Conventional Concrete for Dam Overtopping Protection
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Conventional Concrete for Dam Overtopping Protection

Conventional Concrete Systems• Continuous layer of conventional or massconcrete placed on downstream face ofembankment dam to provide a flow surface• System must remain intact, without defects• Guide walls along sides may be required• May be located on portion or entire face• Must protect abutment groins andunderlying embankment from rapid erosion

Case Histories• Limited examples of concrete spillwaysconstructed on embankment dams cited:– Box culvert service spillways at Meeks Cabinand Currant Creek Dams (Reclamation)– Open chute emergency and service spillwaysat seven U.S. dams (various owners)• Similar in concept to concrete overtoppingprotection, using reinforced concrete slabsand generally waterstopped joints

Box Culvert Spillways on DamsMeeks Cabin Dam, WYCurrant Creek Dam, UTBox culvert 30’ wide by 15’ highDesign discharge 6,250 ft 3 /sBox culvert 20’ wide by 10’ highDesign discharge 850 ft 3 /s

Design Considerations• Crest length sized for design flood• Guidewalls sized for water depths on slope• Joint details provided to minimize potentialfor uplift pressures and foundation erosion• Flow velocities checked for cavitationpotential and required surface tolerances• Underdrains provided to control seepage• Consider future embankment settlement

Uplift Pressures on Slab• Uplift (stagnation) pressures can developbeneath a concrete slab due to waterentering cracks and joints during releases• Pressure results from offsets into flow andpartial conversion of flow velocity head• Hydraulic jacking and failure of slabs mayresult from poor drainage and net uplift• Alternatively, foundation scour may result

Uplift Pressure DevelopmentJoints provided in concrete slabs to control cracking

Uplift Pressure EstimatesMean uplift pressure, sharp-edged geometry, sealed cavity,⅛-inch gap (Bureau of Reclamation, 2007)(Note – Uplift pressures for vented cavity were lower)

Sources for Uplift Problems• Cracks, offsets due to differential settlement• Delamination at joints due to thermal stress• Alkali-silica reaction and concrete spalling• Freeze-thaw damage at concrete surface• Frost heave at slab foundation• Sulfate attack and concrete deterioration• Small, sharp-edged offsets into flow attransverse joints can produce highest uplift

Differential Settlement• Less likely if placed onan existing damembankment that hasalready settled anddeformed, than for anewly constructed dam• Excessive settlementcan fail waterstops andreinforcement at joints

Delamination ThreatDeer Creek Dam, UT

Big Sandy Dam, WYFrost Heave

Uplift Failure Case StudyBig Sandy Dam, WY - 1983

Defensive Design Measures• Waterstopped joints• Transverse cutoffs• Reinforcing bars ordowels across joints• Anchor bars• Filtered underdrains• Rigid insulation• Keyed joints (not shown)• Slotted joints with sealant (not shown)

Role of Underdrains• Provide a perviousunderdrain layer withpipe outfalls to limituplift pressures• Include a filter zone tocontrol potential loss offines through opencracks or joints• Design drainage systemto handle seepage flowUnit discharge for joint/crack, sharpedgedgeometry, ⅛-inch gap (Bureauof Reclamation, 2007)

Uplift Pressure Failure Sequence► Spillway releases occur► Open joints, offsets, and/or cracks exist► Significant uplift pressure occurs► Defensive design measures ineffective and/or non-existent► Chute slab jacking occurs►Unsuccessful intervention►Headcutting leads to breachNOTE: Foundation erosion leading to structural collapse not shown.16

Cavitation Potential for Slabs• Cavitation is the formation of vapor cavities inhigh velocity flow at surface irregularities(cracks, joints, blocks, offsets) which collapse,causing high-pressure shock waves• Cavitation damage can occur on flow surfacesexposed to high velocities for long durations(days) but is less likely to cause dam failure• Smooth surfaces and aeration slots canreduce potential for concrete damage

Case History – ConcreteOvertopping Protection foran Embankment Dam

A. R. Bowman Dam• A.R. Bowman Dam is an embankment damon the Crooked River, about 20 milesupstream of Prineville OR, completed 1961• Embankment consists of a wide centralcore, narrow transitional zones, and rockfillshells; with structural height of 245 feet• Spillway consists of 20-foot-long ogee crestand concrete chute on right abutment, withdischarge capacity of 8,100 ft 3 /s

A. R. Bowman Dam

A. R. Bowman Dam• Flood routing for 1988 PMF indicated damwould overtop for 4.5 days with max depthof 20 feet; threshold flood – 500 year event• Concrete overtopping protection wasoriginally identified as the preferredalternative for passing larger floods, basedon cost and environmental considerations• Concerns for potential development ofstagnation pressures beneath concrete slab

Overtopping Protection Systems• Two alternatives were considered – RCCoverlay and continuously reinforcedconcrete slab (CRCS), with similar costs• CRCS alternative selected as preferredalternative due to better seepage control• Minimum slab thickness of 12 inches overentire downstream face; thickened atchange in d/s dam slope from 2:1 to 4:1 forpotential debris loads on face

CRCS PlanWidth of CRCS slab tovary from 840 feet attop of dam to 380 feetat toe of damMaximum unitdischarge of 280 ft 3 /sper foot at dam crest

CRCS Overtopping Protection• Reinforced concrete cap on dam crest,with concrete blocks on u/s and d/s edges• Anchored concrete block at d/s toe• Slab underlain with crushed rock drainagelayer – drained to line of drain outlets• Aspirating drain outlets located just abovethe toe of the hydraulic jump, and weepholes with flap valves below water surface

CRCS Dam Crest Detail

CRCS Downstream Toe Detail

CRCS Overtopping Protection• CRCS slab anchored into bedrock on theleft abutment and restrained by a concretegravity wall on the right abutment• A 1:48 scale hydraulic model was used foranalysis and design of the CRCS• Model study was to:– identify areas vulnerable to erosion thatshould be protected with concrete– provide guidance for shaping the abutmentsto provide smooth flow lines

CRCS Hydraulic Model Study

CRCS Model Study Discharges

CRCS Water Surface Profiles

CRCS Slab Pressures – 20’ OTFew uplift pressure spikes beneath hydraulic jump

Structural Design for CRCS• Continuously-Reinforced ConcretePavement (CRCP) computer programdeveloped at Univ of Texas-Austin (1977)• Design for crack width of 0.003 inches at32 degrees F, and minimum crack spacingof 2 feet for reinforcement design• Seepage rates for drainage design basedon flow depths and crack assumptions

A. R. Bowman Dam - Conclusions• Hydraulic model study and slab designindicated that CRCS was feasible anddam safety concerns could be addressed• However, overtopping protection at A.R.Bowman Dam was never constructed dueto a reduction in hydrologic loads and risk• These studies provide guidance andanalyses needed to design this type ofsystem for future projects


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