Workers constructing the shell of the Karori Tunnel. Ref: 1/2-038256-F. Alexander Turnbull Library
The old brick portals at each end need to be braced so we will be constructing new reinforced concrete walls and buttresses at both entrances. We'll also be constructing a major new retaining wall on Glenmore Street at the Kelburn end of the tunnel.
The Council's Transport Portfolio Leader, Councillor Andy Foster, says the work has to be done because the tunnel is immediately adjacent to the city's main fault line and structural assessments show the existing portals would not fare well in a major quake.
"While the tunnel itself is likely to withstand a reasonably large earthquake, the adjacent slip-prone hillside and portals could well come down and block this critical route," he says.
"As well as the obvious danger this poses to tunnel users, it could put the tunnel out of action for months."
Karori tunnel, which opened in 1900, is the city's oldest road tunnel. It was built in the days of the horse and cart but is now used by 17,000 vehicles daily on average, including more than 400 buses.
"In many respects, it has stood the test of time remarkably well," Cr Foster says. "But the Council is progressively strengthening key transport routes."
Over the last 10 years, we've been targeting high-risk routes, strengthening road and pedestrian bridges and building new retaining walls that are designed to perform well in earthquakes.
Major strengthening work has been carried out on the Ngaio Gorge Road to secure an alternative route out of the city and future work is planned on Churchill Drive, the Hataitai bus tunnel and Seatoun and Northland tunnels.
The work on the Karori tunnel is expected to take up to nine months because of the difficulties associated with managing traffic flows and noise, and will mean disruption and delays for tunnel users at times. We plan to keep the tunnel operating during the day but it is likely to be closed for about six hours each night after the last bus goes through.
A start date and work hours will be agreed once a contractor is appointed in December, but it looks likely work will start early next year and be in full swing from late February. The trolley bus wires through the tunnel will be switched off around that time and Go Wellington plans to operate diesel buses.
Our intention is that the tunnel will operate in both directions at peak times, however for a number of hours during the day it will be down to one lane. Temporary traffic lights will be installed, traffic will be delayed at both ends and tunnel users will be encouraged to use alternative routes.
The work is expected to be considerably more disruptive than the upgrade work that has been happening in the Terrace and Mt Victoria tunnels this year. While that work has mainly happened inside the tunnels, this will happen at either end and much of it will be too noisy to do at night. As well as demolition, the contractor will need to drill a large number of deep holes so the new retaining wall and tunnel portals can be securely anchored to the hillside.
Council staff will be talking with nearby residents and businesses in coming weeks as planning progresses.