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project type: article [map]
object:Marmaray type:railway-tunnel beyond Bosporus
location:Istanbul state:Turkey
architect:Taisei, Japan (general contractor) materials:Oyak Beton, Istanbul (concrete)
published:tunnel 06/2013 page:12 - 19
crane lowering a precast element (© Robert Mehl / Taisei)
Precast dockyard in the port of Fenerbahçe. With the docks on the left in which the base plate and the outer walls were cast; the cover slab for the floating precast segments were concreted on the right (© Robert Mehl / Taisei)
Historic section drawing with the design of a submarine tunnel viaduct in the Bosporus (© Robert Mehl / Taisei)

Marmaray project with Bosporus tunnel in Istanbul

Immersed tunnel

The Ottoman rulers dreamed of a rail tunnel beneath the Bosporus as far back as 1860. More than 150 years later at the end of this year, this vision of linking the European part of Turkey with its Asian pendant underground will be accomplished.
Wheras other metropolises display concern that their commuter transportation systems might suddenly come to a halt, this situation is a daily one in Istanbul. Even on normal weekdays, it is advisable to schedule more than 90 minutes outside the rush hour for a distance which is comparable with say travelling from Spandau and Friedrichshain in Berlin. Public commuter transportations, in spite of the ambitious manner in which it is being developed at present, can be described as practically non-existent for a city of this category. There is an urban transit line, which at some point transforms into a Metro. It connects the airport on the European side with the central Taksim Square. And there is a tram line, which constantly intersects this route that has to be used to reach the old part of the city, still the actual centre. Owing to the huge number of motorists, the trams using this single line are only able to negotiate the busy roads at snail’s pace. The route passes the city’s oldest university, the historic bazaar, the striking Blue Mosque with the Hagia Sophia next to it, then the old Ruler’s Palace finally ending at the Main station immediately in front of the ferries bound for Asia. The Bosporus represents a mixed blessing for the city. On the one hand, the strait is the reason why such a powerful centre was established here back in ancient times. At the same time, this dividing element is the main obstacle in this city of more than 15 million inhabitants. Although there are two bridges spanning the Bosporus, they are by no means sufficient!
The foundation stone for a third bridge was laid in early summer. The “Marmaray” rail tunnel will follow towards the end of the year but even this is inadequate. A second road tunnel is to be set up only a few km to the south of the first one.
Transport technical concept of the tunnel
The Bosporus Tunnel fulfils two different requirements. Firstly, the European access tunnel passes beneath the old part of the city. Currently additional Metro station are being produced in the old part of the city, designed to relieve the busy tram system and link the Asian part on a high frequency basis. As a result, several underground stations are being built in the Asian suburb of Kadiköy. The tunnel will also be used for internal goods transportations. As a result, the tunnel rack gauge as is essentially the case throughout Turkey corresponds to the European norm. The tunnel ramps possess the permissible gradients and are connected up to the international mainline network. Currently it is intended to mainly use the tunnel during the day for public commuter transit purposes and allow goods trains to run at night. The old tracks, some of which date back to the colonial times, are being used or upgraded for this purpose. The Haydarpasa Station still exists on the Asian side. This is where the Bagdad Railway inaugurated under the German Emperor Wilhelm II once set off. The classical building is located roughly across from the European side of the Bosporus, which is also linked up historic tracks that are still in use today, where the tunnel ramp begins after a few km.
Tunnel construction process
The tunnel bores for the Marmaray project were not produced alongside one another under the Bosporus. It would have been necessary to tackle them at a deeper level to ensure sufficient stability and tightness in the rock. Instead the engineers of the Japanese general contractor Taisei Corp. dedicated on a “cut-and-cover” solution. Eleven giant precast sections were produced for crossing the Bosporus, which is 1,600 m wide at this point. These 10 x 150 m and 100 m long rectangular elements – like giant shoe boxes were made – in some cases, floating – in a dockyard located only a few km away to the south. Subsequently they were transported by tug to above their final position, lowered with absolute precision and connected with each other with permanent seals comparable with standard reinforced concrete tubes. The docklike facility for producing the 11 tunnel segments was set up epically for this purpose in the industrial port of Fenerbahçe, an Istanbul suburb located on the Asian side.
The concrete workers first produced the base plates on the working platform and than began to build the outer side walls upon them once they had set. Conventional steel reinforcement was applied inside. Then concrete was added. Spacers as traditionally used in concreting were also deployed. It was not necessary to secure the shuttering structure at any point to the external formwork layer. Generally speaking experts concede that the proper application of formwork oil and in turn the effect that was desired and actually took place largely contributed to success during this production phase. For after producing the base plates and the side walls two cavities in the concrete precast segments foreseen for installing the tracks were completely filled with floating elements. For this purpose – as in case of a ship – it was decided to establish bulkheads so that the individual sections could be regulated completely independent of each other. The engineers then flooded the formwork dock and the combined buoyancy of all the floating elements raised the tunnel elements from the dock wall formwork anchored to but not firmly attached to the ground. The giant precast segment rose and was pulled to a floating dock by a tug, where it continued to float whilst its cover slab was installed.
Assembly along the tunnel route
While the 11 elements were being produced in Fenerbahçe, large dredgers dug a roughly 7 m deep and approx. 20 m wide trench in the up to 60 m deep bed of the Bosporus. The giant precast segment rose and was pulled to a floating dock by a tug, where it continued to float whilst its cover slab was installed. Although the sheer size of the project might indicate a certain amount of generosity relating to tolerances, the structural elements actually had to be lowered and put in place with absolute precision. This was only accomplished on certain days, which could not be predicted long in advance and depended on the current and weather conditions. As a result, there were regular waiting periods, during which everything had to be kept ready for assembly. In addition, the strait possesses a pronounced warm current on the surface of the water which flows from the Mediterranean to the Black Sea. However, only a few metres lower down an equally strong current of Black Sea water flows in the opposite direction. The resultant eddies made it even more difficult to lower the tunnel segments with great precision.
The Taisei Corp. engineers devised the notion of a "slewing" motion. For this purpose they aligned the floating tunnel elements parallel to the current and thus crosswise to the waterway bank, in their approximate position. By releasing the air from the tanks, they lowered the elements still suspended from their guide cables gradually till they were some 7 m above the bed. Then they turned the element by some 90° and lowered it entirely into the trench. The air tanks were then activated to ensure it was properly positioned. Towards this end, air from the outside was forced into the individual pressurised tanks, which were separated from each other and released as required. At the same time, tugs were employed to pull the guide cables in the proper direction. A digital GPS-based monitoring system was used to check everything together with corresponding position transmitters, which were attached to the segments.
Bonding the individual elements
The 11 precast elements, which were in part comparable with conventional tubes, possess sealing units at their head ends - albeit substantially larger on account of the sheer dimensions and the greater water pressure. These connecting sockets with several special seals placed consecutively are more than half a metre deep. Pressure was required to connect two elements. As soon as the engineers had placed two segments together and they began gliding into position, they resumed pumping out the water, which had in the meantime re-entered the segments. This resulted in under pressure, which was faced with a considerable external water pressure. This immense difference in pressure caused the 2 structural elements to marry. When all the 17 elements had been installed, the dredgers covered the concrete precast segments with earth. Although the sealing layer is made of concrete, the tunnel would not be visible from a submarine for instance.
Set-up of the access tunnels
The submarine tubes were only accessible via a temporary shaft when the tunnel segments were being laid and the initial phase that followed. This gave the appearance of an artificial island and was linked to the Asian mainland by an almost 100 m long jetty. The tubes Ieading to the submarine tunnel route were first being produced from the mainland at this point in time. The objective was to avoid contact with seawater and to first reach the strait with the four tunnels (two on each side) once the central tube unit was completed and it was possible to pass through to the dry cavities with the TBM with great accuracy. The engineers placed underwater concrete at the connecting points to ensure the tightness of the intersections between the mainland and the submarine double tunnel tubes. This concrete as well as the material applied for the giant segments was produced by the Turkish cement manufacturer Oyak Beton. This company, which is active throughout Europe, has experience with ¢submarine tunnelling. In fact the concrete recipe that was used was originally developed tot the underwater section of the Öresund Crossing between Denmark and Sweden opened back in 2000. This roughly 3.5 km long section was also produced by means of the caisson method.
Conclusion
The tunnel's caisson construction method enabled it to be built economically and at the same time efficiently on the seabed. Construction time was saved and substantially longer access ramps avoided especially as trains cannot overcome major gradients. lt would have been necessary to have tunnelled at a considerably greater depth for both stability and sealing purposes to drive through the Bosporus. The depth of the Bosporus in fact caused a rail tunnel project to fail in the past. In 1860, German engineers planned an underwater viaduct to ensure that trains did not have to travel at too great a depth. A steel tunnel tube was to be elevated to cross the strait at a water depth of roughly 15 m. For in those days it was not technically possible to build such high bridges with wide spans to allow the sailing ships of the time with their high masts to pass beneath. Consequently the idea reminiscent of Jules Verne never came to fruition - although it gave rise to the concept of the tunnel immersed in the seabed.
Robert Mehl, Aachen