AnnualConference:AssociationofStateDamSafetyOfficials,Seattle,WA. Schmunk,R.andMigdal,A.,2020, Humanerror‘clearestcontributingfactor’indeadlyClevelandDamincident: Preliminaryreport:CBCNews.Posted:October8,2020, 9:54AM(PT).https://www.cbc.ca/news/canada/britishcolumbia/cleveland-dam-deaths-human-error-1.5755380 Stroman,W.R.;Beene,R.R.W.;andHull,A.M.,1984,Clay shalefoundationslideatWacoDam,Texas. FirstInternational ConferenceonCaseHistoriesinGeotechnicalEngineering, Vol4: InternationalConferenceonCaseHistoriesinGeotechnicalEngineering,St.Louis,MO,pp.578–586.
U.S.DepartmentoftheInteriorBureauofReclamation (USBR)–U.S.ArmyCorpsofEngineers(USACE),2019, Bestpracticesindamandleveesafetyriskanalysis:U.S.Army CorpsofEngineersRiskManagementCenter. Electronic document, availableat https://www.rmc.usace.army.mil/ Library/RMC-Publications/
KeyTerms: RoughRiver,USACE,Karst,Grouting, CutoffWall,DamSafetyModification
ThispaperisacasestudydocumentinghowtheRough RiverDamSafetyModificationProjectevolvedover time.RoughRiverDamisanembankmentdamlocated inwest-centralKentuckythatisownedandoperated bytheU.S.ArmyCorpsofEngineers(USACE)for floodriskmanagement.Theprojecthasarightabutmentcut-and-coveroutletconduitfoundeduponkarstic limestone.In2012,aDamSafetyModificationReport (DSMR)wascompletedthatidentifiedmultipleinternal erosion-relatedpotentialfailuremodesrequiringmitigation.TheDSMRrecommendedamulti-phasedapproach.ThePhaseIprojectincludedconstructionofa workplatforminordertoperformexploratorydrilling andgroutingfortwopartialgroutlines.ThePhaseII projectwastoconsistofgroutlinecompletion,constructionofafull-lengthcutoffwalloverandaroundtheexistingoutletconduit,groutingwithintheoutletconduit, andconstructionofadownstreamfilter.Basedonencounteredconditions,thePhaseIgroutingcontractwas modifiedtocompletebothpartialgroutlinesforslurry controlpriortocutoffwallconstruction.Duringdesign ofthePhaseIIcutoffwall,theexistingoutletconduit wasdeterminedtobestructurallyinadequatetosupport thecutoffwall.ASupplementalDSMRrecommended anewleftabutmentoutletworkstopermittheexisting conduittobeseveredandtoallowforconstructionofthe fullcutoffwallacrossthedamfoundation.ThePhase IIdesignwascompletedinthefallof2021.Thispaper summarizesthekeyinformation,findings,anddecisions thatinformedthefinaldirectionoftheproject.
RoughRiverDamislocatedinFallsofRough,KY, ontheRoughRiverapproximately60mi(100km) southwestofLouisville,KY,andapproximately120mi (200km)northofNashville,TN.Theprojectisan earthenfloodriskmanagementdamworkingincon-
*Correspondingauthoremail:Steven.w.shifflett@usace.army.mil
junctionwiththreeotherdamsintheGreenRiver Basintoreducefloodimpactslocallyandalongthe OhioRiver.Theembankmentis130ft(40m)talland 1,590ft(485m)longwitha40ft(12m)crestwidth. Asoriginallyconstructed,theoutletworksisinthe rightabutmentandconsistsofacontroltower,cutand-coverconduit,stillingbasin,andretreatchannel. Theoutletworksistheonlymeansofcontrolledflow conveyancefortheproject.Inthefar-leftabutment, thereisanuncontrolled,open-cut,rockspillwaythat is65ft(20m)wideand1,800ft(550m)long.
RoughRiverDamislocatedonthenorthwestern portionoftheMississippianPlateauinKentucky.The RoughRivervalleyisarelativelynarrowentrenchment cutintorockofthelowerandmiddleChesterSeriesof Mississippianagethatdipstothesouthwestatarateof about85ft/mi(16m/km).SeeFigure1forasummary ofthestratigraphiccolumnandgeologicdescriptions forproject.Theprimaryrisk-drivingfailuremodesfor thedamareassociatedwiththeuntreatedBeechCreek Limestone(BCLS)andtheuntreatedHaneyLimestoneunits.Theseunitsarepresentonbothsidesof theincisedvalleybuthavebeenfullyerodedatthevalleycenter.Bothunitsarehighlykarsticwithpinnacledupperrockcontactelevationsandcontinuoussolutionfeaturesalongjointsandbeddingplaneswhere theunitsareexposedtoweathering.TheHaneyLimestoneisknowntocontainmassivekarstfeaturesover 50ft(15m)inwidth.TheBCLSrangesfrom10to15ft (3–4.5m)thickandisknowntocontainkarstfeatures over6ft(2m)thick.Thevarioussandstoneandshale unitsunderlyingthedamgenerallyexhibitreducedhydraulicconductivitywithdepthandarenotconsidered toberisk-drivingfailuremodesforthedam.
Abovethebedrockintheincisedvalley,thereisnaturallydepositedalluvialmaterialreferredtoas“overburden”or“foundationsoils”interchangeablyinthe constructiondocuments.Thedamfoundationsoils varyoneachsideoftheriverchannel.Leftoftheriver channel,thedamwasfoundedon40to50ft(12–15m)ofleanclay,silts,andinterfingeredsandswith sandyclays.Totherightofthechannel,thefoundation
materialisupto30ft(9m)thickandisgenerallycomposedofpoorlydrainingcohesivematerial.
ConstructionofthedambeganinNovember1955 andwascompletedinJanuary1958,withthedam placedintoservicein1960.Thealluvialsoilswereonly excavatedaspartoftherightabutmentoutletworks constructionandwerenotexcavatedduringconstructionofthedam.Thedamwasconstructedwithno coretrench,foundationgrouting,ordentaltreatment acrosstherivervalleybetweentherightabutmentoutletconduitandthelowerleftdamabutment,withthe exceptionofa6ft(2m)inspectiontrenchexcavated intothealluvium.TheoutletworkstowerandapproximatelytwothirdsoftheupstreamportionoftheoutletconduitwerefoundedontheBCLS.Theremaining downstreamportionoftheoutletconduitandtheoutletworksstillingbasinwerefoundedonshale.During foundationmappingfortheoutletconduitexcavation intheBCLS,linearkarstfeatureswereobservedand mappedwithintheoutletconduitexcavation.These featuresweretreatedwithdentalconcrete.Karstfeaturesthatexitedtheexcavationorwereotherwisecoveredwithshalewerenottreated.
Theoriginaldamdesignerswereconcernedwithembankmentstabilityassociatedwiththepoorlydrainingsoilsontherightsideofthevalley.Thedesign
incorporateda2-ft-thick(0.6m)upstreamfilterblanketconstructedabovethepoorlydrainingmaterialextendingtowithin50ft(15m)ofthedamcenterlineto addresstheseconcerns.Downstreamofthedamcenterline,aninclinedfilterand2-ft-thick(0.6m)filter blanketweredrapedoverthefoundationsoilsacross thedownstreamtoeofthedam,andthesealsoextendedtowithin50ft(15m)ofthedamcenterline. Seriesof368sanddrainagewells(wickdrains)were advancedthroughbothblanketstothetopofbedrock betweentheriverchannelandoutletconduit(Figure 2a).Thesanddrainagewellsare12in.(30cm)indiameterandspacedat13ft(4m)fromcentertocenter.
Thereservoirisusedforfloodriskmanagement, andthereservoirsurfacecyclesbetweenhighheadwaterandtailwaterlevelsaspartofroutineoperations. Theupstreamblanketallowsreservoirpressurestobe transmittedtowithin50ft(15m)ofthedamcenterline,chargingthesanddrainagewellsincontactwith theBCLS(Figure2b).TheverticaldrainsinadvertentlyfunctionasinjectionwellscapableoftransmittingcyclicalhighhydraulicgradientswithintheBCLS frombothupstreamanddownstreaminfluences.Once insidethekarstnetwork,thesewaterpressurescan betransmitteddirectlytofoundationsoilsindirect contactwithkarstfeatures.Headlossacrossthedam foundationiscontrolledbythealluvialsoilabovethe BCLSbetweentheblanketdrainsthatisindirectcontactwiththesekarstfeatures.
Piezometersintheembankmentandfoundation strategicallytargettheupstreamblanket,thealluvial overburden,andtheBCLSforfoundationmonitoring (Figure2b).Afterfloodeventsin2007and2011,instrumentsintheupstreamalluvialfoundationbegan todecline,whileinstrumentsintheupstreamblanketmaintainedhistoriclevels,readingapproximately 3ft(1m)belowthereservoir.Instrumentslocated
downstreamofthedamcenterlinetippedintheBCLS alsoexhibitedtrendchangesafterthe2011recordpool event.Thechangesindicatedareductioninheadloss acrossthedamfoundationandincreasedreservoirinfluenceactingonthedownstreamdamfoundation.In 2012,anautomateddataacquisitionsystem(ADAS) wasinstalledatRoughRiverDam.Duringthe2012 stillingbasindewatering,significantandinstantaneous dropsinpiezometer(PZ)levelsoccurredalongtheentirelengthoftheoutletconduitalignment.Theautomatedinstrumentswerealsoabletoinstantaneously
detectgatechangeoperationsinthedamfoundation 35ft(10.5m)upstreamofthedamcenterline,orto within15ft(4.5m)oftheupstreamhorizontalblanket. Thisdatasetwaskeysupportingevidencefordevelopmentofconnectivitybetweenthekarstfoundation, thealluvialfoundation,theupstreamblanket,andthe drainagewells.
In2012,aDamSafetyModificationReport (DSMR)wascompletedthatidentifiedfourinternal erosion-relatedpotentialfailuremodes(PFMs)across thedamfoundationandafifthPFMlocatedalongthe outletconduit.TheDSMRrecommendedexecutinga seriesofcontractstorelocateStateHighway79from thedamcresttotheupstreamslope(PhaseIA),toperformexploratoryfoundationgrouting(PhaseIB),and toemplaceslurry-controlgroutingandinstallacutoff wall(PhaseII)tomitigateprojectrisks.Thescopeof thefuturePhaseIIprojectwascontingentuponthe resultsofthePhaseIBgrouting.Thegroutingwas intendedforfutureslurrycontrol,andthedamwas notgroutedtoclosure.TheDSMRincludedplansfor thefuturecutoffwalltobeinstalledacrossthedam foundationandaroundtheexistingconduit,withadditionalgroutinginsidetheconduit.
Theexploratorydrillingandgroutingproject,referredtoasRoughRiverDamSafetyModification PhaseIB,wascompletedinaccordancewiththe DSMRrecommendationsbetween2015and2017.The goalofthePhaseIBprojectwastoexploreandevaluatesubsurfaceconditionsandtoprovideadditional informationtosupport/validatecutoffwalldesign. Phase1BwasawardedinApril2015.Thebasecontractrequiredtwopartialgroutlinestotaling1,670ft (509m)inlengthtobeplacedparalleltoandoffset fromthedamcenterlineby7.5ft(2.3m)upstreamand 10ft(3m)downstream.Groutholeswereangled20 degreestowardeachabutment,crossinginthehistoric valleyarea.Primaryholeswerespacedat20ft(6m) oncenter.Criticalareaswheretheembankmentwas incontactwiththekarsticlimestoneweresplit-spaced downtotertiaryholesspaced5ft(1.5m)oncenter.Inthebasecontract,thecontractorwasrequired toperformopticalandacousticteleviewer(O/ATV) surveyingforprimaryboreholesandverification boreholes.
Waterpressuretestingandgroutingoccurredinprescribedstagesinthebedrockrangingfrom15to30ft (4.5–9m)withtheintenttoisolatespecificformations tothegreatestextentpossible.Theoreticalgrouting pressuresforeachstagewerecalculatedatthemidpointofthestagebasedon0.5psi/ft(11.5kPa/m) ofsoiland1psi/ft(23kPa/m)ofrock.Groutinguti-
lizedbalanced,stablegrouts,whichwerethickenedas appropriatetoallowthetargetpressuretobeachieved. Therefusalcriterionforeachstagewasdefinedas maintainingaflowoflessthan1gal/min(3.8L/min) heldfor10minutesatthemaximumpressurespecifiedforthegroutstage.Refusalforgravitygroutstages wasdefinedasmaintainingequivalentgravitygrout pressureforthestageasmeasuredfromtheground surfaceelevationwithnogrouttakeforaperiodof 1hour.
Theprojectrequiredtheuseofinstrumentedpackersforallwaterpressuretestingandgroutingoperations.Thedatageneratedbytheinstrumentedpackers weremadeavailablebyanautomatedgroutingmonitoringanddatacollectionsystem,whichenabledrealtimedatacollection,monitoring,reporting,andevaluationofdrilling,grouting,waterpressuretesting,and instrumentationresponses.Duringtheperformanceof theexploratorygroutingprogram,anintegratedelectronicrecordssystemwithageographicinformation system(GIS)interfacewasusedtomanagedataas theywerecollectedtoaidininterpretationanddata management.Thereal-timedatarecordsanddaminstrumentationavailablefromtheADASsystemwere criticalforeffectivereal-timemonitoringoftheembankment,foundationsoils,andbedrockduringconstruction.Severalcriticalpiezometersweremonitored duringoperations,whichresultedinhalteddrillingand groutingoperations.
AlldrillingandgroutingconformedtoU.S.Army CorpsofEngineers(USACE)EngineeringRegulation (ER)1110-1-1807—DrillinginEarthEmbankment DamsandLevees(USACE2014)andEngineering Manual(EM)1110-2-3506—GroutingTechnology (USACE2017).Thegroutprogramwasdesignedto effectivelytreattheweatheredrockinterfacezone whileminimizingtheriskofinducingdamageto thedamembankmentandfoundationbyisolating thesoil/bedrockcontact.Thiswasaccomplishedby isolatingthesoil/bedrockcontactusingagroutinflatedgeotextilebarrierbagacrossthecontactrather thanusingalow-mobilitygroutsocket,andbyfully treatingtheupper15ft(4.5m)ofanygivenbedrock formationincontactwiththeembankmentatgravity pressurebeforeallowingpressuregroutingtooccur inthatarea.Thedrillingandgroutingproceduresfor anygivenboreholewereasfollows:
1.EmbankmentDrilling:Thedamembankmentand underlyingoverburdenweredrilledusingresonant sonicdrillswithaprimary6-in.-diameter(15cm) steelcasingadvancedfirst,followedbyanouter
7-in.-diameter(17.5cm)overridecasing.Theouter casingprotectedtheembankmentandfoundation soilswhiletheinnercasingwasremovedandsampleswereextruded.Thisprocesswasrepeateduntil a4-ft-long(1.2m)socketwasemplacedintocompetentbedrock.
2.InstallationofCasingandRockSocketTreatment: Thetemporarysteelcasinginstalledfromtheresonantsonicdrillingwasrequiredtoremaininplace untilthepermanentcasingwasgroutedintothe borehole.Thepermanentcasingconsistedofa6in.-diameter(15cm)schedule80polyvinylchloride(PVC)multiple-portsleevepipe(MPSP)that wasinsertedinsidethesonicdrillcasingtothebottomoftheboreholesocket.Aninflatablegeotextile barrierbagwasattachedtotheMPSPusingdoublepunchlockclampsateachendofthebag.The barrierbagwasusedtoisolatetheembankmentat therocksocket.TheMPSPwastrimmedsothat thebarrierbagwascenteredatthebedrockcontact withsufficientportsintheMPSPbelowthebag,at thebag,andabovethebagtoaccommodategroutingoftheMPSPintoplace.
3.BarrierBagInflationandCasingGrouting:The barrierbagwasinflatedusingneatcementgrout andadouble-packersetup.Theannularspace aroundtheMPSPpipewasthenbackfilledabove thebarrierbaginstagestonotexceed90percentofthetheoreticalhydrofracturepressure,determinedinaccordancewithER1110-2-1807— DrillinginEarthEmbankmentDamsandLevees. OncetheMPSPwasbackfilled,thesteelcasingwas pulled,andtheannularspacearoundthecasingwas toppedoffwithgrout.
4.RockSocketInterface:Therocksocketwasgrouted atgravitypressurethroughopenportsbelowthe barrierbagusingasingle-packersetup.
5.15ft(4.5m)GravityZone:Oncethegroutreached strengththresholds,therocksocketwasredrilled alongwithanadditional15ft(4.5m)sectionof rockbelowtherocksocketusingwater-actuated, down-the-holehammersorstandardrotarydiamondcoringdrillsinsertedthroughtheMPSP. Theholeswerethenwashed,selectholesweresurveyedusingO/ATVdownholeequipment,andthe holeswerethengravitygroutedtorefusal.Any givenrockformationthatwasexposedtothefoundation/embankmentsoilwasrequiredtobefully isolatedbycompletingallgravitygroutingzones acrosstheexposedformationbeforepressuregroutingwasallowedtocommence.Thedamwasthereforeseparatedbyformationforthepurposeofsequencingthework.
6.RockDrillingandPressureGrouting:Afterthe15 ft(4.5m)gravitystagesmetageandstrengthre-
quirementsforanygivenrockformationexposure, downholedrillingandupstagepressuregrouting couldcommence.Atthispoint,theremainderof theholewastypicallydrilledtofulldepth.Thehole wasthenupstagegroutedinprescribedstagesrangingfrom15to30ft(4.5–9m).Asinglepackercould beusedforthedeepestgroutstageatthebottom ofthehole,butadouble-packerassemblywasrequiredforsubsequentwaterpressuretestandgrout stages.Primaryholeswerecompletedfirst,followed bysecondaryandtertiaryholes.
RoughRiverwasoneofthefirstUSACEprojectsto requiretheuseofaninstrumentedpackeraspartofthe projectspecifications(Figure3).Thecontractordevelopedinstrumentedpackersthatallowedforadirect pressuremeasurementatthepointofdischargebelow thepackerwithinthefluidcolumn.Theinstrumented packerswereusedduringallbarrierbaginflation,casingannulargrouting,waterpressuretesting,androck groutingoperations.Thisinformationwascompared toavailablepiezometricdataforanygivengroutstage toexpediteeffectivepressurecalculationsandtodeterminehydrofracturinglimitsandtargetgroutingpressures.Thisprocesswasmoreefficientthantraditional methodsofmodifyingheadergaugepressuresbased ondynamiclinelosses,particularlywhenmultiple holeswerebeinggroutedsimultaneously.Anotheradvantagewasthatequivalentgravitypressurecouldbe quicklyappliedusingtheinstrumentedpackersrather thanfillingtheentireboreholewithgrout,whichsaved productiontimeandmoney.Theinstrumentedpacker wasalsoabletoquicklydetectpoorpackersealsand groutblow-byinrealtime,resultinginreducedrisksto thedamembankmentandfoundation.
Duringthebasecontractconstruction,USACEdeterminedthekarsticfoundationconditionswarranted expedientcutoffwallconstruction.ThePhaseIBcontractwasmodifiedtocompletebothpartialgrout linesforslurrycontrolinadvanceofthefuturecutoffwalltobeconstructedduringPhaseII(Figure 4).TheprojectwascompletedinApril2017,having successfullyinstalledatotalof308productiongrout holesand20verificationholes.Theseholesrequired 32,422ft(9,882m)ofoverburdensoildrilling,7,477ft (2,279m)ofrockcoring,26,058ft(7,942m)ofrock drilling,andatotalof28,444cf(805.4m3 )ofgrout.
TheBCLSwasfoundtohavehighhydraulicconductivityandcorrespondinglyhighgrouttakeswhere theformationwasunprotectedbyupperrockunits, particularlyinthevicinityoftheexposedbedrock outcrops.Highlyweathered,solutionedbedrockand karstwithclayseams,voids,andinterconnectedpathwayswereencounteredinmostoftheBCLSboreholes
shownonFigure5a.Inmostinstances,theinfilledlayersvariedfromafewinchestoafewfeetthick(severalcentimeterstoameterormore).Thesoilinfillbelowtheupperrockroofwascomposedofverysoft, saturatedfinesiltandclayparticleswithweathered limestonefragments.Theconsistencyofthismaterial andthelocationwithintherockmassareconsistent withfine-grainedfoundationandembankmentmaterialtransportedintotherockmassfromlow-stress zonesincontactwithlimestonesolutionfeatures.
Whileadvancingtheangledgroutholeleftoftheoutletconduit,returnfluidwaslostwhilecoringthrough therocksockettreatmentzone.Perprotocol,thistriggeredmandatorydownstagegroutingatgravitypressuretobackfilltheholeandtoprotectthedamembankment.Duringthisgrouting,apiezometerlocated 72ft(22m)downstreamofthedamcenterlineand tippedintheBCLSwasgroutedup.Thislocationcoincidedwiththeareaofhistorictailwaterconnectivitypreviouslyreferencedfromthe2012dewatering.It wastheorizedthatakarsticconnectionlikelyexisted thatlinkedwaterpressurefromthereservoirtoan outletsomewhereinthevicinityofthestillingbasin. Thewaterreleaseswerereducedtozerobeforeadditionalgroutingwaspermittedinordertoverifyifthere wasadownstreamexitpoint.Theboreholewascored againuntildrillfluidwaslostandgravitygroutingresumed.Within30minutesofinitiatingthenextgravity groutstage,groutemerged500ft(152m)downstream inthestillingbasinapron.Instrumentationresponses totheeventaresummarizedonFigure5b.Inlate 2017,PZ-80wasinstalled25ft(7.5m)downstream ofthedamcenterlinetippedatthealluvialfoundation soil/BCLScontactalongthispathway.Thelower4ft (1.2m)oftheboreholeencounteredgroutabovethe bedrockcontact.Thiswasinterpretedtosuggestthat groutfilledapre-existingvoidpresentatthesoil-rock interfacealongthepathway.
Aftercompletionoftheangledholes,threevertical holeswereinstalledat5ft(1.5m)offsetsalongthe conduit.Holestotheleftoftheconduitadvancedinto theBCLSencounteredinfilledvoidsrangingfrom4to 6ft(1.2–1.8m)thickcharacterizedbynoresistance duringadvanceoftheroto-sonictooling(Figure5c). Theclayinfillpresentdemonstratesthelimitationsfor high-mobilitygrouttodisplacesoilwithinkarstand thehighvolumeofinfillmaterialthatcanremainin placeevenwithsuccessfulgroutingefforts.Itshould alsobenotedthatgroutwasencounteredatthesoilbedrockinterfaceabovetheBCLSsimilartoPZ-80.
DuringgroutingoperationsinboreholeDXP2262, theinstrumentedpackerdetectedcoredrillingoperationsinboreholeDXP2162,located100ft(30m) away.Thecoringoperationwashaltedsoworkcould proceedsafely.Thehole-to-holecommunicationisevidencethatfluidpressurefromtheabutmenthasdirect connectiontotheBCLSbelowtheconduit.Theuntreatedwindowbelowtheconduithasthepotentialto circumventthegroutcurtaininadditiontoconcentratingflowneartheconduit.
TheHaneyLimestoneispresentonbothupper abutmentsofthedamandwasdeterminedtohavea
highconcentrationofkarstfeatures.Mostholesonthe leftabutmentbetweenStations10+00and12+10were downstagegroutedduetofluidloss,holecommunication,and/orholecollapse.Thiswasattributedtoa complexnetworkofkarstfeatureswithintheformation.Thelargestsolutionfeatureencounteredinthe HaneyLimestoneformationwasclayfilledbetween Station10+45andStation10+90andextendingapproximately47ft(14m)deepontheupstreamsideof thecrest(Figure6).AsecondsolutionfeaturewasencounteredonthedownstreamgroutlinebetweenStations11+75and12+10,anditwasestimatedtobeapproximately35ft(11m)deep.Bothsolutionfeatures werefilledwithsoftsandyclaywithatraceofgravel. Otherboreholesintheformationrevealedthatalarge lateralnetworkofinterconnectedvoids(referredtoas asubdrainagenetworkonFigure6)existsinthemiddleandlowerpartsoftheformationwithcavesupto 10ft(3m)indiameter.Thisnetworkislikelypartially hydraulicallyconnectedtotheuppersolutionfeatures andiscapableoftransmittingwaterandpotentially movingsoil.Twogroutoutbreaksoccurredintheright abutmentofHaneyLimestoneabout20ft(6m)below thedamcrest.
TheDSMRassumedconservativecutoffwalllimitswitha1,700ft(518m)rectangularwallextending acrossthefulllengthofthedamfoundationtoatotal
depthofapproximately200ft(61m)intotheSample Sandstoneunit.Thephasedapproachrecommended intheDSMRallowedforthegroutingresultstoinformandrefinethecutoffwalldesign.Evaluationsof boringlogs,waterpressuretestresults,andgrouting resultsconcludedthatgenerally40–60ft(12–18m)of caprockwouldbesufficienttogreatlyincreaserock qualityintheabutmentswherecaprockwaspresent. Thisallowedthecutoffwallgeometrytobemodified intoasteppedconfiguration,resultinginsignificant costsavings.Thegroutingresultswerealsousedto informthelimitsofcriticalareasforthecutoffwall withinthedamfoundation.ThefoundationareabelowtheconduitincontactwiththeBCLS(designated CriticalArea“A”)presentedthemostcomplextechnicalchallengefortheproject,asdetailedinthefollowingsections.
TheDSMRincludedseveralalternativesforconstructingthecutoffwallaroundtheexistingoutletconduitatCriticalAreaA.USACEassembledateam ofexpertscomposedofrepresentativesfromUSACE andexternalconsultantstoevaluatetheDSMRalternativesforinstallingthecutoffwallatCriticalArea Aandtodevelopadditionalmethodsthatcouldbe implemented.Evaluationcriteriafactorsincludedlife safety,constructability,andcost.Theteamofexternalconsultantsrecommendedanalternativetoplace aconcretepluginsidetheconduitandthenuseahydrocuttertoremoveaconduitsectionandconstruct aseepagebarrierthroughtheconduit.Oncethewall wascompleted,theconduitwouldbere-establishedby miningouttheconcreteplugintheconduitandinstallingastructuralliner.USACEfurtherdeveloped theconceptofliningtheconduitanddiscoveredseveralfatalflaws:
1.Theexistingconduitwastheonlymeansofcontrolledflowconveyance.Theconstructionduration tolinetheconduitwouldtakeapproximately1year. Thisperiodoflimitedtonoreleasesthroughthe outletconduitwouldinevitablyresultinsustained uncontrolledreleasesthroughtheunlinedspillway, increasingbothbreachandnon-breachrisksforthe dam.
2.Theexistingconduitwaseggshaped,andfabricated steellinersectionsareround.Theresultinglinerdiameterthatwouldstillallowforastructuralconnectiontotheexistingconduitwouldresultinapermanent40percentreductioninavailabledischarge capacity,increasingriskfordamfailure.
3.Theconduitsidewallrangedinthicknessfrom12in. (30cm)atthecrownto18in.(45cm)atthebase, whichprovidedonlymarginalthicknessthrough whichtoinstallproperlyembeddedsteellineranchoragesnecessarytowithstandthefullhydrostatic loadingcondition.
Basedontheseconsiderations,constructionrisks forthisapproachweredeterminedtobeunacceptable. USACEelectedtoproceedwithdesignofacontinuousconcretepanelconstructedaroundtheconduit combinedwithgroutingtotreatthebedrockbelowthe conduitoutlet(Figure7aandb).Earlyinthedesign, athree-dimensional(3D)finiteelementanalysiswas performedtoevaluatetheexistingconduitcondition. Thestructuralanalysisandsoilloadingswerecalibratedandverifiedtobackcalculatecrackingobserved
immediatelyaftertheconduitwasplacedintoservice. Theoriginalconduitdesignmadeaflawedassumption thatthestructurewouldbeloadedsymmetricallyas anarchanddidnotconsiderthe3Deffectsfromthe rockoutcroptotherightoftheconduit,whichresulted innon-uniformstressregimesactingonthestructure. Theanalysisconcludedtheconduitwasoverstressedin itsexistingconditionanddidnotmeetmoderndesign criteria.
Theteamdevelopedaseriesofrestrictionsand designfeaturestoensuretheconduitwouldnotbe overloaded.Nostructuralreinforcementcouldbecut within2ft(0.6m)oftheconduitsidewalls;thiswas criticaltomaintainingthestructuralintegrityofthe conduit.Thedesignteamalsoanalyzedhowtheconduitwouldbeloadedandunloadedduringexcavation andconcreteplacementtoensureworkableconstructionsequencesthatwouldnotoverstresstheconduit. Thisinformationwasusedtodevelopasteelbracing designtoensuretheconduitwouldnotfailduring excavationandcutoffwallconcreteplacement.This bracingwouldbeplacedinsidetheconduitpriortoexcavationandremaininplaceuntiltheconcretepanel aroundtheconduithadfullycured.Thebracingsystemitselfwouldbeheavilyinstrumentedtomonitor theappliedstressesthroughconstructionandconcrete curing.ThePhaseIIdesignwascompletedinOctober 2017.
DuringthefinalSafetyAssuranceReview(SAR) fortheproject,additionalstructuralcommentsand recommendedchangestotheconduitanalysiswere received.Therewerealsosomeconcernsabouthow thelong-termincreaseinthehydrostaticloadingupstreamofthecutoffwallwasfactoredintothedesign.Inresponsetotheseconcerns,thebracingdesignwasfurtherenhanced,andconcreteliftheights abovetheconduitwerelimited.Theresultinghorizontaljointswouldrequireadditionaltreatmentbetween liftstoensureacompletecutoff.WithinUSACE, additionalconcernswereraisedbasedonrecentinclinometermeasurementsmonitoringthecutoffwall installationatEastBranchDamnearJohnsonburg, PA,whichindicatedmeasurablegrounddisplacements duringcutoffwallinstallation.Thesegrounddisplacements,iftheyweretooccuratRoughRiverDam, wouldresultinadditionaleccentricloadingonthe conduitimposedbythecutoffwallandpotentiallyincreasethestressregimebeyondpredictedlimits.The finalconsiderationinvolvedanincidentwheregroutingnearaconduitcauseda1-in.-thick(2.5cm)steel concretelinertobuckle,resultinginsignificantdamageatthatproject.
Theimplicationsofthesefindingswerecarefully evaluatedanddiscussedwithinUSACE.Theconcernsallcenteredonpotentialdamagetotheexisting
conduit,whichwastheonlywayofmakingcontrolled releasesfromthereservoir.Iftheconduitweretobe damagedduringconstructionorbedamagedbylateral movementofthecutoffwallafterconstruction,there wouldbelimitedmeanstomakeaneffectiverepair. Inaddition,theserepairswouldtakeaconsiderable amountoftime,resultinginriskssimilartothoseidentifiedpreviouslyassociatedwithtakingtheconduit outofcommissiontolinetheconduit.Basedonthese considerationsforconstructionandpost-construction risk,USACEdecidedtonotadvertisethecompleted design.TheapprovedDSMRplanwasthenconsideredincomplete,withadditionaldesignmeasuresrequiredinordertosuccessfullyinstallthecutoffwall forriskmitigation.
Theprimaryriskreductionalternativerequiredfor RoughRiverDamwasthecutoffwall.Additional measureswererequiredinordertodevelopacompleteplantosafelyinstallthecutoffwallacrossthe damfoundationandintheareaoftheconduit.These designmeasureswouldsignificantlyincreasethecost fortheproject.ThisrequiredUSACEtore-initiate theplanningphasethroughaprocesscalledaDSMR Supplementinordertogetthenewdesignmeasures approved.TheDSMRSupplementwasessentiallya planningdocumentdetailingconceptualdesignsofalternativemeasuresandengineering/damsafetyanalysesanddocumentation.Theadditionalmeasures wouldthenbeevaluatedbyUSACE,andselectedmeasure(s)wouldbeofficiallyaddedinconjunctionwith theprimaryriskreductionalternative.Thisrevised TentativelySelectedPlan(TSP)wouldthenbecomparedtothealternativesfromtheoriginalDSMRto makeanewrecommendationforthemostappropriate alternative.
TheProjectDeliveryTeam(PDT)initiallydevelopedfourdesignmeasures,whichincludeda rightabutmentoutletworks,aleftabutmentoutlet works,aspillway-controlledoutletstructure,anda re-examinationofliningtheconduit.Eachoneof themeasureswasdesignedtothesamelevelofdetailsothattheycouldbedirectlycomparedwiththe sameamountofriskanduncertainty.Thetwooutletworksmeasuresalsoincludedtheevaluationof multiplealignmentstoensurethatthemostefficient alignmentwasevaluated.ThePDTinitiallyconsidered straighttunnels,butthroughevaluationofsiteconstraints,theseconceptsresultedinmuchlongertunnels,greaterexcavationquantities,andhighercosts. Ultimately,itwasdeterminedthatsignificantcostsavingscouldbeobtainedbyutilizingacurvedtunnel alignmentfortheleftandrightabutmentoutletworks
measures.Beforeimplementingsuchasolution,a prototypeprojectwasidentifiedatYatesvilleDamin easternKentucky.ThePDTcompletedasitevisitto thisprojecttotalktooperationsanddamsafetypersonnel,tourthefacility,andgatherfeedbackregarding operationandmaintenanceofthestructure.Toexpeditetheplanningprocess,USACEcarefullyplanned andexecutedanenhancedconstructabilityevaluation anddesigncharrettetoevaluateeachdesignmeasure withrepresentativesfromthePDT,USACEtechnicaladvisors,costengineers,operationspersonnel,and constructionpersonnel.Thegroupworkedtorefine eachmeasure,workingthroughmultiplescenariosand constraintsforeachdesignmeasure,andimplementing designchangesinrealtime.Costengineersupdatedestimatesinrealtimebasedonestimatedquantitiescalculatedduringthemeetingasthemeasuresevolved.
Eachofthemeasuresincludedtheinstallationof acutoffwallthatseveredtheconduit;therefore,it wasagreedwithinUSACEthattheprojectriskat completionwasessentiallythesameforeachmeasure.Measureswereultimatelyevaluatedbasedoneffectiveness(technicalfeasibility),acceptability(environmental/realestateimpacts),andefficiency(project cost,lifecyclecost,andcutoffwallimpacts).Theleft abutmentoutletworksmeasureandthecutoffwall weredeterminedtobethepreferredalternativeand underwentariskevaluationfordirectcomparisonto theothernon-structuralalternativesidentifiedinthe originalDSMR.Thepreferredalternativewasthen comparedtothealternativesevaluatedintheoriginalmodificationstudy.Theprimarycomparisoncategoriesusedtojustifythepreferredalternativewere projectcost(adjustedforinflation),performancerisk, andbenefit/costanalysis.TheUSACEDamSafety SeniorOversightGroup(DSOG)endorsedthepreferredalternativeinNovember2018.Duringthemeeting,DSOGchangedtheDamSafetyActionClassification(DSAC)from2toDSAC3basedonimproved instrumentationresponsesobservedduringthePhase Igrouting.TheSupplementalDSMRrequiredthedesigntobedevelopedtothe30percentlevelandacertifiedcostestimatetobeobtained.TheSupplementalDSMRwascompletedin2020,approvedbyUSACEHeadquartersinFebruary2021,andendorsed bytheAssistantSecretaryoftheArmy,CivilWorks (ASA[CW])inOctober2021.Thefinaldesignwas completedinSeptember2021,andtheprojectiscurrentlywaitingonappropriation.
ThePhaseIIprojectnowincludesconstructionof anewleftabutmentoutletworksfollowedbyinstallationofafull-lengthcutoffwallthatwillseverthe
existingoutletconduitandmitigatePFMsassociated withthekarsticfoundationbedrockbelowthedam (Figure8aandb).Thecutoffwallcanbeconstructed withhydromillpanels,secantpiles,oracombination ofthetwomethods.Completionofthenewoutlet workswillrequireanewapproachchannel,control tower,servicebridge,outletconduittunnel,stilling basin,concrete-linedapron,andretreatchannel.The newoutletworksmustbeconstructedandoperational beforethecutoffwallcanbeinstalled.Completionof thecutoffwallwillrequireabandonmentoftheexistingrightabutmentoutletworks,constructionofa workplatform,constructionofthecutoffwallacross thedamfoundationthroughtheexistingoutletconduit,backfillingoftheexistingretreatchannel,andrelocationofStateHighway79onthedam.
Acrossamajorityofthedam,instrumentationdata generallysupporttheconclusionthatheadlosshas improvedacrossthedamfoundationaftercompletion ofPhaseIgrouting.Figure9summarizesthebeforeandafter-groutingconditionsfortheright-sideBCLS unit.InstrumentslocatedinthedeepvalleyfoundationsoilswheretheBCLSdoesnotexistshowedno changeinphreaticsurfaceaftergrouting.Profile1on Figure9showsinstrumentstippedinthefoundation soil20to33ft(6to10m)upstreamofthedamcenterline,withinstrumentationresponsesbeforegroutingshowningreenandthoseaftergroutingshownin blue.Priortogrouting,PZ-24andPZ-36hadadrawndownphreaticsurfaceindicatingtheywereinfluenced bytheinterconnectedkarstnetwork.Aftergrouting, therewasanincreaseinthephreaticsurface,indicatingthatthedownstreamconnectionhadbeenpartially disrupted.Notethattheinstrumentationstillreflectsa drawn-downconditionthatislikelyinfluencedbyremainingconnectivitytokarstwithinthedamfoundation.Thisconnectivityislikelyinfluencedbythearea belowtheconduitwheregroutingcouldnotbeperformed,whichallowsseepagetocircumventthegrout curtain.
Onprofile2,located25to47ft(7.5to14m)downstream,thephreaticsurfaceindicatesconcentrated flowneartheconduitasmeasuredbyPZ-80,whichis tippedatthealluvialfoundation/bedrockcontactlocated25ft(7.5m)downstreamofthedamcenterline. TherearenoreadingsforPZ-80priortogrouting,but only8ft(2.4m)ofheadlossisnotedbetweenPZ-80 andPZ-36acrossthedamcenterline.Thelower4ft (1.2m)ofPZ-80encounteredgroutabovethebedrock contact.Therewasalsogroutnotedabovethebedrock contactneartheconduitatthedamcenterline.This suggeststhataseepagepathwayexistsbothintoand
outofthebedrockandalongthefoundationcontact. Clayinfillingandalluvialsoilincontactwithgrout areatrisktoformvoidsandnewpathways.PZ-80is consideredtorecorddirectevidencethatinternalerosionhadinitiated,continued,andprogressedpriorto grouting.Theareaofnogroutingbelowtheconduit concentratesflowwherethedamismostvulnerable.It isimportanttoconsiderthatgroutingisonlyatemporarymeasureanddoesnotpreventthemechanisms thatledtotheinitiationandprogressionofinternal erosion;groutingsimplyarrestsdevelopedpathways.
EffectivelyplacedinstrumentationintheRough RiverDamProjectindicatedacontinuousupstream todownstreamconnectionfromthereservoirpoolto tailwaterbelowthedam.Thepresenceofextensive karst,progressinginternalerosion,andthepotential foranunfilteredexitwereconfirmedthroughfoundationgroutingobservations.Massivekarstwasconfirmedtoexistintheupperleftabutment.Themoderngroutingmethodshavebeentemporarilysuccessfulatfillingsomeopenvoidsandincreasingheadloss acrossthedamcenterline.Thenotedimprovementin instrumentationresponseisconsideredtobetemporaryduetotheextentofclayinfillingandkarsticvoids documentedinandabovethekarsticlimestoneformations.Post-groutingevaluationsindicatedthatpreferentialseepagepathwaysstillremainwithinthekarst, atthefoundationcontact,andwithinthezoneofno
groutingbelowtheconduit,whichwillexpeditefuture erosion.Ifahighpoolevent(asoccurredin2007or 2011)weretooccuragain,itisanticipatedthatinstrumentationdeclinesintheupstreamfoundationinstrumentswouldonceagainresult,potentiallyatafaster ratethanpreviouslyobserved.Theprojectrequires afull-lengthcutoffwalltopermanentlyreducerisks tobelowtolerableriskguidelinesasestablishedby USACE.
Consistentcommunicationandcoordinationto makethebestdecisionspossibleinthemomentare keyfactorsincompletingasuccessfuldamsafetymodificationproject,especiallywhendealingwithchanges inscope,schedule,andbudget.Whilethedecisionto notsolicitthe2017cutoffwallprojectwasdifficultto make,itwasthecorrectdecision,anditisanexampleoftheUSACEprocessworkingasintended.The revisedprojecthasincreasedinscope,schedule,and budgetbutcannowbeconstructedinamuchsafer manner.Therevisedprojectalsohasreducedlongtermoperationandmaintenancecosts,removesthe needforplannedfutureremediation,andstillachieves thesame,orbetter,levelofriskreductionupon completion.
ER1110-2-1807,2014,DrillinginEarthEmbankmentDamsand Levees,USArmyCorpsofEngineers
EM1110-2-3506,2017,GroutingTechnology,USArmyCorpsof Engineers
MARKS.ELSON*
U.S.ArmyCorpsofEngineers,NashvilleDistrict,1109thAvenueSouth,RoomA520, Nashville,TN37203
STEVEND.WIDINCAMP
U.S.ArmyCorpsofEngineers,SavannahDistrict,100WestOglethorpeAvenue, Savannah,GA31401
KeyTerms: SonicDrilling,EmbankmentDam,Levee, DrillingMethod,SoilSampling,RockCoring
Sonicdrilling,augerdrilling,andcable-tooldrilling arethepreferredmethodsutilizedtodrillthroughleveesandembankmentdams.Sonicdrillingisthemost recentlydevelopedofthethreeandhasdistinctadvantagesovertheothermethods.Notably,sonicdrilling offersahighrateofadvancethroughmostmaterials whilestillprotectingembankmentsfromerosionandhydrofracturing.Themethodisnotwithoutlimitations,as costandspaceconstraintspreventitfrombeingthemost effectivechoiceinsomecircumstances.
Theembankmentdrillingguidancedocumentsfrom theFederalEnergyRegulatoryCommission(FERC), U.S.BureauofReclamation(USBR),andtheU.S. ArmyCorpsofEngineers(USACE)identifythree preferreddrillingmethodsforadvancingborings: hollow-stemaugers,cable-toll(churndrills),andsonic (FERC,2016;USACE,2014;USBR,2014).Ofthese, sonicdrillingisthemostrecentlydevelopedmethod andoffersseveraladvantagesoverhollow-stemauger boringsandcable-tooldrilling,althoughitdoeshave limitations.Itisalsonotedthatcable-tooldrillingisa relativelyarchaicmethodandisnotgenerallyusedfor geotechnicalexplorations.
Sonicdrillingutilizeshigh-frequencymechanicaloscillationsthataredevelopedinthedrillhead.Theseoscillationsaretransmittedasresonantvibrationsand, witharotaryaction,aretransferredthroughthetoolingtothebittopenetratesoilorrock(Figure1). Withintheboring,vibratoryactionfluidizessoilparticles,reducingshearstrengthandpushingtheparticles
awayfromthedrillbitandalongthesidesofthedrill string(TerraSonicInternational,2022).
Thedrillingprocedurebeginswithadvancingthe corebarrelfromthegroundsurfaceintotheformation(Figure2).Typically,nofluidsarenecessaryto advancethecorebarrel.Thisinitialadvancementis limitedtothelengthofthecorebarrel,approximately a10-ft(3-m)run.Largersonicdrillrigscanadvance uptoabout20-ft(6-m)runs.Leavingthecorebarrel inplace,alargerdiameteroutsidecasingisadvanced aroundthecorebarreltothesamedepthasthetipor bitofthecorebarrel.Alimitedamountofwatermay beusedtolubricatethecasing’sinteriortoreducefrictionagainstthecorebarrel.Whenneeded,drillfluidis pumpedthroughthesonicheadviaawaterswivel,like aconventionalrotaryrig,andcirculatedintotheannularspacebetweencorebarrelandcasing.Drillfluid pressureismonitoredbyagaugeandcontrolledbya valveattheoperator’spanel.Sincetypicallylowquantitiesofdrillfluidareusedforlubrication,thefluid usuallydissipatesintotheformationandrarelyreturns tothesurface.Despitethelowvolumeofwater,the drillerwillstillneedtomonitorpressurescloselyto preventhydrofracturingtheformation.Thecorebarrelcanthenberetrievedandthesampleextrudedwithoutconcernoftheholecollapsing.Afterretrievingthe sample,thecorebarrelisloweredbackdownthehole andfurtheradvancedintotheformation;theabove processisrepeateduntilthefinaldepthoftheboring isreached(TerraSonicInternational,2022).Ondeep holes,thisprocesscantakesometimebecause,foreach drillinginterval,theentirecoreroddrillstringhasto beremoved(trippedout)andthenreinserted(tripped in)intheholeforeachrun.
Sonicdrillinghasseveralbenefitsoverotherpreferredembankmentdrillingmethods.Theseare,notably,thedrillingrateofadvance,theabilitytodrill throughmostmaterials,precisionofthedrilledhole,
*Correspondingauthoremail:mark.s.elson@usace.army.milThehighrateofadvanceofsonicdrilling(compared toothertypesofdrillingmethods)isabigadvantage oftencitedforselectingthismethod.WhenN-values fromastandardpenetrationtest(SPT)orother insitu testingorsamplingarenotrequired,theholecanbe advancedasfastasthecorebarrelcanpenetratethe formation.Sincethecorebarrelisusuallyadvancedin 10-or20-ft(3-or6-m)runs(intervals),thefrequency thatdrillingisstoppedtoaddadditionaldrillrods ishalforevenone-fourthasoftenascomparedwith adding5-ft(1.5-m)flightstoastringofhollow-stem augers.Requirementsforfrequentsamplingwillsignificantlyreducetherateofadvancementbutisnearly alwaysfasterthanotherdrillingmethods.Usingdrill waterwhenadvancingthecasing,asdescribedabove, willhelptomaintaintherateofadvancement.Ifuseof drillwaterisprohibited,therateofadvancementwill likelybereduced,especiallyindensesandandclay.
Thecombinationofbothvibrationandrotationallowsthedrillbittopenetratealmostanykindofsubsurfacematerial.Thiscanbeasignificantadvantagein rockandrandomfillwithlargeboulders,whichcould oftenmeanhollow-stemaugerrefusal.Theuseofdrill waterwhileadvancingthecasingwillmakepenetrationeasier,butdrydrilling(usingnodrillwater)isnot likelytocauseprematuredrillingrefusal.
Frictionbetweencasingandcorebarrelcanreduce casingadvancementpenetrationratesandcauseother concerns.Soniccasingandcorebarrelsareavailablein numerousdiameterswhichallowfordown-telescoping ofthedrillstringtoreachgreaterdepthsandmitigate difficultdrillingconditions.
Drillingandmaintainingastraightdrillholecanbe criticalwhendrillingnearstructureswithintheembankment,suchaspipepenetrations,drains,anddischargetunnels.Locatingdrillholesnearthesestructurescanbeimportantwhenmeasuringsoilpropertiesandinstallinginstrumentstoevaluateassociated potentialfailuremodes.Hollow-stemaugershavean especiallydifficulttimemaintainingastraightverticaldrillhole,astheirconnectionsaretypicallyloose andtheyarevulnerabletodriftinganddeviationswith changesinsoilconditions.
Sonicdrillingusuallywillmaintainastraighterdrill holethanotherfavoredembankmentdrillingmethods.Duetothevibrationusedtoadvancethedrill hole,drillrodsusedinsonicdrillingmustberelatively thickandthethreadedjointsmustbetighttotransfer thevibrationsthroughthedrillstring.Becauseofthis, sonicdrillingequipmentproducesastiffdrillstring thathelpsmaintainastraightboring.Inaddition,the rotationofthedrillrodsisrelativelylow,whichkeeps torquetoaminimum,therebyreducingthetendency forthedrillstringtocorkscrew.
Oncethecorebarrelisextractedfromthehole,the sampleisextrudedbyvibratingthecorebarrel.This causesthecollectedsampleinthecorebarreltoslide outthebottom.Themostcommonmethodtocollect thesampleisinplastictubing,butpolycarbonatelinerscanalsobeinsertedintothecorebarrelforsample collection.Rockcorescanbeextrudeddirectlyintoa troughorcoreboxforlogging.
Sonicdrillingcanprovideacontinuouscoreofsoil thatinsomecasespreservesmuchofthesoilstructure,althoughitisconsideredadisturbedsampleand
cannotbeusedforundisturbedlaboratorytests,such astriaxialtests.SeeFigure3foranexampleofarecoveredclaysample.Whendrillinginnon-cohesivematerials,steelandplasticcorebasketscanbeusedto augmentsampleretention.Looseningorexpansionof samplescanbeobservedinsomematerialsasa10-ft (3-m)runcouldyielduptoa12-ft(3.7-m)baggedsample.Ifstrengthtestsarerequired,itisstillbesttorely onothersamplingmethods.
Fortunately,mostsonicdrillrigscanperformnumeroustypesofspecializedsampling, insitu testing, andinstrumentinstallationsthatarecommonlyused ingeotechnicalexploration.ThesesamplingandtestingmethodsincludeSPT,Shelbytubesampling,flat dilatometertest,pressure-metertest,andothermethods.Inaddition,sonicdrillscandrillandsample angledholes,whichisasignificantadvantageover hollow-stemaugersandcable-tooldrills.
Rockcorescanbetakenwithsonicdrilling,but thevibrationoftendamagesthecores.Althoughusuallymorefracturedthanwithwirelinecoring,recoverycanbeverygoodandthelackofcirculatingdrill waterallowsforrecoveryofsoftmaterialsthatmay fillvoidsintherock.Foranexample,Figure4shows rockcoredwithasonicdrill.Thecorerecoveredis morefracturedthanthe insitu rock,makingconditionsappearworsethanreality.Coresrecoveredfrom sonicdrillingshouldnotberelieduponforcalculating rockengineeringpropertiessuchasrockqualitydesignation(RQD),rockmassrating(RMR),orQ-system, sincethevibrationfromthedrillstringwillusually induceadditionalfractures.However,downholegeophysicalmethodscanbeusedtoprovidesomeofthe
informationlostfromthesonic-drill–damagedcores. Whenusingwirelinerockcoringmethodsbeneathan embankment,casingwillbeneededtosealthetopof rocktoprotecttheembankmentfromthedrillwater return.Withasonicdrill,youcanpenetratethetopof rocktogetpasttheweatheredzoneandgroutinPVC casingbeforeswitchingtoolsforwirelinerockcoring andtoprotecttheembankment.
Incircumstanceswheredetailedcoreloggingofthe rockisnotaconcern,sonicdrillingmayprovidearock coreofsufficientqualitytoidentifyappropriatesensingzonesforapiezometerorverifythattherockisof sufficientqualityforsocketinganinclinometerintothe rockfoundation.
Whileretrievingacontinuoussampleisanadvantageofothermethodsofdrilling,itisimportantto rememberthatsamplesfromthesoniccorebarrel aredisturbedsamples.Inaddition,fieldteststomeasuresoilstrengthshouldbeusedwithcaution,ifnot avoidedaltogether.PocketpenetrometerandTorvane testsonsamplesretrievedbysonicdrillingarelikely tohavelowervaluesfromthevibrationofthedrill looseningthesamplethanwouldberecordedifmeasuredfromasamplerecoveredinasplitspoonor Shelbytube.Inaddition,theupperportionsofSPT andShelbytubesamplestakenfromsonicboringsmay notproviderealisticstrengthvaluesifthevibration fromadvancingthesoniccorebarrelhasdisturbed thezoneofsoilimmediatelybelowthebit.Thesevaluesarelikelyadequateforgeneralanalysisandrelativestrengthbutmaynotbesuitableforhigh-precision analysessuchasliquefactionpotential.
Sonicdrillingisoftenconsideredadrymethod. Whiledrillwaterdoesnothavetobeused,itdoeshelp withlubricationandpreventsexcessiveheatatthedrill bit.Thisisespeciallytruewhendrillinginverydense sands,clays,androck.Theamountofdrillwaterand pressureneededtokeeptheannulusclearisdependent ontheformationandfeedpressuresusedbytheoperator.Whiledrillinginmostformations,drillwaterpressurestypicallyremaininthe35–50psi(241–345kPa) range,buttheycanspikeupto200–300psi(1,379–2,068kPa)whendrillingthroughclaysordensesands. Thesamecanbesaidwiththeamountofdrillwater; agoodaverageis20–30gallons(76–113.6L)whileadvancinga10-ft(3-m)lengthofcasingbutthiscanget ashighas100gallons(378.5L)inloosesands.Drill
waterusageneedsmustbekeptinmindwhendevelopingadrillingprograminornearadamorleveeembankment,andallowablefluidpressurescalculated,in casedrillwaterisrequired.
Full-size,truck-mounteddrillrigsareveryheavy andrequirealotofspace,withalargesupporttruck usuallypositioneddirectlybehindthedrillrigholdingtheheavydrillcasing.Evensmaller,track-mounted sonicdrillrigsneedtobepositionedonarelatively levelsurface.Thisworksoutfinewhendrillingfrom thecrestortoeoftheembankmentbutmaynotbe idealwhenboringsarelocatedontheupstreamor downstreamslope.Positioningadrillrigonaslope usuallyrequiresjackingthedrillrigupontimbersor constructingalevelaccessroadontheslope.Theembankmentownermaynotallowadrilling-accessroad tobeconstructedintoorontheembankmentandlevelingalarge,heavydrillrigwithtimbersrequirescarefulplanningandisverytime-consuming.Figure5, left,showsatrack-mountedsonicdrillatthetoeof anembankmentandthespaceneededforsupporting drillingequipmentandtools.Ontheright(Figure5),a smallerrotarydrillisleveledusingtimbers,something thatmaynotbepossiblewiththelargerandheavier sonicdrill.
Sonicdrillingtendstobecostly.Asonicdrillcan costamilliondollars,whileatypicalrotarydrillmay beabouthalfthatcost.Thedrillitselfisexpensiveand requiressupportequipmenttohandletooling,especiallythelargerdiameters.Whileithasarapidpenetrationrateinawiderangeofmaterials,shallowholesare generallycost-prohibitiveduetoincreasedsetuptime withsonicdrills.Inaddition,obtainingfrequentSPTs, Shelbytubesamples,orother insitu testingorsamplingwilldecreasethedrillingefficiencyandreducethe advantageofrapidholeadvancementandshouldbe consideredwhenselectingthedrillingmethod.
Sonicdrillinghasbecomeago-tomethodfor drillingindamandleveeembankments.However, carefulplanningandevaluationoftheconditionsare stillrequiredtodeterminetheappropriatedrilling method.Thedrillingprogramdesignerwillneedto weightheneedsoftheprojectagainsttheprosand consofthedrillingmethodsbeingconsideredfor use.High-precisionrequirements,aneedtoexpedite theprogram,and/orchallengingconditionsmaybe thedeterminingfactorsthatnecessitatetheuseof sonicdrillingversusotherdrillingmethods,despitethe higherper-footcost.Anattempttoexecuteachallengingdrillingprogramwithalessexpensivemethod couldfailtoachievetheprogram’sgoalsorevenplace thestabilityoftheembankmentatrisk.Ineithercase, furtherexplorationlikelyusingsonicdrillingmaybe necessary,thuscostingtheprojectevenmoreintime aswellasmoney.
Whenevaluatingpotentialdrillingmethodsforan explorationprogram,thedesignershouldbeencouragedtodevelopacomparisonofcost,schedule,and riskimpactstotheprojectforeachdrillingmethodbeingconsidered.Whilethecostofsonicdrillingmaybe higher,scheduleimpactsorriskstothestructuremay outweighthecost,makingsonicdrillingthepreferred methodinmanycircumstances.
Projectschedule,drillaccess,cost,samplingtype andfrequency,andwhetherwaterwillbeallowedmust allbeconsideredwhendevelopingthedrillingprogram.Inmanycases,sonicdrillingwillbethemost effectivemethodtoobtaintheneededinformationbut willnotbetheanswerforeverysituation.
TheauthorsgratefullyacknowledgeM.M.Goff (U.S.ArmyCorpsofEngineers[USACE],Headquarters),G.D.Rogers(SchnabelEngineeringSouth), andW.D.Mackie(USACE,NashvilleDistrict [LRN])fortheircontributionstothispaper,andto TerraSonicInternationalforuseoftheimagein Figure2.
Bruce,D.A.andDespres,D.L.,2004, Drillingandsampling ofembankmentsusingthesonicdrillingmethod: Association ofStateDamSafetyOfficials(ASDSO)AnnualConference, Phoenix,AZ,September26–29,2004.
FederalEnergyRegulatoryCommission(FERC),2016, Guidelinesfordrillinginandnearembankmentdamsandtheir
foundations:Electronicdocument,availableathttps://www. ferc.gov/sites/default/files/2020-04/guidelines.pdf
TerraSonicInternational,2022, Howsonicdrillingworks:Electronicdocument,availableathttps://www.terrasonicinter national.com/tsi-sonic-technology/how-sonic-drillingworks/
U.S.ArmyCorpsofEngineers(USACE),2014, Drillinginearth embankmentdamsandlevees,ER1110-1-1807:Electronic
document,availableathttps://www.publications.usace.army.mil/ Portals/76/Publications/EngineerRegulations/
U.S.BureauofReclamation(USBR),2014, Guidelinesfordrilling andsamplinginembankmentdams: Electronicdocument,availableathttps://www.usbr.gov/tsc/techreferences/mands/ mands-pdfs/GuidelinesForDrillingSamplingInEmbankment Dams_April2014v2_508.pdf
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Exploratory drilling at the downstream toe of the TVA’s Little Bear Creek Dam, Alabama. Courtesy of Mark Bauer. See article on page 17.