Greece's giant Rion Antirion bridge is taking shape reports Adrian Greeman.
It was all change this summer on the Rion-Antirion bridge project in western Greece, as focus switched from the 'offshore' phase, constructing and placing the four oil-platform-size pier foundations, to the superstructure.
In February this year the last of the four 60m high concrete base units left the 'wet dock' where it had been cast, towed out by three tugs into the narrow entrance channel to the Gulf of Corinth. There the hollow cylindrical pier - complete with centrally mounted tower crane - was placed on to the huge 90m diameter, 11m high, cylindrical foot which serves as foundation (NCEI July 2001).
Project director for the seven firm Franco-Greek Gefyra construction consortium Gilles de Maublanc, was completely satisfied with the four float outs.
'Towing went well, ' he says. The most off-centre unit was only 50mm from target position, not bad for a bridge with a 2,252m long main deck which will extend for over 3km in length.
Each of the piers rests on a 2m thick gravel bed, above an area of seafloor alluvium strengthened with a pattern of around 200 tubular steel piles. These stiffening 'inclusions', designed to protect against seismic liquefaction, do not connect with the structure above. The area of the Pelapponese is highly seismic and analysis showed that large, shallow foundations on a reinforced sea bed were the most satisfactory solution.
'At the end of the four hour tow out we found the best method was to add enough ballast so that the pier was only just buoyant and had a touch of friction against the sea floor, ' says de Maublanc. 'This meant if the tugs released tension the pier stopped immediately, which allowed fine positioning.'
Seawater now fills the piers which rise 3m above the water level. The water will be pumped out as the structure rises but at present gives the piers a load equal to that of the full bridge.
This surcharging will speed up settlement of the sea bed, allowing more accurate adjustments at deck level, explains Peter Iley, who leads the Maunsell supervisory engineering team. 'There has been some 80mm [of settlement] so far, ' he says.
The major construction effort is now focused on building up the piers to their full height, which on the two central units is 53m above sea level.
The cone-shaped piers taper into a transition ring at sea level, above which is an octagonal section. A further section flares outward to form the square, deck level platform from which four 85m inclined pylon legs spring.
The design calls for very heavy reinforcement both in the pylon legs and the uppermost section of the piers; the four pylon legs will transfer not only normal loads but also potential seismic forces.
'The problem is not just the density of the steel, but the shape and positioning, ' says de Maublanc. 'I worked on a nuclear shield with dense rebar but it was very regular - here it criss-crosses and is inclined.'
High bar cutting and bending accuracy is needed, he said, then tight logistical control to deliver exactly the right steel to site at the right moment. A barge moored alongside each pier serves as as a working platform but 'storage is very limited' so the packages are made up on shore. A single delivery barge services all four piers.
Each of the upper pier sections is also heavily reinforced with a prestressed ring at deck level. And to add to the complexity, there are 20 cable ducts to fit among all the steel.
Concreting of the pylon legs has begun on the most advanced piers. Reinforcement cages are assembled in the shoreside production yard and delivered by barge.
The remainder of the yard is being restructured for assembly of the 27.2m wide deck sections with three pairs of low concrete walls providing production line supports. Steel for the deck section is being fabricated by Cleveland Bridge in the UK and first delivery was due in August.
Although delays meant that only part of the order was expected to arrive on time, 'there is enough for the assembly to go ahead, ' says de Maublanc, who expects that the overall schedule will be maintained. Concreting of the deck sections will also now be carried out in the yard, following a major change to the erection procedure of the composite steel and concrete units.
Originally the steel deck frames were to be lifted from barges by pairs of small sheer leg cranes mounted on each of the growing deck cantilevers and concreted once in place. Working out from two piers this meant four operational points and eight cranes.
Now Gefyra has decided to work with Smit International's large floating sheer leg Taklift 7.
With a 130m long boom, it can lift a maximum 390t, the weight of a complete deck unit, which allows concreting to take place on shore.
The advantage is safety, explains Iley. Seismic factors are important during construction as well as during the bridge's 125 year life span. The floating crane is not as vulnerable.
A special 'quick fit' mechanism has been devised for the deck top to transfer the load and support the section before it is attached to its cables.
The Taklift brings another advantage in the construction of the 11 steel huge steel boxes which form the pylon tops and anchorages, each 7.5m by 2.7m by 2.5m deep and weighing around 20t.
The plan had been to lift the boxes with the Potain tower cranes located on each pier to handle rebar and concrete, rerigged to cope with the heavy lifts.
'But that means a total of 44 lifts, ' says de Maublanc. 'And there are not that many days in the sea channel when wind speed drops below critical.'
Re-configured with a 160m long boom and 15m fly jib, the Taklift can handle 220t, even at the pylon height of 158m above sea level. This enables several boxes to be welded together on shore, says de Maublanc, and then lifted in as one unit.
The sequence will involve six boxes, then four and then a final single box lift, 'which is a much reduced risk', he says.
Owner: Republic of Greece
Contractor: Gefyra Consortium (Vinci Construction, J&P Hellas, TEV, Helleniki Technodomiki, Athena, Proodeftiki, Sarandpoulos)
Structural engineer: Gefyra Consortium
Checking engineer: Buckland & Taylor
Construction supervision: FaberMaunsell
Steel construction: Cleveland Bridge & Engineering
Architect: Berdj Mikaelian