Gilgel Gibe II - TunnelTalk
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Collapse of headrace tunnel after grand opening Feb 2010
Paula Wallis, TunnelTalk
Reports of a major collapse in the headrace tunnel on Ethiopia's newly inaugurated Gilgel Gibe II hydro project are confirmed by Italian contractor Salini.
According to a news release by the company, "an unforeseen geological event provoked a 'cave in' and a huge rock fall involving about 15m of the 26km headrace tunnel".
The project is constructed in the Great African Rift Valley region with the tunnel, cutting through heterogeneous and complex geological formations. Managing the conditions required the highest construction standards and the project is considered an outstanding piece of engineering achievement.
The hydro scheme , which was handed over six months ahead of the contractual milestone and recently inaugurated, will undergo a two-month maintenance period, which the client, EEPCO, has entrusted to Salini Costruttori, in the hope of recovering full operational capability. The press release of last week (February 4) said that "highly qualified personnel are already at work to ensure the earliest resumption of energy production."
The admission comes two weeks after the official inauguration of the nation’s largest power plant on January 13, 2009. According to a TunnelTalk source the collapse occurred on the third day of the three-day celebration to commemorate the opening of the plant and happened about 10km from the outlet portal. This information has not been officially confirmed, as calls to Salini, Seli and the owner had not been returned on going to press.
This is the latest chapter in an extremely difficult project that experienced large mud/water inflows, unstable faces of raveling/running and
Collapse specifics and recovery update
Seli managers have added further detail to the collapse event in the Gilgel Gibe II headrace and its recovery.
The cover above the point of the collapse on the long headrace TBM drive is about 800m. When the TBM passed through during excavation, the rock was hard but fissured. The TBM bored through the section with good average performances but with some blocks falling from the face damaging some of the cutters and disturbing the advance. The faulty material that collapsed into the tunnel was not crossed or detected during excavation. It is suspected that the fault that initiated the collapse was just a few meters above or beside the tunnel wall. Investigative probes yet to be drilled will confirm the geological formations and conditions above the tunnel. The exact quantity of material that came into the tunnel is still yet to be measured but a rough first estimate is about 8,000m3.
The exact plan for the repair is yet to be confirmed but there is no possibility and no need to excavate an intermediate access adit. It is possible to repair the tunnel on its existing alignment and possible also to excavate a short bypass around the 15m collapsed zone if that is considered most appropriate. The decision will be taken depending mainly on the estimated time required for the two alternatives.
The 26km long Gilgel-Gibe headrace is considered one of the most difficult tunnel project ever undertaken, due to the critical, and in some reaches, exceptionally adverse, ground conditions. After a major tunnel inundation event during excavation in 2006, the recent collapse event occurred at chainage 8.980 from the outlet portal and three months into operation of the tunnel.
The client, EPCO, has entrusted Salini, Seli, and a team of expert consultants, to investigate the causes of the event and consider the repair measures needed. After a visit to Ethiopia to inspect the dewatered tunnel, Seli managers confirmed that the rest of the tunnel, including the very critical section of the first geological event during tunnel excavation, is in good condition.
Fig 1

Gilgel Gibe II foreseen geological profile

blocky ground, high rapid convergence of tunnel walls, high ground loads on the two TBMs in the 26km long dirve and the hexagonal precast concrete segmental lining, and very hot water and gas inflows.
Seli was awarded the €37 million contract to excavate the 26km bored headrace in March 2005. In October of 2006, a little more than a year into construction, disaster struck. The intake portal TBM hit a fault about 4.2km into excavation that spewed mud under 40 bar pressure. The massive inflow pushed the TBM backwards crushing the last seven segment rings behind the TBM. After a two-year delay, the recovered TBM started up again a few hundred feet behind the fault to excavate a 230m bypass. The second TBM, driving from the outlet portal, faired better despite passing through several fault zones and experiencing hot water inflows of up to 54°C.
Fig 1

Breaking under the pressure

The two TBMs met on June 6, 2009 and the tunnel was ready for hydraulic testing in September 2009. According to owner, The Ethiopian Electric Power Corporation (EEPCo), the new hydroelectric plant will increase the nation's power generating capacity by 50%. It will provide power to a million new users and will contribute to achieving a national goal of extending access to electricity to 20% of Ethiopia's population by 2010. The Gilgel Gibe II hydroelectric project generates power by exploiting the drop between the basin created by the Gilgel Gibe I dam on the Gilgel Gibe River and the River Omo below.
The extent of the recent collapse will not be known until the full system is dewatered and engineers can get inside and inspected the tunnel.
References
Success after mammoth struggles in Ethiopia - TunnelTalk, Nov 09

Salini Costruttori
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