The technique of boring using shielded tunnel boring machines (TBMs) and lining the tunnels with concrete segments is tried and tested. It has been employed successfully in hundreds of tunnel projects worldwide. Examples include building the tunnels under the English Channel and Stora Bælt in Denmark as well as the tunnels for the Copenhagen Metro.
The two TBMss will be assembled and start boring at the open shaft at Holma. The machines will then work northwards toward Triangeln and then on to Malmö C where they will be dismantled.
The boring machines can be closed at the front. In this way the boring can be done under full ground water pressure without water penetrating the machines during the process.
A tunnel factory
A better name for a tunnel boring machine would be a tunnel factory. A TBM bores, seals with concrete segments and removes the waste out of the tunnel.
The boring machines used in the Citytunneln project are around 120 metres long and comprises many units that include the cutter head, service trucks, conveyors and control cabin. The front part of a machine, the shield, is 10 metres long.
The tunnel grows step-by-step within the boring machine’s shields. The concrete segments are assembled here to make rings that, together, form a watertight concrete tube with an internal diameter of 7.9 metres and an external diameter of 8.6 metres.
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The TBM-shield and the first two sections of the Back-up unit. |
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The rest of the back-up unit. The TBMs total length is 120 meters. |
The boring machines are pushed forward with the help of hydraulic jacks. The rock is broken up and crushed to small pieces by a number of round discs reinforced with hard metal cutters being pressed against the rock with great force. The waste material then passes through the cutter head’s openings to a chamber. A transport screw then carries the boring mass for further transport out of the tunnel using conveyors.
Gradually as the boring machine progresses, the tunnel segments of concrete will be transported into the tunnel and joined together to form tunnel rings. As the segments are joined together, injection cement is pumped in between the tunnel rings and the surrounding limestone rock. The tunnel rings then form watertight tunnel tubes. The method is referred to as lining. Approximately 15 people per shift work inside the machine at the same time.
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The two TBMs, Anna and Katrin, used in the Citytunneln project. |
The TBM is driven by electricity, wich requires a transforming plant in the back-up unit. The power reguirement at full operation is about 4 MW: that's teh equivalent of 67,000 light bulbs or the power of 40 ordinary cars.