Success after mammoth struggles in Ethiopia
Success after mammoth struggles in Ethiopia Nov 2009
Ilaria Driussi, Technical Writer
Pic 1

Beles headrace breakthrough

Five months after completing the difficult Gilgel Gibe II tunnel in Ethiopia, the Italian company SELI has completed the last stretch of the 12km long Beles headrace tunnel. After boring the last 7.5m and installing the last five rings of segment for the 12km long headrace, the double-shielded SELI TBM broke through on November 8, 2009.
The tunnel is part of the Beles II hydropower scheme located in the Amhara region in northwest Ethiopia which is designed to channel water coming from Lake Tana Beles into the penstock of an underground powerhouse and into the Blue Nile (Fig 1).

Fig1. Plan of the Beles hydro project

As well as the tough conditions experienced on the 26km-long Gilgel Gibe II headrace tunnel, the double shield EPB dual-mode TBM on the Beles headrace was able to cope with different geological formations and conditions by adapting its configuration and operational mode to the ground characteristics and behaviour.
The tunnel ran through a variety of ground formations with about 10km passing through hard granite rock of up to 350MPa in compressive strength and intersected by local faults, and another 1.8km of loose soils, basically lake deposits, through which the machine advanced with a daily output of about 15-20m/day. The tunnel is lined with 30cm-thick precast concrete segments in 6+1 configuration and the annulus backfilled with pea-gravel and mortar.
Configuration change from double shield mode to EPB mode took about three weeks to accomplish. At about half way through the headrace tunnel a massive failure of the face blocked the TBM cutterhead and brought progress to a standstill. Among the many options available to deal with the difficulty, SELI chose to employ polymer resins and a three-stage process to secure the heading and resume TBM advance.
Technical data of the Beles headrace TBM
Boring diameter: 8.10m
Cutters: 52 x 17in
Max. cutterhead thrust: 11.440kN
Cutterhead speed: 6 rpm
Cutterhead drive power: 2.100kW
Pic 1

TBM assembly site

First, the void ahead and above the cutterhead was filled with high expansion polymer resins that support the void to prevent further material falling. TISfoam DRY K, a high expansion ratio organo-silicate resin, was injected through a set of appropriately positioned pipes through the TBM cutterhead and at sufficient volume to completely fill the cavity.
Secondly, the crown and face area was consolidated using TIS-sil STAB, a low expansion ratio organo-mineral resin. TIS-sil STAB has exceptional mechanical resistance and adhesion and was injected again through cutterhead and via pipes positioned appropriately to consolidate the heap of loose rubble in the face and around the cavity.
Following that a hand-mined heading was advanced out above the shield to remove rocks and free up the cutterhead, being assured at all times that all ahead is stabilized and made safe.

Fig2. Managing rock falls ahead of the cutterhead and samples of resin void filling and consolidation

The headrace tunnel is part of the Beles II multipurpose project. This is an Engineering Procurement and Construction contract, that was awarded in 2005 by EEPCo, Ethiopian Electric Power Corporation, to the Italian construction company, SALINI Costruttori Spa, as a €467 million Engineering Procurement and Construction contract.The project includes construction of a single stage hydropower plant in the Amhara region, about 150km from the town of Bahir Dar on the south-western bank of Lake Tana. The hydro-power plant has an installed capacity of 460MW.

Road to the tunnel portal


A face of good hard rock

Tunnelling work was subcontracted by SAKINI to SELI for a total of €48.7 million. The project's total length of 20km is completely underground and comprises a 12km headrace; a 7.2km tailrace; an underground powerhouse; a 270m penstock shaft; and a 90m surge shaft (Fig 1).
SELI also excavated the 7.2km long tailrace tunnel using a second 8.07m diameter double shielded TBM. It too was lined with an open type parallel ring rhomboidal segmental system with a pea gravel and mortar annular fill. Excavation of the tailrace began in June 2007 and was completed in May 2008, after less than 12 months of TBM progress and to achieve a final breakthrough 46 days ahead of the contractual deadline.
The headrace TBM, after its recent breakthrough, is to be dismantled and the finishing works completed by January 2010. Breakthrough of the Beles tunnel drives follows completion of the 26km long Gilgel Gibe II headrace tunnel. Subcontracted again to SELI by Salini Costruttori and also for the Ethiopian Electric Power Corporation (EEPCo), the tunnel was bored by two 6.98m double-shield TBMs advancing from the opposite portals and lined with 25cm thick hexagonal precast concrete segments.
Work started in mid-2005 and proceeded according to schedule despite the generally adverse geological conditions until in October 2006, after boring 4.2km of tunnel, the intake portal TBM encountered a fault that spewed flowing mud under 40 bar pressure that pushed the TBM back and crushed the last meters of lining installed. Two years of hard work and special measures were necessary to recover the situation. The rescue work included 230m of bypass tunnels; removal of 39,600m3 of mud out of the tunnel, and drilling of 1,600m of drainage/exporatory holes
Pic 13

The finished product

Eventually, the TBM had to be dismantled and reassembled a few hundreds meters behind the fault. From there, it started again in August 2008 and followed a new alignment to bypass the fault.
In the meantime the second TBM driving from the outlet portal advanced regularly for more than 18km, despite hot water inflows of up to 54°C encountered and passing through several fault zones.
The two TBMs finally met on June 6, 2009 and the tunnel was ready for hydraulic testing in September 2009. 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. Considering the very adverse geological conditions - the 40 bar mud pressure encountered in the major fault, the hot water inflows and the consequent tough working conditions - the Gilgel-Gibe II tunnel is considered one of the most difficult tunnelling projects ever excavated. SELI is well pleased to have tunnelling on both projects now complete.


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