Water Power and Dam Construction. Issue April 2009

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DESIGN

cost for all cont racts combined with the c onsultant services are

expected to be about US$365M.

The proj ect works are being constructed under the contracts

shown in Table 1.

EKEZE PROJECT DESCRIPTION

Tekeze hydro power project is comprised of a double curvature (log-

arithmic spiral) concrete arch dam and appurtenant works, two river

diversion tunnels, a 75m high power intake structure, power water-

ways (headrace tunnels, vertical pressure shaft, manifold, and four

steel lined penstocks and four tailrace tunnels), an underground

powerhouse containing four 75MW Francis turbines, outlet works,

230kV substation and a 105km long double circuit transmission

line, connecting to the Ethiopian national grid at Mekele.

At 188m high, the Tekeze Arch Dam ranks as the highest dam in

Africa, eclipsing the previous record height for an African dam of

185m held by the Katse Arch Dam in Lesotho.

IVERSION SCHEME SIMPLIFICATIONS

There were two diversion tun n els constructed around the Tekeze

Dam s ite. T hese t unnels and as sociated cofferdam s were sized to

only pass the smaller flows that occur during the dry season. In order

to pass the monsoonal floods, the original design of the dam pro-

vided two temporary openings 12.2m high by 8.5m wide. The invert

of these openings was to be at the normal river bed level (EL. 971).

The temporary openings (see Figure 1) were to be fitted with bulk-

head gates for diversion closure and reservoir impoundment.

In order to expedite the dam construction schedule and to avoid

the additional costs of forming and bulkhead gates, MWH Engineers

decided to delete these tempor ary sluice openings. Flood waters

would then be passed over selected low blocks of the dam. This is

similar to the methods for passing floods adopted on two dams in

the Sta te of California in the US; namely Mont icello Dam (circa

1956), and New Bullards Bar Dam (circa 1969).

During the Tekeze River floods of 2006, the arch dam acted as a

gravity struc ture, and passed a 10-year 2000m

/sec flood on 9

August 2006. That flood can be seen in Figure 2. Except for flood-

ing of the lower galleries and so me missing forms, there was no

damage to the permanent structures. Nine days aft er the flo od

passed, the dam ga lleries were pumped dry and the C ontractor

resumed placements without any further delay.

Prior to the floods of 2007, MWH Engineers purposely kept two

of the dam blocks lower then the rest of the struct ure. When the

2007 seasonal floods arrived, the reservoir quickly impounded with

the dam acting as a gravity arch, and as such, the contraction joints

were grouted prior to this stage of the impoundment. This allowed

the dam to resist water loads by a combination of cantilever and

arch action. Despite the spectacular flows that resulted over two of

the dam’s low blocks, there was less than 20mm of cavitation and

erosion damage to the concrete toe of the dam. This second over-

WWW.WATERPOWERMAGAZINE.COM APRIL 2009 21

Table 2: Civil works

(arch dam, underground

powerhouse etc.) –

Lot 1B/2/3

Arch dam - Features Dimension/ Unit

Quantity

eight 188 m

Crest length 420 m

Crest elevation 1,145 m

Maximum thickness at base 28 m

Thickness at crest 5.6 m

ow level outlet radial gates –

4 No. at el. 1,057m 8.0 x 5.6 m

pen excavation 310,700 m

Underground excavation 14,800 m

Concrete (mass and structural) 1,034,000 m

Unit Cost of Concrete

(incl. cement, not forms) $100.24 US$

einforcing steel 7,000 tonnes

Embedded cooling pipes 204,000 m

Concrete placement period 36 months

Maximum concrete placement rate 46,800 m

/month

Average concrete placement rate 28,700 m

/month

Cement Content (Dam mix including pozolans) 240 kg/m

Average dam concrete strength (150mm

maximum aggregate mix - 97% std. dev.) 30.2 Mpa

Power Complex - Features Dimension/ Unit

Quantity

Intake structure height 75 m

Upper headrace tunnel, 7.25m lined diameter 320 m

Vertical shaft, 6.75m lined diameter 120 m

Lower tunnel 6.75m reducing to

4 - 4.0m steel lined penstocks 408 m

Powerhouse cavern 95l x 18w x 38h m

Tailrace tunnels,

4 No. x 5m diameter x 73m long 292 m

Francis Turbines 4 No.

Plant Maximum Flow 220 m

/sec

Unit Design Head 155 m

River Diversion Works - Features Dimension/ Unit

Quantity

Diversion Tunnel No.1 – invert lined,

dia. 7.8m 430 m

Diversion Tunnel No.2 – fully lined,

diameter 7.0m 370 m

Upstream cofferdam – Mass concrete

Crest length 75 m

Top elevation 978.5 m

Height 18.5 m

Concrete Volume 10,443 m

Tunnel No. 2- Duel High Pressure

Bonneted Slide gates 2.54w x 2.67h m

Figure 2 – First construction seasonal flooding of dam site on 9 August 2006.

Estimated flow 2000m

/sec

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DESIGN

topping of seasonal floods lasted 81 days, from 13 July 2007 (see

Figure 4) to 2 October 2007. The combined diversion capacity (esti-

mated to peak at a maximum of 1313m

/sec) spilled over the two

dam low blocks with s mall releases fro m the Diversion Tunnel

Bonnet Gate, as shown in Figures 5 and 6.

TAGED DAM CONSTRUCTION AND IMPO UNDMENT

The basic design of arch dams is founded upon standard principles

whereby a series of rigid structures – blocks or cantilevers – are con-

nected together by grouting of the contraction joints to form a single

rigid structure that can resist water loads in arch action. For ana-

lytical purposes, the Engineer assumes that the weight of the struc-

ure or dead load is transferred vertically within each cantilever to

the foundation. The reservoir or live load is then added after dam

completion and all contraction joints are grouted. These loads are

then distributed to the foundation by a combination of both arch

and cantilever action.

During the initial design phase of most arch dams, the final con-

struction schedule, with the associated economic issues, are usual-

ly not clearly defined to the dam designer. The dam designer usually

assumes that the dam cantilevers are constructed, the joints grout-

ed, then the water loads applied, in that order. The impacts due to

any deviations from usual design practice must be carefully weighed

by the des igner. With larger arch dams (such as at Tekeze), that

have construction periods that span three years, the designer may

have to consider the staged placement of concrete, the staged grout-

ing of the contraction joints, and even the staged filling of the reser-

voir. The single purpose of the Tekeze hydr o p ower project is to

generate electricity. To achieve this purpose in the shortest possi-

ble p eriod of time, EEPCO expected the res ervoir to be filled as

early as possible. In doing so, additional dam analyses are manda-

tory, and necessary to quantify the effects of stress re-distribution

within the dam and foundation.

After considerable studies and reanalysis, in May of 2007, nearly

two years prior to completion of the concrete arch dam, the river

diversion was closed, resulting in over 3Bm

of water retained early

during the 2007 and 2008 wet seasons. This allowed the project to

begin generation sooner than would have been the case had the

reservoir impoundment started after the completion of the entir e

dam, and grouting of the contraction joints. The first filling of the

reservoir represents the most complex and challenging activity of

the entire implementation period. During this t ime, structur al

responses from the various structures constantly change as reservoir

loads are applied, foundation conditions constantly change as the

surrounding rock is subjected to groundwater for the first time in

possibly millions of years, completion works will change geometry

of structures and their ability to carry reservoir loads, and design

assumptions for all structures must be confirmed. Some adjustments

are inevitable. All these activities were safely and successfully car-

ried out, concurrent with reservoir impoundment, to achieve the

single purpose of generating electricity early at Tekeze.

It is not until the completed structures are put into service that

the res ults of the efforts of EEPCO, the Dam Engineers, and the

Contractor will be truly confirmed. For the dam, powerhouse and

all appurtenant structures, this confirmation occurs when the reser-

voir is impounded and live loads are applied to all structures.

Previous studies of the effects of staged grouting of the Aurora dam

(done by the USBR in 1977) indicated a re-distribution and

increase of compressive stress in the dam’s cantilevers of approxi-

mately 4%. This redistribution of dead load reduces the confining

stress in the lower blocks of the dam and impacts the resistance of

the dam to cracking at its base. Next, because the ultimate reser-

voir or live loads are carried to the foundation by arching action,

partial filling of the reservoir prior to completion will initiate the

arching action of the uncompleted dam, resulting in possible clo-

sure of the upper un-grouted joints and redistribution of both can-

tilever and arch stresses throughout the dam and foundation. After

the 2008 impoundment at Tekeze, some reduct ion of th e upper

contraction jo int openings did occur, making it m ore difficult to

Table 3: Mechanical and electrical

works in the Powerhouse –

Lots 4 and 5

Feature Dimension/Quantity Unit

Francis Turbines 4 No.

Minimum net head 120.0 m

Maximum net head 162.8 m

Unit Design Head 155 m

Power output at maximum net head 75.0 MW

Manufacturer – Dongfang China

enerators and auxiliar y systems 4 No.

Stator core inner diameter 5,500 mm

Shaft diameter 750 mm

ated output with class F temperature rises 86.7 MVA

ated voltage 13.8 kV

ated power factor 0.9 p.u.

Table 4: Reservoir and hydrology

Reservoir and Hydrology Dimension/ Unit

Quantity

Catchment Area 30,390 km

Mean Annual Rainfall 850 mm

Mean Annual Inflow 3,750 Mm

10 year return peak flood 1,940 m

Maximum Normal Operating Level 1140 m

Minimum Operating Level 1096 m

Total Storage (@ EL. 1140) 9,310 Mm

Active Storage 5,343 Mm

Spillway Capacity (4 LLO Radial Gates @ EL. 1140) 4,416 m

/sec

Estimated Reservoir Annual Evaporation Losses 1,921 mm

Firm Annual Energy 981 GWh/yr

Average Annual Generation 1,393 GWh/yr

Figure 3 – View of the dam from upstream,

taken during the May 2007 diversion tunnel closure

24 APRIL 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DESIGN

grout. For these reasons, close control of th e concrete placement

and contraction joint grouting methods is critical to the dam’s

future performance.

When the MWH dam designers included the effects of staged

grouting coupled with staged filling of the reservoir, the maximum

principle compressive stresses in the dam did not increase above pre-

vious studies and remained at 7.5MPa. There was, however, an

increase in tensile stresses at the upstream heal of the dam. With dam

concrete strengths averaging over 30 MPa; the designers use d a

factor of safety of 3.0 and a design value stress limit of 10 MPa for

the maximum allowable compressive stress in the dam and

dam/foundation contact zones. The increase in tensile stresses

upstream may result in some increase in leakage. The leakage was

onitored during impoundment; and was minor.

From a design point of view, the objectives for dam construction

sequencing can be summarized as follows:

• The best results for joint grouting will be achieved when concrete

placements directly above the joint to be grouted have reached the

highest level possible.

• The best structural response of the dam will be achieved when joint

grouting is completed as high as possible before arching action of

the dam under reservoir loading is initiated.

• The arching action will commence in the dam when the reservoir

live load is applied. From this point, the arching action steadily

increases until the reservoir reaches the dam crest. Likewise, the

rigidity of the dam and its ability to resist the arching action steadi-

ly increases as higher joint grouting is completed.

As noted earlier, no structure is perfectly water tight and some seep-

age in and around the dam is to be expected. To confirm the water

tightness of the dam as designed, seepage into and around the dam

is being monitor ed and measured t hroughout the impoundment

process. To collect this seepage and maintain the dam in a reason-

able dry condition, gutters with monitoring weirs were provided in

each of the dam galleries that in turn are connected to a sump. In

addition, the Tekeze Dam has a comprehensive series of instruments

with real time monitoring and data acquisition eq uipment.

Continuous monitoring of foundation extensometers, piezometers,

temperatures, leakage, and vertical pendulums is on-going during

the final filling of the reservoir.

ONTRACTION JOINT GROUTING BY THE

‘P

ULL

-P

IPE

’

METHOD

The standard method used in the past for the grouting of vertical con-

traction joints was to use a grid of 25mm thin wall tubing riser pipes

with grout outlet boxes. The Contractor proposed an alternate method;

called the “Pull-Pipe” method. On the lead blocks, a half round grout

groove is formed between each joint key. Then on the follower block,

a 25mm PVC flexible hose is pressurized, expanded with compressed

ir, and set into the grout groove. Then after placement, the Pull-Pipe

is withdrawn from the block, leaving a 25mm formed riser (see in joint

grooves, Figure 5). The Engineer considered the Pull-Pipe method supe-

rior, and adopted the method for the contraction joints at Tekeze.

ENEFITS OF STAGED IMPOUNDMENT

• By beginning impo u n dment two years early; 554Mm

of water

was ret ained in 2007. That storage increased dramatically to

3100Mm

by the end of 2008.

• 2008 was a very dry year for Ethiopia; had it been an average

water year, an additional 1200Mm

could have been retained and

power generation started in September, 2008.

Right: Figure 4 – First overtopping of the dam’s ‘Low Block’ on 13 July 2007

Below: Figure 5 – Construction activities continue during the major floods of

2008. Note the “Pull Pipe” grooves and the spherical contraction joint keys.

PVC waterstops were protected from the flood waters with steel cover plates

WWW.WATERPOWERMAGAZINE.COM APRIL 2009 25

DESIGN

• The early impoundment of 3100Mm

is equivalent to a genera-

tion of 1 190GWh f rom the Tekeze plant; based on the present

wholesale value of power, that savings of that water to EEPCO is

worth an equivalent of about US$40M.

• The elimination of the temporary openings formworks, approach

walls, and stoplogs expedited the dam construction schedule and

saved an additional US$1.9M.

THER

EEPCO

PROJECTS

In order to alleviate the current shortage of electricity in Ethiopia,

EEPCO expects to complete, prior to 201 0, the following three

major hydroelectric projects:

• 300MW Tekeze project, described above, complete and ready to

begin generation with the arrival of the monsoon rains in July 2009.

• 420MW Gi lgel-Gibe II project, 92% completed, 4 x 105MW

pelton units, 26km TBM tunnel, penstocks with surface power-

house. Construction by Salini Costruttori, an d Engineering

Management by Coyne et Bellier.

• 460MW Beles project, 75% comple ted, taps water from Lake

Tana and conveys it via a 11.9km concrete lined tunnel and 270m

deep vertical shaft to an underground plant with four pelton units

with an estimated annual capacity of 2051GWh per annum. This

turnkey project was awarded to Salini Costruttori; the pelton tur-

bines are furnished by VA Tech Hydro (now Andritz Hydro).

With the addition of the above 1180MW, the total generation

capacity of EEPCO will increase by 140% and will most certainly

revolutionize Ethiopia’s power sector. EEPCO expects to begin

exportation of surplus pow er to the neighbouring countries of

Djibouti and the Sudan.

EEPCO has also started construction of following two projects:

• 1870MW Gilgel-Gibe III project, 28% completed, total invest-

ment cost 1.47B Euros, 240m high RCC dam, power waterways

and associated structures, 10 x 187MW turbines; construction by

Salini Costruttori.

• 100MW Fincha Amerti Neshe Project, 20% completed, earthfill

diversion dam and tunnel to a 100MW pelton plant, Engineering

Management by MWH, and construction by CGGC of China.

In addition, Ethiopia is planning to develop additional new plants

over the next decade by invitation of Financiers, Contractor’s,

Investors, and Independent Developers. Three such projects are:

• Ashegoda Wind Farm Project, 30MW

• Halele Worabesa Project, a 440MW hydroelectric project.

• Chemoga Yeda Project, a 278MW hydroelectric project.

James R Stevenson, Chief Concrete Dam Engineer,

MWH International, Inc., Ethiopia.

USA Office: MWH Americas, Inc.,

175 West Jackson Boulevard,

Suite 1900, Chicago,

Illinois 60604-2814

Email: James.R.Stevenson@MWHGlobal.com

Ato Mihret Debebe, Chief Executive Officer (CEO),

Ethiopian Electric Power Corporation (EEPCO)

Email: eepco.engdgm@ethionet.et

Above: Figure 6 – View from the powerhouse tailrace platform of the August

2008 overtopping of both ‘Low Blocks’ combined with the discharge from the

diversion tunnel bottom outlet bonneted slide gate

Left: Figure 7 – View of the completed Tekeze Dam on 20 February 2009.

Compensation flow and cooling water releases are 2.5m

from the fixed-cone

dispersion valve

IWP& DC

26 APRIL 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DESIGN

structure at elevation above 130m asl. This means the operational

water level in the Motyginskaya reservoir shoul d not rise above

127m asl as this will reduce the head, and consequently the power

output, of the Boguchansk aya plant. It is also essential that the

reservoir provides enough safe freeboard to avoid da m a ge t o the

bridge during a flood events.

The normal and minimum reservoir levels a s desc ribed above

results in an available operational reservoir volu me of 400 x

. Although the project may be operated without peaking or

storage op erations, this volume provi des flexibility in o peration.

This small live storage ma y be useful in daily or possi bly weekly

evening releases from the Boguchanskaya hydro power plant. The

operational volume of the reservoir is equivalent to the operation

of the Motyginskaya hydro po wer plant at f ull capacity for

27 hou rs.

ESERVOIR OPERATION DURING EXTREME

FLOOD EVENTS

The design c riterion for the maximum flood reservoir level has

been defined at elevation 127.50m asl. This relatively small volume

of approximately 300 x 10

is equivalent to the spillway design

discharge flowing for about 2.5 hours. Thus, the 0.5m rise in water

surface elevation allows for short-term adjustments in spillway dis-

charge capacity [2, 3].

OTYGINSKAYA

The proposed dam is a concrete face rockfill dam (CRFD) [Figure

5]. The structure is 35m high from the bottom of the excavated

river bed and 19m high from the toe of the concrete face elevation

111m asl, where it is connected to a borepile diaphragm 30m high

by means of a pl inth. The dam is located perpendicular to the

HE 1250MW Motyginskaya hydroelectric power plant is

located on the Angara River in Siberia, Russia. The project

has been designed to create a reservoir impoundment of

approximately 4800 x 10

and will consist of the fol-

lowing main structures: i) CFRD with a borepile diaphragm - cut off

wall; ii) gated spillway; iii) powerhouse with 10 x 125MW Kaplan

type units; iv) switchyard and transmission line; v) navigation lock;

vi) fish pass; vii) access roads; and viii) offices and housing. A layout

of the project is shown in Figure 1.

Motyginskaya reservoir is expected to have an area of approxi-

mately 478km

(Figure 2). The project’s average width is around 3km

and its length is almost 180km, comprised of the Motyginskaya dam

site at km 151+600 and the confluence of river Mura into the Angara

at km 383+000 (Figure 3). Key parameters of the reservoir are:

• Maximum flood level 127.50m asl

• Normal operating level 127.00 m asl

• Minimum operating level 126.00 m asl

• Flood storage volume (EL. 127.00 - 127.50) 300 x 10

• Effective, useful or life storage volume

(el. 126.00 - 127.00) 400 x 10

• Dead volume (EL. 98.80 - 126.00) 4400 x 10

• Total storage volume 4800 x 10

• Reservoir area at EL. 127.00 m asl 478.00 km

• Reservoir area at EL. 126.00 m asl 459.00 km

ORMAL OPERATION

As shown above, the normal and minimum reservoir operation levels

have been defined at elevations 127 and 126m asl respectively.

The normal reservoir operating level was determined by the

water level in the tailrace of the Boguchanskaya hydro power plant,

whi ch is currently under construction (Figure 4), and the future

construction of a bridge in the township of Yarki, with the super-

Motyginskaya CFRD –

creating an impoundment

The following paper reviews the design of Motyginskaya concrete faced rockfill dam on

the River Angara in Siberia. By Cesar Alvarado-Ancieta

Figure 1a: Aerial view of the Motyginskaya dam site Figure 1b: Dam arrangement

Riprap

The fac es of both u pstream and down stream sh oulders will have

approximately 1m to 1.5m of riprap measured perpendicular to the

slope. The stone size is determined on the basis of depth-averaged

local flow velocity, taking into account the width of the river diver-

sion gap [1]. The upstream slope of 1:2.4 is affected by ice, there-

fore the tot al thickness of the upstream riprap layer has been

increased by app roximately 50%. The transition zone from the

riprap to the rockfill requires special attention in order to provide

graded transitions between impervious and coarser material and a

stable base for the riprap.

Dredging material

After excavation, the dredging material is transported in suspension

in beach pipes. The mixture is 25-50% solids by volume. The fill-

ing procedure will start inside the existing rockfill shoulders. The

coarse material will settle close to the discharge point while the finer

material is carried to the outlet. The predominant material is sand

and gravel, silts and clays will be limited.

Once dredging is complete, vertical geo textile drains w ill pen-

etrate the hydraulic fill on a layer of gravel material to reduce

excess pore pressure development during subsequent dam filling

stages. After placing a geotextile layer, the top filling will be under-

taken in stages allowing for dissipation of the remaining excess

pore p ressure.

Borepile diaphragm – cut off wall

The lower part of the dam is waterproofed by the borepile

diaphragm wall, which will be constructed fr om the crest of the

upstream roc k-fill shoulder. In addition, pipes are to be install ed

into the secondary reinforced piles in order to allow cement grout

to be injected into cracks in the rock below the pile footings, if

required.

A reinforced concrete beam connects the piles on the top and

serves as a platform for the plinth of the concrete face.

Concrete facing

The upper side of the ups tream dam s houlder has a con crete

facing which consists o f monolithic slabs, 10 to 30m

each. The

slab th ickness is variable, b ased on t = 0.40 + 0.002H i n metres.

Only nominal reinforcement is required, about 0.5% concrete

area in each direction. Water tightness is ensured by copper water

stops [4, 6, 7].

Plinth

Concrete faced rockfill dams require a footing or plinth to be con-

structed around their upstream edge. The plinth is made from con-

crete and serves as a connection to the borepile diaphragm wall and

as a initial point for the concrete face slab [Ref. 4, 6, 7].

Main zones of the dam body

The main elements have been designated according to current trends

for rockfill embankments: 1 for soil materials, 2 for processed gran-

ular materials and 3 for rockfill zones [4, 5, 6, 7].

Zone 1 – Zone 1A is a si lt or fine sand and acts as face slab or

perimeter joint healer. Zone 1B supports zone 1 material.

Zone 2 – Zone 2A is a processed fine filter <20mm and will limit

leakage when a water stop fails and can heal. Zone 2B, the face sup-

port zone, is crusher run <75mm.

Zone 3 – This zone is a quarry run rockfill. The differences in A, B

and C are principally in layer thickness, size and type of rock. Zone

3A provides compatibility and limit void size adjacent to zone 2B.

Zone 3B provides mass, resists the water load and helps in limiting

face deflection. Zone 3C receives little water loading, and settlement

is essen tially du ring cons t ruction. The 2m-thic k layer in zone 3C

accepts large size rocks, is more economical to place and its lower

density saves rock volume.

DESIGN

Angara river valley in a symmetric canyon with a concrete face of

approximately 55000m

, and a total dam body volume of around

2.5Mm

. The dam is comprised of three sections: i) left dam sec-

tion connecting the powerhouse with the le ft river bank, with a

total length of approximately 810m, ii) main dam section between

spillway and navigation lock approximately 960m long; iii) right

dam section next to the navigation lock to the right abutment at a

length of 90m. The total crest length of the CFRD is 1860m.

The dam shoulders – embankment type cofferdams – will be

founded on rock, whereas the central part of the d am will be

founded on the alluvium riverbed soil. The dam sealing will be sup-

ported by a borepile diaphragm wall, which will penetrate the

upstream ro ckfill shoul der (cofferd am) into the bedrock (central

ealed rockfill dam). Hence, it serves as a barrier due to the imper-

meability of the diaphragm wall through the rockfill into the

underlying bedrock. Concrete facing is used for the upper part of

the dam as a surface sealing.

Design work has been carried out considering the geometry of the

area, the minimum dam volume, safety aspects and optimum con-

struction methods. As a result, the following parameters have been

applied in the dam design:

• Dam type CFRD dam + borepile

diaphragm - cut off wall

• Crest length 1860m

• Maximum height 35m

• Crest elevation 130m asl

• Freeboard 3m

• Top width 8m

• Dam base 95m asl

• Dam base width 160m

• Upstream slope Concrete face slab 1:2,

rockfill 1:2.4 (V:H)

• Downstream slope 1:2 (V:H)

• Downstream berm width

(for instrumentation,

monitoring and inspection) 8m

• Base foundation Rock

• Depth of borepile diaphragm

under base foundation 15m

AIN

LEMENTS

Rockfill shoulders - cofferdams

The are composed of gravel or rockfill for underwater placement.

Fines are removed in order to guarantee an adequate control of seep-

age flow. The filling procedure has to be carried out caref ully to

accommodate the high velocities which are expected to be present

in the narrow gap during the second phase of river diversion [1].

WWW.WATERPOWERMAGAZINE.COM APRIL 2009 27

130

125

120

115

110

105

100

478km

459km

4400 x 10

4800 x 10

Normal operation level 127.00m asl

Minimum operation level 126.00m asl

Elevation [m asl]

600 500 400

0 1000 2000 3000 4000 5000 6000

300

Area [km

]

Volume [10

]

200 100 0

Figure 2: Curve volume-area of the Motyginskaya r eservoir

WWW.WATERPOWERMAGAZINE.COM APRIL 2009 29

DESIGN

Only rockfill will be used in this zone. The fill will be derived from

crushing hard, durable, angular rock, free from deleterious matter

such as clay lumps and organic matter.

Every layer of rockfill material will be compacted by a vibrating

roller or equivalent compaction method. Compaction features such

as water content limits, layer thickness, compaction equipment and

number of passes wil l be evaluated based on the data of existin g

rockfill material prior to commencement of works.

Gradations of materials to be placed in the embankment zones

are defined by roughly parallel curves on the standard grain-size plot.

Lift thicknesses are in the range from 50-80cm, depending on the

zone and the number of compaction passes.

In Zone 3B the maximum lift height will be 50cm and the maxi-

mum grain size will be 35cm. The maximum lift height in zone 3C

will equal 80cm, with a maximum grain size of 60cm.

An app ropria te range of gra in-size distributi on is given in

Figure 7 taking into account the material of existi ng quarries

shown in Figure 6.

THER CONSIDERATIONS

Freeboard

The top of the dam should have sufficient freeboard to prevent

waves from splashing over the dam at the highest flood water level.

The required minimum freeboard was determined as the sum of the

maxima for floodwater rise, wind tide, wave run-up and possible

Seiche effect. Th e freeboard was determined to be equal to 4.5m

from the maximum flood level.

Camber

The camber has been conceived to cover expected settlements allow-

ing a certain margin of safety. For well-compacted fills of good qual-

ity rock, founded in rock, a camber of about 0.5-1% is found to be

satisfactory. The top of the core is given the same camber as the top

of the dam. The camber adopted was !h = 0.5m.

As defined above, the top of the dam is at elevation 130m asl, but

the reac h of the dam at the main body shou ld be at the elevation

130.5m asl in a length of approximately 300m. From the edges of

this reach the dam should reach the elevation 130m with a slope of

0.001 and then remain at 130m at the abutments [5].

500

450

400

350

300

250

200

150

Elevation [m asl]

ake Baikal Irkutskaya

Bratskaya

st-Illimskaya

Boguchanskaya

Motyginskaya

Confluence with Yenisey River

2000 1500 1000

ength [km]

500 0

Above – Figure 3: Aerial view of the Motyginskaya reservoir (source: Google Earth

2008). Right – Figure 4: Top – in-line profile of dams in river Angara; Bottom –

Boguchanskaya rockfill dam under construction, upstream of Motyginskaya

140

130

120

110

100

Elevation [m asl]

-80 -60 -40 -20 0

Base width [m]

20 40 60 80

Normal operation level

Concrete face slab

Plinth

Rip-rap

Rockfill for

underwater

placement

Rockfill for

underwater

placement

Graded transition

impervious

coarser

Sand/gravel

dredging

material

Rockfill

3B1:1 1:1

3B 2:3 2:3 3B

Rock foundation Rock foundation

Borepile

1:2.4

1:21:2 1:2

1:2 1:2

Minimum operation level

Maximum flood level

Maximum

tailwater

level

Coping wall

Dam axis

Figure 5: Cross section of the Motyginskaya CFRD

30 APRIL 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DESIGN

Crest width

As a design parameter, the dam crest will reach el. 130m asl,

taking into account the freeboard. A crest 8m wide, with a stretch

fo r restrictive and slow tra ffic, will help provide the freeboard.

The supporting layers of this stretch consist of 30cm-thick coarse

granular material as a sub-base above the rockfill body, and a

well-compacted surfacing layer 20cm-thick as a supp orting sur-

face o f the stretch.

In the berms of the crest, up and downstream and along the traf-

fic stretch, coarse material and selected rocks will act as a support

transition to the slopes and the rip-rap pro tection. Tak ing into

account settlement of the dam, a 0.5m over-heightening of the rock-

fill body along the axis, prior to the execution of the specific-detail

crest operation, has been considered [5].

Access

The crest access will be designed to allow for operation and main-

tenance, and surveillance, of the dam and installations.

Berm

Access to the berm, located at el. 111m asl, will be restricted to nec-

essary personnel for inspection and instrumentation operation pur-

poses. A concrete coping wall will be constructed at the upstream

slope to provide protection against maximum wave height.

Upstream and Downstream Cofferdams

The construction of the dam r e q u i r e s the implementa t i o n of two

cofferdams, one upstr eam and the other downstream of the pro-

jected dam-axis, which will have a crest width of 8m over the ele-

vations 111m asl. The upstream and downstream slopes for both

cofferdams will be 1:2.4 and 1:2 respectively.

Both cofferdams will be constructed in phases to reach the area

of the gated spillway, which would be constructed previousl y to

allow the routing of the design flood through a 190m-wide section.

The design flood has been estimated at 14,000m

/sec for a 10-year

return period. The powerhouse would be also implemented and pro-

tected with coff e rdams of similar elevation in a preliminary river

diversion stage.

IVER DIVERSION

To allow construction of the powerhouse and navigation locks, the

iver will be diverted by temporary cofferdams. At the time of cof-

ferdam closure, construction of the main dam can start with end-

dumping of the gravel/rock shoulders under water for 1/3 of the

length of the dam. Dredging material will be used within the rockfill

shoulders. The remaining 2/3 of the dam will be constructed follow-

ing drawdown of the river water through the gated spillway and the

navigation lock. After closure of the gravel/rock shoulders and com-

pletion of dredging in the central part, the main dam body will be

constructed using rockfill in predetermined lift heights dumped by

the usual methods using the roller compaction method [5].

EMARKS

Recent experiences in CFRD have focused on strengthening concrete

face slabs in the central side taking an asymmetric canyon into con-

sideration. However, for Motyginskaya, this is not so important due

to its symmetric cross profile and length. The implementation of soft

vertical and horizontal joints are expected to minimize compressive

stresses in the face slabs.

César Adolfo Alvarado-Ancieta

Civil Engineer, Dipl.- Ing., M. Sc.

Hydropower, Dams, Hydraulic Engineering

Strauss 799, San Borja, Lima 41, Peru

cesalv@yahoo.com

IWP& DC

References

[1] Alvarado-Ancieta, Cesar A. Diversion Works. International Water Power

& Dam Construction Journal, ISSN 0306-400X, Volume 60, Number 12, pp.

34-36, Progressive Media Markets, Kent, UK, December 2008.

[2] Alvarado-Ancieta, Cesar A. Aprovechamiento hidroeléctrico Angara en

Siberia - Infuencia de los atascos y barreras de hielo en la Central

Hidroeléctrica de Motyginskaya (Spanish). Revista de Obras Públicas,

Colegio de Ingenier os de Caminos, Canales y Puertos, ISSN 0034-8619,

Year 155, Issue 3494, pp. 7-22, Madrid-España, December 2008.

[3] Alvarado-Ancieta, Cesar A. The Angara cascade hydropower

development scheme in Siberia and preliminary estimation of the design

flood of Motyginskaya HPP under the influences of ice jam. 15th

International Seminar on Hydropower Plants, Hydropower Plants in the

Context of the Climatic Change, ISBN 978-3-9501937-4-9. Volume 15, pp.

555-574, Vienna University of Technology-Austria, November 2008.

[4] ICOLD - Committee on Materials for Fill Dams. Concrete Face Rockfill

Dams - Concepts for Design and Construction, November 2004.

[5] Norwegian University of Science and Technology. Hydropower

Development in Norway. Number 10. Rockfill Dams, 2002.

[6] ICOLD - Bulletin 70 - Rockfill Dams with Concrete Facing - State of the

Art, 1989.

[7] American Society of Civil Engineers - Concrete Face Rockfill Dams,

Design, Construction and Performance, October 1985.

Note: Table and Figures herein presented have been performed by the author.

Top – Figure 6: Grain size distribution curves of available material in the

vicinity of Motyginskaya dam site

Bottom – Figure 7: Grain size distribution curve of dam body material - gravel

and rock fill for underwater placement

C1-1

C1-2

C2-1

C2-2

C3-1

C3-2

C5-1

5-2

C6-1

C6-2

C9-1

C9-2

100

8” 8” 3” 2” #4 10 20 40 60 140 200

”

Cobbles

Gravel

coarse

Fine FineMedium

Coarse

Sand

Fines

% Percent passing

1000.000 100.000 10.000 1.000

Size [mm]

0.100 0.010 0.001

8” 8” 3” 2” #4 10 20 40 60 140 200

”

Cobbles

Gravel

coarse

Fine FineMedium

Coarse

Sand

Fines

100

% Percent passing

1000.000 100.000 10.000 1.000

Size [mm]

0.100 0.010 0.001