Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Full bore The New Austrian Tunnelling Method tunnelling has had a chequered past. NCE looks at its use on current projects in Europe and previews plans for tunnelling on the CTRL.

Sugar free

Adrian Greeman checks progress on the Alptransit railway in Switzerland.

Switzerland's much delayed Alptransit trans-European rail links could be further frustrated this autumn if a national referendum on its financial restructuring cancels the 57km world record length Gotthard tunnel.

But the good news for railway engineers is that a 5.5km long test tunnel at Faido in the south part of the route has been completed early after finding better than hoped for rock conditions. And at Sedrun, 1,350m up in the Swiss Alps, sinking of a vital deep shaft using rock anchors and shotcrete has begun.

The test tunnel result is 'almost miraculous', says Peter Zbinden, engineer for SBB Swiss Federal Railways. The tunnel was being driven into the Piora Mulde syncline, a narrow but potentially difficult band of rock where so-called 'sugar Dolomite' was expected. This decomposed sedimentary rock has the texture of running sugar and would need extensive treatment.

Two years ago a drive 350m higher than the main base tunnel route encountered this rock, and a 100bar pressure inflow of rock and water filled the bore with fine white sand. Designers feared the worst in the 230m thick band lower down.

But four major test drillings, curving to horizontal down from the end chamber, all proved the opposite. The rock is 'a stable Dolomite marble or Dolomite/ anhydrite' and 'not subject to water pressure'. So good was the outcome, that an extra shaft and chamber were cancelled as was further drilling for investigation and grouting.

Engineering attention has therefore switched to the Sedrun valley halfway along the route of the Gotthard base tunnel - a second area of anticipated difficulty through the 3km wide Tavetch sedimentary zone. Sandwiched between two much thicker massifs, the rock is heavily fractured and unstable and will not be easy for the TBMs planned for 50km of the 57km long twin main

drives. More careful drill and blast excavation is anticipated.

To make an early start, a deep shaft is already being sunk. The working chambers at the bottom will become the start point for several tunnel faces working outwards. A preparation contract began two years ago, which included a 1km long access adit, formation of a high shaft head chamber, and drilling of a ventilation shaft through to the other side of the mountain.

The Swiss AZS joint venture contractor of Murer Zschokke Locher, CSC, and Loretz used an Atlas Copco H135 boomer for the drill and blast tunnel and chamber. A 2.27m diameter Wirth TBM made a pilot tunnel for the 45degrees air shaft now being widened by further drill and blast.

The contractor is also responsible for building a base camp which includes accommodation, a concrete batching plant, offices and a tourist information centre for Swiss Railways. There is also a small railway to bring in materials and avoid disrupting the picturesque Sedrun village.

A temperature conditioning chamber for workers has also been built. 'We expect that rock temperatures will be high at the base tunnel level,' explains Swiss Railways' Werner Beerle, 'possibly as much as 48degreesC. Even with cooling, working temperature will be about 28degrees' (NCE 17 September).

But in winter, workers will come off shift to temperatures as low as -30degrees. 'They need to adapt to the outside like a diver changing pressures,' explains Beerle.

Work on an 800m deep, 8.3m diameter shaft from the central chamber has already begun and will provide the main access for the tunnelling. South African specialist Shaft Sinkers, which has 30 years' experience in the gold and mineral fields, has been brought in.

'Actually 850m is not so big,' says operations director Peet Nel. 'We have done shafts in southern Africa down to 2,650m.' Rock anchors and shotcrete provide temporary support before the shaft is permanently lined with reinforced concrete.

His only problems, he says, result from the job being 'far away from our normal suppliers'. Over the years the firm has developed specialised items of equipment, many supplied by South African firms, and some 60 containers of equipment have had to be delivered.

'Much of this is not necessarily high technology, just things like more robust hose connections,' he explains. 'We use two inch (50mm) airlines and you can't afford to have those go down there with 27 men working.'

A drilling and explosives crew is working with a six boom vertical jumbo drilling rig developed by Shaft Sinkers with Atlas Copco which Nel expects will give an advance rate of up to 6m a day. The jumbo is also used to advance grout a 42m deep spiral of holes, fanning 11degrees outwards.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.