Norwegian contractor Mesta has holed through on a drill and blast drive for the 7.78km long Eiksund subsea tunnel after more than two and half years of excavation.
Breakthrough in early February was at a depth of 245m, 42m above the tunnel’s deepest point. Work on the west coast project started in June 2004 and is on track for completion this December, being on budget so far, project manager Jonny Madsen told T&TI. The contract price agreed in 2004 was US$54M (2004 currencies).
Almost half the tunnel runs below the sea, from the nearby Steinnes on the mainland to the island of Eika via another island, Yksnoya. A 405m long bridge will link from Eika to the town of Eiksund on the mainland, which gives the tunnel its name.
The tunnel descends from the east (Steinnes) side for 4.7km at a grade of 7.6%, the cross section being typically 63m² to allow for two contraflow lanes of traffic.
On the west side the length is about 3km and the grade steeper, at 9.7%, which exceeds European norms. As a result, the typical cross section along that stretch is 87m² to allow for three lanes of traffic – one lane descending and two rising to cater for heavier, slower vehicles.
Excavation began on the east side, near Steinnes, of the longer, shallower bore. A year later, Mesta began to drive the second face from the Eiksund side to open up the wider, steeper tunnel stretch. The tunnel was driven mainly by drill and blast through gneiss with some hammer excavation in parts.
Madsen said the tunnelling conditions were mostly good, and there being only two main stretches of weaker, fractured rock totalling about 100m in length. There were also a few short stretches with a little clay present in the fractures. Of the relatively little groundwater experienced, it was encountered at the west side below the islands.
In the best week almost 73m was excavated. The faces typically advanced 5m per day in good rock, reducing to 2m-3m in poorer conditions. The number of holes per round varied from 100 on a face up to 140 for the bigger section, there having been 1800 blasts and 1250t of explosive used.
Tunnel lining was about 16,000m³ of steel fibre reinforced shotcrete, varying from 70mm-300mm in thickness, and about 40,000 rockbolts typically 3m-4m in length, though up 6m was used in places.
