Construction method - tunnelling
Tunnelling proceeds from the west with two tunnel boring machines working in parallel. The machines, which were manufactured specifically for this project, are operated by specialists.
Facts and figures
|(for the tunnel boring machines)|
|Length:||approx. 86 m|
|Weight:||approx. 1,990 t|
|Plate diameter:||10.87 m|
|Driving force:||87,000 KN|
|Driving power:||3,800 KW|
A disc cutter equipped with special bits rotates slowly while pressure is applied to the rock mass. The external diameter of the plates for the Finne Tunnel is around 11 metres. The space created by the machine is immediately converted into a tunnel tube using prefabricated concrete segments. The groundwater is either not lowered at all or only in the form of a funnel in the immediate working area. After the section is completed, the original groundwater level is restored. The excavated material is transported out of the tunnel by means of pumps or conveyor belts and prepared for depositing.
In order to be able to deal with the different rock formations, a combined hydroshield/hard-rock tunnel boring machine is used.
In loose rock: The first 1,500 metres or so, where the rock is loose and there is mountain water, are dealt with using a liquid-supported hydroshield tunnelling machine. A steel tube behind the cutting wheel hermetically seals the machinery's working parts from the rock. As a result, water management is not necessary. The loose soil is pumped out. The supporting liquid is separated from the excavated material and fed into the machine again.
In hard rock: After operating in hydroshield mode, the machines are converted for hard rock. The groundwater level is then lowered for the tunnelling machines by means of bore wells up to 98 metres deep. The material is transported away by conveyor belt. In the last 850 metres, the groundwater is below the level of the tunnel floor so water management is not necessary.
Tubing rings are installed in the tunnel behind the tunnelling machines.These consist of prefabricated concrete segments reinforced with steel and weighing around 12 tonnes. They are assembled to form a ring by an automatic machine, the erector. A single-skin tunnel is formed that secures the void created and ensures that no water gets in. Once the tubes have been created, the cross-passages, the concrete floor of the tunnel and the required infrastructure such as empty conduits and water supply lines for fire-extinguishing purposes can be installed. An electric narrow-gauge railway is being used to supply the tunnel with tube lining segments and other material and take the tunnel workers in and out.