World Tunnelling > Global News > ‘Unforeseen geological event’ causes hydro station closure

Global News

A tunnel collapse has shut down Ethiopia’s newest and biggest hydropower station only weeks after it was opened. The collapse at the US$600 million Gilgel Gibe II Hydro Power Plant Project is reported to have occurred in the 26km headrace tunnel that feeds water from the new dam to four enormous turbines.

A notice on the website of Salini, the Italian contractor that built the power station, said that an unforeseen geological event caused a ‘cave-in’ and a ‘huge rockfall’ along roughly 15m of the tunnel. Handed over six months ahead of schedule, the project will now require a two–month remedial period which Salini will undertake for client EEPCO – Ethiopian Electric Power Corporation.

The project’s complex geological setting – in the Great African Rift Valley – has caused problems throughout the construction period. These have included intense ground loads on the two SELI TBMs, unstable tunnel faces, high rapid convergence of tunnel walls and large mud, gas and hot water inflows. Indeed, it is reported that the tunnel crosses a fault line at 26 different points.

Two 6.98m diameter double-shield TBMs were used to bore the headrace tunnel excavating along an alignment that comprised basalt, rhyolite, trachyte and dolerite. Boring began in March 2005 and took 25 months. The tunnel was lined with 250mm thick hexagonal precast concrete segments resulting in a 6.3m internal diameter.

Critics of the project have alleged that the no-bid contract was awarded in 2004 without the proper feasibility studies or environmental permits usually associated with such projects having been carried out.