The Gilgel Gibe Hydropower Project in EthiopiaIn Ethiopia, about 250 km to the southwest of Addis Abeba plateau, along the Omo river and its tributary Gibe, the planning and construction of an important hydroelectric system is under way: Gibe I with an installed power of 184 MW, Gibe II (420 MW) and Gibe III (1,870 MW). The national power grid, including the Gibe scheme, is developed by the Ethiopian Electric Power Corporation (EEPCo). Click here
to see the map of the project.Unable to meet the growing electric power demand, EEPCo introduced a power rationing programme two months ago. According to EEPCo, the country's power demand has increased by 15%. With a population of 80 million, the majority of Ethiopians do not have access to electricity. At the moment, access to electricity is about 22%. The government has launched a universal electricity access programme (UEAP) to be implemented by EEPCo with the view to enhance the access to 50% within five years. At the moment, EEPCo's maximum electric generating capacity is 814 MW, 80% hydro and 20% geothermal and thermal. However, because of various reasons, including water shortage, the corporation generates only 600 to 700 MW. At the moment the power deficit is 80 MW.EEPCo is undertaking massive hydropower projects: Gibe II (420 MW), Gibe III (1,870 MW), Tekeze (300 MW), Beles (460 MW) and Fincha Amenti Neshe (100 MW). The corporation plans to construct more than ten hydropower plants. The feasibility study of Gibe IV has been finalised. Gibe IV will have the capacity to generate 2,000 MW.Gilgel Gibe IThe project area was located 260 km to the southwest of Addis Abeba, the national capital. It is a conventional hydroelectric facility comprising a 40 m-high rock fill dam, a 9 km 5.5 m-diameter concrete-lined headrace tunnel, a 113 m-high 14 m-diameter concrete-lined surge tank, a 165 m-high vertical penstock, a cavern powerhouse able generating 184 MW of electric power and a substation. All these structures were planned by Studio Pietrangeli, Enel and ELC Electroconsult. Visit www.enel.it
The project was divided into eight contracts and construction was carried out by several contractors. Lot 1 (headrace tunnel) and lot 5 (dam) were built by a joint venture of Salini Costruttori of Italy and Necso of Spain (now Acciona). Trevi was entrusted with the impermeabilization works of the dam. The German company Zueblin installed the powerhouse, shafts and tailraces. Visit www.salini.it
The construction of Gigel Gibe dam started back in 1984 by North Korean engineers but ended only in February 2004 after works stopped in the first years of the 1990s. It is Ethiopia's largest dam with its 917 million cu m reservoir capable to generate 184 MW of electric energy.According to the different types of support planned, several cross-sections were defined, ranging from 32 sq m (I and II), 34 sq m (III), 37 sq m (IV) to 45 sq m (V). Cross-sections I, II, III and IV were characterized by a horseshoe shape, but the narrow 3 m-wide floor was a very important constraint which obliged to select specific machinery and to construct niches and enlargements every 500 metres to allow U-turns and crossing of trucks, as well as the machinery to stay on standby. The bored diameters range from 6.60 m for cross-sections I and II to 6.64 m-6.70 m-7.30 m for cross-sections III, IV and V. The tunnel was excavated from four attacks: the intake (2,152 m), Adit 1 (1,198 m excavated upstream and 1,941 m downstream), Adit 2 (798 m upstream and 1,799 m downstream) and Adit 3 (1,192 m). Full-face tunnel driving was always adopted, normally using the drill-and-blast method, except in the sections where it was possible to dig with other mechanical means. In the more complex geological zones, where basalt appeared to be mixed with soft soil, the hard rock was blasted and the soft ground dug by mechanical means. Drilling was performed by semi automatic robotic operation, using Tamrock jumbos (two Para 206-90 and one Maximatic HS 205T) and one Atlas Copco H 188, all of them with two drilling booms and basket boom. In poor quality ground, crews did not use explosives but four Schaeff ITC 312 heading and loading machines used for mucking-out and rock scaling. Visit www.miningandconstruction.sandvik.com
Diesel trucks transported the rock and earth to the dump sites situated near the adits. Basically, 20 Dumec DP5000 dumpers, each with 6 cu m loading capacity, were employed but ten conventional Astra 64.26 trucks were also used as complement, with bigger loading capacity (16 cu m). The other means used included three ancillary loaders (two FIAT Allis FB7B and a FIAT Hitachi W 170). Visit www.dumec.com
Wet-mix shotcreting was systematically implemented using three CIFA shotcreters and a Meyco Robojet. The same jumbos used to drive the tunnels were re-employed for bolting. Grouted bolts were installed from four Dieci Samson ET40.11 telescopic handlers, which were also used to place the steel arches around the tunnel contours. Visit www.cifa.com
Gilgel Gibe IIGilgel Gibe II is a continuation of the Gilgel Gibe I project inaugurated in 2004. The new plant does not need a dam to be constructed. Instead, it will use the water discharged by the Gilgel Gibe I plant, channeled through a 25.8 km tunnel under Fofa mountain, to the Omo river valley. The water head difference is 490 m. The water that comes out of the tunnel will run down a hill in two big penstocks into a power house where there will be four 105 MW turbines. In addition, the project includes a dam, an intake structure, a surge tank, a power station for four Pelton turbines, a bridge, an electric substation, and camps. Because of the project, 80 km of asphalt and all-weather road has been constructed.The power project lies on an ever green mountain and gorge in the Sokoru locality between Oromia and the Southern Nations, Nationalities and Peoples Region (SNNPR). Once the Gilgel Gibe II hydropower plant becomes operational, it will generate 420 MW and increase the generating capacity of Ethiopia by 30-40%.The Gilgel Gibe II construction site was officially launched by Ethiopian government officials and representatives from the Italian Development Cooperation on 19th March, 2005. The scheme has been planned by Studio Pietrangeli on behalf of the main contractor Salini Costruttori, as part of the ELC Electroconsult contract.The tunnel is built by SELI for Salini Costruttori from two drives. Its bored diametre varies between 7 and 8 m while the inner diameter equals 6.3 m. SEA Consult was contracted by EEPCo for the geological and geotechnical work supervision and client support for excavation of the Gilgel Gibe II tunnel on behalf of ELC Electroconsult. Visit www.selitunnel.com
The other underground structures are a reinforced-concrete steel-lined cut-and-cover access stretch at the start of the tunnel (approx. 50 m, 6.3 m in diameter), a 300 m-long 6.3 m-diameter adit (Adit 26) at the end of the tunnel and a 95 m-high 18 m-diameter surge tank.As of early June, about 90% of the work has been finalised. The inlet, the penstock and power house have been built. The turbines and the power controlling machines are installed. Of the 25.8 km tunnel, 17.1 km has been drilled and built. In a culvert at he bottom of the tunnel where now the tracks are laid for the muck train, optic fibres will be installed to allow connection between the inlet and outlet.GeologyThe geological formation crosses basaltic rock in its first half (about 13 km), with varied grade of alteration. Where the path meets faults, the terrain becomes semi fluild loamy material. On the second half of the route, the geology consists of trachitic formations.Tunnelling methodologyThe lateral access adit (Adit 26) and the first 150 metres of each drive is drilled and blasted. For the remaining length of the tunnel, two double shield TBMs (models 0698-108 and 0698-109) are being used from each end (inlet and outlet). The TBM is 240 long and the drilling head's diameter is 6.98 metres. The machines are equipped with 17" (432 mm) cutters. The cutter head is powered by six engines able to supply 1,890 kN (6 x 315 kW) and rotates at a speed of 0-7.5 rpm.The plan was to drill both sides of the mountain with TBM 1 and TBM 2 from west and east direction. Work on the east side is progressing. TBM 2 drilled at least 13 km. However, TBM 1, which was drilling the tunnel near the inlet, has encountered difficult ground.
TBM 1 has been drilling the rocky mountain but after it drilled four kilometres it came across an unconsolidated layer (loose ground). The drillers discovered a huge hole filled with mud that started to flow into the tunnel. First they tried to excavate the mud and to cover the area with concrete. But all was in vain. The machine was unable to proceed with the drilling because of the loose earth and the mud inflow. Highly trained and experienced soil experts were hired to solve the problem but to no avail. It has been a year since TBM 1 ceased operation. SELI dismantled the machine. It took three months. A decision has been made to divert the direction of the tunnel to avoid the loose earth. So now SELI is preparing to resume the drilling in a new direction. Re-assembly of the machine has started since early June and it is expected to take a month.
Muck transportIn the sections dug with traditional means, the excavated rock is mucked away with loaders (Caterpillar 866 and 886), trucks (Astra 3648) and dumpers (Caterpillar 769 D) while locos and wagons behind the TBMs are used in the other sections. Visit www.cat.com
Support and liningWhere traditional means have been employed (including the surge tank), crews applied a temporary support consisting of a 5 to 20 cm shotcrete layer with SIKA admixtures (VicoCrete 3062 dosed at 5.2 l/cu m, Plastiment VZ at 1.6 l/cu m and Sigunite L accelerator at 60 l/cu m), with or without fibres, implemented by means of CIFA 307 and Aliva 263 pumps. Where needed (portals, rock sections Class 3 and 4 or in the presence of instable wedges, the ground had to be reinforced with welded mesh and grouted bolts anchored by a Tamrock bolter. The permanent lining is reinforced concrete or, at some spots, reinforced shotcrete. Visit www.sika.com/stm.htm
The TBM launch chamber
The TBM-driven tunnel is supported and lined by precast reinforced-concrete hexagonal segments (four per ring), 25 cm think minimum. The annular space between the extrados and the segmental rings is filled with gravels and cementicious mortar is subsequently injected under pressure. In the 120 m outlet stretch downstream the surge tank, the tunnel is built with traditional means and steel lined. The gap between the excavation and the steel lining is filled with concrete. Click here
to read more about Gilgel Gibe I and II.Gilgel Gibe IIIThis third hydroelectric facility, also planned by Studio Pietrangeli on behalf of the main contractor Salini Costruttori, as part of the EPC Electroconsult contract, involves an approx. 240 m-high RCC dam on the Omo river, a power station to house 10 Francis turbines, a bridge, an electric substation, roads, camps and secondary works.The project requires, above all, the construction of two headrace tunnels and penstocks to the power plants PT1 and PT2. These two tunnels, 1,155 m and 963 m long respectively with bored diameter of 12 m and lined diameter of 11 m, comprise an intake structure, a long horizontal stretch (935 m / 779 m), a surge tank, a vertical section (108 m / 110 m) and a horizontal outlet to the stations (112 m / 76 m).
There are also three water deviation tunnels (TL, TC and TR), respectively 970 m, 1,025 m and 1,220 m long, with different bored diameters (8 m for TL and 14 x 15 m horseshoe-shaped for TC and TR) and finished diameters (7 m for TL and 13 x 14 m horseshoe-shaped for TC and TR).There are two working faces in each tunnel. TC is accessed thanks to an adit near the outlet in the valley. Other underground structures include two vertical surge tanks (about 110 m high, 20 m in diameter) located along PT1 and PT2 and numerous exploratory and pilot tunnels.Excavation of tunnels PT1 and PT2 was scheduled to commence in May 2008 to end in May 2009. Work at tunnels TL, TC and TR commenced in November 2006 and was planned to be concluded between July and November 2007.However, poor progress has hindered the finalisation of the project. Only 23.8% development has taken place at mid-June. Since July 2007 up to the present month, it has registered only a 68% performance compared to the plans for the year. EEPCo had announced that the commissioning of Gibe III plant would take place in the year 2012.
GeologyThe tunnel goes through a geological formation made of tachytes, generally little weathered and hardly fractured (RMR Class II-III), but locally much more weathered.Tunnelling methodology and mucking-outExcavation is performed using traditional means and for the tunnels of more than 8 m in diameter, adopting two sequential phases: top heading and benches. The rock is removed by loaders and dumper trucks. Support and liningThe temporary support is made of a 5 to 25 cm-thick shotcrete layer prepared with SIKA admixtures (VicoCrete 3062 dosed at 5.2 l/cu m, Plastiment VZ at 1.6 l/cu m and Sigunite L accelerator at 60 l/cu m), reinforced with fibres and welded mesh, 4.6 m bolts at variable intervals according to the rock class, with placement of steel arches where needed (Class IV-V).The permanent lining is reinforced concrete (0.5 m thick in tunnels TL, TC and TR or 0.8 m thick in tunnels PT1 and PT2) and steel lining in the downstream high pressure sections and surge tanks. 26/08.