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Seeing double

Twenty years ago in Brisbane on Australia's east coast, a concrete arch bridge was built. Now an
identical structure is being built alongside it. Adrian Greeman reports from Queensland's capital.

The Gateway project in Brisbane would be considered a major scheme for its central bridge construction alone – the duplication of a high concrete arch bridge six lanes wide and with a 260m main span.

But the scheme also involves major widening work on 12km of motorway to the south with a complex traffic diversion programme. To the north there is an even more difficult project: building a completely new 7km loop of motorway across soft marshy ground. For this, extensive and varied ground improvement is required.

"This is really three schemes in one," says Leighton Abigroup joint venture deputy project manager Mark Palmer. It is carrying out the A$1.88bn (£878M) motorway upgrade in a design, build and maintain package. Design is by Maunsell-AECOM and Australian firm SMEC and the client is Queensland Motorways, which has the concession for the single-point toll crossing and motorway from the Queensland Government.

The centrepiece is the main bridge: a dual, three-lane crossing that was deliberately built as an icon for the city in the 1980s. Its high, slim form stands 65m above the Brisbane river, necessary to allow cruise ships and some freight vessels to navigate the 9m deep channel below, with a 57m clearance. "Even then there is only about 1.5m to spare with the biggest boats," says Gerry van de Wal, project manager for the Gateway Alliance, which is building the new £164M bridge.

The new bridge will be almost identical to the old, a choice forced on the designer because it must give the same clearance and keep its profile low to avoid interfering with flight guidance radar at Brisbane airport. "That ruled out most alternatives," says Palmer.

There are variations. The bridge will be wider, at 28m instead of 25m, allowing for
a combined pedestrian and bicycle path on top of the six-lane highway that it duplicates.

Also, vantage platforms and water provision points at intervals give people the chance to admire the long views over the estuary and the city skyline.

Two of the approach pier positions are slightly different too, cutting out an irregular 88m span just before the main bridge so that the 10 northern and five southern spans will all be 71m.

Balanced cantilever construction is being used for the entire length of the bridge, just as before, but this time modern technology is being emplyed. For the approaches therefore, 750m on the north side and 350m on the south, this means precast segments – the first use of match casting in Australia, claims Van de Wal. They will be erected using a lifting gantry which stretches between, and a bit beyond, two spans.

The gantry has crab lifters which will lift a rotating sequence of four segments around the pier top to extend two spans in balance and connect to the next. The separate three-lane approaches will eventually be joined longitudinally with a concrete stitch to form a single six-lane road deck.

Segments are being cast in a factory built by the main contractor on the north bank side of the main crossing. They are mostly around 70t, although some expansion joint units are more than 200t. These will also be lifted by the gantry, says Van de Wal, but with strand jacks. However, main bridge construction must be done insitu because it is simply too large for gantry work, adds Van de Wal.

The main spans will have a double box section rather than the original's single box, which at 12m wide and 15m deep at the piers is still the largest in the world, although the 260m main span of the Gateway it supports has since been overtaken. The double box will be 15m wide at its base.

That comes later, however.

For the moment attention is focused on the foundations and piers, particularly for the main crossing. As before the piers will sit on a pilecap at the top of an array of bored piles, although there are fewer piles for the main piers than before, just 24 at 1.8m in diameter compared to 48 at 1.5m in diameter.

"There were some initial concerns about the adequacy of these," says Peter Rotolone from the client and one of the engineers on the original crossing. A special testing regime was instigated for the piles, including the use of Osterberg cells in two special sacrificial test piles to ensure their strength.

"The rock head is variable, which was also of concern," says Rotolone. "And then there is a requirement for a 300-year design life on this project, which is unusually long."

A core was taken for each pile to see where the rock began, so that each could be properly socketed. Bores were made with steel casings up to 35m-long to keep them stable in the soft silts and marine clays above the rock. The casings will remain in the ground as part of the
300-year life.

To give access for the work, which began in February last year, the Gateway Alliance created two rock islands in the river. These are just downstream from the first bridge, putting the new one 50m westwards from the old.

Foundations for the approaches have also been complex, particularly on the north side where the ground is marshy and soft. For most of the approach piers this means substantial piling, though in this case with large octagonal-section driven concrete piles. These go through soft alluvium to a gravel layer at about 35m deep.

But the problems do not stop there. An entire spectrum of foundation works has been necessary for the 7km motorway beyond the bridge. This is a new section, which doubles the route capacity by taking some traffic away from the city onto a new alignment closer to the airport. It too has a number of large structures.

Almost the entire length is across the river estuary's flood plain, with soft marine clays up to 30m deep, giving the contractor numerous groundwork issues. To handle them it formed a second alliance, the Gateway Piling Alliance, with the Keller Group, which includes Piling Contractors, Franki Pile, Vibro Pile and Keller Ground Engineering. Around 20 big piling rigs have been required at peak for the work over the past year.

Click here for new motorway cross section and map of scheme

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