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

Dashing through the snow

An advanced form of the 'white-tank'waterproof concrete system is helping speed up tunnelling work on the new Swiss high-speed railway. Adrian Greeman reports.

As the first snows fell in Switzerland last month Meier & Jäggi, contractor on the new Thunstetten railway tunnel, remained unperturbed. Warm water in the concrete mix, antifreeze additives and insulated curing covers mean work can continue down to -10ºC.

Nor does the weather affect important crack sealing work under way some distance inside the tunnel. Temperatures within the cut and cover tunnel are above freezing and with backfill already in place above, are likely to stay suitable for injection of a special acrylic sealant for a while yet.

'If anything, the cool conditions are better because the concrete contracts and opens up the cracks to their full extent, ' says Martin Smith from specialist concrete firm Rascor. 'Crack sealing might have to stop for excessively frosty conditions but it is not on the critical path. '

Both operations are important however. The new tunnel, an 8. 75m high double arch enclosing two 890m parallel lengths of twin-track line, is being built by the so-called 'white-tank' method which creates a completely impermeable concrete for direct waterproofing. No membranes or other sealants have to be applied.

To limit cracking from shrinkage, concrete quality both in the mix and the placing is critical for white-tank. Recent engineering standards in most German-speaking countries, where the system has been developing, limit cracks to 0. 3mm per metre length. No crack should exceed 0. 2mm.

At Thunstetten, the basic process goes a step further. Rather than rely on reduced cracking, the cracks which are present in even the most carefully made concretes are deliberately induced at fixed points.

These are permanently sealed with a special flexible resin, injected into the concrete joints along embedded channels.

Client Schweizer Bundesbahn (SBB - Swiss Railways) is watching this new method with interest. It is responsible for numerous tunnels, old and new. Making and keeping them watertight is difficult but important to avoid numerous operational and maintenance difficulties.

Water spray from ceilings can affect overhead catenaries, causing excessive wear; dangerous icicles, sometimes metres long, can build up in tunnels; and flooding of the floor can reach dangerous levels.

Most of SBB's new tunnels are on a 46km stretch of line for the Bundesbahn 2000 project, a programme of line renovations and some new sections for 200km/h trains. The project will cut journey times on many of the major routes in Switzerland, particularly between Geneva, Bern, Basel and Zurich.

One or two lengths are mined tunnel but most are shallow cut and cover sections. For these SBB is using a number of different waterproofing systems including bitumen, bentonite-fabric and PVC membranes.

Thunstetten, in the Emmethal valley, is the only double tunnel section, where a local line is also being taken underground. The advantages of the new white-tank method are already clear says Smith, an expatriate British engineer now working in Switzerland.

'The system is much faster and less cumbersome than covering the tunnel with membrane, ' he says. 'It takes waterproofing off the critical path because injection, though it must wait for the concrete to age sufficiently, can then be done at almost any time. '

Because the work takes place inside it is more or less weatherproof. Laying and welding fabrics requires special formwork roofing to put operations under cover and rain can stop work.

For Meier & Jäggi the advantages are speed and flexibility, says project manager Walter Kummer. His firm, working with joint venture partner Zschokke Locher, has a SFr39M (£16M) contract for a 3km stretch of line including the tunnel.

In particular, he says, the earthmoving can be done much more easily because intermediate work stages are not needed on the tunnel.

'We can reduce or even eliminate rehandling and stockpiling of fill because the backfill operation takes place just three sections behind the formwork, ' he says.

Lining and then placing protection over the easily damaged waterproof membrane before backfill would interfere more with operations.

'One problem with linings is that backfill can damage them, ' adds Smith. 'And if you do get a leak it is hard to find the source because water travels along the void under the lining and may emerge a distance away. '

Kummer says another limitation is also removed. 'Conventional' white-tank is limited to 12m long pours because of concrete shrinkage. But at Thunstetten he has been able to use an 18m pour, increasing the distance achieved with a oneweek reinforcement and pouring cycle.

'We would have used 24m but wanted to be cautious on this first use, ' he says. But for the next project he will be more confident.

These time savings mean the contractor's lump-sum earthworks deal will pay off. And Kummer believes the contract will come in on time, despite a six-month delay at the beginning because of concrete equipment supply problems.

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.