Viaduct gantry works towards meeting end-of-year target

The construction gantry towers over the replacement Newmarket Viaduct southbound lanes of the Southern Motorway. Photo / Brett Phibbs

Motorway traffic is expected to start using southbound lanes of Auckland replacement Newmarket Viaduct before the end of this year.

The $215 million project's giant blue gantry, which each night lifts 80-tonne concrete segments into place, is now straddling Newmarket's main street as it works its way along the new viaduct's 690m passage between Gillies Ave and St Marks Rd.

About half the southbound viaduct structure has been completed, and the Transport Agency is confident three of its four motorway lanes will be available to traffic by November.

That will give it the equivalent traffic capacity of the existing southbound carriageway, which will be demolished once the new structure is completed.

This will create make a gap into which the northbound side of the new viaduct will be built.

But drivers will have to wait until February to use the fourth southbound lane, to enable completion of complicated tie-in operations with the rest of the motorway.

These include a new $18 million auxiliary lane, which is being added to the 1.7km stretch between the viaduct and the Greenlane interchange.

That will be completed in ample time for the Rugby World Cup, meeting an agency commitment to provide extra traffic capacity to serve that event.

The agency's northern highways manager, Tommy Parker, said during a site inspection that the fourth southbound traffic lane would provide the project's major capacity benefit.

Although the main reason for replacing the viaduct is to strengthen the most seismically-challenged section of Auckland's motorway network, to be able to withstand a once-in-2500-years earthquake, the new northbound carriageway will have only three traffic lanes.

But because the two carriageways will be joined, Mr Parker acknowledged that their median barrier could theoretically be adjusted later to allow four lanes in each direction.

But he said that was highly unlikely, because of the need to align the viaduct with the tight capacity of Spaghetti Junction and because of the tendency of drivers to weave between lanes while jostling to position themselves for motorway exits north of the viaduct.

The new viaduct towers 20.5m over Newmarket at its highest point, and is from 50cm to 2m higher than the existing structure, along which 160,000 vehicles pass daily.

This allows it to overlap the old 1965 viaduct, rather than straying outside the area of land designated for the motorway.

Once the new southbound structure is completed, the agency's construction alliance will cut concrete connecting the carriageways of the old viaduct to start demolishing the southbound structure while keeping traffic flowing on the northbound lanes.

Tonnes of temporary supporting steel will be required to brace the northbound carriageway and stop toppling while the southbound structure is being demolished.

Each new carriageway will comprise 234 concrete pre-fabricated sections.

These are being "post-tensioned" throughout the construction process by multiple steel cables capable of resisting a combined force of 540 tonnes on the viaduct.

The concrete sections are being made at a yard in East Tamaki which has already fabricated enough to complete the southbound carriageway, and has begun producing components for the northbound structure.

The Transport Agency and its construction partners are planning an open day for the partly-built viaduct to coincide with other Newmarket celebrations on May 16.

IN THE PLAN
* Southbound lanes of the new Newmarket Viaduct should be in use before the end of the year.
* The last southbound lane will not open until February.
* The existing southbound carriageway will be demolished once the new structure is completed.
* This will make a gap so the northbound side of the new viaduct can be built.