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Sizing up the challenge


Tunnels are becoming bigger and bolder, and three record breaking undersea projects will be grabbing the headlines over the next few years. Dave Parker reports on the very different challenges and solutions being called for as engineers push technology to the limit in Norway, Turkey and China.

Eiksund, Norway

FIRST OUT of the starting blocks is the £32M, 7,765m long, classic hard rock single bore Eiksundconnection road tunnel under the Vartdalsfjord in western Norway.

Main contractor Mesta has already drilled and blasted more than 1,200m into hard banded gneiss, but at a rate of progress around 50m a week it will be more than 15 months before the tunnellers reach their deepest point, nearly 300m below the surface of the fjord. It will go down in tunnelling history books as the deepest ever sub-sea mark.

In practice, travellers using the crossing after it opens in late 2007 will face less of a rollercoaster ride than first appearances suggest. Despite the depth, the tunnel has a maximum gradient of just 10%.

Mesta project manager Svein Skeide says that everyone is aware of the risks involved in tunnelling at such depths.

'We drill up to eight control holes 30m or so ahead of the working face and check for water penetration. If it's above a minimum we drill more holes and inject grout.

'Then we carry out four drill and blast operations, which move us on about 18m before drilling another set of control holes.' Minimum thickness of rock between the floor of the fjord and the tunnel will be around 50m. Skeide says that so far water ingress has been much less of a problem than expected. Rock bolting and high pressure grouting needs are lower than planned for, as is the consumption of sprayed concrete for the lining.

Surprisingly, perhaps, in the wake of recent tunnel fire disasters, no sprinklers will be fitted. But polypropylene fibres are included in the sprayed concrete lining to minimise explosive spalling and resulting damage to the lining integrity in the event of fire.

To aid evacuation of vehicles from the tunnel, the bore will be widened to three traffic lanes where gradients near 10%, while two lanes are considered sufficient where the gradient is less than 8%. This is intended to provide drivers with sufficient room for three-point turns, and to accommodate different vehicle speeds as drivers make for the safety of the portals.

As with most hard rock tunnels the only way out will be back along the bore: there will be no refuges or escape tunnels.

But again turning points are provided: three for heavy goods vehicles, and 15 for smaller vehicles.

Chongming, China

IN SIX years time Shanghai, will be home to not only the largest ever bored tunnel but one of the largest tunnels of any type in the world. With twin 15m diameter bores and an overall length of 9km, the Chongming South Channel Tunnel is part of the 25km long Shanghai-Chongming Expressway linking the fast growing megacity to the underdeveloped island.

Chongming is China's third largest island, and creating a fast, six lane motorway link to 1,000km 2 of potential lebensraum will be the municipality's biggest ever infrastructure project, estimated to cost £775M.

Making up the link will be the 10km Chongming North Bridge, and a 6.5km stretch of elevated motorway across the intervening Changxing Island.

Tenders have just been invited for the supply of the massive earth pressure balance tunnel boring machines (TBM) needed, which will be working in soft estuarine silts with as little as 9m separating the crown of the tunnel from the swirling waters of the Yangtze River.

Design sponsibility for the project falls to a multinational joint venture involving Halcrow, Parsons Brinkerhoff (Asia), Shanghai Tunnel Engineering & Rail Transit Design & Research Institute (STEDI) and The Third Harbour Engineering Investigation & Design Institute (THEIDI). Halcrow is also involved in a different design joint venture with the Chongming North Bridge (NCE 30 September 2004).

Exact details of the proposed fi re precautions and means of escape have not been finalised by Halcrow. But the twin bores will be cross linked at regular intervals, and in an emergency people will escape into the other bore.

From there they could be rescued by emergency vehicles running either on the main three lane carriageway or in an emergency lane below.

Precast oncrete segments, 650mm thick, will form the bore. The soffits, potentially the areas most vulnerable to a vehicle fire, will be protected by false ceilings and ventilation ducts.

Marmaray, Turkey

AT 60m below sea level, the undersea section of the Marmaray rail tunnel across the Bosphorus may not be as deep as Eiksund, but this will be the deepest water in which an immersed tube tunnel will ever have been assembled.

Japanese-Turkish joint venture design and build contractor Taisei-Kumagai Gumi/Gama Nurol is drawing up detailed plans in Singapore which are still largely under wraps. But Capita Symonds director of civil engineering Richard Lunniss, who headed up the design team for one of the unsuccessful Turkish/Japanese joint venture bidders, says the challenges are formidable.

'The 1.8km immersed tube section is connected to conventional bored tunnels on land at each end - and these connections have to be made in the Bosphorus. The sea bottom is very soft and prone to liquefaction during earthquakes, so it will have to be improved before the trench for the tunnel can be excavated.' After that, he says, the units have to be sunk and placed in a very busy waterway 'with strange currents and variations in salinity with depth'.

Normally, a surface layer of less dense brackish water flows south from the Black Sea towards the Marmora Sea, where sea level is up to 750mm lower.

Above the sea bed a denser, more saline layer is driven north by the density differential. Where the two layers interface is a substantial zone of turbulence.

Even units up to 90m long and weighing several thousand tonnes will not sink in a totally predictable pattern.

It might be necessary to 'winch down' each unit via precisely located anchorages, Lunnis says, adding: 'At 60m divers have limited endurance.

We would have considered using robotics as much as possible.' Capita Symonds planned to make the tricky connections between the bored tunnels and the immersed tube just offshore.

Massive blocks of concrete would have been cast around each end of the immersed tube section, linking them to trenches on each shore. These blocks would then have been bored out from the land to form the fi nal link.

These 'underland' tunnels are no mean projects on their own, with a total length of nearly 12km. The project also includes the upgrading of 63km of existing suburban networks either side of the Bosphorus.

So far the only news coming out of the Singapore design office is that the contractor has opted for a US style steel/ concrete composite design for the immersed tube section.

Escape routes will almost certainly be through connecting doors into the adjacent tube. Fire protection may involve adoption of a new sprinkler system developed to control fires in liquefied petroleum gas (LPG) tankers and installed in tunnels on the new Betuweroute Freight Line in the Netherlands.

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