Wellington fast-tracks retaining wall construction after flurry of slips adds to work

Wellington City Council is fast-tracking construction of new retaining walls while also upgrading its current ones in poor condition.

The city has been in recovery mode after getting plummeted by 670 slips in seven weeks this winter.

Eighty five high-priority slip sites from that period were prioritised down to 27 for action – seven of those will be fast-tracked for the next year and 20 the year after.

Of the almost 3000 retaining walls keeping things upright across Wellington’s hilly city, 206 are in poor or very poor condition.

But there is one which is front of mind for council transport and infrastructure manager Brad Singh.

Karori Retaining Wall

A 200-metre-long wall holding up Northland Tunnel Road was rated in average condition. Photo: RNZ / Samuel Rillstone

At Chaytor Street, on the way to Karori from the city, a 200-metre-long, almost 8m-high wall holds up Northland Tunnel Road.

While it was rated in average condition, there were risks, Singh said.

“It does have limitations when it comes to seismic movement, and so at some point the wall needs to be strengthened in order to make sure it can handle any seismic event.”

Despite the wall being a top priority for expensive strengthening, Waka Kotahi this year declined council’s bid for extra funding.

Karori slip

Wellington suffered 670 slips over winter this year. Photo: RNZ/ Samuel Rillstone

Brooklyn, Khandalah, Kelburn and Johnsonville all have among the most walls or accessway walls in either poor or very poor condition.

But Wadestown and Wilton together had 15 in bad nick.

Wadestown, which neighbours Wilton, is also a key alternative route out of the city in an earthquake – if State Highway 1 is compromised.

It was a priority area, but winter had added more to council’s plate, Singh said.

“We’ve constantly got to prioritise our workload … obviously post those slips we’ve had to reprioritise because some of those sites are more urgent than the planned walls we had on our list to build.”

Wadestown wall slip

Wadestown is considered a priority area for retaining wall upgrades and construction. Photo: RNZ / Samuel Rillstone

But the issue for Wellington is not just where retaining walls are, it is where they are not too.

The capital has cut and filled into the hills for years, building roads and properties above or below them.

DTCE Structural Engineering director Marlo Bromley said those roads did not need consents like buildings did and it was risky.

“All the roads around Wellington, all have cut faces that are just at an unsafe angle, it’s just too steep,” Bromley said.

“So if you have a house, and it’s close-ish to a cut face that’s over the top of a public road, then you might want to have it looked at by a geotechnical engineer.”

The ground took decades to settle into place after developments like roads and properties so in the long term things should ease, Bromley said.

Chaytor Street in Karori, Wellington, was down to one lane after a slip fell on the road.

A slip came down on Chaytor Street in Karori, Wellington in October. Photo: Supplied / Wellington City Council

Over the next decade, the city council is spending more than $18 million on retaining wall upgrades and much of that will be within the first few years.

Councillor Tamatha Paul, chair of the environment and infrastructure committee, said that was necessary.

“Climate change demands that of us, because there’s nothing we can do about the fact that the weather is changing – our budget, I think, reflects that,” Paul said.

“I know that there’s ambition around the table to make as much budget commitment deliverable as we can.”

The city council will try to get more funding for the Chaytor Street wall from Waka Kotahi next year.

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