Repair of limited collapse in Ethiopia
Repair of limited collapse in Ethiopia Mar 2010
Patrick Reynolds, Freelance Reporter and Shani Wallis, TunnelTalk
Crews have completed laying track and installing ventilation, air, water, and power services to the repair zone of the failed headrace tunnel on the Gilgel Gibe II hydropower plant in Ethiopia. In January this year, about 8,500m3 of material, ranging from silt and mud to blocks of rock up to 2-3m wide, crashed through a 15m long section of the 8.10m diameter TBM driven segmentally lined tunnel, bringing operation of the facility to a standstill just three months after inauguration. Recovery operations are divided into three phases - drilling of geological investigation boreholes; clearing of the collapsed material from the tunnel; and reconstruction of the waterway.

Route of the 26km long headrace

The collapsed zone lies about 9km from the outlet portal and under about 800m of cover. There is no possibility, and need according to engineers, to drive a lateral access. Recovery will be on-line and built either as a small tunnel mined through the collapse debris and enlarging to the full cross-section; or as a 140m by-pass starting about 50m before the collapsed zone and rejoining the headrace tunnel about 50m beyond. The final solution will be based on the estimated time to complete each options and borehole investigation of the rock mass.
An initial program of two boreholes into the roof, two each into the side walls, and two more all the way through the collapsed debris to drain build up of ground water on the upstream side, will identify the need for further investigations. Estimates are that clearance, intervention and rebuild of the tunnel will take five or six months.
Excavated by Seli as a subcontract to main construction contractor Salini Costruttori, the 26km long TBM drive through complex geological formations of the Great Rift Valley region of Africa is recognized as one of the most difficult TBM tunneling projects on record. Large mud/water inflows, unstable faces of ravelling/running and blocky ground, high rapid convergences, high ground loads, and very hot water and gas inflows were conditions faced by the crews of two double shield machines working towards each other from the portals. The intake portal drive was on hold for about two years while a 230m bypass was excavated to rescue the TBM after it hit a fault that spewed mud under 40 bar pressure into the heading, pushing the TBM backwards and crushing the seven rings of the lining behind the TBM. After starting in October 2006 the machines met in June last year and the tunnel was watered-up in September.

Longitudinal section of the 26km headrace

According to Seli engineers, the poor rock conditions that broke into the tunnel subsequent to inauguration were not crossed or detected during excavation. When the TBM passed through, the rock was "hard but fissured" and only blocks falling from the face and damaging some of the cutters interrupted good average performance.
Man-entry inspection of the tunnel after dewatering confirmed that the inrush of debris created a collapse hole that extends about 30m-40m up into the rock mass overhead. A 20m length of collapsed tunnel plus a further 10m of damaged lining on the downstream outlet side is in need of repair.
Lining designs
As well as planning the alignment options, the design of the lining through the repair zone is being developed by the Lombardi engineering firm. According to information from Seli, the current plan is to line the repaired section with steel ribs and thick shotcrete.
The first geological event, encountered during excavation, was overcome with a bypass excavated by the TBM and lined with the hexagonal segmental lining used throughout the tunnel - a system that is said to perform well under the difficult geological conditions. Even under the conditions of the recent geological break-in, the lining did not collapse along the joints. "Each segment in the hexagonal lining system is in contact with six other elements and this helped to avoid a domino effect and limited the failure to only 15m.
"Like any other segmental lining, the hexagonal system can be bolted or connected with connectors, guiding bars, or a combination of these. The need to bolt or not is dependent not on stability of the lining but on the segment sealing system selected. For tunnels under high cover, as is the case of Gibe, the external groundwater pressures can be so high that no rubber seals can resist the pressures. Rubber seals also make it impossible to execute high-pressure consolidation grouting and prevent the filling of the joints with the grout mortar, which is detrimental for the structural resistance of the lining along the joints.
"For most cases of tunnels under high cover, rather than rubber gaskets, the joints are sealed with a special mortar that completly fills the joints with grout mortar. This ensures the structural continuity of the lining and allows the possibility of consolidation grouting at 10-15 bar pressure.
"More than 500km of tunnels have been lined with the hexagonal lining system and this is the first time such a collapse has occurred. The reason for the collapse cannot be attributed to inadequacy of the lining. Otherwise other sections of this very long tunnel under extreme ground conditions would have collapsed. Inspection of the collapsed zone confirms that all the joints and the joint sealant were sound."
After a visit to Ethiopia to inspect the dewatered tunnel, Seli managers confirmed that the rest of the tunnel, including the critical section of the first geological event during initial excavation, is in good condition.

Endurance under pressure

"The reason for the collapse is the suspected presence of a major fault, perhaps an old river bed, hidden behind a thin diaphragm of hard but fissured rock. The pressure of this fault - and remember we measured 40 bars during the first geological event under much less cover - must have been so big as to break the rock diaphragm and apply a high concentrated load on a few segments" causing them to crush.
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. Owned by The Ethiopian Electric Power Corporation (EEPCo), the new plant increases the nation's power generating capacity by 50% and will provide power to a million new users. Construction of the Gilgel Gibe II project is part of wider plans for exploitation of the Gilgel Gibe and Omo Rivers and was supported financially by the Italian Development Cooperation in Ethiopia. The Environment and Social Impact Assessment for Gilgel Gibe III, the next in the masterplan, was completed in mid-2009 and construction of the 243m high roller-compacted concrete dam and its associated 1,870 MW power station has started. The Gibe cascade also includes the planned Gibe IV (1,472 MW) and Gibe V (560MW) dams and power installations.
Collapse of headrace tunnel after grand opening - TunnelTalk, Feb 2010
Ethiopian Electric Power Corporation (EEPCo)
Salini Costruttori

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