ARMA-01-0951_Design and Analysis of Foundation Modifications for a Buttress Dam

8/12/2019 ARMA-01-0951_Design and Analysis of Foundation Modifications for a Buttress Dam

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RockMechanicsn the National nterest,Elsworth,Tinucci& Heasley eds), 2001 Swets& ZeitlingerLisse, SBN 90 2651 827 7

Design nd nalysisf foundationodificationsora buttressamG.A.Scott, J.T.Kottenstette J.ESteighnerU.S.Bureau fReclamation,enver, olorado, SA

ABSTRACT:spillwaytillingasin as riginallyxcavatedownstreamfseveraluttressestPuebloam.Geologicnformationndicatedhepresencefweak haleeamsn he oundationhat aylightn hestillingbasin xcavation.he eservoirad ot illed ompletelyince riginalonstruction,nd hereforehe oundationwas ot ully ested. nalysesndicatedhe oundationould ave very limmarginfsafetyf the eservoirfilled to normal evels. Due to the largepopulationt risk downstreamrom the dam, herewas strongjustificationo reducehe isk, ndmodificationsereundertakeno improveoundationtability. hemodificationsonsistedf buttressinghe oundationithroller-compactedoncreteRCC)and ockbolts.Additionalrainageas lsonstalled.lthoughhedesign as ompletedsingimitequilibriumechniques,uniquespectsf hegeometryerenvestigatedsingDEC nd DAanalyses.oth onfirmedhe esignprovideddequateesistance.his aperocusesn he esignnd nalysisf he oundationreatmentystem.I INTRODUCTION

Pueblo Dam is a compositeearthfill and concretemassive head buttress dam (see figure 1) on theArkansasRiver just upstreamof Pueblo, Colorado.The dam, completedn 1975, is about53.3 m high. Aspillwaystillingbasin,168 m long and 13.7 m deepwas originallyexcavatedat the downstream oe ofButtresses through14, which form the uncontrolledoverflowspillway or the dam.

2 GEOLOGY AND POTENTIAL MODES OFINSTABILITYThe central overflow section of the buttress dam isfounded on Dakota Sandstone, onsistingof nearlyhorizontally bedded sandstonesand shales, withoccasional arbonaceousartings.The excavationorthe spillwaystillingbasincreated free surfacentowhichweak oundation hale ayersandopenbeddingplanescoulddaylight. This was recognizedduringoriginal construction, nd additionalexplorationandexcavationwasperformed o identifyandremove hedownstreamportion of weak layers found to belaterally continuous ear the foundationcontactofsomebuttresses.However, he possibility f potential

slidingplaneshatstepalonga vertical oint from anupper treated discontinuity to a lower untreateddiscontinuity, r slidingplanes ormedby shale ayersin combination ith openbedding lanes, aylightingin the stilling basin,was not considered.

Figure 1. Concrete ectionof PuebloDamGeologic information obtained from originalmapping,alongwith that obtained rom exploratorydrilling (bothduringconstructionnd aterdamsafetystudies) and geophysical esting, was plotted ongeologic crosssections hrough each of the sevenexposedbuttresses.Figure 2 shows he crosssectiondeveloped hroughButtresses and 9. A thick shalelayer is present under the upstream portion ofButtresses 8 and 9, and to a lesser extent under

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Buttresses 0 and 11. Thinnershale ayersarepresentunder he downstream ortionof Buttresses and9,and under the upstreamportionsof Buttresses 0through 4. An opensandstoneedding lanepartingwasmapped crossheentirestillingbasin xcavation.A potentialoundation lidingplane, ontinuouslongbeddingplanediscontinuities nd daylighting n thestilling basinexcavation, ould be identifiedbeneatheachof the sevenspillwaybuttresses. hesepotentialsurfaces aylight airly low (about levation1437.1m)in the stillingbasinexcavation or Buttresses and9(see Figure 2), but at a higher elevation (aboutelevation 1443.2 m), where the continuous eddingplanepartingwasmapped, or Buttresses 0 through14.

AXIS OF DAM

TOE PLUG//// \S CONTRACTION

JOINTSFigure 2. Typical section,Buttresses and 9

The maximumhistorical eservoir urface rior othis evaluation was elevation 1489.97 m. This is awater depth about 1.60 m below normal pool and3.15m below the spillway crest. Two-dimensionallimit equilibrium analyses indicated that thefoundationwouldbe marginallystableagainst lidingif the reservoir rose to these levels (Trojanowski,2000). A risk analysis concluded there wasjustification o reduce he risk to the argepopulationrelativelyclose o the dam downstream.

3 REMEDIATION ALTERNATIVES

Conceptual designs were prepared for severaltreatment lternatives,ncluding ddedweightbetweenthe buttresses, tendons, buttress extensions, andconstructionof a roller-compacted oncrete RCC)plug and toe block,anchoredwith rock bolts, n thespillway stilling basin to block daylightingbeddingplanes,as shown n Figures2 and 3. The leastcostandmost fficient isk eduction ltemative, onsistingof the RCC plugand toe block,wasconstructed. he

lower "plug" portionblocksmovement f the lowerdaylightinglanes tButtressesand , and heupper"toe block"portionsupportshe foundation here hehigher daylighting planes occur at Buttresses10through 4. The design ndanalysis f thissystemsdescribedn the followingsections.SPILLWAY AXIS OF DAMCREST-L

ROCK BOLTS

OPEN BEDDING PLANEFigure 3. Typical section,Buttresses 0 through 4

4 SHEAR STRENGTH PARAMETERS

Results rom testson shalesamples onductedn 1967and 1997 are shown in Figure 4. Although thedisplacementates for tests n 1967 were not slowenough o represent rained onditions,heresults reconsistent with the tests performed at slowerdisplacementates n 1997. Peak strengthswere notconsideredappropriate or a potentiallysofteningmaterial like the shale. Similarly, the residualstrengths rom repeatedshearingwere thought o betooconservative. herefore, post-peakriction ngle(9) of 17 degreeswas selected for the stabilityanalyses. ost-peak trengthsemainedairlyconstantat sheardisplacements reater han about 10 to 20percent f thespecimen idth.Samples f sandstoneiscontinuities ere estedn1968 and 1997. The character f thesamplesestedn1968 is not clear. Therefore, the 1997 tests on 102-

;1967ESTS 1997ESTSHi i i i J i i i i i i IJ_I_LJ._LI I I 1 Ill IIlll TIIIIIIIIIlillll/I I I It I illitllllllllliiiii]lllllllllllll i111111111111111111ll IIIII111111 IllIfil Illlllllllllllllllllllfi" ,'1 I I I I I I I I I I I I"1 I I I I I I I I I I I I I I I I I I I I-J.-&-L IIII_..ou IIIII IIIIIIIIIIIIIIIIII Illll[11111111lll Illll ...... ''''''''''''11111111llllllllllv MAX. NORM. STRESS II I real II Ill i I I mF4:4]OFNTEREST..... lm,1 H-H-m-H-m

_. I I I I I i I I I '. I I l_l I i ) [ 1_] IIIIIIIIII Illlllllt 111lllllllllll' O'O IIIIIIIIII I I I I I I II'FI I I I I I I I I[I I I i I L.-I"M17, u.vo I I I I I I I I I I I I ll.--r16t%b'&b,l I II I J,.4>TI . I, IIIIIIllll .l i i i i i i i i i i I I-'1 I I I il i I IIIIlllll i i i i i i i i J JJ.-,-l i I I I I &H-ql i I I I LI LI ] i i i i 1 i i i i L..+'T"ff IIl'lnl iZk/(/) -F[~F[ I IXl I I :al:;l I I I I I ? I ',' ,,.' , .--;; I I I I J/I I iJr 1111-It11 I I I I I I I I 11 ] J_L_LL i i iJl 1114 LJrl I I I I I I I I I I I I I I I I/ I I I I I ELI ts x.-.l--e I IllIll I I I I III Ill 1111 I [Ill llJ1111 IIl[1111111111111111111111IlllI I I I I I I0.69 1.38 2.07

NORMAL STRESS (MPa)

Figure4. Direct shear est esults or shalesamples952


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mm-diameter core were used to characterize thestrength f openbedding lanepartingsn sandstone.Thesesamples onsistently roduced frictionangleof about41 degrees.The shear strengt, of the RCC is critical insupportinghe oundation.A highcementitious CCmx with bonding mortar between lifts wasrecommendedor the application. Horizontal ointsbetween RCC lifts would be the weakest surfaces inshear or thismaterial.Experienceromotherprojectsindicated hat about85 to 100 percentof the surfaceswould be bonded,with a cohesion C) of about2.3 to2.4 MPa, and friction angles between 40 and 55degrees.The consultanteviewboard or the projectrecommendedafety actors f 3 and 1.5 be applied othe cohesion nd tamp friction angle), espectively,whenused n a passiveesistanceonfiguration. heselecteddesignvalueswere a cohesionof 0.62 MPaand a friction angle of 30 degrees for RCC liftsurfaces.

5 UPLIFT

Lnesof uplift pressure ipeswereoriginallynstalledunder hemassive eadportionof Buttresses and12.In addition, everal iezometers ere nstalledn 1997duringdam safetyexplorations. he pressuresnderButtress have been consistently igher han underButtress12, as shown n Figure 5.*PIEZOMETERS - HISTORIC MAX=BUTTRESS 12 - HISTORIC MAXo BUTTRESS 7 - HISTORIC MAX,, BUTTRESS 7 -TOP OF DAM (estimated)

II11111111 IIIII]| I111i111&Ill I IIIIIII I I I I I I I [IllILl I 111111W[[ 1111 III I I I 11 I /I I ' I I I Ii i II I IllIIIl[[Itllll[lik1493.5-H-IJ-H- I ,,,,i,[[[t i ],"'['[rlJl j illl IJ I Jllllltllll*l Ill IJlJlJl I LJllllll I1'111111111, .JlJlJ [ I I t J t Lt I I I I I I I I i i 1 II -/ I II II1[ JillIll I[ IJllllll I I J ' Ii11111111 1[[111111 i i i i i i i i jl [ I I I ,I I I I LJ J J J I I I I I I J IZ J Jl Xll II / I I II IllIll II IIII J J]O J 11 gllllJ I I [ IIII IlllJllllllJJ lJlJil I Jt J [1 I I Illlllllll11111111lgE l,]111Jl I I I I I I I I I I I I I I I I I I I ] II111 Illl I 1111,ffi4JJ I I I III I I I I J I I I I I/ I I J III I I I LJ I III I 1 I I I I I I]]qqJJ I I I I I I I I i I I I I I I III I I1'11 I I I J J I I I I IJJJ J I I I I I I I] 'lq$l I I I 11--$-4. - - &lllJ[1111111-- I I I I I I I I I I I I ['l'l'tl't't'r I' l I III[I II I IIIIIIIll IIII II Jill ii /I I IIIII Illll I I Ill I Ill I I I J IllIll I I Illlllllllllllllll III1' -.2 ' ' .2 36.DISTANCE FROM DAM lS (m)Fgure . Uplift pressureistributions

The historic maximum reservoir elevation was1489.97m. The top of dam s elevation 501.14m.The pressure istribution nderButtress at thehstoricalmaximum eservoir levation,supplementedbythenewpiezometricata,wasusedor thedesignandanalysistudies. iezometricdjustmentsoother

reservoir levationswere madeusing he differentialhead atio DHR), definedn equation .

DHR - (1)

where P is the piezometric evel, R is the reservoirlevel andTW is the tailwater evel. Adjustmentsoother eservoir levations re henmadeaccordingoequation2.NP = DHR( NR- NTW) + NTW (2)

whereNP is the new piezometric evel,NR is the newreservoir evel, and NTW is the new tailwater level.

6 LIMIT EQUILIBR ANALYSIS AND DESIGNLimit equilibrium analyseswere used or the initialdesign. n working with the consultanteview boardfor the project, the following factor of safety (FS)requirements ere establishedo meet he desired iskreduction:

At reservoirelevation1493.12 m (spillwaycrest):FS 2 2.0 for q0sm, 17,q0s^sTo 41,q0Rcc30,andCgc = 0.62 MPa.;FS > 1.5 for q0si = 17,q0s^sTo 41,q>gcc30,andCuc = 0; andFS > 1.3 for q0sm, 17,q0SAmSTON= 41,andusingonly he weightof new RCC above hebedding laneand orces rom rock boltscrossinghebeddingplanein computinghe resistance fferedby the treatment.The last wo requirements nlyapplied o the oeblockportionof the design Buttresses 0-14), as the plugportionof the treatment ompletely locksmovementfrom upstreamo downstreamor daylighting eddingplanesat Buttresses and 9.At reservoirelevation 1501.14 m (top of dam):FS 1.0 or q0si = 17,q)SANDSTONE---1,q0acc 30,and CRcc 0.62 MPa.Analyses f theexistingbuttresses ereperformedtaking nto account orceandmoment esolution.ForButtresses 0 through14, the moment nducedby thereservoir ctingon the dam increaseshenormal stresson the stronger ownstream andstoneortionof thepotential lidingplanes, ndreduceshenormalstresson heweakerupstream haleportions f theplanes atButtresses and 9 the entirepotentialslidingplane scomposed f shale).

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7.2 UDEC StudiesThe toe block geometry ssociated ith Buttresses 0through 14 was investigated singa UDEC model.Deformable blocks were used for the model shown inFtgure7. The entire dam was not modeled n theseanalyses; nly a portionof the foundation lock andsttllingbasin reatmentwas modeled.

BLOCK 1//,.,,/__..CN OINTS

Figure7. UDEC modelThe approach sedwas to ramp a horizontal oadonto the upstreamportion of the foundationblock(Block 1), by applying a constantvelocity, until thetreatmentailed. This provided heultimate esistanceof the foundation treatment,which was compared othe resistance ecessaryo provide he required actorof safety rom the imit equilibrium nalyses. erticalmovementof the foundationblockwas suppressedosmulate he weight of the dam above he block. Thetensile strength across all concrete joints wasconservativelyassumed o be zero. However, thecohesion on these surfaces was assumed 1.97 MPa, a

2 NORMALORCE................-0 -2 .......... SHEARORCEO-4LL

BOUNDARYORCE '"-.......",,,,o ' ' 'X 10E+04 CYCLES (i.e. TIME STEPS)

Figure8. Toe blockpredicted ehaviorromUDEC model(without rockbolts)

value hatmore loselyesemblesheiractual trength.The [fictionanglewasconservativelyeft at 30The model was run with and without rock bolts.The sum of shear forces and normal forces on theprojectedsliding plane (through he RCC), and thehorizontal forces on the upstream side of thefoundation lock Block 1) were racked hroughheanalysis,as shown n Figure 8 (without rock bolts).The sumof the shear orceswas typicallyslightlylower han he horizontal orceson the upstreamacein all cases. The sum of shear forces was thereforeused to estimate the ultimate resistance. Without therock bolts, he ultimatestrength f the toe block wascalculated o be only 88 percentof the targetvalue.However, with the rock bolts, the ultimate strengthexceeded he targetvalue by about 14 percent. Therock bolts provide an active stabilizing force andincrease he tensilecapacityat critical ocations. t isa tribute o the experience f the consulting oard hatthe simplestrength eduction actor or design,basedon an assumed uniform shear stress distribution andcohesion, loselyapproximateshe strength f the toeblock calculatedusingUDEC.7.3 Probabilistic StudiesThe limit equilibrium quations ereprogrammedntoa spreadsheet, nd Monte-Carlo simulationswereperformed using commercially available software.Buttress 10 was selected for these simulations since ithad someof the lowestdeterministic actorsof safety,and the potentialslidingplane geometrywas simple(nearlyhorizontalat a constant levation)and welldefined.Triangular istributions ereassumedor theinput parameters, hich ncluded he RCC cohesion,frictionangle, ndpercent ond; he rictionanglesorthe shale and sandstone; he drain effectiveness; andthe rock bolt force. The lower bound,upper bound,and best estimate or each parameterwas in somecases based on test results, and in others based onjudgement nd experiencerom otherprojects.Thedistributionof outputFS from the simulationswasevaluated o determine he probabilityof a FS < 1.0.In cases here heprobability fFS < 1.0 s remote,a distribution needs to be assumed to make an estimateof this likelihood. The distribution assumed can havea significant ffecton the results.The CentralValueTheorem suggests hat results tend to follow alognormaldistributionwhen they are the result ofmultiplyingor dividingothervariables, nd a normaldistributionwhen they are the result of adding orsubtracting ther variables. Since both operations reinvolved n computing S, Chi-Square,Kolmogorov-Smirnov,and Anderson-Darling oodness f fit tests

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Distribution or Factor of Safety

0.107 ... -' ...0.;, .......0.054 ......0.027 - -

'l '

,I18,29 24.18 30.08 35.98 41.87 47.77 53.66Values in 10A-1Figure 9. Resultsof Monte-Carlosimulationwereperformedor bothdistributions sing heoutputresults. The reliability ndex, [3, s calculated romequation for a normaldistribution, ndequation fora lognormaldistribution.

FS4- 1.0fi, = (4)o'

]LN 'In FS IJl+CoV (5)a]ln(1Co 2

whereFS is the meanFS, o is the standard eviationof the output FS, and CoV is the coefficient ofvariation, defined as o/FS. Standard probabilitytables can be used to estimate the probability ofFS


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Figure 11. Tensioning ock boltsconfigurationwill actually perform better than theoriginal.

9 CONCLUSIONS

Nearly horizontal halebedsandopenbedding laneswere identified in the foundation of the concretespillwaysection f PuebloDam. Theseplanes ormedpotential stepped" lidingsurfaceshat daylight n thestilling basin excavationat the base of the concretedam buttresses. n-depthdiscussions nd analysisofthe risk indicated ustification o reduce isk to thelargepopulation mmediately ownstream f thedam.The risk evaluationprocess efinitely benefittedandenrichedheproject. The design f the modificationsrequired close coordination between geologists,geotechnical ngineers, tructural ngineers, ydraulicengineers,materialsengineers, nd field constructionstaff.

Strengtheningf the foundation onsisted f fillingthe stilling basinwith RCC, andconstructing n RCCtoe block, anchoredwith rock bolts. The designwasbased on traditional limit equilibrium methods,andwas predicated n reducing he cohesionof the RCCby a factor of 3, and the friction angle tangentby afactor of 1.5. The slopinggeometryof the existingstilling basin and resultingblock geometry equiredmoredetailedanalyses f the treatment ehavior.Both DDA andUDEC analyses ereperformedostudy heeffectsof the slopinggeometry.Both setsofstudiesshowed he designed reatmentprovides herequired resistance. However, since these studiesmodeled localized stress distributions and behavior,unfactored ohesion nd/or riction angleswere used.The strength actorsused or limit equilibriumdesignwere shown o be appropriate. Care must be takenwhen performing imit equilibriumdesign o accountfor unusual eometry ndstress istributions.Specialdetailedstudiesmaybewarrantedo ensure hedesigns

will perform as intended. Factorsused o reduce hestrength f the RCC for designat PuebloDam wereintendedo accountor the ollowing:1. It is difficult o predictwithcertaintyusthowgoodRCC construction ill be. Theremaynot becomplete100% bond,andbondmay be less n someplaces han other.2. The resistances passive. The resistancewillnotbe mobilized ntilsome isplacementakes lace.The amount of needed displacements somewhatuncertain, s s the strengthhat may be mobilizedonthe potential foundation planes at a givendisplacement.3. The actual shear stresses ill likely not beuniform; however, the designcomputationsmayassume a uniform shear stress distribution. Localized

overstressn an areaof high shearstresses an ead toprogressive nstability.4. Some ensilestresses aybe generatedocallydue to the geometryof the structurehat couldreducethe shearstrength n thoseareas.

ACKNOWLEDGMENTS

The PuebloDam modification rojectwassuccessfuldue to the many qualifiedand talentedpeoplewhowere nvolved. Their contributiono design, nalysis,and construction of the project is gratefullyacknowledged.

REFERENCES

Kottenstette, .T. 1999. DDA analysis f theRCC modificationforPueblo am. Proceedings,a nternationalonferenceon the Applicationof DynamicDeformationAnalysis.Vail,Colorado:127-132. Alexandria, irginia:AmericanRockMechanics Association.Trojanowski,J. 2000. RCC used o stabilizePuebloDam.USCOLD Newsletter: Issue No. 120.

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ARMA-01-0951_Design and Analysis of Foundation Modifications for a Buttress Dam