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One of the country's most distinctive engineering features, the sweeping Newmarket Viaduct on Auckland's Southern Motorway, may be replaced in time for the Rugby World Cup in 2011.

The $150 million project has cleared legal hurdles earlier than expected, meaning Transit NZ will consider if it can be accelerated, even though it is not due for completion until 2013.

Transit northern manager Peter Spies said yesterday that a consent order issued by the Environment Court without need for an appeals hearing meant the project was now "buildable" and his organisation had applied to Land Transport NZ for funds for design work likely to take six to nine months.

He said Transit's board would consider at its July monthly meeting whether the three-year construction project could begin before a planned start in 2009-2010.

Asked whether there was a chance of completion in time for the world rugby extravaganza, Mr Spies said: "Certainly that is one of the opportunities that would be looked at."

The 700m viaduct, up to 20m high in places, was completed in 1966 for $2.26 million on New Zealand's first pair of balanced cantilevered bridges.

Transit intends replacing the viaduct parallel to its existing alignment but to far higher earthquake-resistance and safety standards.

Mr Spies said its rating to withstand only a one-in-500 years magnitude quake made it the most seismologically vulnerable structure on Auckland's motorways.

Transit's website warns that parts of the viaduct could collapse in just a "reasonably moderate earthquake", making it unacceptably risky by international standards, given its role as Auckland's main link to the south.

A lot of debris also falls from the viaduct, and in 2004 a truck driver was lucky to escape serious injury when his vehicle plunged 15m off it and landed on its roof beside St Marks Rd.

Although Transit has yet to decide whether to build the new structure from steel or concrete, it will be designed to withstand a one-in-2500 years earthquake and will have safety barriers and handrails up to 1.5m high.

That is almost twice as high as the existing barriers, which have gaps through which debris can fly. Transit says it will consider other safety options as a condition of satisfying an appeal from the Auckland Regional Council to protect a viewshaft from the viaduct to Rangitoto Island.

Mr Spies said all five appeals to the Environment Court had been settled without need for a hearing, including one from retail developer Westfield, which wants to build large numbers of new shops right up to the replacement viaduct.

The new structure will be 13m to the east, or seaward side, of what it is replacing and 3m wider after the addition of a fourth southbound lane.

Transit has developed a cunning engineering strategy to build it in stages, with little or no impact on the 200,000 vehicles carried each day across the existing viaduct, which is busier than even the harbour bridge.

Construction will start with four new southbound lanes, followed by the demolition of the existing three southbound lanes.

These will then be replaced in the same spot by three new northbound lanes, allowing the rest of the old structure to be demolished.