WORK HAS started on the main access shaft for tunnelling on the 57km long Gotthard base tunnel of the Alptransit railway in Switzerland.
Swiss joint venture contractors Murer, Zschokke Locher, CSC and Marti are building the 8.3m diameter, 800m deep shaft at Sedrun in the Swiss Alps, along with South African specialist Shaft Sinkers, which has 30 years of experience in sinking deep shafts in the gold and mineral fields.
The shaft is being sunk through the heavily fractured and unstable phyllites, schists and gneisses of the 3km wide Tavetch formation and will not be easily tackled by the TBMs planned for 50km of the 57km long twin main drives. Instead, drill and blast will be used, with excavation expected to take substantially longer.
The working chambers formed at the bottom of the shaft will be the start point for several tunnel faces which will work outwards. The most difficult and slower section will be 2km of the twin bores northward, while somewhat more easy ground will take the drive 4km southward.
A preparation contract began two years ago, including tunnelling of a 1km long access adit, formation of a high chamber for the shaft head, and drilling of a ventilation shaft through to a valley on the other side of the mountain.
Initial work for the shaft proper is being carried out from a single deck platform suspended by cables running from temporary wind- ing motors via a steel frame over the shaft head. This involves excav- ating an 80m deep section and lining it with concrete at a rate of around 1.5m a day, which will make room for a much bigger 90t weight five stage platform.
This has a cable and equipment store at the top level, three work decks for the concrete shaft lining and a bottom deck for wall cleaning and preparation. It will move on a hoist which will later be used for the main excavation
In a 20m deep space below, the drilling and explosives crew will use a six boom vertical jumbo developed by Shaft Sinkers with Atlas Copco to place explosives that will give 3.2m to 3.5m advances. During blasting, the platform is pulled about 60m to 100m back up the shaft and the men come back up to the surface.
The Jumbo will also be used to drill and grout up a 42m deep spiral pattern of nine cased holes, fanning 11degrees outwards. The shaft is progressed 36m before each grouting.
It is hoped that once work is under way, an advance rate of 4.5m to 6m a day can be achieved, if the rock is sound and bolting can be limited. A rock bolt and shotcrete treatment will provide initial support with an insitu shutter poured lining above. For this a fast setting concrete is needed, giving 6N/mm2 in eight hours - because the shutters may be stripped in 12 hours.
Finally there is a temperature conditioning chamber for workers, needed because of anticipated high rock temperatures at the base tunnel level, possibly as high as 48degreesC. Even with cooling working temperature will be about 28degreesC. But in winter, temperatures can fall as low as -30degreesC.