Sitting at the crossroads of Europe and Asia, the ancient city of Istanbul has seen thousands of years of trade, battles and invasions. Now it is the scene of one of the most audacious engineering projects in the world.
The Marmaray Rail Tube Tunnel, due to open in 2010, will not only be the deepest underwater tunnel ever constructed. It willalso pass within 16 kilometres of one of the most active geological faults in the world. A major earthquake is not only expected, but imminent. No wonder the Turkish government is calling it the project of the century.
Istanbul is divided by the Bosporus strait that connects the Black Sea to the north of the city with the Sea of Marmara to the south (see Map). Part of the city lies in Europe, on the western side of the strait, while the rest is in Asia. Two road bridges cross the strait and there are plans for a third, but ever since the Ottoman sultan Abdul Mecit suggested it in 1860, city leaders have dreamed of building a tunnel to link the two halves of the city. Last year, a mix of technical expertise, foreign investment and national pride finally came together to make the sultan’s dream a reality.
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This time, the plan is not so much to unite an empire as to deliver modern Turks from traffic hell. Today, crossing the Bosporus means either a 3-hour trip by rail and ferry, or braving gridlock in narrow, 2000-year-old streets and the two overcrowded road bridges. The Marmaray project, which takes its name from the Sea of Marmara and “ray”, the Turkish word for rail, aims to ease the strain by replacing car traffic with an upgraded rail service that will whisk commuters between Europe and Asia.
The plan is first to improve the existing railways on both sides of the strait and then extend them to the coast via tunnels bored through bedrock. The centre section, under the Bosporus, will be a 1.4-kilometre tube made up of several shorter sections that will be built on land, floated into position and sunk into place (see Diagram). End to end, the tunnel will be 12 kilometres long.
It might sound straightforward, but the project engineers face a major geological hurdle. Twenty kilometres south of Istanbul lies the North Anatolian fault (NAF), where the Anatolian plate that underlies Turkey, Greece and the north Aegean is being squeezed to the south and south-west by the surrounding Arabian, Eurasian and African plates. The result is what geologists refer to as a right-lateral strike-slip fault, similar in size and type to the San Andreas fault in California. The NAF runs for 1600 kilometres across northern Turkey, and the abutting plates move about 2 to 3 centimetres relative to each other every year.
Shaky ground
Earthquakes along the NAF are common. In the past seven decades Turkey has endured seven quakes of magnitude 7.0 or greater. While some earthquakes release the stress that has built up on a fault, seismologists have come to realise that others simply shift it along the fault, leaving it even more prone to slip. Almost every quake along the NAF in the past 100 years seems to have set up a larger one, to the west. The process appears cyclic: quakes march along the fault in sequence until stress falls below a certain threshold, and then start again after a period of quiet.
In 1997, geologists studying the most recent cycle predicted that the next shock would hit near the port city of Izmit, 80 kilometres east of Istanbul (Â鶹´«Ă˝, 28 August 1999, p 5). Sure enough, a major quake of magnitude 7.4 struck close to Izmit in August 1999, followed by another in DĂĽzce in December, together killing over 18,000 people and causing $10 to $25 billion of damage.
Seismologists agree that the most recent quakes on the NAF have shifted the stress steadily closer to Istanbul. Now the question isn’t if a major earthquake will strike the city, but when. Recent estimates by the US Geological Survey, the University of Tokyo and Istanbul Technical University estimate that the probability of a strong quake hitting Istanbul is up to 44 per cent in the next decade and as much as 77 per cent in the next 30 years. A major earthquake and accompanying tsunami are considered inevitable within a generation.
Geoffrey King, director of the Tectonic Laboratory at the Paris Institute for the Physics of the Globe in France was among those who predicted Izmit was at risk. “Istanbul is in great danger,” he says. The quake will likely be even bigger than at Izmit, and since Istanbul’s population density is 10 times greater than that of Izmit, “a catastrophic event is likely to end the lives of a significant fraction of its current inhabitants”, he adds.
Despite the prospect of a quake anywhere up to magnitude 7.5, the Marmaray team is undeterred. “To build a tunnel in a seismically active area is not something new,” says Steen Lykke, project manager for Avrasya Consult, which is managing the construction. He points out that San Francisco’s Bay Area Rapid Transit (BART) rail tunnel and the Osaka South Port Tunnel in Japan were both built through quake-prone regions. The BART tunnel rode out the magnitude-7.1 Loma Prieta quake in 1989, and the Osaka tunnel was hit by a 7.2 quake in 1995 while it was still under construction, yet escaped with minimal damage.
The crucial factor that lets the tunnels withstand quakes of this magnitude is the fact that both are “immersed tubes”. In this design, engineers dig a channel into the seabed and float the prefabricated sections into position above it before sinking them and covering them over. The Marmaray tunnel will use a similar approach.
“The trick is to build sufficient strength, flexibility and ductility into the structures in combination with flexible joints where the stiffness or the cross sections change,” says Lykke. Special flexible joints made from thick rubber rings reinforced by steel plates will be installed where the tunnel is most vulnerable – at the linkages between the rock-bored sections and the immersed tube, and between the tunnel and the three underground stations it will connect.
These joints will act “like big gaskets” in the event of a tremor, says Lykke, allowing the sections on each side of the joint to move as the ground shakes. There will also be floodgates at both ends of the immersed section to protect the rest of the tunnel if the midsection is breached.
As well as being more resilient in earthquakes, immersed tubes are generally faster and less expensive to build than traditional bored tunnels. For starters, problems with one section of the tunnel won’t necessarily hold up the entire project, and because they can have a rectangular cross-section, they are a more efficient shape than a circular tunnel for packing in railway lines side by side. And while a bored tunnel is usually considered stable only if its depth under the seabed is at least equal to its diameter, an immersed tube can sit immediately under the sea floor, allowing shallower approach gradients.
Equally important as reinforcing the tunnels is preparing for what could happen to the seabed during an earthquake. Saturated sandy soils have a tendency to liquefy when shaken strongly, as the grains move freely past one another due to a sudden increase in water pressure. Liquefaction on land can cause buildings to collapse and roadways to fracture. Anything buried under the sea might sink deeper into the sediment or, if it is buoyant enough, may even rise to the surface. To counteract this, the foundations of the immersed tunnel will extend about 16 metres below the seabed and the soil up to another 9 metres below that will be stabilised with injected mortar.
The construction effort got under way late last year, and the 11 tunnel sections are currently being built on land, each 135 metres long and weighing 18,000 tonnes. The first immersion is planned for February 2007. Meanwhile, engineers are dredging up some 1 million cubic metres of rock, sand and soil to form the trench in which the tunnel will sit.
This winter a pair of tunnel-boring machines, each wider than the fuselage of a 747 airliner, will start digging from the Asian side to connect the immersed tube with the overground rail lines. Another pair will leave from the European side in the spring.
This last part might not be so easy, however. While digging the foundations for the new Yenikapi railway station on the European side of Istanbul, engineers stumbled across the remains of the 4th-century port of Theodosius, the busiest in ancient Istanbul – which was in turn capital of the eastern Roman, Byzantine and Ottoman empires and known for many centuries as Constantinople. Archaeologists had predicted where the port would turn up, but it wasn’t until buildings were removed to make way for the new station that their suspicions were confirmed.
Excavations revealed the remains of dams, jetties and no less than eight wooden boats, including the first known Byzantine naval vessel. Investigators also found anchors, lengths of rope and personal items such as candle holders, hairbrushes and sandals.
“From a historic point of view, the excavation is of utmost importance,” says Robert Ousterhout, professor of history at the University of Illinois at Urbana-Champaign, who has visited the site twice. “Although Constantinople was the most important city in the world for a thousand years, modern Istanbul has witnessed virtually nothing that might be called urban archaeology. What we know about the city comes from texts.”
Archaeologists are therefore keen to explore the site properly before handing it over to the construction project, but the size, cost and national prestige of the tunnel project create pressures of their own. “The site is huge,” says Ousterhout – almost a kilometre in length and as large as several football pitches. Exploring it properly could seriously delay the project, adding to its estimated $25 billion price tag.
Project managers are trying to figure out how to proceed with construction without jeopardising what has turned out to be a major archaeological find. Some artefacts will undoubtedly be reburied, while others may be displayed in museums incorporated into the tunnel project itself.
Meanwhile, construction continues within Istanbul at breakneck speed. In July the mayor of Istanbul province, Kadir Topbas, announced that the project team was digging a record 35 metres of tunnel per day throughout the city, and that they was planning to speed things up. Even so, says Lykke, the Marmaray engineers have their work cut out. “There is as far as we know no other tunnel where the sum of the challenges is of the same nature” as in Istanbul, he says.
“The question isn’t if a major earthquake will hit Istanbul, but when”
Whether or not it meets the 2010 deadline, the Marmaray tunnel may ironically turn out to be one of the best places to be if and when the next big one hits. “The tunnel will certainly get shaken by the next earthquake,” says King, “but tunnels are very strong. The faults that will move will not cut the tunnel, so it will probably be safer than the above-ground parts of the railway.”
