Engineers size up tricky bridgework

The $195 million replacement for the Newmarket Viaduct will be more earthquake-proof than the existing structure. Photo / Dean Purcell

Engineers preparing to start digging in the first of 24 columns this month to support a $195 million replacement Newmarket Viaduct believe their trickiest task will be demolishing the existing structure.

Tonnes of temporary supporting steel will have to be erected to allow the southbound carriageway of the 690m viaduct to be demolished without toppling the northbound side, which will remain open to motorway traffic until towards the end of project, in 2012.

A 140m launching gantry, the same one used by the Transport Agency and its contracting alliance to build a viaduct across the Waiwera River on the Northern Gateway toll road, will be erected above the Newmarket skyline for both the demolition and construction phases of the project.

The plan is for a replacement southbound structure to be erected in the first instance, 13m north of the existing viaduct and with four traffic lanes, a gain of one lane in that direction.

That new structure will be available for traffic, and then the southbound half of the old 1965 viaduct demolished, by the end of 2010 - in time for Auckland to host the Rugby World Cup the next year.

A replacement northbound carriageway of three lanes will then be built in the gap left by the old southbound structure, by the end of March, 2012.

The remainder of that year will be taken up in demolishing the old northbound lanes.

Although the elaborate sequencing will allow the entire viaduct to be replaced on piers up to 18m high with minimal disruption to the more than 160,000 vehicles using it daily, there will have to be some closures of local roads as well as motorway on and off-ramps.

These will include a fortnight-long closure from Saturday of the St Marks Rd on-ramp, for preparations to move that structure to make way for the wider viaduct.

A supporting steel truss will also be manouevred into place over Easter, to brace the existing viaduct at its seismically weakest point, directly over the main trunk railway line.

Transport Agency northern director Wayne McDonald said last night that though increasing traffic capacity was the main "driver" for the project, there was also a need to improve the seismic robustness of such an important piece of the motorway network to withstand a once-in-2500-year earthquake - five times the existing rating.

As well as serving traffic needs, the project will include a new pedestrian link beneath the northern edge of the replacement viaduct, between the Gillies Ave ridge and Broadway in Newmarket.

Engineers preparing to start digging in the first of 24 columns this month to support a $195 million replacement Newmarket Viaduct believe their trickiest task will be demolishing the existing structure.

Tonnes of temporary supporting steel will have to be erected so the southbound carriageway of the 690m viaduct can be demolished without toppling the northbound side, which will remain open to motorway traffic until towards the end of the project, in 2012.

A 140m gantry - previously used by the Transport Agency and its contracting alliance to build a viaduct across the Waiwera River on the Northern Gateway toll road - will be erected above the Newmarket skyline for the demolition and construction phases of the project.

The plan is for a replacement southbound structure to be erected first, 13m north of the existing viaduct and with four traffic lanes, a gain of one lane in that direction.

Once traffic is using the new structure, the southbound half of the old 1965 viaduct will be demolished.

The project is expected to reach this stage by the end of next year - in time for the Rugby World Cup the next year.

A replacement northbound carriageway of three lanes will then be built in the gap left by the old southbound structure, by the end of March, 2012.

The remainder of that year will be taken up in demolishing the old northbound lanes.

Although the elaborate sequencing will enable the entire viaduct to be replaced on piers up to 18m high with minimal disruption to the more than 160,000 vehicles using it daily, there will have to be some closures of local roads as well as motorway on- and off-ramps.

These will include a fortnight-long closure from Saturday of the St Marks Rd on-ramp for preparations to move that structure to make way for the wider viaduct.

A supporting steel truss will also be manouevred into place over Easter, to brace the existing viaduct at its seismically weakest point, directly over the main trunk railway line.

Transport Agency northern director Wayne McDonald said last night that although increasing traffic capacity was the main reason for the project, there was also a need to improve the robustness of such an important piece of the motorway network to withstand a once-in-2500-year earthquake - five times the existing rating.

The project will also include a new pedestrian link beneath the northern edge of the replacement viaduct, between the Gillies Ave ridge and Broadway in Newmarket.