Prestressed Concrete Dams1934 - 1964

by A. C. Roemermann*

Two methods of prestressing damshave been in use since their incep-tion in the early 1930's:

1. The Coyne method of pre-stressing with tendons.

2. The Freyssinet method of pre-stressing without tendons, butwith inflated flat-jacks.

The purpose of this report is to de-scribe the Coyne method, and onlybrief mention will be made of theFreyssinet method.

Prior to the turn of the century,it was common practice to designgravity dams without any allowancefor uplift pressures, such dams de-pending solely upon their weightfor stability against sliding and over-turning. Uplift pressures, when per-mitted to build up, have in timeresulted in increasing a dam's buoy-ancy, thus decreasing its effectiveweight. Disregard for uplift has con-tributed, in large part, to the failureof some old dams, and to the threat-ened failure of other dams.

It was not until about 1910 thatuplift in dams began to receive mer-ited recognition. Since 1910, seri-ous attention has been directedtoward corrective measures for keep-ing uplift within allowable limits.Such measures include grouting and

*Structural EngineerFormerly associated withAbbott, Merkt & Company, Inc.New York, New York

drainage of dam foundations. Fur-thermore, considerably more atten-tion has been devoted to the selec-tion of suitable dam sites. In suchselections, geologic investigationsand rock mechanics now play moreimportant roles than in the past.

The importance of uplift in hy-draulic structures was fully recog-nized by the United States Bureauof Reclamation at an early date.At the conclusion of an extensiveresearch program undertaken in1915, the Bureau reported that:"Water under pressure was foundat every point under all U.S.B.R.dams".

In 1935, foundation uplift pres-sures were first recorded in the 726ft. high Hoover Dam, which wasthe world's highest concrete gravitydam until 1960. These tests showedincreases in uplift until 1939, whenfurther corrective measures werecarried out by additional founda-tion grouting. Through World WarII and into the 1950's the groutingwas continued. Final foundationtreatment on the Hoover Dam wascompleted in 1953.

While uplift in the design andconstruction of dams had been en-tirely neglected in the past, it is nowfully provided for in all dams. Theassumed allowance for uplift in thedesign of new dams, as adopted bythe United States Government agen-cies, is usually about two-thirds of

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the upstream hydrostatic pressure,decreasing uniformly to zero atdownstream tailwater. Such upliftis usually assumed to be effectiveon 100% of the base area. However,should special conditions warrant,those assumptions must of coursebe varied.

In the early 1930's, French engi-neers, confronted with failures andpotential failures of gravity dams,chiefly in North Africa, undertookto make studies and tests forstrengthening dams. These damshad been constructed in the 1880's,without allowance for uplift. Thelate Andre Coyne, noted Frenchconsulting engineer, and formerPresident of the International Com-mission on Large Dams, met thischallenge by devising an ingeniousmethod for stabilizing weak gravitydams by anchoring them to founda-tion rock with post-tensioned verti-cal tendons. This method likewisepermitted increasing their heightwithout the customary addition ofnew masonry on the downstreamface.

Among various methods for height-ening of gravity dams, the one mostfrequently adopted has been to addand bond new masonry on the ex-isting downstream face, to increasethe base width—usually a difficultand costly procedure. The Coynemethod has made it practical andeconomical to heighten dams with-out adding downstream masonry. Itis necessary only to add a smallvolume of masonry atop the dam'screst, and to depend upon post-tensioned cables to restabilize thedam for the additional height.

In the early 1930's the late Eu-gene Freyssinet, noted French con-sulting engineer and pioneer in theprestressing of concrete, invented amethod of prestressing dams by in-

flation of flat-jacks without utilizingpost-tensioned tendons. The Freyssi-net method, owing chiefly to itsrather complicated character, hasnot been too frequently utilized indam construction. However, theCoyne method, since its inceptionin 1934, has been quite frequentlyutilized in both new and old damsin various countries for strengthen-ing and heightening. The Freyssi-net method has usually been appliedto new dams of the multiple archand buttress types, constructed, forthe most part, in Algeria.

Since the purpose of this report,as noted above, is to describe theCoyne method, only brief mentionwill here be made of the Freyssi-net method. In both methods, arti-ficial forces are created; the Coyneforces being applied in the upstreamportion of a dam, while the Freys-sinet forces are applied in the down-stream buttresses, as follows:

In the Freyssinet method, thrustsusually of 10,000 to 20,000 tons perbuttress, but exceeding 30,000 tonson occasion, are produced by inflat-ing, under pressures up to 2,000 psi,flat steel pouches or flat-jacks. Whenthe desired runout (maximum = 1in. per jack) is attained, thus induc-ing precompression in the buttresses,the jacks are usually grouted into remain permanently in the struc-ture. These thrusts serve to counter-balance large predetermined por-tions of the active water pressure,usually about one-third. Many ad-justments of the flat-jacks are madeat regular intervals normally overperiods of several years to compen-sate for creep, shrinkage and tem-perature changes in the concreteand rock, and also to compensatefor changes in reservoir water level.

In 1934-1940, the Beni-Bandelmultiple arch dam was constructed

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in Algeria for irrigation and powerpurposes. Construction was nearingcompletion in 1937 when it was de-cided to supply the City of Oranwith drinking water. To meet thisincreased demand, it was necessaryto heighten the dam by 24 ft. to188 ft., thereby increasing the over-turning moment by 50%. Thanks toFreyssinet's ingenious invention, ma-jor revisions in the plans were notrequired. The solution adopted wasto assume a design load per but-tress of 22,000 tons for reservoir fulland 11,000 tons for reservoir emptyfor inducing thrusts. The number offlat-jacks required per buttressranged from 14 for the lowest to40 for the highest buttresses.

The following recent examples ofthe Freyssinet method are cited:

Erraguene (Dien-Djen) Dam:Multiple-arch, 256 ft. high, con-structed in Algeria 1955-60. In addi-tion to thrusts of 8,800 to 37,400tons induced by flat-jacks at thebase of the 11 buttresses, the arch-barrels were post-tensioned bymeans of 36 Freyssinet cables perbarrel, each cable having 36-wires0.276" in diameter. This dam isclaimed to be one of the world'slargest multiple-arch dams.

Menjil Dam in Iran: Buttresstype, 350 ft. high, bolstered by 23self-supporting buttresses. Thrustsof 30,000 tons were induced in eachof the 13 highest buttresses. Pre-stressing with tendons is also uti-lized in this dam. Each sector-gateis designed to sustain 1870 tons max-imum water pressure. Prestressedconcrete girders, post-tensioned withFreyssinet cables, are used to sup-port these gates. Menjil Dam,claimed to be the world's highestbuttress type, was completed in1962.

COYNE METHOD

In 1934-1935, the first practicalapplications of the Coyne methodwere carried out in the rehabilita-tion of the stone masonry Cheurpasand Fergoug (El Habra) Dams,constructed in Algeria in the 1880's.At this time almost all installationswere dependent for continuous op-eration upon a single source of wa-ter supply. The Coyne method madeit possible to strengthen and restoresuch dams without accompanyingdrawdown of reservoirs, and with-out interruptions in service for irri-gation, power and water supply.

The Coyne method consists ofplacing steel cables, consisting ofmany small high tensile wires usu-ally 0.2" in diameter, in verticalholes drilled through the dam andinto rock near the upstream face.The lower ends of the wires areleft exposed and loosened, and aresecurely anchored into foundationrock by grouting. The upper wire-ends are spread out in bouquet fash-ion, and well embedded into heav-ily reinforced concrete headblockswhich are approximately cylindricalin shape. Tensioning of the cables,including 10% initial overloading,is done with hydraulic jacks, mount-ed on the crest, beginning at leastthree weeks after homing in. Thelarger cables are tensioned in stages,while the smaller ones require onlya single stressing operation. On com-pletion of the tensioning, the cableelongation is usually held by wedg-ing steel cylinders filled with con-crete between crest and headblock.Finally, the cables receive a protec-tive coating of grout, injected un-der pressure. Following is listed therange of cable sizes and spacing,that have been used to date:

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Minimum MaximumCable O.D. (in.) 13/s UnknownNo. of wires 37-0.2" dia. 400-0.276" dia.Capacity (tons) 78 1570Spacing (ft.) 3-6 13-21Vert. hole (in.) 21/2 14Drill method Percussion Rotary

In dams where excessive upliftpressures have developed, as in In-dia's Tansa Dam, a minimum cablespacing of 18" was required and,in addition, the cables were placedin two rows.

The following briefly describesthe restoration in 1934-1935 of twoAlgerian gravity dams constructedof stone masonry in the 1880's:

Cheurfas Dam: Rehabilitationafter failure was carried out by plac-ing 37 Coyne cables 6" in dia. x 1,100tons, spaced 13 ft. cc., and locatedabout 7.5 ft from the upstream face.Each cable was assembled on thecrest from 630 galvanized, parallelwires (0.2" dia.) wrapped in sail-cloth and bitumen and placed in10" dia. drill holes, enlarged to 15"dia. to form an anchorage for thegrout plug. At least three weeks af-ter homing in, each cable was ten-sioned to 1100 tons in 3-stages (to330, 770 and 1100 tons) by 3-440ton hydraulic jacks mounted on thecrest. Tensioning time required percable was one week. Maximumlength of cable was 164 ft., and pen-etration into limestone foundationrock was 32 ft. The combined force,40,700 tons, of the 37-cables wasequivalent to increasing the dam'sweight by about one-third, therebymaking it possible to increase theoperating head by 10 ft. Tests madeon some of the cables in 1944 after9 years of service showed an aver-age loss of prestress of only 4.5%.This loss was restored by re-jacking.

Fergoug (El Habra) Dam: Inflash floods of 1927, Fergoug Dam's

weak crest section was torn awayto depths of 13 to 33 ft. below thecrest. Restoration was carried outin 1934 by capping the remainingstone masonry with concrete an-chored thereto by means of Coynecables similar to but smaller thanthe above-mentioned Cheurfas Damcables. The new concrete was placedin 13 to 17.5 ft. sections separatedby copper sheets. Each new blockwas anchored into place by one ca-ble placed at its midpoint. Cablelengths varied from 29 to 59 ft.,and capacities from 138 to 315 tons.The cables, in this case, served onlyto tie the new concrete to the exist-ing masonry without penetratingfoundation rock. The maximum pen-etration into stone masonry wasabout 40 ft., and the grouting lengthof the anchorage was about 16 ft.

Plant, equipment and site-laborrequired to carry out the early ap-plications of the Coyne method werequite extensive. Since about 1953,the Coyne method has been mate-rially streamlined to reduce the costsof such items. Prefabrication tech-niques have been used, and therehas been a trend toward usingsmaller cables. For example, the ca-bles required to stabilize India'sTansa Dam, and to heighten SouthAfrica's Steenbras Dam were only13/8" diameter, shop-fabricated from37 wires 0.2" in diameter, trans-ported to the job site in reels, andplaced in 2W' dia. drill holes.

Implementation of the Coyne orsimilar methods is generally carriedout by specialist firms, such as TheCementation Company Limited inGreat Britain, Soletanche and Son-dages, Etanchements & Consolida-tions in France. In the past ten years,the British firm's activity has beendevoted, in large measure, to theprestressing of both existing and new

February 1965 79

dams, especially in South Africa andAustralia.

In designing gravity dams, oneof the usual criteria specified is tohave the resultant of forces actingon the dam intersect its base withinthe middle third. When post-ten-sioned vertical cables are placednear the upstream face of a weakdam, its resultant line of pressure isshifted upstream to a position morefavorable than that of a convention-al dam without cables. Such addi-tion of cables is equivalent to in-creasing the dam's weight, and,thereby, its stability against slidingand overturning. In the case of newprestressed concrete dams, the addi-tion of upstream cables is effectivein reducing the weight of masonryrequired in the downstream face.Savings in costs of 15 to 20% havebeen realized in the recent con-struction of such new dams.

Since 1951, new gravity damshave been designed and constructedas all prestressed concrete structuresin various countries. In the UnitedStates, prestressing in connectionwith dams has recently taken adifferent pattern. Since about 1960,our Government agencies and othershave applied prestressing to newconstruction of the appurtenantworks of dams and power plants.As anticipated, such prestressing hastaken a form in which prefabrica-tion and precasting have played animportant role in achieving sub-stantial reductions in site-labor andequipment costs over conventionalmethods. Some recent examples arecited below:

Markland Locks and Dams: TheCorps of Engineers, U.S. Armyhave, since the late 1950's, frequent-ly utilized prestressing techniquesin the construction of concrete piersfor spillways, chiefly to reinforcethe trunnion anchorages for tainter

(radial) gates. Subject to the con-tinuous pressure of flowing water,such anchorages are now usuallypost-tensioned to induce sufficientprecompression in the concrete toprevent the formation of cracks.The trunnion anchorages for taintergates 100 ft. wide by 42 ft. high,placed in the Markland Dam, arethe largest utilized to date by theCorps of Engineers. Details of thiswork are shown in Figs. 1-3. Theanchorages were assembled byshop-fabrication in eleven units,each 83 ft. long, made up of 56-lY4'

Fig. 1—Markiand Dam—Post-tensioning

Fig. 2—Markiand Dam-83 Ft. Fabricated Bar As-semblies

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Fig, 3—Markland Dam—Spillway Pier

dia. high-tensile Stressteel bars,spaced at 6" vertically and at 7"horizontally. The assemblies wereplaced in the piers by crane. Thebars were post-tensioned to 100,000psi simultaneously in symmetricalpairs by separate hydraulic jacks.Each assembly was designed to de-velop a prestress force of 3425 tonsto resist the maximum water pres-sure.

Wanapum Dam: This dam wasrecently constructed on the Colum-bia River for Grant County (Wash-ington) Public Utility District. Inits spillway, 11 concrete piers sup-port trunnion anchorages for gates59 ft. wide x 65 ft. high. Each pieris reinforced with a 12-ton assembly,prefabricated in a jig 5 ft. wide x10 ft. high x 75 ft. long. Each as-sembly consists of 14-90 wire ten-dons (with ¼" dia. wires) sheathedin undulating ducts, and post-ten-sioned to 4,400 tons by the SwissBBRV prestressing system. Thewires have cold-formed buttonheadends. The 11 assemblies were pre-fabricated in Seattle, complete andready for installation, trucked tothe site and placed by crane. Each12-ton assembly is designed to re-sist water pressures up to 3,750

tons. Dense, almost impervious andcrackless concrete obtained throughpost-tensioning is now frequentlyutilized to withstand these highpressures and vibrations. It isclaimed the 65 ft. high tainter gates,each weighing 224 tons, are theworlds' highest. Some details areshown in Figs. 4-6. In 1961-1962,prestressing was also utilized inWan a p u m Dam's intake workswhere 78 large vertical tendons,about 74 ft. long, were placed in16" dia. holes drilled into founda-tion rock. Each tendon is made upof 4 groups of 90 wires 1/4" in diam-eter. Four 500 ton hydraulic jackswere required to post-tension eachgroup of 360 wires to 1270 tonsafter grout anchorages were per-mitted to harden for two or threeweeks. In pull-out tests with amaximum force of 1950 tons pergroup, about 1.5 times the designload, neither the wires nor the bondof the grout could be broken.

In recent applications, the Ten-nessee Valley Authority has utilizedprestressing for stabilizing the wallsof cofferdams during construction,and in other minor construction ap-plications.

Listed below in 4-groups are some

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Fig. 4—Wanapum Dam—Downstream Face

Fig. 5—Wanapum Dam—Downstream Face of Two Spillway Piers

Fig. 6—Wanapum Dam—Assembly Being Lifted into Place

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of the dams that have been strength-ened, heightened or constructed asnew prestressed concrete structuressince about 1948. Those dams indi-cated by an asterisk will be brieflydescribed:

Group 1 includes some of the newprestressed concrete dams con-structed from 1951 to date:

Ernestina Dam, Brazil (1951-54)Freyssinet

"Allt-Na-Lairige, Scotland (1954-56)Lee-McCall

*Tourtemagne, Switzerland (1957-58)Freyssinet

Swallow Falls, So. Africa (1956-58)Coyne ,

°Catagunya, Tasmania (1959-61)Coyne

*Meadowbanks, Tasmania (1964-Coyne

Group 2 includes some new damsdesigned for future heightening,with cables omitted in the presentconstruction. However, ducts orshafts have been left in the originalconstruction to sheath the futurecables when increased water de-mand must be met. For example:

°Girotte Dam, So. France (1946-48)Multiple-arch, to be raised36 ft.

'Avon Darn, Great Britain (1954-58)To be raised 13 ft.

Erfenis Dam, So. Africa (1959-61)To be raised 30 ft. inthree stages.

Group 3 covers the strengtheningof existing gravity dams without in-crease in height:

°Tansa Dam, India (1953-55)2399 cables @78 tons

Walwhan Dam, India (1960-61)292 cables @224 tons

°Mahinerangi, New Zealand (1960)20 cables @300 tons

Waitaki Dam, New Zealand (1960-61)46 cables @300 tons45 cables @179 to 224 tons

February 1965

Group 4 includes existing gravitydams that have been heightenedand restabilized by means of Coynecables:

°Steenbras Dam, So. Africa (1953-54)Raised 6.5 ft. plus 5.5 ft.flood surcharge

Witbank Dam, So. Africa (1957-59)Raised 18.5 ft.

Compies Dam, So. Africa (1958-59)Raised 23 ft.

Hume Dam, Australia (1959-60)Raised 6 ft. and 24 ft.gated spillway added

Mazoe Dam, So. Rhodesia (1960-61)Raised 10 ft.

*Argal Dam, Great Britain (1960-61)Raised 10 ft.

GROUP 1

Tourtemagne Dam

The Tourtemagne Dam, locatedin south Switzerland, is 108.3 ft.high, and is claimed to be the firstnew arch dam to be prestressed. Itwas constructed to impound Alpineglacial waters in a reservoir that canbe entirely emptied in winter sea-sons, and alternately filled andemptied several times in summerseasons. In the upper three-fifths ofits height this dam is only 3.9 ft.thick. This slenderness and fluctuat-ing reservoir levels combine to sub-ject it constantly to temperaturestresses. Its stability is assured with-out recourse to tensioned cables bysharp horizontal curvature withradii only 66 ft. and 98 ft. long, andcrest length of only 377 ft.

Both vertical and horizontal pre-stressing by Freyssinet cables of 12wires 0.276" in diameter, and Freys-sinet flat-jacks was utilized toneutralize temperature stresses, andto eliminate all tensile stresses withthe reservoir either full or empty.Sixty vertical cables, spaced 3.3 ft.in the center section and 4.9 ft. atthe ends, were placed in a 230 ft.

83

Fig. 7—Catagunya Dam—Cable Heads and Homing Wheel

section of the 377 ft. crest length.Prestressing without tendons was

carried out by inflating Freyssinetflat-jacks 16 1/x" in diameter. In eachof the vertical expansion jointsseparating the concrete monoliths,12 Freyssinet flat-jacks, spaced 3.3ft. vertically on the dam's centerline, were placed and inflated. Oncompletion of adjustments, the jackswere grouted in place.

Allt-Na-Lairige Dam

The Allt-Na-Lairige Dam, locatedin Scotland, is 83 ft. high x 1360 ft.long, and is claimed to be the firstdam in which high tensile bars in-stead of wire cables were utilizedas tendons. The tendons consist of28 vertical Macalloy bars 1'/a" in di-ameter x 109 ft. long placed in 47shafts 4'-0" in diameter and spaced21'-0" on centers. The prestressedsection extends over 987 ft. of thedam's 1360 ft. crest length. The barswere spliced at two points bythreaded couplers and post-ten-sioned individually to 47 tons usingthe Lee-McCall system after anchor-age by grouting into 26 ft. ofgranite rock. The estimated savingin the concrete placed over that ofa conventional gravity dam was

about 40%, and the saving in costwas 15%.

Catagunya Dam

The Catagunya Dam (Figs. 7 and8), located in Tasmania, is 150 ft.high. This newest and highest of thenew prestressed concrete dams,completed in 1961, is provided withan over-hanging spillway section425 ft. long designed for 18 ft. ofsurcharge. The spillway cantilevers24 ft. upstream from the verticalupstream face, and discharges intoa high-level bucket dissipator. Itwas stabilized with 412-3 in. dia.Coyne cables, each of 224 ton capac-ity. .The cables were assembledfrom 102-wires 0.2" in diameter andplaced in two rows. Holes for thecables were 4" in diameter, andwere percussion-drilled into doleriterock. Transite pipe sheaths 45/s" indiameter were used in passingthrough concrete. Maximum lengthof cable was 170 ft., and groutanchorage in dolerite was 13 ft.The estimated saving in cost over aconventional gravity dam was about20%.

Meadowbanks Dam

In January 1964, construction of a

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PCT journal

new prestressed concrete dam ofthe buttress type, 130 ft. high x 830ft. crest length, with a 250 ft. spill-way, was begun by the Cementa-tion Company Limited in Tasmania.Prestressing with tendons will becarried out later when 14,000 lin.ft. of cable will be placed in thedam at 35° to the vertical. Each ofthe 140 Coyne cables will consist of72 galvanized wires, 0.276" in di-ameter and will have a capacity of300 tons. Heretofore, post-tensionedcables placed in dams have, in mostcases, been vertical or nearly verti-cal. In this case, the cables will be

sloped to provide both vertical andhorizontal stabilizing forces. Thespillway will be provided with twosector gates 119 ft. wide x 15 ft.high. The Cementation Company'scontracts for preloading and grout-ing of foundations, and of placingthe post-tensioned cables will totalabout $560,000.

GROUP 2

Avon (Dartmoor) Dam

The Avon Dam in Great Britainis presently 108.5 ft. high. Theproposed future increase in height

Fig. 8—Catagunya Dam—Partially Complete Showing Line of Cable Heads

February 1965 85

of 13 ft. was not known at the timethe design plans were completedand the contract awarded in 1954.However, provision for 34 futurecables was made at small additionalcost in the present construction. Asconcreting proceeded in 6 ft. lifts,ducts required to sheath futurecables were formed by means of 21"dia. concrete pipe in 6 ft. sections.In foundation rock, 24" dia. x 15 ft.holes, temporarily capped with 2ft. of concrete, were provided forfuture anchorage. Had Avon Dambeen originally designed in pre-stressed concrete, a considerablesaving in volume of concrete on thedownstream slope could have beenrealized.

Girotte Dam

This multiple-arch dam, 114 ft.high with 18 inclined arch-barrelssupported by buttresses spaced78.7 ft. on centers, was designed for36 ft. future heightening. When theincreased water demand must bemet, this dam will be restabilizedby means of post-tensioned verticalcables placed at buttress centers inshafts provided in the original con-struction (one shaft per buttress).In the future addition, the 18-bar-rels will be extended up 36 ft.vertically, and securely anchored tothe original inclined barrels.

GROUP 3

Tansa Dam

The stone masonry Tansa Damin India now 133 ft. high was con-structed from 1886 to 1891 to aheight of 118 ft. of greenstone(basalt) masonry laid up in surkhi(lime mortar). It was designed,without uplift allowance, for 17 ft.higher head, and was heightenedto 133 ft. in 3-stages—in 1914, 1925and 1946. For years the City of

Bombay had depended solely uponthis unstable dam for its water sup-ply. It was kept under close observa-tion, and was investigated at varioustimes by such prominent hydraulicengineers as the late Andre Coyneand the late Louis F. Harza, formerPresident of the Harza EngineeringCompany of Chicago. By the early1950's, measures for stabilizing it,complicated by a post-war popula-tion explosion, had become extreme-ly urgent.

In 1953-55, under Andre Coyne'ssupervision, the stabilizing was car-ried out by the Cementation Com-pany Limited. In this project morethan 240,000 lin. ft. of 1 1/8" dia. cableswere placed in 2399 holes 2W' indiameter drilled through the dam5 ft. from the upstream face. Pene-tration into basalt rock was 25 ft.The cables were shop-fabricatedfrom 37 wires, 0.2" in diameter,placed in coils 1200 to 1400 ft. long,transported to the site, passed intothe holes, and anchored into groutplugs 12 ft. long. Maximum cablelength was 180 ft. The upper endsof the cables were anchored intoMeehanite cast iron headblocks,and post-tensioned by means ofhydraulic jacks mounted on thecrest. At least 3 weeks after homingin and grouting in, each cable wastensioned above its working load to90 tons. After a lapse of an addi-tional 4 weeks, each cable wasfinally tensioned so that a minimumdesign load of 78.4 tons was per-manently held. In sections whereuplift was excessive, the cableswere placed 18" apart in two rows.

Rehabilitation of Tansa Dam, car-ried out over a 28 month periodterminating in March 1955, was byfar the largest project of its kindever undertaken. This work wasdone with the reservoir full, and

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without interruptions in water sup-ply.

Mahinerangi Dam

The 140 ft. Mahinerangi Dam inNew Zealand was strengthened in1960 by means of 20-300 ton verticalcables of 72-parallel stranded wires,0.276" in diameter, placed in 4"dia. drill holes 3'-3" from the up-stream face. Anchorage length ofthe cables in rock was 25 ft., andmaximum length was 160 ft. Thewires were arranged in three con-centric circles around 1 1/8" I.D. x 6"central ferrules spaced 6'-O" oncenters. The cable headblocks, coni-cal in shape, were poured in placeand embedded in the dam just be-low the crest.

GROUP 4

Steenbras Dam

The 80 ft. Steenbras Dam inSouth Africa was heightened by6.5 ft. and, in addition, designed tosustain 5.5 ft. flood surcharge asfollows: Concrete in the existingcurved crest section was first cutdown about 3'-6", and a 21" rein-forced-concrete cantilever wall 9'-3" high having the same upstreamface was superimposed on the low-ered crest. The new wall was secure-

ly anchored to the existing dam bysteel dowels. Required for stabiliz-ing were 163-13/s" dia. semi-endlesscables, each of 37 wires, 0.2" in di-ameter, placed in 326 drill holes2½" in diameter, and anchored bygrouting. The cables were bent overprecast semicircular saddles mountedon the crest and post-tensioned to154 tons (77 tons per leg) by 224ton jacks. The design was based onthe following assumptions:

1. Uplift pressure = 0.67 of fullstatic head at the upstreamface, decreasing uniformly tozero at the downstream toe.

2. Effective area of uplift-100%of base.

3. Coefficient of friction for bal-ancing the horizontal forces =0.75, assuming shear in con-crete = 0.

Interesting to note in connectionwith the rehabilitation of SteenbrasDam was the fact that the proceedsfrom the sale of water obtained fromthe enlarged reservoir during thefirst summer of 1955 more than paidfor the entire cost of the project.Provision for further heighteningwas made in the present construc-tion by so arranging the currentlyplaced cables to allow sufficientspace for future cables.

Fig. 9—Argal Dam

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Fig. 10—Argal Dam—Cables Bent Over Steel Cages

Argal Dam

The Argal Dam (Figs. 9-10) inGreat Britain was constructed in1939-40 and heightened by 10 ft.in 1960-61 to meet an increased de-mand for water. This marks the firstapplication of the Coyne method inGreat Britain. After the existingcurved crest section was cut downby three feet, the new concrete wasplaced in two lifts of 7 and 6 feet.In the first lift, the crest washeightened by 7 ft. to form the baseand face for placing 47 verticalcables spaced 5'-6" on centers in4" dia. holes percussion-drilled to27 ft. minimum below the founda-tion. Each cable is about 3" in di-ameter and is fabricated on the siteof 102 wires, 0.2" in diameter, ar-ranged in circular cross-sectionaround a central ½" dia. grout pipe.Cables were anchored by grouting13 ft. into foundation rock, and inreinforced-concrete cylindrical head-blocks on the crest. At least 3 weeksafter homing in, each cable waspost-tensioned to 224 tons, afterinitial overloading by 10%, by meansof 3-112 ton hydraulic jacks mountedon the crest. Cable elongation wasmaintained by means of steel tubes

filled with concrete. On completionof the post-tensioning, the secondlift, 6 ft. of new concrete, wasplaced to form the new crest sec-tion and encase the cable head-blocks.

I.C.O.L.D.

At the 6th Congress of the Interna-tional Commission of Large Damsin New York in 1958, under Ques-tion No. 20, various ways of height-ening dams, including prestressing,were fully considered. Of 29 paperspresented with discussion, 6 paperswere on prestressing of dams.

It is deemed impractical and un-economical to construct new pre-stressed concrete dams higher than200 ft. This has generally been ac-cepted as the ceiling, and has beenconfirmed in a discussion at the 7thCongress of ICOLD in Rome in1961.

ACKNOWLEDGEMENT

The writer is indebted to theCementation Company Limited ofLondon, Stressteel Corporation andJoseph T. Ryerson & Sons, Inc. formuch of the information containedin this report.

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