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Project introduction

Works are underway to repair State Highway 1 through Mangamuka Gorge after heavy rains in July 2020 caused a number of slips. The road is currently closed between Victoria Valley Road and Makene Road, with a detour in place via State Highway 10.

Project update 1 July 2021

The Mangamuka Gorge has been opened again.

State Highway 1 can now be travelled by all vehicles except over dimension vehicles.

We encourage everyone to drive carefully as in some places road alignment has changed and road shoulders have been reshaped.

We were able to open the road through the Gorge on schedule on Wednesday 30 June. Thank you for going on this journey with us and for your patience and understanding during a major and challenging piece of work.

  • Project type

    Road maintenance

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Background

Heavy rain over 17 to 18 July 2020 caused several slips through the Mangamuka Gorge along State Highway 1 (SH1).

At the main slip site on the northern side of the gorge, 6 to 7 thousand cubic metres of material fell from under the road to the river below, leaving the road unsupported to the centreline. The earth is brittle and unstable and heavy rain could bring down more of the hillside.

There is also an historic slip directly above the road that is complicating repair efforts.

Repairs to main slip

Planning for repair of the main slip has been challenging and very complex.

Remedial work to stabilise the main slip is underway. This stage of the work involves installing concrete piles under the road to support it.

The next stage will involve cutting into the bank above the slip to realign the road and restore it to two lanes.

This work will take several months but should be complete by mid-2021.

Road closed

State Highway 1 is closed between Makene Road and Raetea North Side Camping Area, south of Kitchen Road.

The heavy machinery required to complete the repairs takes up the full width of the remaining road, so there’s no room for vehicles to pass.

As works progress we will look at any opportunities there might be to partially open the road.

Detour route map

The official detour route is via State Highway 10 and will add an additional 20–30 minutes to the journey. This route is safe for all vehicles, including heavy and freight vehicles.

The Twin Coast Discovery Route is also open and provides a scenic alternative for light vehicles and campervans.

Mangamuka detour map

Download detour map [JPG, 497 KB]

Further information about planning your journey through the area(external link)

Frequently asked questions

  • Why is the repair taking so long?

    Designing and planning for the repair has been challenging and very complex due to the instability of the main slip site and the presence of an old slip above the road that could reactivate. See the image below (from the 1950s).

    Because the earth beneath the road is so brittle, we had to insert temporary piles just to strengthen the road enough that it could support the larger machinery required to fix the road.

    This larger machinery (a 20-tonne drill rig and a crane) helped us install 49 20-metre concrete piles under the road, allowing us to save the road that remains at the main slip site—about one lane.

    We now need to install anchors and a capping beam to complete piling works.

    We will then cut into the bank above the road in order to realign the road and create enough space for two lanes of traffic. This will have to be done very carefully to avoid destabilising the bank above.

    The large machinery has moved 500 metres up the road to install piles at the site of another slip.

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  • Why can’t you allow vehicles past while you do the work? What about convoys?

    While work to install the piles is underway there is no room for vehicles to pass safely.

    Once the piles, anchors and capping beam are in place and the road is supported, we will be working to widen it by cutting into the bank. When designing this work we will be looking for opportunities to allow vehicles past if at all possible.

    One option could be convoys. However, it can easily take crews more than an hour to clear the site for vehicles to pass, and the same amount of time to set up again afterwards. If each convoy takes about three hours out of a work schedule (for example), and there are two convoys a day, over the course of several weeks this would add considerable time to the works programme.

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  • When will we be able to use the road again?

    The road should be fully operational by the middle of 2021.

    We are carrying out the repairs in unstable conditions. This means we have to adjust our design and construction methodology as we go, making it difficult to provide firm commitments or timeframes.

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  • How will rain affect the repair works?

    In heavy rain crews must stop work as it is not safe to work in very wet conditions.

    We assess the impact of any significant rainfall to see whether slips have moved and to make sure it’s safe to continue working.

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  • Why don’t you just build a tunnel or an alternative state highway?

    Both of these options would require significant investment.

    On average, the state highway through Mangamuka Gorge carries only about 1300 vehicles per day, as compared to somewhere like the Brynderwyn Hills, which carries around 10,000. This means the business case for such a large investment would not meet the criteria for Waka Kotahi funding.

    While SH1 through Mangamuka Gorge is an important route for Northland, it does not currently feature in national long-term resilience planning. We have previously invested to fix some of the resilience issues and in an emergency, there is an alternative state highway (SH10) that can provide the same level of service.

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