Northland Tunnel strengthening

From January to November 2017, we are doing earthquake strengthening work on the Northland Tunnel entrances and adjacent retaining (wing) walls.

Delay in Northland Tunnel seismic strengthening

Our seismic strengthening on the Northland Tunnel has had a slight set-back as we discovered cables which weren’t on our service plans – and probably date back to the days of the trams. Our health and safety regulations meant work had to be stopped while we confirmed the cables weren’t live. We apologise for the convenience, and estimate the work to be finished early November (not mid-September, as was originally planned).

Strengthening work being carried out at Northland Tunnel.

Northland tunnel

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We are doing this work because structural assessments show that while the tunnel itself is likely to withstand a reasonably large earthquake, the banks above the tunnel could slip and damage the concrete portals and retaining walls – potentially blocking the road and tunnel. 

The strengthening work will reduce this risk and make it more likely the tunnel will be able to be used following an earthquake.

This work is part of the Council's ongoing programme to strengthen important transport routes and earthquake-prone structures, and is not related to the November 2016 earthquake. Similar work has already been carried out on Karori Tunnel and the Hataitai bus tunnel. The NZ Transport Agency has also upgraded the Mount Victoria Tunnel.

What we are doing

  • building new retaining walls on the Northland side in front of the two existing retaining (or wing) walls that are on either side of the tunnel entrance
  • constructing a new reinforced concrete beam behind the top of both tunnel entrances which will help secure the portal to the underlying rock
  • enlarging the two existing vertical buttresses on the Karori side and securing them to the hill with long steel rods called rock anchors
  • clearing vegetation and building three new buttresses on the Karori side in front of the existing retaining wall to the right of the tunnel entrance
  • drilling and installing 36 rock anchors
  • repairing cracks, plastering and repainting the tunnel entrances
  • reinstating and planting the banks above the tunnel.

See the Northland Tunnel Seismic Assessment Report (35.6MB PDF)

Progress so far

Work on the Northland Tunnel is progressing well with 40 percent of work completed. The strengthening is taking longer than expected as we had to change the design to deal with unexpected ground conditions behind the portal wall. The limited availability of specialist sub-contractors following the Kaikoura earthquake, led to more delays. We’ve got extra resources now and expect to complete work by mid-September. Here is a summary of what we have done so far:

  • completed construction of reinforced concrete beam behind the top of Karori side of the tunnel
  • 16 of the 36 rock anchors have been drilled, installed and tested
  • built two new columns on the Karori side of the tunnel
  • completed all crack repairs in the two existing retaining (or wing) walls
  • looked through other concrete repairs needed and prepared the site for further repairs.

See the Northland Tunnel work update report (18KB PDF) | Text version (26KB RTF)

What to expect

Traffic changes

The tunnel will be down to one lane, Monday to Friday, 9am to 4pm. Temporary traffic lights will be installed and traffic management will be in place for drivers and cyclists.

You’ll be able to travel through the tunnel in both directions but there will be short delays during the day when only one lane is open. There will continue to be some delays on bus routes at these times.

Two lanes will operate at peak times, at nights and during the weekends.

The speed limit in the vicinity of the tunnel will be reduced to 30km/h for the duration of the project.

Pedestrian access

The footpath in the tunnel may be out of action at times, but an alternative safe route through the tunnel is being provided for pedestrians at all times. 

Parking changes

Please do not park in this area during working hours including the wider berm/car parking at the corner of Northland Tunnel Road/Northland Road on the western side of the North Portal. If your car gets in the way it might have to be towed. If your car cannot be returned to where it was originally, it will be parked in the nearest available space. Please check nearby streets if your car is not where you left it before contacting the contractor.

Other things

No night work is planned at this stage.

Scaffolding will be going up on both portals, so work can happen on both sides of the tunnel as required.

Construction work is a noisy, dirty business but please bear with us. We will be doing all we can to minimise the impact on your household and business. Your cooperation and patience will help us get the job done as quickly as possible

The contractor is Fulton Hogan, the same company that strengthened the Karori Tunnel. You can phone them on 04 576 1293 or 027 839 5823.

Background

The tunnel was built in 1927 and is listed as a heritage structure in our District Plan.

More information

Contact Team Leader Structures, Faiz Tawfeek.

Email faiz.tawfeek@wcc.govt.nz