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High above Newmarket some amazing sleight of hand is unfolding – building a new viaduct, dismantling the existing one, and all the while allowing dense traffic to keep flowing over the twin-bridge structure. BY GAVIN RILEY
Such magical sleight-of-hand by a peak workforce of 150 is being made possible by two factors: an ingenious four-part plan and use of an overhead launching gantry. The hardest part, the yet-to-come dismantling of the existing structure in an ultra-tight urban corridor, is claimed to be without global precedent. The $215 million project, known as the Newmarket Connection, began in the middle of last year and is due to be finished at the end of 2012. It is being carried out by the Northern Gateway Alliance (NGA) – the team responsible for the much lauded Alpurt B2 Orewa-Puhoi roading project north of Auckland. Members of NGA Newmarket are the New Zealand Transport Agency, Fulton Hogan, Leighton Contractors, Beca Infrastructure, VSL New Zealand, URS New Zealand, Tonkin & Taylor, and Boffa Miskell. Planning began as far back as 2002 to replace the 1965 Newmarket Viaduct, which cost $2.26 million and consisted of the first pair of balanced cantilever bridges in this country. It was decided an alliance was required to build the replacement because of the complexity of the works, made more difficult by the need to keep the six traffic lanes open throughout.
These advantages are acknowledged by project manager Ian Harbeck (Leighton Contractors). “We’re fortunate to have retained much of the crew that worked on the launching gantry as part of the Northern Gateway toll road project [Alpurt B2],” he says. “The retention of such a vast amount of skill, knowledge and experience gives me great confidence in the ability of my team to complete this incredibly complex and high-risk project in a safe, considerate and timely manner.” NGA Newmarket is constructing a new southbound-traffic bridge next to the existing viaduct on its northeast side, a shift requiring realignment of the motorway on- and off-off ramps at Gillies Avenue and the on-ramp at St Mark’s. The new northbound bridge will be built in the space left by the dismantling of the existing southbound structure to create a 30-metre-wide twin-bridge viaduct, three metres wider than the old structure and slightly higher. The old northbound lanes will then be removed. NGA’s construction/deconstruction plan is:
Constructing the new viaductAfter consideration had been given to building the new viaduct as a steel or concrete box structure, it was decided to construct it using precast match-cast segemental balanced cantilever methodology. This involves the deck being built out simultaneously and equally either side of a pier using the overhead launching gantry. These spans are extended out until the midpoint is reached, whereupon each is attached to an identical half deck span built out from an adjacent pier. At the two end spans, where no adjacent pier exists, the spans are completed using temporary supports.
A self-launching piece of equipment, it is powered by a 150kVA generator and is designed to launch for a 62-metre span. After Alpurt it was stored at Newmarket, where it was serviced and painted. It was assembled during December and January, had hydraulics fitted, received a “warrant of fitness” from an independent expert from the United Kingdom, and began its launching work in February. Tommy Parker, the Transport Agency’s state highways manager for Auckland and Northland, has noted the importance of Big Blue to the project’s construction efficiency and the ability to keep the southern motorway open. He has also pointed out that use of the gantry will enable stage one of the project to be completed before the 2011 Rugby World Cup. The first 48 bridge segments were craned into place before Big Blue took over to place a further 169 segments required to complete the new southbound bridge. A total of 468 segments, weighing up to 80 tonnes, will be needed for the entire viaduct. They are being fabricated at the NGA’s precast facility at East Tamaki and are being post-tensioned by multiple steel cables capable of resisting a combined force of 540 tonnes on the viaduct. Already all the southbound segments have been completed and work has started on the northbound sections.
The new viaduct, which will stand up to more than 20 metres above Newmarket, is being built to resist a one-in-2500-year earthquake. It will feature better noise protection for the surrounding community, thanks to noise-absorbing asphalt, solid concrete crash barriers, and partial noise walls. Urban-design improvements will also be part of the project, such as a volcanic-themed walkway from Gillies Avenue down towards Newmarket. Dismantling the old viaductBig Blue’s key role in building the new viaduct will be repeated in reverse in the dismantling of the old viaduct, considered to be the most difficult part of the project. “The immediate proximity to motorway traffic and such a busy retail environment further complicates an already highly complex deconstruction schedule,” project manager Ian Harbeck says.
Extensive analysis and computer modelling have been carried out to predict what will happen at each stage of the deconstruction process. Determining the forces which exist in the viaduct is complicated by its having experienced the effects of creep, shrinkage and temperature variations over the past 45 years. It has also been internally strengthened on several occasions. Detailed investigations and inspections have been carried out to gain a better understanding of the structure’s condition. Core samples have been taken to verify the strength of the concrete and holes have been drilled into the bridge deck to inspect the condition of the pre-stressing strand. A variety of methods will be used to monitor the viaduct throughout the dismantling process to confirm it is behaving as expected. These include precise three-dimensional surveys, measurement of strains and relevant movements at critical locations, and checks on the loads in temporary works. All the dismantled material will be recycled. NGA Newmarket is looking at a total of 26,000 tonnes for the 11,000 cubic metres of concrete in the old structure. Other materials for recycling will include 900 tonnes of steel, 500 tonnes of steel cable, 2400 tonnes of asphalt, 150 tonnes of median grating, and 50 tonnes of bridge railing.
As part of NGA Newmarket’s commitment to raising the bar in New Zealand construction practice, the alliance wants to find innovative ways to increase the value of the recycled product rather than see it used simply as fill on other projects. One possible application is for a noise wall on the southern motorway. The alliance has received Transport Agency funding and applied for a Ministry of Environment grant to pursue recycling opportunities and alternatives. It has also invited contractors to tender efficient and innovative solutions. Given the complex tasks which lie ahead, NGA Newmarket will be hoping the sun continues to shine on its project as it has done this past summer. Certainly, Ian Harbeck is fully aware how high his team is flying. “These unprecedented challenges make the viaduct replacement a genuinely world-first project,” he says.
Contractor Vol.34 No.4 May 2010 |
One of the balmiest summers in living memory has speeded progress on one of Auckland’s trickiest roading projects – the staged replacement of the 700-metre-long Newmarket Viaduct while still allowing it to be used by more than 160,000 vehicles daily as an indispensable section of the city’s southern motorway.
The NGA was chosen for the knowledge and experience gained in building bridge structures on Alpurt B2, the savings in time and cost it delivered on that project, and the expectation of similar savings at Newmarket through the transfer of workforce and equipment.
Stage one – Building of the new southbound bridge (now just past the halfway point, with expected completion in November). A fourth lane is being added to the bridge and is being continued as far as the Greenlane exit 1.7 kilometres away to accommodate the 16 percent of traffic which exits the motorway at that point. The continuation beyond the viaduct is an $18 million contract being carried out by Downer EDI Works (expected completion in February 2011).
The gantry makes this method of construction economical and practical in an area crowded at motorway and ground level (Newmarket’s Broadway carries 30,000 vehicles a day). The 800-tonne, 140-metre-long monster, known as Big Blue, is owned by NGA member and bridge-building specialist VSL and was brought to this country from Singapore.
NGA Newmarket has also finished all the piling and grouting work for the southbound piers, which are up to 18 metres high. Footing work is expected to be completed before mid-May. All 12 southbound piers have been erected and the last one will be flared about now. Half the northbound piers have been completed, three pours have been carried out to close the gap where spans meet in the middle, a noise wall at Mt Hobson Road and a mass block retaining wall at Gillies Avenue have been erected, and a Gillies Avenue overbridge is underway.
The old structure has deck spans ranging from 33.5 to 61 metres. Temporary steel bracings have been installed not only so segments can be cut out and lifted away using the overhead launching gantry but to improve the seismic ability of the bridge while it is still in use.
It may be possible to crush concrete on site, but given the space constraints within a highly built-up area the more likely solution is that the materials will be transported off-site.