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Skim the text to find out why the drainage will be difficult.






 

The Gotthard Base Tunnel passes through difficult geology, and at great depth beneath the Alps. Working simultaneously from five worksites will reduce construction time to less than 10 years, with the first train due to run through in 2011.

Sedrun’s access shaft was the scene of the first blasting work for the Gotthard base tunnel on February 4.

Air horns sounded, prompting workers and invited guests to cover their ears. Jakob Blickenstorfer, chief engineer for the Sedrun section, pressed the red button. We felt shockwaves in the cavern floor as 80 m below explosives detonated. The deafening rumble gave way to the applause of everyone present – Alptransit’s Sedrun access shaft was formally under way.

Although excavations were started in april 1996, the first blast on February 4 1999 marks the start of construction proper on the Gotthard Base Tunnel. Not only is this now one of the most expensive railway projects under way anywhere in the world, but also one of the largest civil engineering projects of recent years.

With a funding package now secured by referendums last year, Alp Transit Gotthard AG has a mandate to pursue the construction of twin north-south rail tunnels. Work on the adits at Bodio and Erstfield which will form the outer ends of the new Gotthard tunnel will follow later this year and in early 2000. Priority is, for now, being given to tackling the hardest part of the Gotthard project – creating running tunnels through some of the most difficult geology in the Alps.

The layer of weak rock known as Piora Mulde had been expected to cause the greatest headaches, but exploratory bores from Faido showed that this fear as unfounded.

The rock is a type of sugar-crystal dolomite, varying enormously in its structure from water-bearing rock, through a thixotropic ooze, to almost liquid. Encountering such rock types could have stoped tunneling dead, and having been bitten once by bad geology during the construction of the nearby Furka base tunnel, the Gotthard engineers were understandably keen to know what they were letting themselves in for.

Working from a short adit into the hillside near Faido, a number of trial bores were drilled down to the area the running tunnels would occupy. These showed that the proposed alignment would miss the worst strata.

Having boosted confidence over the Piora Mulde, there remained the matter of the Tavetsch Massif – a thick section of heavily-faulted and unstable rock through which the tunnel would pass, and which could not be avoided. Four trial bores were made from around the site of the proposed access shaft at Sedrun, and these suggested that tunneling, though difficult, would not be impossible, and injection grouting should give sufficient strength to the surrounding rock to use either blasting or boring machines.

Arbeitsgemeinschaft Schacht Sedrun was set up as a consortium to built the 800 m deep Sedrun shaft. ASS comprises South African mining specialists Shaft Sinkers International working with Swiss tunneling firms Murer, Zschokke Locher, Marti Tunnelbau and CSC Impresa Costruzioni.

A 990 m access and gallery, header cavern and 80 m deep hole beneath it have been machined into the heart of Alp Tgom on the south side of Las Rueras from Sedrun. Work began at this site as long ago as April 1996, but with construction work proper now beginning, the work site has been expanded and upgraded.

 






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