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. These reinforcing structures will be about half a metre thick and the same height as the existing walls but not as long.
- constructing a new reinforced concrete beam behind the top of both tunnel entrances. You won't be able to see these horizontal beams but they 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 about 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)
What to expect
The speed limit has been reduced to 30km/h in the vicinity of the tunnel for the duration of the project and temporary traffic lights are in place. The tunnel is down to one lane at times during the day.
You can drive or cycle through the tunnel in both directions but expect short delays when it is down to one lane.
Two lanes are operating at peak times and other times.
Work hours are 7am-6pm, Monday to Saturday. However, we're not planning to do work that will affect traffic flows at peak times, 7am-9am, and 4pm-6pm, Monday to Friday.
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.
The contractor is Fulton Hogan, the same company that strengthened the Karori Tunnel.
No night work is planned at this stage.
A site office and an area to store gear and equipment has been established on the Northland side of the tunnel. This means some on-street car parking spaces will not be available until the work is finished.
Scaffolding will be going up on both portals, so work can happen on both sides of the tunnel as required.
Drilling the holes for the 36 rock anchors will be noisy at times, but will happen during the day.
The tunnel was built in 1927 and is listed as a heritage structure in our District Plan.
Contact Team Leader Structures, Faiz Tawfeek.