Wellington's oldest tunnel will not be safe in a major earthquake.
Wellington City Council has admitted it is unlikely the Karori tunnel entrances would withstand an earthquake greater than magnitude 7. However, strengthening work is to begin later this year.
The Karori tunnel, which opened in 1900, has cracks and leaks, and the hillside above the Kelburn entrance is unstable.
Transport assets manager Deven Singh said independent structural engineers had assessed the tunnel.
"The portals [entrances] could well fall in in a major quake, but they are perfectly fine in normal situations," Mr Singh said.
"The barrel itself should withstand a [magnitude] 6 or 7 earthquake, but if there's a really significant event, like an 8, then all bets are off.
"Of course, it depends where the earthquake is and how deep it is."
The Wellington fault runs under the Karori tunnel and an earthquake along that fault could result in damage to the hillside and tunnel entrances.
Mr Singh said the Karori tunnel could be closed for up to three months if the portals collapsed.
However, the council's plans to strengthen it include bracing the portals with metal rods, fixing the cracks and leaks, and building a retaining wall along the hillside near the Kelburn entrance.
GNS spokesman John Callen said tunnels generally performed well in earthquakes, although the portals could suffer damage.
"Our view is that the Karori and Northland tunnels would probably survive all but the biggest quakes reasonably well."
Mr Singh said work on the tunnel would start after the Rugby World Cup, once engineers had determined how to do so without causing too much disruption to traffic.
Council engineers have also assessed the Seatoun tunnel and Hataitai bus tunnel, and the portals of both are considered an earthquake hazard. It has been recommended they be braced with steel plates and bolts.
Work on the Hataitai tunnel is scheduled for 2013-14 and on the Seatoun tunnel for 2016-17.
The Northland tunnel portals also needed strengthening, Mr Singh said. Northland tunnel will be reassessed during the 2011-12 financial year and strengthening work is scheduled for 2014-15.
The Terrace and Mt Victoria tunnels are the responsibility of NZ Transport Agency.
NZTA spokesman Anthony Frith said both tunnels were seismically sound, but were undergoing maintenance work.
Both tunnels' fire-fighting, ventilation and drainage systems are being upgraded.
Work on the 80-year-old Mt Victoria tunnel is being carried out in two stages.
The first stage began last week and is expected to cost $17.5 million.
The second stage is expected to be completed next year. The cost of that has not been finalised.
The $50m Terrace Tunnel refurbishment is expected to be completed in September.
Work on the three-lane, 446-metre-long tunnel began in December.
- The Wellingtonian