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Slip repair to start on SH1 through the Mangamuka Gorge

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Waka Kotahi NZ Transport Agency says it has a plan to open State Highway 1 through the Mangamuka Gorge to light vehicles by mid October, but much will depend on the weather and work about to start on repairing a significant slip on the northern side of the summit.

Northland System Manager Jacqui Hori-Hoult says all other slips on SH1 from the July 17 weather event have been cleared. Where the road is damaged, it can be safely opened to one lane for light vehicles.

“Our priority is to get a solution in place that will secure the road that is left and open it in some form past the last big slip. That may involve convoys at set times and it will only be for light vehicles.”

“We’re looking to award a contract for this first phase of work in the first week of September and for work to begin as soon as possible after that. The work will take at least six weeks, depending on the weather.”

The plan is to construct a wall of 20 metre concrete piles to support the road where the rock underneath has fallen away. The wall will be built from north to south to ensure the safety of the work crew and equipment.

“We’re dealing with a very challenging and complex slip in a difficult environment. The continued rain isn’t helping, but that’s out of our control.”

On July 17 the ground under the road fell about 300 metres to the river below, leaving the road surface unsupported to the road centreline. In the first few days, engineers drilled into the road surface to test the stability of the rock that remained.

“They found the ground was unstable both under and above the road and the concern remains that any repair work could reactivate old landslips.”

In the last month, the continued heavy rain has resulted in parts of the hillside continuing to move. The slip investigation also found an ancient slip above the road which is complicating efforts to establish if a new road alignment can be cut into the hillside.

“For that reason, building the concrete pile wall is a first phase in restoring the road to two lanes. We’re essentially designing and building as we go and breaking the build into phases. For example, we’ll have to install retaining walls above the road but we’re still working out how much will be required.  The team is really conscious of the natural environment and we want to preserve it.”

“We’re very fortunate that data collected during the geotech investigation identified the ancient slip above the road and the significant safety risk it presents to the repair effort. Without that technology we wouldn’t have known it was there.”

For now, SH1 through the Mangamuka Gorge is closed between Victoria Valley Road and Makene Road. SH10 is the detour with motorists advised to allow an extra 20-30 minutes for their journey.

“The rock structure through the Mangamuka Gorge is weak and prone to slips. In recent years, we’ve put a lot of time and effort into building the resilience of SH10 as an alternative route. That work is now paying off, given our current slip challenges on SH1.

Waka Kotahi acknowledges the disruption to the local community and the extra time and cost of having to travel on SH10.

“We thank motorists for their patience and understanding. We’re working as quickly as possible in difficult conditions to repair and reopen the road.”

Road slipping down bank with repair vehicles at the top on the road.

The slip that has fallen away under the road, closing State Highway 1 through the Mangamuka Gorge.

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