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Sitting out the earthquake

Rion-Antirion

The massive concrete piers of a new bridge in Greece will not be joined to their foundations. If an earthquake strikes, they will be free to slide.

Adrian Greeman reports.

Sea depths of up to 60m, strong tideraces and high winds face builders of the RionAntirion motorway bridge across the 2.5km stretch of water which separates the Pelopponese peninsula from mainland Greece at the entrance to the Gulf of Corinth.

None of these constitute the biggest problem, however. Nor does the superstructure, even though the bridge will be among the world's longest cable stay structures, with three central 506m spans and a continuous bridge deck of 2250m.

The problem is the ground.

Tectonic history and current activity in the southern Alpine region create both seismic and geological difficulties.

Earthquakes in Greece can measure more than seven on the Richter scale, like the one which devastated the southern town of Kalamata 15 years ago. Local tectonic plate movements mean the peninsula is heading east relative to the mainland.

All this makes for interesting design at the abutments, where a 2.5m movement will occur over the bridge's 125-year lifespan.

But the challenges do not end there, explains geotechnical engineer Lena Kanakari with project supervising engineer Maunsell Greece: the two land masses have left only alluvium between them.

Though the Greek mountains loom close to the crossing on either side, 'within the channel you cannot find any bedrock for well beyond 100m down'. Liquefaction adds to the general difficulties of tying down any kind of structure to the seabed.

In these conditions one of the world's most dramatic bridges must be built. Not only will it connect the small ferry port of Rion with its mainland twin Antirion, it will also link into the main Patras-Athens-Thessalonkia highway (PATHE), Greece's main spine route.

The bridge is categorised as one of 14 critical connections in the European Union and Euro loans will help pay for the Dr248bn ($6.2bn) project. Other finance sources for the build operate transfer scheme include a Japanese bank and equity funds from the Franco-Greek consortium Gefyra undertaking the project.

Its seven member firms are Groupe GTM, J&P (Hellas), Elliniki Technodomiki, Volos Technical Company, Athena, Proodeftiki and CI Sarantopoulos. The same group has also formed a separate design and build joint venture. The consortium will operate the bridge for 35 years. The contract is supervised by General Secretariat of Co Financed Public Works.

Total bridge length will be about 2.8km including approach viaducts, with a main crossing of five cable stayed spans supported by four high piers. As well as three 506m central spans there will be two 286m long side spans.

To allow ship clearance of 50m, the tallest piers will be 114m high, simply to rise through the sea and reach deck level, with another 87.6m of pylon section to support the cable fans above that.

Founding such large bridge piers is difficult enough. But it is even more problematic because the main bridge deck is supported entirely by cable with no side connections. This means pylons must resist very high torsional forces, especially during construction as the decks extend outward.

'Spans are supported from the towers and, unlike most cable stay bridges, do not tie into the viaducts, ' explains GTM-Dumez engineer and JV project manager Gilles du Maublanc.

'With no back span connection for stability, the worst case analysis, such as for an accident creating one-sided loading, reveals very high loads.'

Rather than battling with an earthquake, the bridge will be allowed to slide. Huge flat-bottomed piers shaped like giant chess pawns will simply sit on the seabed, free to move if there is sudden ground acceleration.

Their shape resists overturning.

Each of these giant structures consists of 90m diameter circular base beneath a narrower 60m high concrete cone topped by a cylindrical neck that widens into a square platform at deck level. Above this, four inclined legs form the pylon support for the cable anchorages and bridge superstructure.

Of course there are foundations for the gravity structures to sit on. But the solution means they do not connect to the large flat bases of the piers.

Preparation work for the seabed is complex. The site must be dredged, precisely levelled and formed with appropriate materials to support the piers. A 3m thick three-layer filter blanket of sand, gravel and crushed rock is spread across a 10,000m2 area.

Beneath the blanket is a grid of piles, 2m diameter steel tubes driven 25m into the seabed - 'though these are not piles as such, they are 'inclusions'', explains de Maublanc. The 25m 'nails', hammered in an array, stiffen the soil during an earthquake.

Like the whole design, this structure was developed with checking engineer Buckland & Taylor based in Vancouver, Canada.

Construction of the four pier units is well under way with the first just about ready for floatout. 'These are like gravity oil platforms, ' says de Maublanc - and production is organised like a North Sea yard, with concrete units built in a 230m long by 100m drydock.

The first unit base was floated out from the drydock in September 2000. The cone stub as been extended to its full 63m height in the wet dock, a dredged area of seabed close to the shore.

Meanwhile a barge, originally used on the UK's Second Severn crossing, has been preparing the seabed sites. This jack-up has been modified into a 'tension-leg' barge using huge 1,250t concrete blocks on the ends of chains to hold it in position.

To place the filter layers to the required tolerance and guarantee even coverage, the contractor invented and manufactured a square frame for the back of the barge. Within it, an X and Y traveller positions a special placing tube with a hopper.

'By monitoring the buoyancy of the placing tube you can tell when the blanket is high enough to exert a force on the tube which gives you an accurate level, ' du Maublanc explains.

'Diving inspections check the work.'

'Getting this level to within 50mm, has been the most difficult task, ' says du Maublanc, 'especially in 1.5m/s currents.'

The tube was also used in its frame to guide the piling hammer precisely to the grid locations where the inclusions were needed.

Once the float-out of the first pier is completed, concreting will continue in its final position to form the neck deck platform, the tower legs and finally the superstructure.

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