Doubling of the Brisbane Gateway bridge could have been seen as an opportunity to explore some of the latest bridge design trends. After all, the original high, slender, concrete arch striding over the estuary on the seaward side of the city was a deliberate statement, intended as an icon for the Queensland state capital. In the 1980s when the bridge was built, Brisbane was transforming itself from a provincial backwater into a world city. The new bridge could have been equally bold.

“But to do that would have meant coming up with something utterly different,” says Mark Palmer, deputy project manager for the Leighton Abigroup Joint Venture which is building the new bridge, now well into construction and rising to its full 64.5m height. “There is nothing worse than a half-hearted or mismatched variation.”

But radical change is also ruled out by constraints at the site. Firstly the bridge must sit more or less alongside its 1986 original, because its function is to widen the existing motorway crossing on the same alignment. More importantly, it must keep within the same tight dimensional envelope that governed the first.

It has to provide a substantial vertical clearance of some 57m. The Brisbane river has a central channel more than 9m deep, which is sufficient for big cruise liners which moor upstream. But even this height means they only just pass below the existing bridge by a metre and a half. Fortunately the river's tidal range is not excessive.

At the same time the height of the structure must be kept as low as possible, because the Brisbane International airport is located at the end of the estuary. The structure cannot be allowed to intrude into either the flight paths of the planes, or the radar beams that guide the planes in. Neither the bridge nor its lighting or gantries must be higher than 80m.

The obvious answer was to duplicate the original, matching its pier locations, approach structures, and the concrete box girder main span, which at 260m was the longest of its type in the world when it opened in 1985. It is believed to still have the largest cross-section concrete box girder in the world.

However there have been some slight variations on the original design. Although the new bridge will add six lanes to the crossing, which is currently a dual three lane highway, it will be 28m wide rather than the 25m of the original. This extra space will accommodate modern requirements such as a combined pedestrian and bicycle path with viewing platforms and drinking fountains at intervals, giving people the chance to admire the extensive views over the flat estuary and the city skyline.

The other main change is in some of the approach piers, which has been done for efficiencies in construction. “We have adjusted the position of two of the approach piers,” explains Gerry van de Wal, project manager for the Gateway Alliance. This organisation is building the main bridge as part of a major upgrade of the entire Gateway motorway. These 'alliances', which are common in the Australian construction industry, are a particularly strong form of partnering contract which are usually formed between the client and the main contractor, to ensure timely delivery of a project. The Gateway scheme is considered large enough for alliances to be formed with some of the main subcontractors too, in this case between Leighton Abigroup and VSL Australia for construction of the bridge, including civil and foundation works, substructure and deck construction, and operation of the precast yard.

The alteration involves an adaptation of the layout of the original approach structures, which consist of a series of concrete piers, ten on the north approach and five on the south approach, which gradually elevate the deck on a 5.5% gradient to the full height required at the main span.

Approaches are being built using balanced cantilever construction with precast segments which will be placed using a special gantry, spanning two piers and lifting the segments in a controlled sequence.

“On the original bridge the final approach span is longer than the others, at 88m,” says van de Wal. “But retaining that layout would have dictated the sizing for all the equipment, and especially the launching gantry we are using.”

It is much more economical to redesign this to the same 71m length rather than make adjustments for one larger span. An alternative would have been the use of large cranes for the last span, but this would have been difficult and expensive too, and might have breached the 80m height limit. So after thinking long and hard, van de Wal says, the team changed the position of the last approach pier.

“Most of the important views of the bridge are oblique, either from the city centre or the airport on the northern headland, and the difference is virtually unnoticeable,” he suggests.

The difference in time and cost will be noticeable however, particularly as the alliance has just three and a half years to complete its work, compared with the five years it took to build the original bridge.

Other changes to the design are less visible. The main spans will have a single deck unit with a double box section rather than the original's single box, which at 12m wide and 15m deep at the piers is believed to still be the largest cross-section in the world, although the 260m main span has been overtaken. The double box will be 15m wide at its base.

By contrast, the approach structures will have twin boxes to carry the deck. “But the double box is still much larger than we could handle with a precast system,” says van de Wal, “so it will be cast in situ using travelling formwork.”

Up until now, attention has been focussed on the foundations and piers, particularly for the main crossing. As in the original design, the piers will sit on a pilecap at the top of an array of bored piles, but once again there are differences; there are far fewer piles for the main piers than before. A total of 24 piles of 1.8m diameter has replaced the original 48 piles of 1.5m diameter.

“There were some initial concerns about the adequacy of these,” admits Peter Rotolone from the project client, Queensland Motorways, the owner and future operator of the crossing. He brings a lot of experience to his role as construction manager for structures, not least because he worked on the original bridge. The reduced pile numbers was one concern, and so was the amended structure of the bridge itself which no longer has pin-joints at the top and bottom of the piers but uses full moment connections.

“The rock head is variable, which was another aspect that concerned us,” he says. “Added to which there is a requirement for a 300-year design life on this project, which is unusually long.”

The contractor carried out a programme of tests to investigate pile capacity: firstly the pile design was verified using relatively modern Osterberg cell methods, which involved boring several full-size sacrificial test piles. The reinforcement cage was made up, including hydraulic ram units, which can be operated via tubes from the surface once the pile is made. Inbuilt instrumentation measures the response of the piles, for both skin friction and end bearing, as load is applied.

In addition, a core was done on each pile to establish where the rock began, so that they could be properly socketed. “And we went to some trouble to inspect each socket with a TV system, as it was made, to ensure cleanliness of the socket,” Rotolone adds.

Piles were drilled by the Gateway Piling Alliance, another alliance, this time between the contractor and various Keller Group subsidiaries, which are carrying out most of the project's groundworks. Bored piles have steel casings up to 35m long to keep it stable in the soft silts and marine clays above the rock. These casings will remain in the ground as part of the 300 year life-span protection.

To provide access for the work, which began in February last year, the contractor created two rock islands in the river, 50m downstream of the first bridge.

“On the south bank we were able to just make an extension of the shore by dozing rock out into the river,” says van de Wal. “But the pier on the north side is further into the water, so we had to create a 120m-long causeway and then form the island at the end of it.”

Some initial dredging to remove river silt was done and a silt curtain installed to prevent too much turbidity in the water, which is still relatively shallow at this point.

The island platforms have eliminated the need for cofferdams and result in a slightly higher level for the big pilecaps which were poured earlier this year. These are each 20m square with a 1070m³ volume, which meant coordinating four concrete pumps for each, fed by concrete deliveries through the day and into the night.

“We needed some cooling for the concrete,” says van de Wal, “which was provided using tubes in the block.” Air pumped through the tubes was sufficient to keep temperatures within limits. The pilecaps were formed inside a precast concrete shell which incorporate stainless steel reinforcement mesh for longevity.

“Without it, we would have needed 150mm of cover over the main reinforcement to achieve the design life in this splash zone” explains van de Wal. “The water at this point, even 4km from the river mouth, is still brackish and chloride-laden.”

For the last couple of months the main piers have been rising, each one consisting of two parallel leaf columns 15m long by 2.5m wide. A climbing form allows 4.5m-tall lifts to be cast; inside void formers create a series of cells for the structure.

At the top of the 45m-high columns some very complex steel fixing is due to start shortly. This will form the 15m-deep pier-top unit for the balanced cantilever, and requires dense reinforcement.

After that, construction of the main and back-spans will start, using mobile form travellers to build the spans with the balanced cantilever technique. Some 30 pours will be needed over 42 weeks to bring the central span to its joining point with the other side and the back spans to the approaches.

At the same time, construction of the approach viaducts has been continuing, with its own challenges. First of these is foundations, particularly on the north side where the ground is marshy and soft and forms part of a wide flood plain for the Brisbane river.

For most of the approach piers this means substantial piling, though in this case using large octagonal-section driven concrete piles, going through soft alluvium to a gravel layer at about 35m depth. “The first of the piers by the river is on bored piles because it attracts a lot of load,” says van de Wal. The others typically have around 50 driven piles.

The 15m jointed octagonal pile sections have been made in a special precasting yard set up on the northern shore. But the primary purpose for this yard was to create bridge segments which will be used both for the approaches and for several other major flyovers being built along the length of the toll road project (see below). It has also been used for smaller bridge beams and other items.

All of the approach spans are being assembled from precast segments, which will be put into place using a large 80t launching gantry. The purpose-designed unit straddles two piers at a time and cantilevers beyond. Lifting 'crabs' raise each segment into position to be glued and stressed onto the growing spans. The lifting sequence works around four segments, one back and one front for either side of the growing span, thus keeping it in balance.

Segments are being match-cast, the first time this technique has been used in Australia. Most will be between 60t and 70t in weight but there are a few 200t heavily-reinforced segments at expansion joints. These will have to be raised using strand-jacks on the gantry, rather than the winch lift.

Initial spans to test the segment sequences have already been assembled using a mobile crane, but the gantry, built in China, has not begun work yet.

Back at the main bridge there will be a series of final tasks to carry out; surfacing and furnishing the new road and fitting it with state-of-the-art tolling gantries. At the pier bases the rock temporary works will find a new use; they will be formed into 'collars' around the new piers and those on the old bridge, providing ship collision protection to modern standards.

Maunsell Aecom and SMEC are the principal designers, with Cardno and AAS-Jakobsen as the specialist bridge designers.

See corresponding feature about the formwork solution here

The whole story

The A$350M bridge is only part of a major upgrade taking place on the Gateway motorway, which is a vital toll road link between the main part of Queensland to the north and the Gold Coast and more populated areas of this south-east corner of the state. It also plays a significant part in the commuter network around Brisbane and Ipswich, further south.

The state has one of the fastest-growing populations of the country, with an estimated influx of some 1,500 people per week, and economic growth is sustained by minerals and coal, particularly those sold to China. This road is particularly important for the Australia Trade Coast area of Brisbane airport, the port and industries.

Road use is already 100,000 vehicles a day and rising. In total some A$1.88bn is being spent on widening a 12km length of motorway to the south of the bridge and to expand capacity in the north with an additional 7km-long motorway link on the north side. The separate six-lane alignment rejoins the existing motorway further on.

To begin with, the new motorway will not use all of the new capacity being created at the bridge but it is anticipated that it will eventually be expanded further.

Client for the project is the Queensland Motorways, which is a private company wholly owned by the Queensland Government. It operates a small network of some 61km of motorway around the city.

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