Bridge under troubled water

42 | NewScientist | 7 October 2006 www.newscientist.com

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, opposite). 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

● 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 will

also 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, opposite). 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.

Even as Istanbul braces itself for a major earthquake, a tunnel joining its European and Asian halves is under construction. Julian Smith sizes up an epic engineering challenge

Bridge under troubled water

The Bosporus Strait looks deceptively calm

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www.newscientist.com 7 October 2006 | NewScientist | 43

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 groundEarthquakes 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 (New Scientist, 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 Düzce 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

Bosporus

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,


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

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

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

The new tunnel under the Bosporus will mean the end of long, slow commutes by road and ferry

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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.

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.” ●

Julian Smith is a travel and science writer based in Santa Fe, New Mexico

“ The question isn’t if a major earthquake will hit Istanbul, but when”


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Bridge under troubled water