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New Forth crossing ends highest risk phase

Construction of the £790M replacement Forth Crossing in Scotland remains on schedule despite delays in installing the bridge’s three, huge steel foundation caissons, project bosses told NCE this week.

The last concrete to form the 25m thick mass concrete underwater plug inside the largest southern tower caisson was poured this week, marking the culmination of the project’s highest risk construction phase.

But creating foundations for Scotland’s newly-named Queensferry Crossing over the Forth Estuary has taken longer than contracting joint venture Forth Crossing Bridge Constructors had planned, eating into the project’s float.

Time will be made up by accelerating erection of the deck in early 2015.

On programmme

“We have experienced challenges during this extensive underwater operation but, thanks to unconnected future acceleration plans, the overall contract remains on programme,” said Forth Crossing Bridge Constructors (FCBC) project director Carlo Germani.

Forth Crossing Bridge Constructors (FCBC) is a joint venture of Hochtief Solutions, Dragados, American Bridge International and Morrison Construction.

“We expected from the outset this would be the most high risk stage and we have just used up some of our own float time,” said Germani.

“Now we are coming out of the water, and are about to work in the dry on all three towers, a major part of the construction risk is over,” added David Climie, project director for Scottish Government client Transport Scotland. “We have worked through the underwater challenges with good collaborative teamwork and I am confident the overall project remains on schedule for completion by the end of 2016.”

Lowering the road bridge’s three steel foundation caissons, the largest of which is 32m diameter, down through 40m of often turbulent water and positioning them to 200mm tolerance in the unpredictable glacial till and sandstone seabed was challenging, with divers working in near zero visibility (NCE 30 May).

Forming this week’s 25m thick mass concrete underwater caisson demanded a fleet of barge-mounted concrete mixers delivering a world-record 17,000m3 continuous pour over 16 days.

Structural concrete foundations will be cast within the dewatered caissons later this year, ready for tower construction to continue through 214. The project team will then claw back time lost though a revised deck erection plan in early in 2015.

Originally deck sections were to raised from the water and erected as balanced cantilevers using two crane gantries operating sequentially over the three towers. But early in the project, the JV proposed an accelerated alternative programme involving six gantries working simultaneously on all three towers.

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