Connecting the dots on the Northern Corridor Improvements Project: Excavation work to join SH1 and SH18 begins


From next week there will be lane changes for people using the Northern Motorway as 16,000m3 of material, the equivalent of six Olympic sized swimming pools, is excavated to create an underpass providing a direct connection between SH1 and SH18 as part of Auckland’s Northern Corridor Improvements (NCI) project.

“The underpass will provide a direct motorway to motorway connection for people travelling from the north heading west. When NCI is complete, it will be the final link of the western ring route”, says Waka Kotahi NZ Transport Agency National Manager Infrastructure Delivery Andrew Thackwray.  

Construction of the underpass began just over two years ago and is being built using a ‘top-down’ construction method which means that the roof and walls are constructed first and then the material under the roof excavated and taken away.

“With the majority of the walls and roof now complete, we have reached the stage where excavation can begin. The remainder of the walls and roof of the underpass will continue to be built in the coming months and will eventually become the lanes of the motorway above for vehicles travelling in both directions on SH1,” says Andy Thackwray.

With the underpass measuring 120m long, it falls just short of the requirements to be classed as a permanent tunnel. However, excavation of the material falls under the tunnelling and mining regulations and requires a crew of 30 people with specialised skills to work in an underground construction environment.

To enable this work to be carried out, there will be several changes for drivers travelling southbound on SH1. From the morning of 12 July, the southbound off-ramp at Upper Harbour Highway will return to its original position approximately 400m further south (see map below). This change means the team can move their work area to the middle of the motorway where they can finish building the roof and walls of the underpass and so that excavation can take place.

The speed limit will temporarily be reduced to 80km per hour from Greville interchange through to Constellation interchange for the duration of work.

“Our team need to enter and exit the work area directly from the motorway live lanes which is a significant safety risk. By lowering the speed limit, it will help keep drivers as well as our team safe,’ says Mr Thackwray.

“While traffic will be switching back to its original layout, we ask drivers to be patient as people get used to the adjustment.”

Once completed, NCI will provide a transport upgrade for the Albany and North Shore community, with the Northern Busway being extended from Constellation to Albany Station. It includes a new motorway connection between SH1 and SH18 and will open up access to the Western Ring Route and travel to the airport. It will deliver more than 7kms of walking and cycling paths and a wide range of local road and park connections. Work is planned to be completed by the end of 2022.

Northern Corridor Improvements project

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