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Bosphorus blockbuster

Next month the second element of the world’s deepest immersed tube tunnel will be sunk in the Istanbul Strait. Jackie Whitelaw reports on construction of the first intercontinental underwater rail link.

The gang’s all here! A bunch of the peripatetic global tunnelling industry are half a mile down a Channel Tunnel Rail Link sized bore gazing enrapt as the teeth of a TBM bite through the rock under Istanbul in Turkey.

There’s a Canadian, an Irishman, an American, a Dane, a couple of Japanese and four Turks, all chatting happily in English about how to improve progress from five rings a day to eight.

The 7.85m diameter tunnel is being driven through Trakya siltstone and sandstone a further 300m to a crossover that will be created using the New Austrian Tunnelling Method (NATM). The TBM will then travel another 300m to an underground station at Uskadar and then another 700m ultimately taking it out under the strait that splits the city to mate with the centrepiece of this enormous project - the 1.4km immersed tube crossing of the Bosphorus.

 

The immersed tube is set to be a world beater on a number of counts. It will be the deepest ever placed - at 58m, the eleven sections are the biggest ever tried at 135m long, 8.75m high and 15.3m wide. When complete it will create the first ever intercontinental rail connection.

 

Parsons rinckerhoff’s roject manager Daniel Horgan therefore has his worries.

 

‘There’s no doubt to me that immersed tubes are much more difcult than bored tunnels. Give me a TBM any day. As long as you pick the right machine you just put it in the ground and off you go.’ There will be five TBMs in the ground at Istanbul - four Hitachi Zosen slurry machines and one Lovat earth pressure balance machine which will be used in an area of softer soil on the European side, creating two twin track rail running tunnels.

 

‘There are just so many risks with immersed tubes, ’ Horgan says. They have to be cast, oated, sunk - in this case in one of the world’s busiest channels with 60,000 ships a year shooting up and down between the Black Sea and the Sea of Marmara. Current velocity in the strait can vary five knots (9.3kph) in a single day and part of the tunnel sits on the Analtolian Fault so must be protected against a 7.5 Richter scale earthquake.

 

An immersed tube crossing of the Bosphorus is the only way at the moment to connect the Asian and European sides of the city by tunnel - a scheme that has been dreamed of since Sultan Abdul Mecit first proposed an ‘underwater bridge’ in 1860.

 

‘Boring a tunnel would have taken it so deep, particularly to get the gentle gradient required for railway, that it would have been impractical’ says PB Tunnelling head Douglas Madsen.

 

‘The stations are already very deep. Sirkeci for instance is 45m below ground level.’ Istanbul has decided on a rail crossing rather than a third road bridge because the city is gridlocked with cars, coaches, trucks and hoards of yellow taxis. Twelve million people now live in this seething, exciting crossroads of a metropolis that joins Europe and Asia.

 

Three lane highways are regularly turned into ve by inventive drivers (Highways Agency network managers take note) and it can take an hour to make a journey that should take 10 minutes.

 

The plan is to encourage people on to the trains to cut the congestion and reduce pollution.

 

The tunnels being created through Istanbul are just part of the $3bn (£1.5bn) Marmaray project. This 76km rail route from Halkali to Gebze should take 1M people off the roads when it opens in 2011. The existing line is being upgraded to triple track, 37 stations are being refurbished, three new underground interchanges created and 13.6km of tunnel, which includes the immersed tube, are going in.

 

But for now there is some way to go. On the immersed tube, the first element of 11 has been successfully sunk in its trench just off Uskadar on the Asian side of Istanbul. The next is due to be placed in July.

 

Horgan’s concerns over the strait’s underwater tunnelare alleviated somewhat by the presence on the team of Walter Grantz. The 77 year old is a PB immersed tube specialist and was involved in the company’s initial studies for the crossing in the mid 1980s.

 

He takes us all down to Tuzla, 40km east of the tunnel site where the rectangular box sections for the immersed tube sections are being cast. On the way he announces: ‘This is the most difcult immersed tube in the world.’ While PB project manager Horgan takes that on board, Grantz explains that this is also why he is still working rather than sitting with his feet up in America. ‘I love immersed tubes and this one is very exciting.’ The elements are vast when you see them. They are being cast in pairs in two dry docks hewn out of hard rock. And the effort it took to create the docks is one reason why only the base slab and part of the walls of the tube elements are being poured there. The half nished elements are oated out to a jetty where the last stage of concrete can be poured. ‘This cut down considerably the amount of excavation required’, Grantz says.

 

Reinforcement is what you most notice in the dry docks.

 

There’s loads; some of it 40mm in diameter. Each element needs 1,400t so the nished cast weight of each is 19,000t.

 

That clearly helps with the earthquake protection. Out at the tunnel site under the 15.3m wide trench where the elements will sit is more protection in the form of compaction grouting.

 

‘The ground under elements eight to 11 contains layers that could liquefy. We’ve injected 2,778 grout columns, up to 8m deep, not an easy operation at depths of more than 30m, ’ Grantz says.

 

A couple of the vast elements are floating broodily at Tuzla, just, (there’s under half a metre draft), eager to be sent out to sea and away from the seagulls currently inspecting the workmanship.

 

They will journey first to a nearby island where the jacks that are used to join the elements together are added.

 

Then each will be taken up to Istanbul and when the weather and currents are right - only 3.5 knots, they will be sailed round and sunk into the strait.

 

‘The contractor has developed a sophisticated programme to forecast 48 hours ahead to give us a window for installation, ’ Grantz says.

 

Back in Istanbul on a jetty near Uskadar with the rst Bosphorus bridge not far away, Grantz and his colleague from Pacic Consultants Claus Iverson are gazing lovingly into the water where their rst element sits safely in place. Iverson is a veteran of Denmark’s Storebaelt crossing and is unfazed by the challenges ahead. Everyone is looking forward to creating their world beating tunnel. They are looking forward to putting this tunnel together. ‘And at the end of it we’ll have made possible the old dream of a railway from Project team and contracts Client for the Marmary Project is Turkey’s general Directorate of Railways, Harbours and Airports Construction (DLH) which is part of the Ministry of Transportation.

 

Engineer for DLH is Avrayconsult, a joint venture of four consultants in association with Parsons Brinckerhoff International. Lead jv partner and PB’s client is Japanese firm Pacic Consultants International. The other partners are Yuksel Proje from Turkey and JARTS and Oriental Consultants from Japan. The project is being nanced by loans from the Japan Bank for International Cooperation and the European Investment Bank.

 

There are three main design and build FIDIC silver book contracts.

 

The Bosphorus Crossing contract (BC1) covers tunnelling, the immersed tube crossing and construction of three big underground stations at Yenikapi, Sirkeci and Uskudar.

 

Contractor is Taisei, Kumagai and Gama. Work started in 2004.

 

The Commuter Rail contract (CR1) covers all the rail and rail systems plus the 37 surface stations. Contractor Alstom Marubeni Dogus began work in May.

 

The CR2 contract for provision of 440 cars of rolling stock (10 per train) is scheduled for award in 2008

Marmaray project

Total length: 76.3km

European side: 19.3km

Asian side: 43.4km

Project cost: $3bn (£1.5bn)

Construction start date: 2004

Completion: 2011

Immersed tube tunnel: 1.4km

Maximum depth of immersed tube: 58m

Immersed tube element dimensions: 135m long, 15.3m wide, 8.6m high, 19,000t weight

Number of elements: 11

Bored tunnel: 9.8km

Cut diameter: 7.97 or 7.85m

Finished Internal diameter: 7.04m

Cut and cover/open cut tunnel: 2.4km

Existing stations being upgraded: 37

New underground stations: 3

New track: 250km

Travel time between Gebze and Halkali

Existing commuter rail and ferries: 185mins

New/upgraded rail route: 104mins.

 

Stations

The three underground stations are being constructed using different techniques.

 

Uskudar is cut and cover with New Austrian Tunnelling Method (NATM) used for the crossover. During crossover construction which is now complete, two minor collapses and surface ground settlement occurred causing minor cracks to buildings and the evacuation of two apartment blocks. The solution was to use longer rock bolts, cast a temporary invert with shotcrete and install foot piles. Yenikapi will be cut and cover.

 

Sirkeci, which is 45m below ground level, will use NATM with an access tunnel located between the running tunnels. Station entrances will be constructed using cut and cover and NATM.

 

Archaeology

Eight sunken ships from ancient Byzantium are the star archaeological find in the rich soil.

 

But their discovery has created headaches for the Marmaray project team.

 

Istanbul, like Rome and Athens, is sitting on aeons of history. The Greeks founded Byzantium in the 7th century BC.

 

It was then conquered by the Romans in 330AD and renamed Constantinople after the Emperor Constantine. When the Roman Empire split it was the Eastern capital until it was won in 1453 by Ottoman Sultan Mehmed II who made it the capital of his empire when the city was renamed Istanbul.

 

The eight ships found at the site of Yenikapi station were a precursor to the discovery of the ancient Roman walls. A decision by the government that they need to be preserved insitu plus the need to dig the site have delayed the project by eight months or two and a half years, depending on who you talk to. It has meant an enormous planned interchange at Yenikapi has had to be dropped and a new, less ambitious design created. There is still discussion on going with the tunnelling contractor over whether it will be able to remove its earth pressure balance TBM at the site as originally planned. Until that is given the ok, that section of tunnelling in the soft ground on the European side has yet to start. The contractor has been issued with an extension of 18 months and cost increases are likely to be resolved through increased European Investment Bank funding.

 

Construction sequence

Element 11 of the immersed tube was the first to be installed and is sitting on jacking frames at each end (NCE 5 April). Subsequent elements will be positioned on guides on the previously placed element and a jacking frame at the outboard end to permit very accurate vertical and horizontal adjustment.

 

The jacking frames are supported on gravel foundation and once elements are in position, a grout mixture will be pumped under them.

 

Each immersed tube section will be joined using a rubber GINA gasket on the elements ends.

 

‘In this way each joint can be entered and cleaned, the bulkheads removed and the joint space completed as the tunnel marches across the Bosphorus, ’ Grantz says.

 

As the water filled ballast tanks in each element are removed their weight is replaced with concrete permanent ballast in the invert.

 

Element E11 was special for two reasons. It was provided with sleeves to receive the TBMs on the Asian side and it carried the single 34m access shaft between the tunnel roof and the surface of the sea to allow men and equipment into the tunnel.

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