The three island communities of Stord, Bømlo and Fittjar were connected to the mainland in the so called "Triangle Link Project" consisting of 3 bridges and the Bømlafjord sub sea tunnel. Bømlafjord is the outer part of the famous Hardangerfjord. Client was Norwegian Public Road Administration, Hordaland (Statens Vegvesen, Hordaland). The Bømlafjord sub sea tunnel is 7.9 km long and on it's deepest reaches 260 meter below sea level. The construction started in September 1997 and breakthrough was almost exactly 2 years later in September 1999.
The Bømlafjord subsea tunnel and the Triangle Link Project.
The part of the tunnel shown red in this figure was excavated by the Norwegian Public Road Administration's in house contractor. The white part was done by contractor NCC-Eeg Henriksen. Preinvestigations consisted of field mapping, seismic survey and coredrilling. Two steered coreholes were drilled prior to the excavation. One 430 meter long from Otterøy, and one 900 meter long from Valevåg (steering and positioning by Devico). In the 900 meter long hole from Valevåg, a massive 60 meter wide moraine zone appeared approximately relative to tunnel chainage 4400 (see figure). The drillstring was then withdrawn 400 meters and a new hole drilled from this position and 30 meter deeper under the problem area. (The coreholes can be seen in the figure as a fork-like line going down from Valevåg). Hole number 2 showed favourable conditions and the tunnel was due to these results lowered another 35 meter. Construction through this area later went on with no technical problems except from minor water leaks of 50 liters/min that was sucessfully grouted. During the construction and in the subsea areas, probedrilling was systematically implemented to cover at all times a minimum of 3 blasts (12-15 meters) ahead of the production face. For every 3 blasts a curtain of 3 to 5 probeholes length 24 meters, were drilled to controle rock conditions, rock cover and water. Geology: The southern 50% of the tunnel with Valevåg and mainland Sveio is dominated by basement rock of gneiss and micaschist. The northern part with the islands of Føyno and Otterøy consists of Cambro-Silurian metamorphic cover rocks; greenstone and phyllites called the "Hardangerfjord cover complex". The whole tunnel consisted of rock with good quality and very little water leak. In a bid with several of Norway's largest geotechnical consultant companies, Tunnel & GeoConsult won the contract as Norwegian Public Road Administration's on site geoconsultant during the 2 year construction period.
Read our article in Tunnels & Tunneling March 2000.
Figure 2: Bottom reached. 260 meter below the Bømlafjord and till then the deepest tunnel in Europe. Construction manager Sigve Martinsen (left) and project manager Henrik Schaatun register this milestone in the project.
Figure 3: Example of support drawings with mapped geology, rock mass classification and permanent support. Each drawing covers a sequence of 200 meter along the tunnel. The geological mapping is done continuously after each blast. The drawings are updated daily and given to the rock works management for implementation.
List of norwegian subsea tunnel: http://no.wikipedia.org/wiki/Undersj%C3%B8iske_veitunneler_i_Norge