Within weeks of Ethiopia’s inauguration of the 420-megawatt Gilgel Gibe II hydro plant by Prime Minister Meles Zenawi in January, a 15-meter section of its head-race tunnel collapsed. Italian engineers have begun reinstalling lights and ventilation to prepare for months of work to get water flowing again.
According to the project’s main contractor, Rome-based Salini Costruttori S.p.A., the rest of the 26-km head-race tunnel is in “perfect” condition. The collapse site, about 10 km upstream of the powerhouse, was deemed to be in “sound rock” during construction, says a company official.
The project, some 250 km southwest of Addis Ababa, was built under an engineer-procure-construct contract between Salini and state-owned Ethiopian Electric Power Corp. Salini let a $51-million tunnelling subcontract to Rome-based S.E.L.I. S.p.A.
In July 2005, S.E.L.I. started tunnelling through basalt, rhyolite, trachyte and dolerite using a pair of 7-meter-diameter, double-shield tunnel-boring machines. The contractor completed the bore last July. It lined the tunnel with 25-cm-thick precast-concrete segments and backfilled with grouted pea gravel, according to Salini.
The tunnel crew bypassed a “major mud flow” a few kilometers from the portal in 2006, says the Salini official, but he reported no other similar mishaps.
The power project is the second in a cascade on the Omo River and a tributary, as they flow toward Lake Turkana on the border with Kenya. Following international bidding competition, Salini won the contract for the first, 184-MW plant on the Gilgel Gibe River. The contractor compled work in 2004.
The contractor then negotiated on an exclusive basis the roughly $680-million EPC contract for the Gilgel Gibe II plant. The job includes construction of the head race from the first plant’s dam, which provides a 500-meter head at the new powerhouse on the Omo River.
According to the European Investment Bank, which provided a $68-million project loan, construction prices for the negotiated contract were based on those of the first plant. They were adjusted for inflation and contingencies and independently audited, reports EIB. Italy’s government provided soft loans covering some 60% of the project cost.
Salini also negotiated the $2-billion EPC contract for the 1,870-MW Gilgel Gibe III plant, about 155 km down the Omo River. At 240 meters, the project’s roller-compacted concrete face will be the world’s tallest when completed in 2013, the contractor claims. Salini plans to start raising the dam this July.
Under another EPC contract, Salini is close to completing the 460 MW Beles hydro project for Ethiopian Electric Power Corp. at Lake Tana, 370 km north of Addis Ababa. The project includes a 12-km head race, which is also machine-driven by S.E.L.I.