First blast launches Gotthard base tunnel

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 80m 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 (RG 2.99 p101), AlpTransit Gotthard AG has a mandate to pursue the construction of twin north - south rail tunnels. Work on the adits at Bodio and Erstfeld 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 was 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 stopped tunnelling dead, and having been bitten once by bad geology during the construction of the nearby Furka base tunnel, the Gotthard engineers were understandibly 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 tunnelling, 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 build the 800m deep Sedrun shaft. ASS comprises South African mining specialists Shaft Sinkers International working with Swiss tunnelling firms Murer, Zschokke Locher, Marti Tunnelbau and CSC Impresa Costruzioni.

A 990m access tunnel and gallery, header cavern and 80m 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.

Rail access

Because of the great depth of the running tunnels at Sedrun (nearly 1 km beneath the level of the FO line), it is impossible to provide the kind of direct rail-based excavation used on the Channel Tunnel. With the need to minimise environmental impact, provision has been made to transport as much material by rail as possible. A 2·3 km branch has been built from a junction with the Furka-Oberalp Railway to the east of Sedrun station. This rack-equipped line drops down sharply into the Vorderrhein valley below the town to sidings alongside the river, serving the construction depot at the mouth of the Sedrun adit.

At the same time, SFr120m has been spent on upgrading the FO and RhB route from Chur to Sedrun for construction traffic, including signalling, power supplies and extra passing loops. The objective is to bring 90% of materials for the tunnel by rail, most of which will be cement. Much of the tunnel spoil will be reused as aggregate for concrete for the tunnel, and for strengthening a number of local bridges, so very little will be brought out by rail.

Accommodation for the 130 workers building the shaft and a site office and exhibition centre have been provided a little below Sedrun town, but as it is a considerable drop down into the valley floor construction site, an SBB-owned funicular railway has been installed to transport workers up and down the hillside.

Once the shaft has been sunk into the rock down to an altitude of 547m above sea level, expected by December this year, the job of creating workfaces for the running tunnels will begin. First, a cavern will be built at the bottom of the shaft to provide logistics space for moving equipment and spoil, and ASS is contracted to have this ready by August 27 next year.

Work will then move on to create the connecting passageways and initial tunnel sections, ready for the start of tunnelling through the difficult geology to begin in early 2002. Working conditions will also be unpleasant - the high ambient rock temperature combined with water seepage will make the workface hot, humid, noisy and muddy. Adequate ventilation and rotating shift working will help minimise prolonged discomfort, but if necessary workers will be supplied with special water-cooled worksuits. Drainage will also be difficult, as water must not only be pumped up the access shaft, but also pumped uphill from the workfaces.

The twin tunnels and cross-passages running 2·1 km north descend at a grade of 0·4%, and are due for completion by 2003; these should connect with an 11 km drive from the Amsteg access sometime in 2006.

Meanwhile, the Sedrun workers will have opened a second face, boring the tunnels south. A 4·5 km section will be dug by 2005, initially climbing at 0·4%, it will then drop at 0·7%, connecting with the 15 km northwards drive from Faido in 2007. Fitting out will have already begun on the outer ends of the tunnel, where construction will be easier and consequently faster. It will move to the central section later - this work should be completed in 2010, ready for opening of the whole base tunnel in 2011.

• CAPTION: Preparatory works have begun for driving twin rail tunnels 2500m below the peaks of the mountains around Sedrun (left). Jakob Blickenstorfer, chief engineer for the Sedrun section, presses the red button to set off the first explosive charges for the 800m access shaft (above)
• CAPTION: Fig 1. The Gotthard Base Tunnel passes through difficult geology, and at great depth beneath the Alps. Working simultaneously from five work sites will reduce construction time to less than 10 years, with the first train due to run through in 2011. In the next 12 months work will get under way at the two access adits of Amsteg and Faido, and at the southern portal at Bodio
• CAPTION: The roof of the cavern deep under Alp Tgom soars upwards to provide the headroom needed for winding gear and other construction plant
• CAPTION: Fig 2. The access shaft at Sedrun will be used to construct a 6·6 km central section of the tunnel, including crossovers, and will eventually be used to ventilate the base tunnel
• CAPTION: Rail access to the construction base at Las Rueras has been provided by building a branch from the Furka Oberalp main line
• CAPTION: A funicular railway (right) has been built to provide easier transport for workers between the construction site around the mouth of the adit tunnel at Las Rueras on the lower level (left) with the accommodation, exhibition and office buildings in Sedrun, further up the side of the valley

Une première explosion pour le démarrage du tunnel de base du Saint Gothard

Le 4 février, lors d'une cérémonie, a eu lieu la première explosion souterraine destinée à marquer le commencement des travaux de l'un des plus grands et des plus coûteux projets ferroviaires du monde. A Sedrun, au plus profond des Alpes suisses, les travaux ont commencé par le percement d'un puits vertical d'accès de 800m de profondeur menant au niveau du tunnel de base du Saint Gothard, long de 57 km. Une cavité devant être excavée au pied de ce puits pour le mois d'août 2000 permettra de démarrer deux chantiers de percement du tunnel principal tandis que d'autres tunneliers commenceront leur travail à partir des portails nord et sud. Andrew Hellawell était invité à la cérémonie de lancement du chantier