More work needed to complete Mangamuka Gorge repairs in Northland

Piling work is under way on the second slip near the Mangamuka Gorge summit.

Work on slips that blocked State Highway 1 through Mangamuka Gorge in Northland is progressing, but there's still a lot more needed to get the road open again.

Waka Kotahi NZ Transport Agency said there has been significant progress on repairing SH1 past the big slip in the gorge. The installation of steel and concrete piles to support the road above the slip has now been completed.

The slips closed the road during a massive storm in July that dumped more than 220mm of rain on Northland in a few hours.

The total estimated cost of the Mangamuka Gorge repairs is $13.8m. This includes the ongoing slip repairs and the cost of the initial emergency response to last July's storm damage.

The road was opened to holiday traffic for three weeks over Christmas and closed again on January 10 so work could resume. Since then, another 16 piles have been drilled, completing the total of 49 required to support and ensure the long-term safety and stability of the road.

"It's been a great effort by the drill crew, but this is just stage one of the repair and there's still a lot more work required before we can open the road to traffic again," Waka Kotahi Northland system manager Jacqui Hori-Hoult said.

The next stage of the repair is to install capping beams that add to the stability of the pile wall. The capping beams are attached to the top of each pile with supporting cables drilled into the bank under the road. Once the road is rebuilt, the capping beams will be part of the road substructure.

Work has also begun on cutting into the bank above the road to widen and restore the road past the slip to two lanes.

"This is another complex part of the repair because of the old slip found above the road during our initial investigation of the storm damage last July. For the safety of our crews, we have to work carefully, monitoring the hillside to ensure there is no new earth movement."

Meanwhile the drilling rig has moved a few hundred metres towards the summit to a second slip under the road where the road surface has slumped by more than a metre.

There will be 20 piles drilled to support the road edge so the road surface can be built up again. Installing the piles will take four weeks, with another four weeks to add capping beams.

"At this stage we plan to have the road restored to two lanes by mid 2021, but it could be sooner depending on the weather and work progress. We're constantly evaluating the work programme with our contractors to look for any opportunity to open the road at pre-arranged times to some vehicles,'' she said.