New bridge saves driving time…and wildlife too

The 256m-long 'eco-viaduct' takes shape at Otanerua. Picture / Martin Sykes

Traffic using the $365 million toll motorway being built between Orewa and Puhoi will cruise past kauri treetops on an "eco-viaduct" towering 34m above a picturesque bush stream.

The 256m-long Otanerua Viaduct, which commands views across regenerating kauri forest towards Hatfields Beach and the Hauraki Gulf, is the first major structure to be substantially completed along the motorway's 7.5km route.

Although it has yet to receive its surface seal and lane markings, the viaduct's concrete deck is already carrying trucks as part of an internal haulage route which is minimising the use of public roads for moving tonnes of construction material.

Far below its dual carriageways, rare semi-flightless fernbirds and other wildlife will be able to move through a bush canopy between the viaduct's 14 giant concrete legs, as native freshwater fish continue to swim up and down the northern arm of the Otanerua Stream.

Although conservationists wanted an even longer viaduct (to cater for two other tributaries of the stream), Transit NZ believes it is providing generous enough wildlife corridors at key points along the route.

The project director for the Northern Gateway Alliance construction consortium, Robert Jones, yesterday pointed out horizontal "ladders" aimed at easing the passage of fish through culverts where motorway embankments will be built instead of bridges.

Fernbirds are shy creatures which are hard to spot, but a fat and noisy woodpigeon flew low over the heads of visitors walking under the Otanerua Viaduct yesterday.

Several initial piers have been constructed through bush for a second slightly smaller viaduct to the south, over the northern branch of the Nukumea Stream. The structure was added at a late stage in a bid by Transit to reduce the motorway's environmental "footprint".

Contractors have also bored 100m so far into Johnstones Hill at the northern end of the project, for the second of a set of twin 333m tunnels which are replacing an earlier plan to carve 60m out of the bushclad ridge.

The Otanerua Viaduct, which includes 80 concrete beams across eight spans and is almost as high as the Auckland Harbour Bridge, will not be the largest structure on the motorway extension.

Pre-cast concrete segments are being turned out at a large workshop in the Waiwera Valley for a set of twin bridges, 537m long, on the motorway's southern approach to the tunnels.

Dump trucks are meanwhile carting away tonnes of earth and rock from a 55m cut into a saddle on Chin Hill, which will remain the highest topographical feature of the motorway route at 157m above sea level, between the Waiwera and Otanerua catchments.

Mr Jones said about 1.2 million cubic metres of material was moved in the last construction season, and he expected similar progress this summer. He was also quick to point out plans to revegetate the route with 1 million native plants.

Another bridge, to carry traffic over the motorway at the end of Hillcrest Rd to the north of Orewa, is also substantially complete and work will start this summer on an interchange at the southern end of the project.

Although the motorway is not due to open until early 2009, Mr Jones said the construction consortium was on track to complete the project by the end of 2008, depending on weather and other considerations.

He said a culture of environmental responsibility had spread to the point that "hardened" construction workers who might once have been the first to bulldoze earth into a stream now appreciated steps taken to protect natural features and wildlife habitat.

Transit proposes to part-fund the motorway with an inflation-adjusted toll (likely to start about $2). It predicts time savings of six to nine minutes, depending on traffic.

* Construction work will be interrupted on Sunday while buses carry 300 people along the route for a rare open day which was booked out almost as soon as it was announced.