Safety evaluation of a slab and buttress dam
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Transcript of Safety evaluation of a slab and buttress dam
Dams – Securing Water for Our Future 1
Safety evaluation of a slab and buttress dam
Marius Jonker1, Francisco Lopez2 and John Bosler3
1 Principal Dams Engineer, GHD Pty Ltd, Level 8, 180 Lonsdale Street, Melbourne, 3000, Vic, Australia2 Senior Dams Engineer, GHD Pty Ltd
3 Principal Dams Engineer, GHD Pty Ltd
This paper describes the safety evaluation and development of remediation options for Clover Dam, a 28 mhigh slab and buttress structure situated in the alpine region in northeast Victoria, Australia. The reviewwas particularly challenging considering the complexity, age and cracked condition of the dam structure,which required the development of an analysis method for this type of dam.Completed in 1953, Clover Dam is one of five dams in the Kiewa hydroelectric scheme. The 76 m long damcomprises a 45.7 m long covered slab and buttress section, supported on each abutment by concretegravity sections. The review was undertaken as a result of severe cracking occurring since the early 1970sand because a detailed design review had not been undertaken since its construction.Current guidelines for the safety review of existing dams provide little detailed information on slab andbuttress dams. Consequently, a methodology was developed to analyse Clover Dam. This methodologycould also be applied in the review of this type of dam in general, and is currently being used for safetyassessments of three other slab and buttress dams.This paper focuses mainly on the dam structural assessments undertaken during the safety review. Thestructural analyses involved 3D finite element analyses for thermal, static and earthquake loading.The outcome of the review was that both the gravity and buttress portions of the dam do not meet currentdesign standards. The development of practicable remediation options was complicated by the operationalconstraints and the restricted access to those areas within the dam where remedial works were required.
Keywords: Slab and buttress dam, 3D finite element analysis, seismic assessment.
IntroductionClover Dam consists of a 28 m high slab and buttressstructure, supported on each abutment by concrete gravitystructures of the same crosssectional profile.
The dam was completed in 1953, and cracks werereportedly first observed in the early 1970s. A formalprogramme of routine surveillance of the dam began inthe early 1990s and included fiveyearly comprehensivecrack surveys of the whole structure, which indicated thatcracking was continuing to develop.
In 2002 a decision was taken that, in view of the dam’s bynow severely cracked condition and because no safetyreview had been undertaken since its completion, it wastimely to conduct a comprehensive assessment of itsstatus with respect to current dam safety standards and toestablish the possible causes of the cracking.A failure modes analysis and full safety review of the damwere undertaken in accordance with ANCOLD guidelines(2003). This paper, however, focuses mainly on thoseaspects of the safety review that identified majorstructural deficiencies in the dam.
At the time of publication a feasibility study of upgradeoptions was being undertaken and the paper closes with adiscussion of those options currently under consideration.
Description of the dam and schemeThe Kiewa SchemeClover Dam is part of the Kiewa Hydro Electric Scheme,
which was developed between 1939 and the 1960s in thealpine region of NorthEast Victoria. The scheme issituated on the east and west branches of the Kiewa Riverand consists of a series of diversion weirs, tunnels,penstocks, power stations and five dams. Clover Dam is avital link in the scheme, incorporating the link to transferwater from the East Kiewa to the West Kiewa River forthe West Kiewa Power Station
The former State Electricity Commission of Victoriadesigned and constructed Clover Dam during the period1948 to 1953. The dam is currently owned, operated andmaintained by AGL Hydro.
General layoutClover Dam comprises a slab and buttress structure withgravity flanks, a bottom outlet pipe for controlledreleases, and an ogee spillway with flashboards.
The nonoverflow crest reaches to a height of 28.55 mabove the lowest foundation level. The 76 m long damincorporates a central 61 m wide spillway. The buttressportion of the dam is 45.7 m long, with the remainingapproximately equal lengths on the flanks made up of theconcrete gravity blocks. The spillway includes the fullbuttress portion of the dam wall as well as part of thegravity blocks, as shown in Figure 1.
Access into the slab and buttress section is via an accessshaft inside the gauge house on the left flank and a gallerythrough the left flank gravity section.
2 IPENZ Proceedings of Technical Groups 33/1 (LD)
Figure 2. View of right bank, spillway and apron
ButtressesThe slab and buttress section includes five buttressesequally spaced at 7.6 m centres, thereby creating six equalspans covered by the face and spillway slabs (seeFigure 1). The buttresses are up to 20 m high and ofconstant thickness (0.76 m) above the foundation, whichis 1.22 m thick.
Each buttress (see Figure 3) incorporates two inclinedcontraction joints between the foundation and upstreamface, effectively forming three inclined columns withinthe buttress, to transfer the water loads to the foundation.Each buttress face is lightly reinforced (19 mm diameterbars each way at 305 mm centres). The bars extendthrough the horizontal construction joints, but terminateon each side of the inclined contraction joints.Partial lateral support of the buttresses is provided bymidheight horizontal struts spanning between thebuttresses, as well as between buttresses and the adjacentgravity abutments. These struts also serve as a walkwaythrough the dam. Access through the buttresses is bymeans of openings in the buttresses at the level of thestruts.
Figure 3. Typical slab and buttress detail
Face slabsThe upstream face is composed of a number of inclinedslabs of varying thickness, stacked on top of each otherbetween adjacent buttresses (or a buttress and the insideedge of a gravity block), with up to six slabs over theheight of the dam at the highest point.
Buttress
Figure 4. Face slab / buttress connection
Each panel rests on half the width (0.38 m) of thesupporting buttresses. Crucially, the slabs are not tied tothe buttresses or each other (refer Figure 4), and are keptin position by their own weight and the hydrostatic
45.7 m long slab and buttress section
Gravity section Gravity section
61 m wide spillwayAccess shaftand gallery
Figure 1. Clover Dam viewed from upstream
Dams – Securing Water for Our Future 3
pressure, as well as a key along the perimeter of the panelto provide interlocking with the adjacent panels. Eachpanel is also keyed into the upstream face of thesupporting buttresses / gravity block. The vertical andhorizontal expansion joints separating the slabs are sealedwith copper waterstops and bitumen.
Spillway, slabs and apronThe 61 m wide spillway includes the six spans of thebuttress section of the dam, and a gravity section at eachend of about the same length as each span between thebuttresses, as shown in Figure 1.
The nappeshaped spillway crest has been provided withflashboards (wooden planks kept in position by steelpipes) to allow operation of the dam above the full supplylevel while retaining the flood capacity.The spillway slab, similar to the face slab, consists ofindividual panels, stacked on top of each other betweenadjacent buttresses (or a buttress and the inside edge of agravity block), with up to six panels over the height of thedam at the highest point. These panels are of constantthickness and shaped to form the spillway crest.
The vertical and horizontal expansion joints separatingthe spillway panels are not sealed.Each panel rests on a 150 mm wide edge of the adjacentbuttresses. The panels are not tied to the buttresses oreach other (refer Figure 5), but are kept in position by anextension of the buttress between the panels,incorporating a key to secure the panels to the buttress,and keys in the upper and bottom edge of the panels.Unlike the upstream face panels, the spillway panels arenot keyed into the supporting buttresses / gravity block.
Buttress
Figure 5. Spillway slab / buttress connection
The apron consists of a 7.5 m long slab over the full widthof the spillway (see Figure 2). The upstream part iscurved to tie in with the spillway slab, with the sides atthe flanks curved to tie in with the natural rock level.
Current condition of the damThe condition of the dam is being assessed throughweekly routine inspections and annual intermediateinspections, in accordance with ANCOLD guidelines(2003). A comprehensive inspection was conducted in
2002 as a basis for the safety review and the failuremodes assessment.
The spillway slab (downstream face) presents only a fewcracks, visible because of efflorescence on the concreteface, and minor spalling. Longitudinal cracks extendalong the spillway crest, apparently along constructionjoints where the crest beams and spillway slabs areconnected. The vertical faces of the training walls andnonspillway walls at the sides of the spillway are coveredwith severe mappattern cracking, and the concreteappears in poor condition.
Seepage has been noted on the right flank emanating fromthe rock at the downstream end of the apron. Closeupinspection has revealed a cavity behind the concrete at thepoint of seepage.The downstream face of the nonspillway section presentssome cracks, visible because of efflorescent on theconcrete face, and minor spalling. Longitudinal crackinghas occurred along the downstream face of the spillwaytraining walls.
Cracks in the order of 5 mm width are present in the shaftand the gallery (see Figure 6), as well as along the verticaland horizontal construction joints on the transverse facesof the gravity blocks on both abutments. These faces arealso extensively covered with mappattern cracking.Seepage emanating from the cracks indicates that it isbypassing the waterstops and that the cracks areconnected to the reservoir. Minor cracking is presentgenerally on all the internal faces of the face and spillwayslabs. Cracking has also been observed in the walkwaybeams between the buttresses.
Figure 6. Cracks in gallery
Horizontal cracks in excess of 5 mm width are present inthe buttresses along the horizontal construction jointsintersecting the openings through the buttresses. Thesecracks extend right through the buttresses and along mostof the length. Horizontal differential displacement (theconcrete above the crack appeared to have displacementin the downstream direction relative to the base) of theorder of 5 mm is notable at the openings through thebuttresses, as shown in Figure 7.
4 IPENZ Proceedings of Technical Groups 33/1 (LD)
Figure 7. Horizontal displacement at crack in buttress
Assessment of crackingMechanismsCertain observations were found common to similarcomponents of the dam, i.e.:The gravity blocks:• Individual, deep cracks up to 2 mm width on the
construction joints, with seepage through these cracks.• Alkali aggregate reaction type surface mappattern
cracking on all faces.The buttresses:• Individual, horizontal cracks larger than 2 mm width
(up to 5 mm in places) passing through the structure,on the construction joint in line with the top of theopening through the buttress, in all five buttresses.
• Alkali aggregate reaction type surface mappatterncracking in localised areas on some of the buttress.
The face slab:• Efflorescent deposits at the horizontal joints between
the panels, and seepage through these joints.• Parallel, individual cracks up to 2 mm width.The spillway slab:• Efflorescent deposits are present on the horizontal
joints between the panels, with some seepage throughthe joints, and the joints have opened up to 2 mmwidth.
• Parallel, individual cracks up to 2 mm width.
It is likely that there are two major mechanisms causingcracking at Clover Dam, i.e., alkali aggregate reaction andtemperature stress. Though they cannot be separated asbeing the only cause of cracking in certain elements of thedam, they can be identified as the predominant cause.
Investigation of significant buttress cracksIn view of the size and noticeable horizontal differentialdisplacement of the cracks like the one shown in Figure 4,it was decided to expose the reinforcing steel at onelocation to assess its condition. Investigations revealed:• The intact concrete around the crack was in good
condition with no evidence of fracturing.• The reinforcing steel was in very good condition and
installed in accordance with the available drawings.
Only minor rust stains were observed, despite the steelbeing exposed in the cracks for some time.
• The referenced displacement of the upper part of thebuttress at the horizontal joint was confirmed by akink observed in the vertical bars (see Figure 8).
• Measured diameters at the kink part of the bars wereslightly less than those in the straight parts, indicatingminor yielding.
Figure 8. Kink in reinforcing steel indicating relativedisplacement between concrete above and below joint
Investigation of alkali aggregate reactionIn view of the extensive mappattern cracking typicallyassociated with alkali aggregate reaction, as observed onall the exposed faces of the dam, investigations todetermine the presence of alkali aggregate reaction in theconcrete have been undertaken.
Alkali aggregate reaction is a general term including thethree types of reactions described below.• Alkali silica reaction• Alkali silicate/silica reaction• Alkali carbonate reaction
Factors affecting alkali aggregate reactionResearch to date has found that concrete deteriorationfrom alkali aggregate reaction is due to the simultaneouseffect of several factors. The eventual manifestation of thereaction in a particular concrete, that is the rate and extentto which the reaction proceeds, depends on factors thatcan be grouped as material properties, external influences,and time.
Materials used at Clover DamAggregate used for concrete production was obtained bycrushing gneiss, diorite and pegmatite excavated fromNo. 4 Head Race and Tail Race Tunnels. A laboratory testsheet of the concrete aggregate, dated August 1951,shows the aggregate classified as gneiss, comprisingmainly quartz (62%), biotite (13.25%) and sillimanite(11.3%), with other elements such as orthoclase,plagioclase, muscovite, cordierite and chlorite in smaller
Downstream movement
Dams – Securing Water for Our Future 5
quantities. The rock was also classified as having amediumgrained texture and faint gneissose structure.
According to Beavis in 1952, none of the minerals wasconsidered as potentially reactive with high alkalicements. Quartzbearing rocks, however, have been foundto be alkali silicate/silica reactive, including quartzite,quartz biotite and gneiss, with the most commonlyimplicated minerals being the siliceous polyphasedminerals and crystalline minerals like quartz and feldspar.Furthermore, gneiss containing strained quartz as animperfection, as well as felspar in an altered chemicalstate, was found to be reactive as well.
Therefore, based on current knowledge, the gneiss isconsidered as likely alkali silicate/silica reactive and thenecessary precaution should have been taken duringconstruction to limit the alkali content of the concrete.
Alkali aggregate reaction investigationsInvestigations were conducted in 1997 and 1999 onconcrete cores from Clover Dam to determine whether thecracking in the concrete was caused by alkali aggregatereaction (Shayan 1997a, 1997b, 1999). The investigationconfirmed that the reaction was present in the concrete,and that the amount of soluble alkali available in theconcrete appeared adequate for the reaction to continue.External influencesThe individual cracking (as opposed to mappatterncracking) is caused by concrete swelling as a result ofalkali aggregate reaction, and concrete failing in tensiondue to restraint to expansion. The moisture availabilityand high summer temperatures are other externalcontributing factors.Photographs taken at Chambon Dam in France (ICOLD1991), which was found to be affected by alkali aggregatereaction, show very similar deep cracks in thelongitudinal direction of the gallery roof.
TimeThe literature indicates that cracks induced by alkaliaggregate reaction may appear after varying lengths oftime. Concrete swelling may be gradual or may occursuddenly at a later stage, and stop after some time orcontinue indefinitely, depending on the type of reaction.
According to Charlwood et al. (1994) the alkali silicareaction occurs for about 30 +/10 years, while the alkali silicate/silica reaction is slower and may continue forgreater than 50 years.It is not known when the cracking started, but ahandwritten inspection report dated 9 February 1979mentions that the cracking in the access tunnel wasobserved in 1972, some 18 years after completion ofconstruction.
Cracks movement records of the last 12 years (it is notknown when crack monitoring started) show that thecracks are gradually opening, indicating that the concreteis still expanding, some 50 years after construction. Thelate development of concrete deterioration, andcontinuing expansion, suggest that the type of reaction isalkali silicate/silica.
Assessment of cracking at Clover DamIn view of the above it was considered that alkali silicate/silica reaction is likely the predominant cause ofthe cracking in the gravity blocks on both abutments,including the spillway gravity blocks, as well as the minormappattern cracking observed in one of the buttresses.The individual horizontal cracks in the buttresses arelikely the cause of thermal stresses, as explained later inthis paper.
Structural assessment of slab andbuttress sectionA detailed structural analysis of the slab and buttresssection of Clover Dam under flood, seismic and thermalloading was undertaken using finite element analysis.Finite element modelsFinite element models were created using solid brickelements to represent the buttresses, plate elements torepresent the face and spillway slabs, translational massesto represent structural masses attached to the dam, andnontranslational masses to represent the hydrodynamiceffects. All models also contained massless brickelements representing the foundation rock.Since the models were not intended for nonlinearanalyses, they do not simulate the actual timedependentbehaviour of cracks, column joints or horizontal joints.The expected behaviour of the structure was interpretedfrom the exhibited linear response of the models.
Single buttress modelThis model, shown in Figure 9, comprised one centralbuttress, with half thickness buttresses on either side, tofacilitate modelling of the distribution of the slab’smasses and their inertial contribution. The model wasused for the normal, flood and thermal analyses.
Figure 9. Single buttress model
Figure 10 is a more detailed view of the FE modelling inthe central buttress, with the half buttresses and the slabsnot shown. The buttress comprises, in effect, threecolumns, separated by inclined contraction joints.
6 IPENZ Proceedings of Technical Groups 33/1 (LD)
Figure 10. Single buttress model showing columnsand contraction joints
Fivebuttress modelDepending on the frequency content of the earthquakeand the variation in effective stiffness between buttresses,the behaviour of the dam is expected to be variable duringcrossvalley seismic excitation, ranging between thepossible limiting cases of all buttresses vibrating in phaseand all buttresses vibrating out of phase.
To effectively model this response it was necessary tocreate a second finite element model of the whole dam,including the five buttresses, the abutment gravitysections and the face and spillway slabs.
Thermal analysisThe thermal analysis of the dam was undertaken using thesingle buttress finite element model. The modelling wasundertaken in two steps.
In step 1 the effect of the column joints was ignored, thatis, full coupling of the elements on either side of the jointswas assumed. This analysis indicated that the principalstresses along the column joint 2 (refer Figure 10)immediately beneath the face slab were tensile and wouldtend to open the joint. Figure 11 illustrates a typicaloutput plot for the magnitude and direction of theprincipal stresses in the buttress due to temperaturedifferential, indicating the zone of most likely opening ofthe joint. The state of stress shown in Figure 11 alsoincludes the effect of the gravity and hydrostatic load.
Figure 11. Magnitude and direction of the principalstresses in the buttress due to temperature differential
In Step 2 the opening of column joint 2 was modelled bycreating a crack in the FE model at this location, so as toobtain a more accurate indication of the magnitude anddistribution of the stresses along the horizontal joints.Figure 12 illustrates a typical plot of the magnitude anddistribution of vertical tensile stresses, indicating the zoneof most likely opening of the joints.
Figure 12. Magnitude of the vertical tensile stressesduring thermal loading
In general, the resulting cracking pattern from the modelmatched the main cracking observed along the horizontaljoint in all buttresses at the level of the access opening.These cracks extend along the horizontal joint up tocolumn joint 2 and along the latter to the upstream edgeof the buttress. It also reproduced the cracking observedalong most other horizontal joints in the buttresses atlocations adjacent to the spillway slabs.
The analysis indicated that a differential temperature of7.5°C between the upstream face slabs and the buttresseswould be sufficient to open a segment of the column joint,and yield the vertical skin reinforcement that runs throughthe horizontal joints of the buttresses. Based on availabletemperature records both inside the dam and outside (datalogger measurements every 30 minutes), this temperaturedifferential is often exceeded.
In the thermal FE analysis perfect coupling of the plateelements in the slabs to the brick elements in thebuttresses was assumed to take account of the frictionalstrength capacity available on the contact area betweenthe slabs and the buttresses, augmented by the existenceof shear keys. The analysis indicated that the developmentof thermal loading induced cracking in Clover Dam couldhave been prevented or at least alleviated if the originaldesign had incorporated the use of a bituminous materiallayer into the buttress/slab contact, as recommended inUSCOLD (1988).
Flood load analysisThe analysis of the dam for the spillway design flood casewas undertaken using the single buttress finite elementmodel. The analysis indicated that no tensile stresses wereexpected along the column joints. Therefore, the completesingle buttress model was valid.
Figure 13 illustrates a typical plot showing the magnitudeof flexural moments on both the face and spillway slabsof the dam during the design flood. The demand imposedon all elements of Clover Dam by the flood loading casecan be safely withstood by the structure, when analysed
Dams – Securing Water for Our Future 7
assuming the dam remains in an “as new” condition.Further assessment of the dam in its current (cracked)condition was held over pending the outcome of theearthquake analyses.
Figure 13. Flexural moments on face and spillwayslabs for flood loading
Analysis of seismic load in an upstreamdownstream directionSpectral analysis – MDE in downstream directionThe single buttress FE model was used to analyse the damfor the maximum design earthquake where the seismicbase acceleration occurs in the downstream direction. Theanalysis indicated that no tensile stresses were expectedalong the column joints. Therefore, the complete singlebuttress model was valid.
Figure 14 illustrates a plot of the magnitude of maximumcompressive and tensile stresses. It was found that thedam could safely withstand the imposed demand.
Spectral analysis – MDE in upstream directionThe single buttress finite element model was also initiallyused to analyse the dam for seismic loading in theupstream direction. A preliminary analysis for themaximum design earthquake indicated that tensile stresseswould develop along the column joints. Therefore, whenthe base of the structure is accelerated in the upstreamdirection during an earthquake, the inclined column jointswould open and the buttresses would not deformmonolithically.
Consequently, three separate finite element columnmodels, as shown in Figure 15, were used to assessstructural response and likely damage. The complete
model, but with the finite elements nulled (deleted) in thezones where serious damage was expected, was howeverused to assess the postearthquake static load capacity ofthe dam.
Figure 14. Maximum principal stresses(MDE upstream direction case)
Figure 15. Separate column models
Figure 16 illustrates a plot of the expected maximumcompressive stress in column 3 and vertical stresses onthe column joint 2 (other column models not shown here).The analyses indicated that the dam could withstand themaximum design earthquake in the upstream directionwithout collapse. Localised damage, however, is expectedto occur at the buttress/foundation interface and someminor damage could occur at the buttress supports for theface slabs spanning between the bottoms of the buttresses.
8 IPENZ Proceedings of Technical Groups 33/1 (LD)
Figure 16. Maximum principal and vertical stresses(MDE downstream direction stream case)
Analysis of seismic load in crossvalley directionA preliminary run of the fivebuttress model subjected tocrossvalley seismic loading established that the sheardemand on the seating between the face slabs and thebuttresses under moderate transverse seismic loadinggreatly exceeded the expected frictional resistance tosliding.
Employing an approach commonly adopted in theanalysis of bridge beams under seismic loading it wasassumed that there was no friction between the face slabsand the buttresses and that each buttress was able todeform out of plane without any force or moment restraintacting on its upstream edge.
Under these assumed boundary conditions the degree ofcoupling between buttresses was greatly reduced and itwas found that the maximum demand on any buttressunder crossvalley loading could be predicted using thesingle buttress FE model.
The upstream face of the buttress was assumed free andtranslational masses accounting for the inertialcontribution of the face slabs were added to the upstreamedge of the buttress. On the downstream edge of thebuttress the spillway slab connection will provide morerestraint to the buttresses than the connection between theface slabs and the buttresses. The spillway slab was
assumed to prevent movement of the downstream edge ofthe buttress in the crossvalley direction.
In order to calculate the maximum demand in the buttressat any time during the earthquake, an envelope ofresponses was obtained using two different single buttressmodels. The first model accounted for the demand on thebuttress while the simply supported walkway strut wasstill in place, creating maximum demand around thewalkway opening in the buttress. The second modelaccounted for the demand on the buttress once the struthad fallen, creating maximum demand on the buttressmembrane at its approximate midpoint between thewalkway opening and the top of the buttress, due to alarger unbraced distance.
Frequency analysesThe deformed shapes for the fundamental modes ofvibration, with and without the walkway strut, areillustrated in Figure 17.
Figure 17. Fundamental mode of vibration (crossvalley) with and without struts
Spectral analyses – Cross valley directionFigure 18 illustrates the expected state and magnitude ofmaximum compressive, vertical and horizontal tensilestresses in the buttress, for the maximum designearthquake once the walkway strut has fallen.
The performance of the dam under earthquake in thecrossvalley direction was found to be unsatisfactory forboth the OBE and the MDE events. The dam would beexpected to suffer generalised minor damage andlocalised severe damage during the OBE. The demandcaused by the MDE event cannot be withstood by thestructure and one or more buttresses would be likely tocollapse, leading to a complete failure of the dam. Even ifcollapse did not occur, the damage in the structure for theMDE would at the least necessitate major repair works.
Dams – Securing Water for Our Future 9
Figure 18. Maximum displacement and stresses in thebuttress for the MDE case without struts
Upgrade optionsGeneral approachThe objectives of a refurbishment program at Clover Damwill include addressing existing damage and ageing,solving identified structural deficiencies and a lack ofcapacity for current loading, enhancing the capacity forfuture loading conditions, or a combination of these. Thesuitability of alternatives would depend not only on thenature of the identified deficiencies in the dam, but alsoon project constraints such as accessibility and the lengthof time that the dam would have to be taken out ofoperation.
Alternatives to address stability and strength problems ofbuttress dams may include external posttensioning of thebuttresses, increasing the cross section of the buttresses,providing lockup of buttress/slab connections and manymore.
Options considered for Clover DamGeneral options to address the structural deficienciesidentified at Clover Dam, comprised strengthening,infilling and replacement. Within each of these optionsare a number of suboptions. The practicability of theseoptions will be significantly affected by access conditions(or rather lack thereof) into the dam where the remedialworks will be required.
StrengtheningStrengthening alternative 1, a reinforced concrete option,consists of constructing a diaphragm wall that providesthe required additional strength to resist crossvalleyseismic loading, and a continuous corbel beam along theupstream and downstream edges of each buttress toaddress the thermal expansion problem, with thesupplementary benefit of providing additional seatingdistance for the slabs.
Figure 19. Concrete strengthening
Strengthening alternative 2, a carbonfibrereinforcedpolymer (CFRP) option, consists of the application oflayers of carbon fibre strips at specific locations on thebuttresses, oriented to provide the required additionalstrength for both the seismic and thermal loading cases.
Figure 20. Carbon fibre strengthening
A third alternative could be a combination of the abovetwo alternatives.
InfillingThis option comprises converting the slab and buttresssection to a concrete gravity dam. This could be achieved
10 IPENZ Proceedings of Technical Groups 33/1 (LD)
by infilling the openings between the buttresses withconventional mass concrete, or demolishing the entiresection and reconstructing it in either RCC orconventional mass concrete.
ReplacementThis option entails demolishing the entire dam andbuilding a replacement. In the case of Clover Dam therewere two alternatives, either a new dam at the samelocation or at a new site immediately downstream of theexisting dam. The latter was considered the preferableoption, in view of reducing the impact on the operation ofthe hydro scheme.
At the time of publication an options study has beencompleted and strengthening alternative 2, the carbonfibrereinforced polymer (CFRP) option, was beingevaluated as the preferred option, based on cost, impacton operation and ease of construction. The gravitysections and foundation would be stabilised by anchoringinto the foundation.
Application of developed methodologySince slab and buttress dams are no longer economicallyattractive, recent literature only touches superficially oncertain aspects of buttress dams, and very limited researchhas been directed to fundamental aspects such as theirearthquake response.The structural analyses required for the safety review ofClover Dam led to the development of a methodology forthe assessment of slab and buttress dams (Lopez andBosler, 2007), which is currently being applied in thesafety review of Junction Dam (a 270 m long, 33 m highslab and buttress dam with 19 buttresses), as well asRubicon Falls Dam and Royston Dams (both 6 m highslab and buttress dams with 12 buttresses each).
ReferencesANCOLD 2003. Guidelines on Dam Safety Management.
Beavis, F.C. 1952. The Geology of Clover Dam. CivilBranch Group. State Electricity Commission of Victoria.
Beavis, F.C. 1962. The Geology of the Kiewa area.Proceedings of the Royal Society Victoria.
Charlwood, R.G.; Solymar, Z.V. 1994. A Review ofAlkali Aggregate Reaction in Dams. Dam Engineering(5)2: 3162.ICOLD 1991. AlkaliAggregate Reaction in ConcreteDams. ICOLD Bulletin 79.
Lopez, F.; Bosler, J. 2007. Methodology for assessmentand refurbishment of slab and buttress dams. Paper to bepresented and published in proceedings of Hydro2007.
Shayan, A. 1997a. Report on the Inspection of Clover andJunction Dams. ARRB Transport Research. Melbourne.Shayan, A. 1997b. Investigation of Concrete Cores fromJunction and Clover Dams for the Identification of AARin the Concrete. ARRB Transport Research. Melbourne.
Shayan, A. 1999. Further Examination of Concrete Coresfrom Junction and Clover Dams for the Characterisationof AARAffected Concrete. ARRB Transport Research.Melbourne.
BibliographyAmerican Concrete Institute (ACI) 2005. Building CodeRequirements for Structural Concrete (ACI 31895) andCommentary ACI 318R95.ANCOLD 1998. Guidelines for Design of Dams forEarthquakes.
Australian Standards 2001. AS 3600 ConcreteStructures.
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Fell, R. 2004. Design of Embankment Dams to WithstandEarthquakes Developments since the ANCOLDguidelines were published. Proceedings of ANCOLDConference Melbourne
Fenves, G.; Chopra, A.K 1988. Simplified EarthquakeAnalysis of Concrete Gravity Dams. ASCE Journal ofStructural Engineering 113: 16881708.
ICOLD 1989. Selecting seismic parameters for largedams. ICOLD Bulletin 72.Jansen, R.B. (ed.) 1988. Advanced Dam Engineering forDesign Construction and Rehabilitation.
USACE 2003. TimeHistory Dynamic Analysis ofConcrete Hydraulic Structures. EM 111026051, USArmy Corps of Engineers.
USCOLD 1988. Development of Dam Engineering in theUnited States. United States Committee on Large Dams.