Northland Tunnel strengthening to start soon

14 December 2016

Earthquake-strengthening work on Northland Tunnel is due to get under way early in the New Year and will take about six months.

Northland tunnel.
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Like the work carried out on Hataitai bus tunnel and the Karori Tunnel in recent years, the project will make the tunnel’s portals and adjacent retaining walls, stronger and more resilient.

The work is part of Wellington City Council’s ongoing programme to strengthen and protect key structures and transport routes, including tunnels, bridges and retaining walls. It has been planned for some time and is not directly related to November’s earthquake. 

Wellington Mayor Justin Lester says the city’s roads and infrastructure came through last month’s earthquake relatively well, but there is no room for complacency. 

“We need to keep prioritising and investing in projects like this which reduce the risk of damage and help ensure we can get the city up and running as quickly as possible after significant shakes.” 

Temporary traffic lights will be installed and the speed limit in the immediate area reduced to 30km/h for the duration of the project. 

Work hours are Monday to Saturday, 7am to 6pm. The tunnel will operate in both directions but will be down to one lane at times during the day. To minimise disruption at busy times, two lanes will operate during peak hours (Monday to Friday, 7am–9am and 4pm–6pm). 

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

Wellington City Council portfolio leader for Transport Strategy and Operations Councillor Chris Calvi-Freeman says structural assessments on Northland Tunnel have shown that the tunnel itself is likely to withstand a reasonably large earthquake. 

“However, there is a risk the banks above the tunnel could slip and damage the concrete portals or walls adjacent to them – potentially blocking the road and tunnel,” he says. “We plan to reinforce these structures in a range of ways and securely anchor them to the rock below to make them stronger and safer. 

“I’m hoping road users will be patient if their journeys are delayed for a few minutes while the work progresses. This work is an essential part of making the city more resilient and keeping everyone safe.” 

Fulton Hogan, the company the Council has contracted to do the work, will be setting up on site from Tuesday 10 January.  All going well, the work will be complete by the end of June. 

A site office and an area to store gear will be 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. 

Work on the tunnel will include:

  • putting scaffolding up on both sides of the tunnel
  • 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. These horizontal beams won’t be visible 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. 

The tunnel was built in 1927 and is listed as a heritage structure in our District Plan. Changes planned will be in keeping with the existing design.