Hydro power in Ethiopia - the staged construction of Tekeze Arch Dam11 May 2009
Upon completion at the end of February 2009, the Tekeze Concrete Arch Dam in Ethiopia became the highest dam on the African Continent. Here James R Stevenson and Mihret Debebe provide further details of the staged construction and filling sequence for the dam, while summarizing and updating the status of the power projects currently underway in the Federal Democratic Republic of Ethiopia by the Ethiopian Electric Power Corporation
The Tekeze Dam is located on the Tekeze River in the Northern Tigre Region of Ethiopia. The Tekeze and Atabara Rivers originate in the Simien mountains of Ethiopia. Each of these Rivers are little known but are key tributaries to the 6650km long Nile River. They are also separate from the White and Blue Nile tributaries. Since ancient times, the Nile has supported the many civilizations that surround it. The Blue Nile combined with the Tekeze/Atabara Rivers provide most of the water and silt in their seasonal runs to the Nile. Most of the rainfall (89%) arrives with the East African Monsoon during the months of July, August, and September.
The White Nile is the longest tributary and rises from the Great Lakes region of central Africa, with the most distant source in southern Rwanda. The White Nile flows north through Tanzania, Lake Victoria, Uganda and southern Sudan, then meets up with the Blue Nile in Khartoum, the capital of the Sudan. The Tekeze River originates in the Northeastern slopes of the Simien Mountains, then flows out of Ethiopia and into the Atbara River which later joins the Nile River at a point approximately 322km downstream of Khartoum, such that it feeds into the Merowe Reservoir.
Ethiopia’s hydro power commitment
The Federal Democratic Republic of Ethiopia is a land-locked republic in the ‘Horn of Africa’. The current republic was formed after the fall of the ‘Derg Regime’ in 1991. The country has a population of approximately 78 million.
Ethiopia is a country whose location and high mountainous areas provide a vast hydroelectric potential that is estimated to exceed 40,000MW. After the ‘Derg’, the government of Ethiopia made its first priority that of overcoming the environmental, financial, and technical obstacles necessary for the implementation of several major hydroelectric developments. To alleviate Ethiopia’s present shortage of generating capacity, EEPCO’s financial resources were focused on the early completion of the Tekeze hydro power project.
Innovative modifications were made during the construction period to expedite the completion of Tekeze Dam, such as a modified river diversion scheme, staged dam construction, and the staged impoundment of the reservoir.
The Tekeze project is being funded entirely by the Federal Democratic Government of Ethiopia. The total project construction cost for all contracts combined with the consultant services are expected to be about US$365M.
The project works are being constructed under the contracts shown in Table 1.
Tekeze project description
Tekeze hydro power project is comprised of a double curvature (logarithmic spiral) concrete arch dam and appurtenant works, two river diversion tunnels, a 75m high power intake structure, power waterways (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, a 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.
Diversion scheme simplifications
There were two diversion tunnels constructed around the Tekeze Dam site. These tunnels and associated cofferdams 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 provided 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 bulkhead 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 temporary 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 State of California in the US; namely Monticello Dam (circa 1956), and New Bullards Bar Dam (circa 1969).
During the Tekeze River floods of 2006, the arch dam acted as a gravity structure, and passed a 10-year 2000m3/sec flood on 9 August 2006. That flood can be seen in Figure 2. Except for flooding of the lower galleries and some missing forms, there was no damage to the permanent structures. Nine days after the flood passed, the dam galleries were pumped dry and the Contractor 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 structure. 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 overtopping of seasonal floods lasted 81 days, from 13 July 2007 (see Figure 4) to 2 October 2007. The combined diversion capacity (estimated to peak at a maximum of 1313m3/sec) spilled over the two dam low blocks with small releases from the Diversion Tunnel Bonnet Gate, as shown in Figures 5 and 6.
Staged dam construction and impoundment
The basic design of arch dams is founded upon standard principles whereby a series of rigid structures – blocks or cantilevers – are connected together by grouting of the contraction joints to form a single rigid structure that can resist water loads in arch action. For analytical purposes, the Engineer assumes that the weight of the structure 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 construction schedule, with the associated economic issues, are usually not clearly defined to the dam designer. The dam designer usually assumes that the dam cantilevers are constructed, the joints grouted, then the water loads applied, in that order. The impacts due to any deviations from usual design practice must be carefully weighed by the designer. 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 grouting of the contraction joints, and even the staged filling of the reservoir. The single purpose of the Tekeze hydro power project is to generate electricity. To achieve this purpose in the shortest possible period of time, EEPCO expected the reservoir to be filled as early as possible. In doing so, additional dam analyses are mandatory, 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 3Bm3 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 entire 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 time, structural 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 carried 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 results 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 reservoir 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 approximately 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 reservoir 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 closure of the upper un-grouted joints and redistribution of both cantilever and arch stresses throughout the dam and foundation. After the 2008 impoundment at Tekeze, some reduction of the upper contraction joint openings did occur, making it more difficult to grout. For these reasons, close control of the 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 previous 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 used 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 monitored 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 steadily increases as higher joint grouting is completed.
As noted earlier, no structure is perfectly water tight and some seepage 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 monitored and measured throughout the impoundment process. To collect this seepage and maintain the dam in a reasonable 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 equipment. Continuous monitoring of foundation extensometers, piezometers, temperatures, leakage, and vertical pendulums is on-going during the final filling of the reservoir.
Contraction joint grouting by the ‘Pull-Pipe’ method
The standard method used in the past for the grouting of vertical contraction 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 air, 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 superior, and adopted the method for the contraction joints at Tekeze.
Benefits of staged impoundment
• By beginning impoundment two years early; 554Mm3 of water was retained in 2007. That storage increased dramatically to 3100Mm3 by the end of 2008.
• 2008 was a very dry year for Ethiopia; had it been an average water year, an additional 1200Mm3 could have been retained and power generation started in September, 2008.
• The early impoundment of 3100Mm3 is equivalent to a generation of 1190GWh from 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.
Other EEPCO projects
In order to alleviate the current shortage of electricity in Ethiopia, EEPCO expects to complete, prior to 2010, 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 Gilgel-Gibe II project, 92% completed, 4 x 105MW pelton units, 26km TBM tunnel, penstocks with surface powerhouse. Construction by Salini Costruttori, and Engineering Management by Coyne et Bellier.
• 460MW Beles project, 75% completed, 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 turbines 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 power 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 investment 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: [email protected]
Ato Mihret Debebe, Chief Executive Officer (CEO), Ethiopian Electric Power Corporation (EEPCO). Email: [email protected]
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