World’s largest and deepest tunnel, the Gotthard Base Tunnel which revolutionised Europe’s freight transport, runs through the Alps in Switzerland. It lies at the heart of the Gotthard axis and constitutes the third tunnel connecting the cantons of Uri and Ticino, after the Gotthard Tunnel and the Gotthard Road Tunnel.

Its purpose was to increase local transport capacity through the Alpine barrier, especially for freight and more specifically to shift freight volumes from trucks to freight trains. This both significantly reduced the danger of fatal road crashes involving trucks and reduced the environmental damage caused by heavy trucks.

The tunnel provides a faster connection between the canton of Ticino and the rest of Switzerland, as well as between northern and southern Europe.

Years of construction proved fruitful at last with Italy and Switzerland becoming closer neighbours post its construction.

Construction
It took powerful drilling machines, precise monitoring solutions, reliable instrumentations, experienced manpower and efficient Tunnel Boring Machines to build the Gotthard Base Tunnel. With slightly different length, the base tunnel comprises two bores with two track crossovers and multiple interconnections for foot access between the bores. To carry out the project, intermediate shafts were sunk to allow for simultaneous tunnelling on several faces to become integrated in the finished structure for servicing and emergency access. Strict environmental control was in place for the work sites, the material supply chain and for the disposal of spoil.

The tunnels were mainly created by tunnel boring machines, accompanied with drilling and blasting. The final breakthrough in the east tube of the Gotthard Base Tunnel took place in October 2010. It took nearly 14 years and 2,500 workers to connect the two ends of the tunnel. This final breakthrough in the west tube was completed in March 2011.

AlpTransit Gotthard AG was responsible for the construction of the Gotthard Base Tunnel and, it is a subsidiary of the Swiss Federal Railways (SBB CFF FFS). The cheapest tunnel design could have been the one having two single tracks for movement in either direction but, then, it wouldn’t have provided any emergency evacuation which is unsafe. Alternatively, three tube systems, somewhat similar to the English Channel could have been used where one of the two tubes would be for each direction while the smaller one for evacuation during emergencies. This small tunnel could have been then connected to the main ones through lateral tunnels.

The cheapest option was unsafe while the safest option was too costly. So, a mid-way design was made that had two separate tubes, one for each direction and connected through passageway every 325 m (1066 ft.). These passageways are the emergency exits.

There are two crossover points at Sedrun and Faido along with tracks trains to switch tunnels. In case of emergencies, the computers in the command centre divert the trains to the nearest emergency evacuation so that the passengers can exit safely. Apart from this, massive ventilators have been installed to pull in fresh air while throwing out the smoke.

To lessen the construction time up to half, tunnelling was done from each direction while four access tunnels were built to start construction from four different places simultaneously. which are: Erstfeld, Amsteg, Sedrun and Faido. The fifth site was also added at a later stage at Bodio. This approach sped up the construction by 6-10 years. Not only this, all the sites had its own base camp, concrete factories, water treatment centre, living quarters for workers, cafeteria and a mass transit system.

24 million tons of rock was excavated while drilling the tunnel which was then sent to the concrete factory at base camps to produce 7.5 million tons of concrete for tunnel lining. To navigate through the mountains’ interior, elevators, trains and buses were used. The drilling, digging and blasting was continuously done 24/7 for 12 long years using extensive tunnel boring machines of 35 ft. in diameter.

Gotthard Tunnel Boring Machine
Four Herrenknecht Gripper TBMs, each over 1,400 ft. long, were used in the construction of the tunnel. Each TBM was composed of 90,000 parts and cost around $21 million. The machines were locked at their place with gripper pads that pressed against the tunnel walls. The hydraulic arms pushed the 9.5 m long cutter head forward. As the cutter head moved forward, it ground the rocks which were pushed outside through a conveyer belt. These rocks were then converted to concrete at base camp concrete factories to be re-used in the inner-lining of the tunnel.

Just behind the cutting wheel, there’s a set of robotic arms to ensure that loose rock does not fall. The TBMs could dig only hard rocks while the soft ones were either drilled through or blasted off.

Safety
Wayside train monitoring systems are fixed close to the track that transmits a range of measurement data to an analysis system to check irregularities in passing trains. If any irregularities are encountered, the information is sent to rail service staff and to the wayside train monitoring system intervention centre in Erstfeld. According to the priority and nature of the irregularity, certain measures are adopted.

There are two stop stations for emergency evacuations. Rolling stock is fitted with emergency features that enable trains to reach emergency stop stations even after a fire has broken out in the train. If the train stops outside the emergency stop station, positive pressure can be applied to the adjacent tunnel tube. Apart from this, there are cross-passages into the adjacent tube every 325 m, which allows for fast access into a safe area.

There are two ventilation units in Sedrun and Faido as well as 24 jet fans at the tunnel entrances to supply fresh air and extract smoke in case of an accident.

Country
1999-2016: 17 Years
57.09 Km
$12,500 Million
40,00,000 cubic meters
3200 Km
20 minutes
AlpTransit Gotthard AG

There is also an open water conduit system in each tunnel tube which is fed with 5 litres of water per second (continuous supply) so that contamination and hazardous substances can be transported to the retention basins in front of the tunnel entrance.

References
» https://www.railway-technology.com/projects/gotthard-base-tunnel/
» https://www.encardio.com/blog/science-behind-the-megastructures-gotthard-base-tunnel/
» https://www.bbc.com/news/world-europe-36416506
» https://ambergengineering.com/references/projects/gotthard-base-tunnel/
» https://www.herrenknecht.com/en/references/referencesdetail/ gotthard-base-tunnel/

By -
Tuhina Chatterjee, Associate Editor - Civil Engineering and Construction Review

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