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Corinthian undertaking

The Rion-Antirion bridge in Greece is on course for completion by the end of 2004. Adrian Greeman visited the site.

Four giant chess pawns are being built in western Greece. They are made of 60m high concrete cones sitting on 90m diameter circular bases. A tubular 'neck' attaches the cone to a widening platform section - the 'head' - completing the pawns which will carry a new, long needed bridge.

By the time the platform sections are complete most of their structures will be hidden underwater. The four pieces, the tallest 114m high, will stand in a line across the Gulf of Corinth.

These piers are the base of a bridge across one of Europe's key straits, the 2.5km wide entrance to the Gulf. The bridge will link the little ferry port of Rion on the Peloponnese with its mainland twin Antirion, but more importantly it will carry a four lane motorway connecting the main Athens-Patras highway on the south with a future west coast route to northern Greece.

It is deemed one of the 14 most important connections in the European Union.

Euro loans will help fund the £520M build operate transfer (BOT) scheme which also has Japanese bank money, and equity funds put up by the seven firm Gefyra concessionaire. The BOT client will operate the bridge for 35 years once its member firms, who make up a separate design and build construction JV, complete the work.

The bridge was never going to be easy to build. The crossing is long and spans a channel with strong tidal flows in up to 60m of water and high winds. Regional tectonic activity creates both seismic and geological problems.

Earthquakes can be over Richter scale 7, like one which devastated Kalamata to the south 15 years ago. And local plate movements mean the peninsula is heading east relative to the mainland. It will move some 2m even during the 125 years of the bridge's lifespan. It makes for interesting design at the abutments.

Moreover, the two landmasses have left behind a channel with only soft deposits between them, explains geotechical engineer Lena Kanakari with project supervising engineer Maunsell Greece, 'Within the channel you cannot find bedrock for much greater than 100m down.'

Rather than resist earthquake forces Gefyra will let the bridge slide. Its design department in Paris devised flat bottomed piers that sit on the seabed and are free to move if there is sudden ground acceleration. Wide bases resist overturning. The designs were refined with checking consultant Buckland & Taylor in Vancouver, Canada.

The piers will carry four legged towers, like two A frames leaning together. These pylon sections will rise a further 87.6m from the deck platforms, which are already 50m clear of the sea.

The top will carry another 25.4m of mainly steel cable anchoring positions for the stays.

The unusual frames are essential to resist high torsional forces, says Gilles du Maublanc, GTM-Dumez project manager at the site at Antirion.

'There are five spans, all supported from the towers and with no back spans for stability. The worst case analysis, such as for an accident creating one-sided loading, demands a very stiff frame, ' du Maublanc adds.

Mainly for seismic reasons too, the road will be carried on a free hanging composite steel and concrete deck that will be continuous for its entire length.

Even though three central spans at 506m each are not the longest in the world, the 2,252m long deck will make the bridge a record holder. Two more spans are 286m long. .

'There will be no structural connection to the deck at the bridge towers, though there will be movement dampers against wind oscillation, ' says du Maublanc But all that comes later. At present the site yard is devoted to production of the four pier units, which have strong affinities with gravity oil platforms.

Production is correspondingly organised like a North Sea yard. First task was creating a drydock 230m long by 100m wide using a cellular sheet piled cofferdam. Two units can be built inside.

Concrete for insitu placing is delivered by two big Schwing pumps from two batchers through fixed lines through the cofferdam. Precast panels are used where possible to speed the work. Like the dense reinforcement cages, these are made up in the workyard; where five railmounted Potain MD350 tower cranes move materials. Later the deck units will be made here.

Unit bases are 9m high with a sloping top slab, and are formed in the shallower 8m deep top end of the dock. Then, for the last three units, the slab and first cone lift waits until the unit moves into the deeper 12m part of the dock.

The first unit was floated out last September. In an innovative touch the contractor did not rebuild the temporary cofferdam seal but used the second, halfformed, unit as a gate: small concrete walls make the seal at the maximum diameter point.

The first unit is now in 'wet dock', a dredged area near the ferry terminal where the cone stub is being extended to the full 63m height. To do this the cells in the base are progressively flooded with each 2.8m lift, sinking the unit synchronously with its extension.

Constant observation and pumping water between cells balances the unit vertically.

According to Maunsell's manager Peter Iley, this first unit will be taken to its prepared site for immersion in May. A barge, brought over from work at the Second Severn Crossing, has in the meantime been preparing the site. Once a jackup, this has been modified into a tension-leg barge using huge 1,250t concrete blocks on the ends of chains to hold it semi-submerged and therefore steady 'The 2m diameter steel tubes we drive are not piles but inclusions, ' says de Maublanc. The 25m 'nails', hammered in an array, stiffen the soil during earthquakes. They are covered with a 3m thick three-layer filter blanket of sand, gravel and crushed rock over a 10,000m 2area.

Once in position the pier concreting will continue, forming a straight 29m high 'neck section', and then a 16m pier 'head'.

On the spot

Name: Lena Kanakari Qualification: Civil engineer, MSc, PE(California) Job title: Senior materials engineer What is the best thing about your current job? The project's diversity and complexity. It is necessary to have a good and long previous experience, I nevertheless have the chance to learn the implementation of new - state of the art - technology in construction materials.

. . . and the worst? Commuting.

There is obviously a very good reason for building this bridge - I live on the wrong side, I am already contemplating of ways to cross it before it's even finished, like pulling some Indiana Jones tricks. I am only dreaming; safety is - and will remain - our top priority.

Best thing about being an engineer? Drawing, designing and building are not childhood toys anymore; it's real life and you get paid for it.

. . . and the worst? The risk of getting isolated in your area of expertise. One needs to try hard, beyond his/her duties to grasp other aspects of a project and ultimately get the whole picture that engineers used to have.

Best projects you have been on? Seismic retrofitting design for dams in California. Dams are my favorite structures. However, bridges do come second.

. . . and the worst? Anything that gets repetitious beyond the point where I can add some improvement to it.

What ambition do you have?

Keep working in different parts of this world, as I have done so far. I have specific places in mind. Being married with a child, however, I may have to somewhat tailor this ambition.

Anything else we should know: I enjoy artistic drawing, simple household constructions, visiting modern and contemporary art museums/ galleries and collecting posters.

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