KEY POINTS:
Builders of sweeping twin viaducts across the bush-clad Waiwera Valley on Auckland's $360 million Northern Motorway extension say they are not presuming to compete with nature.
Transit NZ and its development partners decided against adorning the 540m viaducts - the largest structures on the 7.5km tolled motorway extension between Orewa and Puhoi - with etched concrete-panel images of flora and fauna used to soften the impact of big urban roading projects such as Spaghetti Junction.
Northern Gateway Alliance project director Andrew McRae said that trying to superimpose images of nature against the natural grandeur of the valley would have created a pointless distraction for motorists.
"We want to keep things as simple as possible so there is no unnecessary extra impact on the environment," he said.
"We are trying to blend in and minimise the impact."
That might sound like a tall order, given the size of the viaducts.
They are being extended rapidly across the Waiwera River by a specially imported 135m "launching" truss to meet a set of twin road tunnels already bored through a steep hillside separating the valley from Puhoi at the northern end of what will be Transit's first tolled motorway when it opens in 2009.
But the viaducts seem surprisingly unobtrusive, despite passing close to a pristine island resplendent with giant native ferns in the middle of the tidal river, as they sweep on a gradual curve towards the tunnel portals poking discreetly out of Johnstones Hill.
Rising to a maximum height of 34m above the river, on seven sets of piers designed to support 365 pre-fabricated concrete box segments ranging in weight from 50 tonnes to 95 tonnes each, the viaducts are about two-thirds complete and should reach the northern embankment in March.
Project structure construction manager Vaughan Robbins said the bridge spans were being built to lengths of up to 76m to limit the number of piers in compliance with tough environmental conditions of resource consents.
That had also enabled the bridge-builders to avoid intruding into a burial mound between the two viaducts which contained remains of three 19th-century settlers who drowned in the river.
Community reference group member Tom Mayne, president of an Orewa residents association, said he was impressed with the visual line of the viaducts and thankful Transit had abandoned an earlier plan to carve a 57m chunk out of Johnstones Hill.
A ministerial advisory group criticised Transit early this year for adding the 333m tunnels to the project instead, at a cost of $35 million, which it said was "not fully justified."
But Mr Mayne said the tunnels were far more acceptable than the original plan to "bulldoze right through the bush" and he hoped they would set a standard for future roading projects in sensitive natural areas.
The overall project is more than 70 per cent complete.
New Rodney District Mayor Penny Webster said that although she had shared the concern of many others that the addition of the tunnels might have caused more delays to a long-awaited and sorely needed alternative to the winding Hibiscus Coast Highway, she believed the environmental result justified the extra cost.
Transit project manager Brett Gliddon said the tunnels had the added safety advantage of allowing the northern end of the motorway to tie back in with the coastal highway at a lower altitude than would have been practicable in the open-cut plan.
Even so, the project partners expect to have moved almost four million cubic metres of earth before the motorway opens, including 1.2 million cubic metres from a 53m cut through the highest topographical feature along the route, Chin Hill, south of the Waiwera River.
The project also includes two other substantial viaducts, across the Otanerua and Nukumea streams between Waiwera and Orewa, aimed at allowing wildlife such as rare fernbirds to pass underneath.
Toll gantries will be built near the southern end of the route to charge motorists just over $2 and truck drivers twice that sum.
