The $56m, 7.8km long Bømlafjord Tunnel is due to be handed over to the client, Statens Vegvesen (the Norwegian National Road Administration) this month, six months ahead of schedule. The tunnel, thought to be the longest undersea tunnel in the world, links the island of Føyno with the Norwegian mainland south of Bergen.
The tunnel was driven from two fronts using conventional drill+blast techniques. NCC Norge, in conjunction with NCC Anläggning AB (NCC’s Norwegian and Swedish construction divisions respectively), constructed the tunnel from the island of Føyno to the mid-point of the tunnel, 260.4m below the seabed. The other half was driven from the mainland by the project division of Statens Vegvesen.
The rock was found to be of good quality, with the exception of a fault that was encountered at an early point in Staten Vegvesen’s contract, leading to a considerable inflow of water. However, grouting with micro cement and reinforcing the rock with shotcrete proved to be adequate measures to cope with the conditions. Otherwise, there were no great inflows of water, but grouting, rockbolts and shotcrete were applied as required. An average advance of 60m/week was achieved, with a maximum advance of 90m towards the end of the drive.
Breakthrough took place on September 2 1999, six months ahead of schedule and the tunnel will be complete in May. When T&T International visited the site, measures were being taken to render the tunnel frost resistant before the electrical installation and the final handover of the tunnel.
It forms part of the Trekantsambandet (Triangle Connection) project that is the largest infrastructure project currently under way in Norway. The scheme will provide a fixed link to the mainland for an island community of 30 000, which is currently dependent on ferry connections.
The $203m Trekantsambandet project, which includes a number of bridges in addition to the Bømlafjord Tunnel, is scheduled for completion in February 2001.
