Rockfill Dam

8/9/2019 Rockfill Dam

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Principles of design ofrocklldams


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Defnition o Rockfll Dam:

A dam that relies onrock, either dumped in liftsor compacted in layers, as a majorstructural element.

An impervious membraneis used as a water barrier and can beplaced either within or on theupstream slope of the embankment.

An impervious membrane can beeither of earthor concrete or asphaltic concrete or Ge

omembrane.


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Rock fll Dam:


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Design o Earth & Rockfll RockfllDams :

Dams !an be constructed on anygiven foundation condition. "oil#rock materialslocally available are used with negligibleprocessing. $se of costly manufactured itemslike cement and steel is eliminated and thus

saving in cost. %mbankment dam is moreresistant to seismic forces and are preferred inareas of high seismicity. %mbankment dam can beconstructed in stages and the dam height can beincreased later on easily, if needed. &ith modernearth moving machineries, the dam can becompleted in less time compared to a rigid dam.%mbankment dams are generally much cheaper.


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Basic requirements o anembankment dam:'o meet the dam safety re(uirements the design,

construction, operation, and modication of an

embankment dam must comply with the followingtechnical and administrative re(uirements)

'echnical re(uirements) 'he dam, foundation, and abutments must be stable

under all static and dynamic loading conditions. "eepage through the foundation, abutments and

embankment must be controlled and collected to ensuresafe operation.

'he freeboard must be su*cient to prevent overtopping

by waves and include an allowance for settlement of thefoundation and embankment.

'he spillway and outlet capacity must be su*cient toprevent overtopping of the embankment by the reservoir.


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Basic requirements o anembankment dam:Administrative re(uirements)

%nvironmental responsibility.

+peration and maintenance manual.

onitoring and surveillance plan.

Ade(uate instrumentation to monitorperformance.

-ocumentation of all the design, construction, andoperational records.

%mergency Action Plan) dentication, notication,and response subplan.

"chedule for periodic inspections, comprehensivereview, evaluation, and modications as

appropriate.


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General Design Considerations:/reeboard)

The term freeboard is applied to the verticaldistance of a dam crest above the maximumreservoir water elevation adopted for the spillwaydesign 0ood. 'he freeboard must be su*cient to

prevent overtopping of the dam by wind setup,wave action, or earth(uake e1ects. nitial freeboardmust allowfor subse(uent loss in height due to consolidation of

embankment and#or foundation. 'he crest of thedam will generally include overbuild to allow forpost construction settlements. 'he top of the coreshould also be overbuilt to ensure that it does notsettle below its intended elevation.


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General Design Considerations:'op &idth)

'he top width of an earth or rockll dam withinconventional limits has little e1ect on stability and isgoverned by whatever functional purpose the top ofthe dam must serve. -epending upon the height of the

dam, the minimum top width should be between 23and 45 ft. &here the top of the dam is to carry a publichighway, road andshoulder widths should conform to highwayre(uirements in the locality with consideration given to

re(uirements for future needs. 'he embankment6oning near the top is sometimes simplied to reducethe number of 6ones, each of which re(uires aminimum width to accommodate hauling and

compaction e(uipment.


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General DesignConsiderations:Alignment)

A7es of embankments that are long with respect to their

heights may be straight or of the most economicalalignment tting the topography and foundation conditions."harp changes in alignment should be avoided becausedownstream deformation at these locations would tend toproduce tension 6ones which could cause concentrationof seepage and possibly cracking and internal erosion. 'hea7es of high dams in narrow, steepsided valleys should becurved upstream so that downstream de0ection underwater loads will tend to compress the impervious 6oneslongitudinally, providing additional protection against theformation of transverse cracks in the impervious 6ones. 'heradius of curvature forming the upstream arching of thedam in narrow valleys generally ranges from 8,555 to 9,555ft


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General DesignConsiderations:%mbankment)

%mbankment sections adjacent toabutments may be 0ared to increase stabilityof sections founded on weak soils. Also, by0aring the core, a longer seepage path is

developed beneath and around theembankment.


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General DesignConsiderations:Abutments)

Alignments: Alignments should be avoided that tie into narrow

ridges formed by hairpin bends in the river or that tie intoabutments that diverge in the downstream direction. Grouting maybe re(uired to decrease seepage through the abutment. Zones ofstructurally weak materials in abutments, such as weatheredoverburden and talus deposits, are not uncommon. t may be moreeconomical to 0atten embankment slopes to attain the desiredstability than to e7cavate weak materials to a rm foundation. 'hehori6ontal permeability of undisturbed strata in the abutment maybe much greater than the permeability of the compacted ll in theembankment: therefore, it may be possible to derive considerablebenet in seepage control from the blanketing e1ects of 0aredupstream embankment slopes. 'he design of a transition from thenormal embankment slopes to 0attened slopes is in0uenced by

stability of sections founded on the weaker foundation materials,drainage provisions on the slopes and within the embankment, andthe desirability of making a gradual transition without abruptchanges of section. Ade(uate surface drainage to avoid erosionshould be provided at the juncture between the dam slope and theabutment.


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Abutment slopes: Where abutment slopes aresteep, the core, lter, and transition !ones of anembankment should be widened at locations ofpossible tension 6ones resulting from di1erentsettlements. &idening of the core may not be

especially e1ective unless cracks developing in ittend to close. %ven if cracks remain open, a widercore may tend to promote clogging. ;owever,materials in the lter and transition 6ones areusually more selfhealing, and increased widths ofthese 6ones are benecial. &henever possible,construction of the top 23 ft of an embankmentadjacent to steep abutments should be delayed untilsignicant embankment and foundation settlementhave occurred.


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"ettlement:#ecause large di$erential settlementnear abutments may result in transverse crackingwithin the embankment, it may be desirable to usehigher placement water contents combined with0ared sections.


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General DesignConsiderations:%arth(uake %1ects)

%eneral:The embankment and critical appurtenantstructures should be evaluated for seismic stability.'he

method of analysis is a function of the seismic 6one as

outlined in %< 888528=5>. -am sites over active faults

should be avoided if at all possible. /or projects locatednear or over faults in earth(uake areas, special geological

and seismological studies should be performed.

-efensive design features for the embankment and

structures as outlined in %< 888528=5> should be used,

regardless of the type of analyses performed. /or projectsin locations of strong seismicity, it is desirable to locate

the spillway and outlet works on rock rather than in the

embankment or foundation overburden.


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General DesignConsiderations:%arth(uake %1ects) &efensive design measures: &efensive design measuresto protect against earth'uake e$ects are also used forlocations where strong earth(uakes are likely, and includethe following to increase the safety of the embankment)? %nsuring that foundation sands have ade(uate densities @atleast 5 percent relative densityB.? aking the impervious 6one more plastic.? %nlarging the impervious 6one.? &idening the dam crest.? /lattening the embankment slopes.? ncreasing the freeboard.? ncreasing the width of lter and transition 6ones adjacentto the core.? !ompacting shell sections to higher densities.? /laring the dam at the abutments


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Seepage Control in RockFoundations :%eneral considerations:"eepage should be cut o$ or controlled

by drainage whenever economically feasible. "afety must be the

governing factor for selection of a seepage control method

(uto$ trenches:(uto$ trenches are normally employed when thecharacter of the foundation is such that construction of a satisfactorygrout curtain is not practical. !uto1 trenches are normally backlledwith compacted impervious material, bentonite slurry, or neat

cement. !onstruction of trenches in rock foundations normallyinvolves blasting using the presplit method with primary holes deckloaded according to actual foundation conditions. After blasting,e7cavation is normally accomplished with a backhoe. !uto1 ofseepage within the foundation is obtained by connecting animpervious portion of the foundation to the impervious portion of thestructure by backlling the trench with an impervious material. nrock foundations, as in earth foundations, the impervious layer of thefoundation may be sandwiched between an upper and a lowerpervious layer, and a cuto1 to such an impervious layer would reduceseepage only through the upper pervious layer. ;owever, when thethicknesses of the impervious and upper pervious layers aresu*cient, the layers may be able to resist the upward seepage

pressures e7isting in the lower pervious layer and thus remain stable.


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Seepage Control in RockFoundations :)pstream impervious blankets: *mpervious blankets

may sometimes give ade'uate control of seepage water

for low head structures, but for high head structures it isusually necessary to incorporate a downstream drainagesystem as a part of the overall seepage control design.

'he benets derived from the impervious blanket are dueto the dissipation of a part of the reservoir head through

the blanket. 'he proportion of head dissipated isdependent upon the thickness, length, and e1ectivepermeability of the blanket in relation to the permeabilityof the foundation rock. A lter material is normallyre(uired between the blanket and foundation.

%routing: Grouting of rock foundations is used tocontrol seepage. "eepage in rock foundations occursthrough cracks and joints, and e1ectiveness of groutingdepends on the nature of the jointing @crack width,spacing, lling, etc.B as well as on the grout mi7tures,e(uipment, and procedures.


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Seepage Control in BeneathSpillways and Stilling Basins:

Ade(uate drainage should be providedunder 0oor slabs for spillways and stillingbasins to reduce uplift pressures. /or soilfoundations, a drainage blanket under the

slab with transverse perforated pipe drainsdischarging through the walls or 0oor isgenerally provided, supplemented in thecase of stratied foundations by deep well

systems. -rainage of a slab on rock isusually accomplished by drain holes drilledin the rock with formed holes or pipesthrough the slab. 'he drainage blanket is

designed to convey the seepage (uicklyand e1ectively to the transverse collector


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Seepage Control AgainstEarthquake Efects:

/or earth and rockll dams located whereearth(uake e1ects are likely, there are severalconsiderations which can lead to increasedseepage control and safety. Geometricconsiderations include using a vertical instead of

inclined core, wider dam crest, increasedfreeboard, 0atter embankment slopes, and 0aringthe embankment at the abutments. 'he corematerial should have a high resistance to erosion.


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Embankment Materials oRock Fill !ams""ound rock) is ideal for compacted rockll, and

some weathered or weak rocks may be suitable,including sandstones and cemented shales @butnot clay shalesB.

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Rockfill Dam