Flying over Newmarket

The replacement of the Newmarket Viaduct is a key element in the freeing up of Auckland’s congested motorway system.  HUGH DE LACY explains.

Newmarket_1.jpgWhen it was completed in 1965 – just six years after the Auckland Harbour Bridge – the six-lane Newmarket Viaduct with its tall, slender piers was something of an engineering wonder, the first of its kind in the Southern Hemisphere.

Forty years on it is a much-maligned contributor to Auckland’s chronic traffic congestion, too weak seismically to withstand the heaviest loaded trucks let alone a severe earthquake, so narrow in the shoulders that any accident stops traffic flow and makes it difficult for rescue vehicles to reach the site, suspect to the high stresses caused by variations in temperature, with edge barriers so flimsy a truck crashed off the southern sections a couple of years ago, and barely capable of accommodating vehicle movements that have trebled to 160,000 a day.

Add to that the distorting effect it has had on the development of the inner-city suburb of Newmarket and the need to free up Auckland’s traffic ahead of the Rugby World Cup in 2011, and the old viaduct was doomed by its own inadequacies.

And even though its replacement will be sited only 13 metres to the east, it is to be pulled down and most of its components recycled.

Newmarket_2.jpgThe new viaduct, on which enabling work was officially launched by then Prime Minister Helen Clark in November of last year, will be built for the New Zealand Transport Agency at a total cost of $215 million. The project will take 49 months, and is due for completion in December 2012, though the four new southbound lanes will be completed, and tied in with an expanded road layout running as far as Greenlane, in time for the Rugby World Cup.

The project has been contracted to the Northern Gateway Alliance, the same group of companies that built the Northern Gateway toll road out of Auckland, and which has been re-dubbed NGA Newmarket. It comprises Fulton Hogan and Australia’s Leighton Contractors as the main builders, backed up by specialist designer URS NZ, environmental planning consultancy Tonkin & Taylor, environmental consultancy Boffa Miskell, engineering and architectural consultant Beca, and large-span bridging engineer VSL.

Newmarket_4.jpgThe significance of NGA being kept on for Newmarket is the similarity in challenge between the two projects, in particular that between the new viaduct and the twin bridges at Waiwera on the northern toll road. The same 680-tonne overhead launching gantry used at Waiwera will be used at Newmarket (pictured right being disassembled). Powered by a 150KVA generator, the 140 metre tall gantry, designed to launch spans up to 62 metres long, is soon to become a temporary feature of the Auckland skyline.

“Transferring the equipment, experience and knowledge from the Northern Gateway project to Newmarket realises a value proposition,” NZTA regional director, Wayne McDonald told Contractor. “The elements we’ve brought into this new project will speed it up - and indeed has already speeded it up through the design processes. It also allows us to benefit from the site and safety management aspects.”

The viaduct will be 686 metres long and stand 24 metres above Broadway at its highest point. It will comprise four southbound and three northbound lanes, though there will be provision for a fourth northbound lane to be added later. It will be built to modern seismic standards, capable of withstanding a one in 2500 years earthquake. It will feature enhanced safety and protection through solid edge barriers incorporating elevated slim-line rails that will also help prevent debris falling over the sides while still allowing motorists some appreciation of the view it will offer over central Auckland.

Newmarket_3.jpgThe surface will be of open-graded porous asphalt (OGPA) to reduce skidding and improve noise attenuation. Replacing the old viaduct was also seen by the planners as an opportunity to enhance land usage below, replacing the plethora of used car lots and temporary parking lots, ordained by current building restrictions, with developments more appropriate to an inner-city suburb.

There is even provision for a volcanic walkway between Gillies Avenue and Broadway that will expose parts of the old Mt Eden lava flow presently covered with rubble and vegetation.

The construction method is the pre-cast segmental balanced cantilever, with the deck being simultaneously built out on either side using the overhead launching gantry. The spans are extended to the mid-joint where they are joined to the cantilevers extended from the adjacent piers by a concrete stitch and pre-stressed tendons. The two end spans, which have no adjacent pier, will be completed using falsework.

The work, including the deconstruction of the present viaduct, will be done in four stages, the first of which is the building of the new southbound bridge on the eastern side of the existing one. Stage two will see the old southbound viaduct dismantled, and stage three the new northbound bridge built in place of the old southbound one. Stage four will see the old northbound viaduct dismantled.

The deck spans on the old bridge range from 33.5 to 61 metres in length, and were early examples of a cast-in-situ balanced cantilever box girder bridge. They will be dismantled in a reverse of the construction method for the new viaduct, in a balanced cantilever sequence using the overhead launching gantry.

Temporary bracing will be needed during the successive dismantling of the two halves of the old viaduct, to ensure the stability of the remaining half and to support the extra weight of the equipment.

The entire project is to be carried out while keeping the motorway open at all times, and while mitigating noise, dust and any other inconvenience to the city’s commuters and businesses. It is also intended to be as environmentally sensitive as possible, from reducing, re-using or recycling waste to using sustainable procurement policies and encouraging staff to use public rather than private transport to get to work.

All of which amounts to a pretty tall order given the location and size of the project and the need especially to get the four southbound lanes operative in time for the Rugby World Cup.   

Contractor Vol.33  No.4  May 2009
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