KevinS.Richards,Ph.D.FederalEnergyRegulatoryCommission

Prof.KrishnaR.ReddyCivilandMaterialsEngineeringUniversityofIllinois,Chicago

September24,2009

PresentationOutline1. Introduction2. Definitions3. StatisticsofPipingFailures4. EarlyWork5. DispersiveSoils6. AdvancesinFilterDesign/InternalStability7. RecentWork8. Conclusions9. OngoingWork

1.INTRODUCTION

Historyofpipingfailuresresultedinprogressivelymorecomplexdesigns

Pipingoccursinthreestages; Initiation Continuation Progression

Thereareanumberofdifferentmodesofpiping

2.DEFINITIONS BackwardsErosionPiping InternalErosion

biologicallyornaturallyinduced withinembankmentorfoundation

Tunneling/Jugging Suffosion Heave

PipingProcess(BackwardsErosion)

UnfilteredExit

PipingProcess(InternalErosion)

UnfilteredExit

GateWell

Conduit

PipingProcess(Tunneling/Jugging)

PipingProcess(Suffosion)

UnfilteredExit

PipingProcess(Heave)

LeveeFill(Clay)

SiltRiverDeposits

SandandGravelRiverDeposits

(highporepressures)

3.STATISTICSOFPIPINGFAILURES 46%ofalldamfailuresareduetopiping1 Basedondetailedanalysis2

Foundationrelatedfailures(15.0%) InternalErosion(49.8%) BackwardsErosion,Suffosion(31.1%) BiologicalActivity(4.1%)

1Foster,Fell,andSpangle,20002RichardsandReddy,2007(267casehistories)

4.EARLYWORK VonRichthofen,1886 Coinedpiping,Loess,Landforms Col.Clibborn,1895 L/H,1898failureofNaroraDam Forchheimer,1900 Flownets Bligh,1910 CreepMethod,Designofspillways,L=cH Lane,1934 WeightedCreepMethod,Designofspillways,Ln=1/3LHoriz.+Lvert.

Terzaghi,1922,1939,1948 Theoryofheave,soilfilters,factorofsafetyforpiping(byheave)

Casagrande,1937 Exitgradientsw/flownets Bertram,1939 Filterexperiments

ImportantFindingsFromEarlyWork

L/HClibborn,1895;importanceofhydraulicgradient L=cHBligh,1910;L=requiredsafeflowlength,c=percolationfactor,variesfrom18(finesiltandsand)to4(bouldersgravelsand),H=hydrostatichead

Ln=cHLane,1934;Ln=requiredsafeflowlengthbasedonweightedlengths,c=percolationfactor,variesfrom8.5(veryfinesandorsilt)to1.6(hardclay),H=hydrostatichead

D15/d854Terzaghi,1939

ImportantFindingsfromEarlyWork Gs=(W/Ue)(Terzaghi,1943;Gsisthefactorofsafetyagainstpipingbyheave,W istheeffectiveweightofmostcriticalsandprism,Ue istheupliftpressureatthebaseofthesandprism

mostpipingfailuresoccurathydraulicheadhc muchsmallerthantheheadhc computedonthebasisoftheory Terzaghi&Peck(1948)

5.DISPERSIVESOILS Aitchison,1960Pipinginvolvesdispersionofclays Ritchie,1963DispersionIndexMethod 1964AustralianSymposiumtoDiscussFailureofSmallDams

Sherard,etal.,1972PrecursortotheDoubleHydrometerTest

RileyandArulanandan,1972 RotatingCylinderTest Sherard,1976PinholeTest 79th AnnualASTMMeeting,1977(ASTMSpecialTechnicalPublicationNo.623)

6.ADVANCESINFILTERDESIGN/INTERNALSTABILITY

Filters BourdeauxandImaizumi,1977Conventionalfiltercriteriawillcontrolleaksthroughdispersiveclay

SherardandDunnigan,1984,1989 Noerosionfilterdesignconsideringbasesoilcharacteristics

InternalStability Kezdi,1979 Internalstability,Suffosion SkemptonandBrogan,1994 Horizontalversusverticaltests;criticalhydraulicgradientofunstablesoils,reductionfactorforconfiningstress

OtherAdvances Terzaghi,etal.,1996

Mostofthepipingfailuresonrecordhavebeencausedbysubsurfaceerosioninvolvingtheprogressive

removalofmaterialsthroughsprings;thisconditioninvalidatesthetheoryofpipingbyheave.Thefactorofsafetywithrespecttopipingbysubsurfaceerosion

cannotbeevaluatedbyanypracticablemeans.

ImportantFindings LimitationsoftheTheoryofHeave RoleofDispersiveSoils EvaluationofInternallyUnstableSoils LimitationsinSoilFilterDesign/SoilClogging

7.RECENTWORK TomlinsonandVaid,2000UniaxialstressvsCriticalGradient.

Briaud,Ting,Chen,Cao,Han,andKwak,2001ErosionFunctionApparatus

WanandFell,2002HoleErosionandSlotErosionTest Fell,WanandCyganiewics,andFoster2003;Annandale,MurthyandBeckwith,2004Timeofpipedevelopment

McCook,2004 Geneticcharacteristicsofpiping,importanceofdamconduitfailures

ReddiandKakuturu,2006;Bonala,1997;Reddi,MingandHajra,2000 Particletransportandfilterclogging

ImportantAspectsofRecentWork Importanceofdifferentpipingfailuremodes Predictionsofpipingbreachtiming Predictionofcloggingoffilters Riskbasedassessmentofpipingfailure

8.CONCLUSIONS Nosatisfactorymethodforcalculatingafactorofsafetyforbackwardserosionpiping

Constitutiveaspectsofpipingareunknown Influenceofseepageorientationhasnotbeenquantitativelystudied

Nolaboratorymethodsfortheevaluationofpipinginnoncohesivematerials

Roleofvarioustypesoffinesisunknown

9.ONGOINGWORK ICODS EvaluationofPipingGuidelines ICODS FilterGuidelines LaboratoryinstrumentationforbetterevaluationofbackwardserosionandSuffosionpotential RichardsandReddy,DevelopmentofTrueTriaxialApparatusfor

DeterminingPipingPotentialinEarthStructures(submittedforpublication)

Assessmentofcriticalfactorsinfluencingpiping RichardsandReddy,ExperimentsinBackwardsErosionPiping

(submittedforpublication)

Newmethodforevaluationofbackwardserosionpipingpotential RichardsandReddy,NewMethodforEvaluationofBackwardsErosion

Piping (underdevelopment)

Questions?

Reference Paper - Richards, K.S. and Reddy, K.R., 2007, Critical appraisal of piping phenomena in earth dams, Bull. Eng. Geol. Environ., 66:381-402