The B�mlafjord 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.
Figure 1: Schematic wiev of the tunnel.
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 Otter�y, and one
900 meter long from Valev�g (steering and positioning by Devico). In the 900 meter long
hole from Valev�g, 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 The drillstring was then withdrawn 400 meters and a new hole drilled from this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 Valev�g). 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 Valev�g and mainland Sveio is dominated by basement rock of gneiss and micaschist. The northern part with the islands of F�yno and Otter�y 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.
Figure 2: Bottom reached. 260 meter below the B�mlafjord and till
then the deepest tunnel in Europe. Construction manager Sigve Martinsen (left) and project manager Henrik
Schaatun register this milestone in the project.
. . . . . . . . . .
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 goes on continuously after each blast. The drawings are updated daily and given to the rock works management for implementation. |
|---|
.
.
.
.
.
.