Northland floods: Road to Kaitaia to open for Christmas, five months after slips

Monday, 19 October 2020

Waka Kotahi NZ Transport Agency Northland system manager Jacqui Hori-Hoult explains the difficulty of opening State Highway 1 south of Kaitāia after the July 2020 storm. (Video first published October 2020.)

State Highway 1 south of Kaitaia is set to reopen in time for Christmas, five months after the devastating floods that caused eight slips in the Mangamuka Gorge.

The road has been closed since the one-in-500-year flood on July 17 and will only be able to open one way to light vehicles, Waka Kotahi NZ Transport Agency’s Jacqui Hori-Hoult said.

The detour through State Highway 10 adds about 30 minutes to a trip to Kaitaia, isolating the small community of Mangamuka and stopping some children from accessing education.

When the Northland storm first happened, Waka Kotahi NZ Transport Agency was unsure how easy it would be to repair the underslips, but the full difficulty of the site has now been revealed.

Hori-Hoult said while most of the road has been cleared to at least one lane, there has been no safe way past the biggest slip on the northern side of the gorge, which saw material plummet 150 metres to the river below.

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Waka Kotahi NZ Transport Agency Northland systems manager Jacqui Hori-Hoult points to slip 7 in the Mangamuka Gorge, which continues to move in wet weather.

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State Highway 1 through the Mangamuka Gorge will remain closed until mid-December, when it will be open one way to light traffic.

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Slip 8 in the Mangamuka Gorge is the most difficult to fix, with a temporary platform needed to make it safe to drill deeper piles.

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SafeTrack foreman Terry Anderson is in charge of traffic control.

“Initially the plan was to open the road in late October by installing a wall of concrete piles to support the road, but the slip repair is complicated with significant safety issues,” she said.

“An estimated 7000 cubic metres of material fell away beneath the road, leaving the road surface unsupported to the centre line.

“The earth beneath the road is brittle and unstable and there is an old slip above the road that could reactivate.

“Due to the instability of the slip site, we have to put in small temporary piles to make sure the road can safely support the large machinery required to install the permanent piles, which will be 15 to 20 metres long.”

The transport agency is hoping to save the existing road and open it to light vehicles in mid-December.

“Our crews will be working 20 hours per day to get this done as quickly as possible, but much will depend on the weather and the stability of the hillside.”

As an example of the impact of the weather, one of the gorge’s eight slips keeps moving every time it rains.

The cost of the first stage of the repair is $3.8 million.

But the finished product will involve realigning the road and building high concrete walls to hold in place an historic slip.

The full cost, plus the date for a full opening, is not yet known.

Works will shut down for three weeks over the Christmas-New Year period to accommodate holiday traffic. Traffic management will be in place throughout this period.

When the road is closed, the detour is on State Highway 10, which adds about 20 to 30 minutes to a journey north to Kaitaia.

The area, also known as Maungataniwha, has great significance to many hapū in the north and the transport agency is consulting closely with them on the work, including how to beautify the concrete walls.