UHE Barra Grande

7/23/2019 UHE Barra Grande

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BARRA GRANDE HYDROPOWER PLANTDESIGN, CONSTRUCTION AND PERFORMANCE

Author: Jorge Magno Vieira Borges


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Main Brazilian Dams III

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BARRA GRANDE HYDROPOWER PLANTDESIGN, CONSTRUCTION AND PERFORMANCE

1. INTRODUCTION

The Barra Grande powerplant is located in the PelotasRiver, at the border of the municipalities of Anita Garibaldi(Santa Catarina State) and Pinhal da Serra (Rio Grandedo Sul State) as shown in Figure 1.

The concession for development of the powerplantwas granted to the consortium BAESA - Barra GrandeEnergtica S.A., an association of VBC Energia S.A.,ALCOA Alumnio S.A., CCCC Construes e ComrcioCamargo Corra S.A. and DME Energtica(Departamento de Municipal de Eletricidade de Poosde Caldas).

Construction works at the site began in July 2001and the project was implemented under a turnkeycontract by the UNEBAR Consortium, formed by CCCC(civil works and erection), ALSTOM Brazil (mainequipment supplies) and ENGEVIX Engenharia S.A.(engineering and substation supplies).

The first studies for development of the Uruguay basinwere undertaken by the Comit de Estudos Energticosda Regio Sul - ENERSUL between 1966 and 1969 guidedby Canambra and reviewed in 1978 by Eletrosul. Thefeasibilities studies were developed by Engevix.

2. LAYOUT

The normal maximum operating level of the reservoiris at El. 647 m, with a drawdown of 30 m for flow regulation.At the dam axis, the average elevation of the rockfoundation is at El. 466 m, resulting in a maximum dam

Figure 1 - Location map

height of 185 m.

The main features of the project layout are as follows:A 185 m high and 650 m long concrete face rockfilldam CFRD provides a total head of 167 m for powergeneration.

The surface spillway equipped with six Tainter gates,15 m wide and 20 m high, was designed for a peakdischarge of 21,800 m/s. The concrete chute is 274 mlong with two aeration steps, to prevent concretecavitation. A stilling basin will provide the dissipation ofhydraulic energy during high flows.

The power structures comprise a (51.30 m high and

30 m wide) concrete intake followed by three (450 mlong and 6.20 m diameter) concrete lined power tunnels.

The indoor powerhouse was designed toaccommodate three 236 MW Francis turbines.

The river diversion scheme comprises two unlinedtunnels, with arched-rectangular section (15 m wide and17 m high) and lengths varying between 816 and 921 m.The high peak floods on the Pelotas river led to theconstruction of 61 m high cofferdam, designed for1:50 year flood (discharges up 7,500 m/s). The first stageof the dam (94 m) was designed for a 1:500 year event or14,000 m/s of peak inflow.

A compensating tunnel (3.0 m diameter) controlledby two valves will maintain an ecological dischargedownstream of the dam during reservoir filling.

The general layout of the Project is presented inFigure 2.

The main structures for the project were tested on ahydraulic model at the CEHPAR laboratory in Curitiba,Brazil.

3. GEOLOGICAL AND GEOTECHNICAL

FEATURES

The hydroelectric power plant is located in the Southportion of the Sedimentary Paran Basin. It comprisessedimentary and volcanic rocks, dominated by basaltsof the Serra Geral Formation, in a mainly subhorizontaldistribution gently dipping to West. In the intervalsbetween flows, aeolian sand sediments from theBotucatu Formation were deposited in restricted basins(intertrappean sandstones).


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Figure 2 - General Layout

In the upper part of the Serra Geral Formation, anacid volcanic sequence occurs in some areas in theSanta Catarina and Rio Grande do Sul States.Thenomenclature to define these acid volcanic rocks,

containing silica amounts greater than 66%, is stilldebated, some of the suggested terms are vitrophyre,granophyre or rhyodacite.

Usually, the majority of the basaltic flows arecharacterized by dense basalts in the lower and centralparts of individual flows, with some vesicular at the baseand a vesicular-amygdaloidal zone in the upper part, witha layer of basaltic breccia formed by basaltic fragments,surrounded and welded by secondary minerals or by silty-sandy materials at the top. Vesicular basalts at the baseand basaltic breccias at the top, may be absent in somelava flows.

The local landscape reflects a pronouncedlithostructural control of the relief, making evident thelayered subhorizontal shape of lava flows. Along thehillsides, the occurrence of steplike shapes of the slopesis associated to flat areas, related to the upper zones ofthe lava flows, intercalated by subvertical slopes relatedto the central zones of the flows, where the rock haspredominantly a vertical columnar jointing zone. The bedand the banks of the Pelotas River present a young erosionpattern, with very steep and deep valleys with a narrow"V" shape, averaging slope inclinations of 30 to 40,without the development of typical alluvial flood plains.At the dam axis, the valley is about 250 m high, the river

bed is 100 m wide and the water flow is about 1.0 m deepin the dry season.

The exploratory drill holes detected a sequence offourteen basalt flows ranging approximately from El. 710

to 420 m, disposed subhorizontally, dipping about 0.5downstream in the West direction. The soil overburden inthis area is shallow, less than 4 m deep and its contactwith the underlying rock is abrupt, without a transitionzone.

The rock mass, near the surface, is generally fractured,with expressive water losses (more than 1l/min.m.atm),associated with relief fractures. In its inner portion, themajority of the contacts between individual lava flows aresealed. There are large water losses in the river bed, morethan 10 l/min.m.atm, at the joint at El. 465 m and at thecontacts between the M/N lava flows (El. 450 to 455 m)and at the N/O flow (El. 435 m). The most significantwater losses (higher than 10 l/min.m.atm) on theabutments were detected in the F lava flow.

The main geotechnical factors of the dam foundationare mainly related to the shallow zones, which are moreweathered and/or fractured, extending to inside of therock mass, which will demand careful dental treatmentsto remove the material of poor geomechanical quality andfill the cavities with concrete. On the left bank, betweenthe El. 615 and 625 m, the rock mass is much fractured(more than 10 fractures/m). In the river bed, the dam islocated over rocks of the M flow, which present goodgeomechanical conditions.


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4. HYDROLOGICAL AND HYDRAULIC

FEATURES

The Pelotas River (tributary of Uruguay River) at theBarra Grande site drains a catchment area of about13,000 km. The long-term average flow is 292 m/s andfloods can occur at any time of the year, but the largestrecorded ones always occur in the period from May toOctober.

Barra Grande is a regularization river plant. Maximumdrawdown is 30 m for pondage. The volume of the reservoiris about 2,286 hm.

The results of the flood frequency analysis areillustrated in the following table:

The probable maximum flood (PMF) resultedin 23,840 m/s.

5. DESCRIPTION OF THE MAIN

STRUCTURES

5.1. River Diversion

The diversion system comprised two cofferdams - oneupstream 66 m high and one downstream 25 m high andtwo tunnels - 15.0 m wide and 17.0 high and with a lengthfrom 816 m to 921 m The diversion design flood was7,500 m/s, with an annual probability of occurrence of1:50. The structures were closed by stoplogs, andconcrete plugs were built subsequently in the intermediatepart of the tunnels.

5.2. DamThe dam has a crest 10 m wide and is located in avery narrow valley. The ratio between crest length andheight is 3.65, with the abutments having an averageinclination close to 45. Total volume of embankmentoriginated from excavations in basalt rock was about of12 million cubic meters, placed and compacted on soundrock foundation.

The upstream and downstream slopes haveinclinations of 1V : 1.3H and 1V :1.2 H respectively. Thetheoretical control slope of both upstream anddownstream faces is 1.3H : 1V. Transitions 2A and 2Bare compacted in layers of 0.50 m thickness, the

embankments 3B and 3D of the upstream and centralzones were compacted in layers of 1.00 m thicknesses,wetted in the ratio of 200 l/m during placement, andembankments 3C and 3D of the downstream zone werecompacted in layers of 1.6 m without wetting.

The average void ratio of the upstream zones was0.24 and average unit weight was 22.1 kN/m, the

unconfined compression tests showed that the strengthof the basalt rock was over 90 MPa. The downstreamthird of the dam section showed an average unit weightof 20.2 kN/m.

For protection of the processed transition and beddingfor the upstream slab, an extruded curb concrete waspoured with an average cement ratio of 50 kg/m.

The perimetral slab (plinth) and the concrete face slabcompose the watertight elements of the dam.

The face slab with a total area of about 108,000 m,was built in 16 m wide strips, separated by longitudinaljoints.

All vertical joints are protected by copper joint sealsat the base of the slab. In the tensile region, near theabutments, the joints are covered with a mastic-filledPVC membrane.

The perimetral joint between the plinth and face slabwas protected by a copper waterstop and a surfacesealant (mastic) covered with a PVC membrane.

The thickness of the concrete face varied from0.30 to about 1.00 m, according to the following formulas:

t = 0.30 + 0.0020 x H (for H up to 100 m) (1)t = 0.0050 x H (for H higher than 100 m) (2)Where:t is the slab thickness; H is the water head in meters.

Double reinforcements were adopted in the first 20 mof the slabs above the plinth, with a rate of 0.5% in bothdirections. In this zone, the reinforcement was shared inthe two slabs surfaces, top and bottom, being 60 % onthe upper face and 40 % on the lower face. Reinforcementof 0.3 % of concrete in the horizontal direction and0.4 % in the vertical direction were provided in the centralzone of the concrete face.

The foundation treatment included consolidation andcurtain grouting under the plinth slab.

The rockfill followed the classical zoning for CFRD,more careful grading and compaction were concentrated

in the upstream third of the dam.Table I gives the physical indexes of rockfill materialsand Figure 3 shows a transversal section of the dam.

5.3. Spillway

The chute spillway, located on the left bank, has adesign capacity of 21,810 m/s and the outflow is controlledby six radial gates measuring 15.0 m x 20.8 m.For maintenance work there is a stoplog made up of sixinterchangeable panels, operated by a gantry crane atthe crest of the spillway structure. A bridge at the crest,made up of precast elements, links the intake structureto the left bank.


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Figure 3 - Transversal section of the dam

The concrete lined chute is 274 m long and 111 mwide and has an inclination of 22% with two aerationsteps. The reinforced slab of the chute is anchored tothe foundation rock by rock bolts with a diameter of25 mm, spaced 2 m in each direction, in holes 3 m deep.The flip bucket located the end of the chute, has a radiusof 30 m and an angle of 20, which directs the jet to adissipation basin 120 m wide. The longitudinal profile ofthe spillway is shown in Figure 4.

5.4. Intake and Power Tunnels

The intake structure is a conventional concretestructure founded in sound rock. It is 51.30 m high and30 m wide.

The concrete lined power tunnels were reinforced tocontrol shrinkage cracks during construction.

Both, the vertical portion and the sloping portion havetwo layers of longitudinal steel 16 mm each 20 cm, withrings of 8 mm at 25 cm spacing, increased to

Table 1 - Dam Materials


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16 mm each 20 cm where crossing weaker flow contactzones in the vertical shafts.

The vertical upper curve has an empirically defineddouble reinforcement of 16 mm each 20 cm both ways.

The lower vertical curve is similarly reinforced by20 mm each 10 cm.

This reinforcing was planned to distribute crackseventually produced by radial and circumferentialdeformation due to water pressure in order to reduceleakages to a minimum.

Immediately upstream from the steel lined section,

the reinforcing was increased to double rings of 20 mmeach 20 cm internally and 8 mm each 25 cm externallyand longitudinal reinforcing of 16 mm each 20 cmexternally and 20 mm each 20 cm internally.

5.5. Powerhouse

The powerhouse is of the indoor type and is 92.0 mlong including the erection bay. The net head is 154 mand the plant is equipped with three generation units,each one rated 236 MW.

The control system of the power plant and substationis of the digital type and in the event of an emergency itcan be controlled and supervised at the local level.

6. CONSTRUCTIONConstruction facilities for Barra Grande included an

industrial yard containing basically: recreation centre,water treatment plant, offices, warehouse, laboratories,repair shop, substation, crushing and batching plants.

7. PERFORMANCE

7.1. Dam

The reservoir reached El 617.50 m on September 5 th,2005, sixty three days after diversion closure. Filling

speed in this period was about 2,15 m/day, due to theoccurrence of heavy rains in the region.

On September 19th, 2005, the flow meter collectingthe water leakage through the dam recorded 220 l/s. Threedays later the leakage increased to 400 l/s. At this time,when the filling of the reservoir reached El. 634 m, about93% of its maximum head, concrete rupture (spalling)was observed along the compression joint, between faceslabs 19 and 20, (see Photos 1 and 2).

An underwater investigation by divers and a robot(used in offshore works) showed the spalling reached a

depth of about 100 m under water.The reservoir was lowered to El 630.00 m and theconcrete slab and the joint above the water level wererepaired. Photos 3 and 4 present the repaired slab.

Filling of the reservoir restarted in November, 2005.The maximum percolation through the dam recorded bythe flow meter located in the downstream cofferdamreached a peak of 1,284 l/s.

In March the first placing of clay-silt material startedin order to seal the joint and the damaged concrete ofthe slabs under water. This initial placing reduced theinfiltration flows to about 800 l/s. In July after the increasein percolation, additional material was placed, totalling

about 22,000 m of clay-silt material. See figure 5.The rupture, which occurred in the compression zone,

presented the characteristics of spalling caused by highcompressive stresses in the transversal and longitudinaldirections of the face slabs.

Settlements are being measured by Swedish boxsettlement devices, placed across the dam at four levels,and by magnetic settlement gauges placed in three verticalsections.

Maximum displacement of the slab until the presentmoment reached about 71 cm near the crest of the damand the specific deformations are varying between

Figure 4 - Longitudinal Profile of Spillway


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Photo 1 - Concrete rupture between face slabs 19 and 20 beforecleaning of slab surface

Photo 3 - Repairing of the slab

2 to 5 per cent. The general displacement and settlementsof the rockfill are shown in Figures 6 and 7, respectively.

The conclusions that can be reached based on theresults of the monitoring instrumentation and theobservation of the behaviour of the dam for the last20 months (after filling the reservoir) are:

Specific deformations obtained are similar to otherCFRD dams constructed with basalt rockfill;

Photo 2 - Concrete rupture between face slabs 19 and 20 aftercleaning of slab surface

Photo 4 - Repairing of the slab conclud


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Figure 6 - Deformation of the slab

Figure 5 - Percolation and Reservoir Level

Settlements observed during construction in theBarra Grande rockfill are very similar to those measuredin similar dams such as Foz do Areia, Segredo, It andothers.

The leakage through the dam recorded on thedownstream flow meter at the present time is about935 l/s, which could be considered acceptable consideringthe dam dimensions and reservoir head and whencompared to other similar dams in operation around theworld.

The analysis of the instrumentation data confirmsthat the dam is completely safe, as it was for the firstfilling, with leakage compatible with the size of thestructure and behaviour of the embankment within theusual range observed in other concrete face rockfill dams.

Spil lway

The instrumentation consisted of six standpipepiezometers, installed from the drainage gallery in thegeological features of foundation rock and at the concreterock contact, six triorthogonal joint meters installed


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Figure 7 - Settlements of rockfill

Mean annual rainfall 1,788 mmOwner BAESA ENERGTICA S.A.Design and constructionBasic and detailed design ENGEVIX Engenharia S.A.Construction and erection Camargo Correa

Comrcio e Construes S.ASupplies:Electromechanical equipment Alstom BrasilSubstation and transmission lines ENGEVIX

Engenharia S.A.

Reservoir

Area at Max. Normal Water Level 92 km

Storage at Normal Water level 5,000 hmStorage at Minimum Water level 2,286 hmMax. Normal Water Level 647.0 mMax. Flood Level 649.17 mMinimum Water Level 617.0 mLength 120 kmWidth 600 m

Tailrace

Max. Normal Water Level 480.0 mMinimum Water Level 478.5 m

FlowsMax. Mean daily inflow 292.4 m/sMax flow recorded July/1992 10,421 m/sMinimum daily flow recorded May/1952 18.8 m/sMean long term flow 292.4 m/s

Dam

Type CFRDMaximum height 185.0 mLength 665.0 mWidth at the crest 10.0 mCrest elevation 651.0 m

between blocks and three extensometers and a flowmeter to measure the seepage flow.

The results of the instruments indicated an adequatebehaviour of the structure. The maximum seepagerecorded was 101.95 l/min.

Intake

The instrumentation consisted of three standpipepiezometers, they did not record any piezometric waterlevel in the foundation.

Powerhouse

The instrumentation consisted of ten standpipe

piezometers, installed from the drainage gallery in thegeological features of foundation rock and at the concreterock contact, nine triorthogonal joint meters installedbetween blocks and four extensometers, two electricaljoint meters, a pendulum and a flow meter to measurethe seepage flow.

The result has been satisfactory up to the present.The maximum seepage recorded was 158.4 l/min

8. TECHNICAL FEATURES

General Location

River PelotasBasin Uruguay River MunicipalitiesRight bank Anita GaribaldiLeft bank Pinhal da SerraStates Santa Catarina and Rio Grande do SulLatitude 27 46' SouthLongitude 51 13' WestBeginning of construction June 2001End of construction October 2005Catchment area 13,000 kmMean annual temperature 16.5 C


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Spillway

Type surface (ski jump)Maximum discharge capacity 23,840 m/sCrest elevation 627.0 mLength 119.4 mMaximum specific discharge 233.9 m/s

GateNumber of gates 6Gate height 20.98 mGate width 15.0 m

Intake

Height 52.3 mLength 24.3 mCrest elevation 651.0 m

Gates

Type fixed wheel gatesNumber of gates 6Gate height 6.54 mGate width 6.2 m

Diversion

Type 2 TunnelsDimension 15.0 m (wide) x 17.0 m (high)Length 816.0 and 920.0 m

Power Tunnel

Type indoor Number 3Internal Diameter 6.9 m

Length 309.2 mReinforced stretch 177.1 m

Powerhouse

Type indoor Height 45.0 mLength 91.8 m

Turbines

Type FRANCIS - vertical shaftQuantity 3Rated power 236 MWRated flow 165 m/sOperating speed 200 rpmManufacturer ALSTOM

Generators

Type Vertical shaftRated capacity 245 MVAVoltage 16 kVFrequency 60 HzOperating speed 200 rpmPower factor 0.95Manufacturer ALSTOMStep-up Transformers

TypeNumber 3+1Rated power 245 MWVoltage 230 kVFrequency 60 HzManufacturer ALSTOM

Substation

Type ConventionalRated voltage 230 kV

The main volumes of civil works are as follows:

Earth excavation 2,000,000 mRock excavation 10,000,000 mTunnel excavation 470,000 m

Rock fill and transition 12,000,000 mConcrete 330,000 mReinforcement 14.000 tons

9. BIBLIOGRAPHY

[1] J. Barry Cooke - Concrete Face Rockfill Dams:Volume - CFRD 2000 - Beijing, China - 2000.


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