Newmarket Viaduct

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The alignment of the viaduct as seen from Mount Hobson, looking west. The viaduct will shift to the right in the future.

The Newmarket Viaduct, sometimes considered 'one of the most distinctive engineering features' of New Zealand,^[1] is a six-lane State highway viaduct in Auckland, the country's largest city. Carrying the Southern Motorway over the Newmarket suburb area southeast of the CBD of the city, the 700 m long viaduct is up to 20 m high in places. Due to concerns about its ability to withstand earthquakes and its increasing inability to cater for peaktime traffic demand, it will be replaced with a stronger and wider structure within the next few years.

[edit] History

Original structure

The viaduct was finished in 1966 for NZ$2.26 million, and was constructed as New Zealand's first pair of balanced cantilever bridges. While considered a successful design, it was built to far lower earthquake standards than which other New Zealand structures now fulfill, being built to survive only a 1 in 500 year return period earthquake. This made the structure the weakest strategic transport infrastructure link in the Auckland area, with a potential collapse cutting off all motorway transport to the South.^[1] Also, during design, the temperature stresses were not considered to a sufficient degree, with the expansion and contraction leading to increased wear on structural elements.^[2]

'Concrete box' structure as seen from below, looking west. A truck once toppled over the left side near here.

Plans for replacement

The limited earthquake stability, together with increasing traffic demands (over 200,000 vehicles a day, more than on the Auckland Harbour Bridge and the most traffic on a section of road anywhere in New Zealand),^[3] led to the planning for a new viaduct structure, which will be able to withstand an earthquake with a 2,500 year return period. Other factors were the very low safety barriers (which once allowed an out-of-control truck to fall onto its roof 15 m below, in 2004), which are also insufficient to prevent debris from falling onto the properties beneath,^[1] and the fact that the existing viaduct is a prohibited route for overweight vehicles, forcing more trucks through the city streets.^[2]

Major maintenance of the existing structure was also considered as an option. However, the structural elements of the existing viaduct did not lend themself to easy strengthening, and even a reconditioned viaduct would only have provided an estimated life expectancy of 40-50 more years. With a proposed new structure costing only around 15% more than a repair, and expected to last for over 100 years, it was decided to go for a full replacement.^[2]

Proposed structure

The new Transit New Zealand structure is estimated to cost NZ$ 150 million, and cleared planning hurdles quicker than expected, leading to hopes that it would be possible to build it in time for the 2011 Rugby World Cup. Originally, the three-year duration project was expected to start in 2009/2010. Factors that influenced the quick decision process were the lack of any archeological evidence in the construction area, as well as the lack of substantial numbers of residential properties affected by the new structure.^[4]

The new structure will be built in a way to minimally affect the traffic flow on this crucial motorway link, by first constructing a new viaduct of four southbound lanes to the north-east of the existing structure, then demolishing the existing three southbound lanes, before constructing the three new northbound lanes in the gap and finally demolishing the three old northbound lanes. It has not been decided on whether to build a 'steel box' or a 'concrete box' structure, however both would look similar to the existing viaduct structure.^[1]^[5]