SH1 through Mangamuka Gorge in Northland to reopen to traffic by Christmas

Work on this massive slip that has closed SH1 through Mangamuka Gorge, in the Far North, is continuing and the road should finally be reopened by Christmas. Photo / Peter de Graaf

It will have taken around five months, but the massive slip that closed State Highway 1 through Mangamuka Gorge should be repaired and open for traffic by Christmas.

The so-called slip number 8 - dropped about 150m to the river below and hollowed out a cavern under the road extending about 1.5m from the edge of the seal into the hillside after the July 17-18 storm dumped 220mm of rain on the region in a few hours.

It was the largest of the eight slips on SH1 through the gorge and Waka Kotahi NZ Transport Agency says it's on track to open the road through the Mangamukas before Christmas - at an estimated cost of $3.8 million - with the slip repair work entering a significant new stage.

The huge slip that closed SH1 through Mangamuka Gorge in the July 17-18 storm that dumped 220mm of rain on the region in a few hours.

The road will be open to light vehicles under stop/go traffic management or traffic lights by mid December and remain open over the Christmas-New Year period to accommodate holiday traffic, Waka Kotahi senior system manager Wayne Oldfield said.

"Waka Kotahi appreciates how important the holiday period is to the local economy and we are doing all we can to open the road. We've installed the first set of piles. More than 100 four-metre and eight-metre piles have been drilled along the road centre line so the heavier machinery can move in to complete the job," Oldfield said.

"A 20-tonne drill rig and a crane are now on site to drill the 49 main support piles that will reach up to 20m into the rock under the road and complete phase one to protect the road.

"The repair teams will be working double shifts [20 hours/day], which will accelerate the work and save 15-20 days on the schedule. Much still depends on the weather, the hardness of the rock we're drilling into and the stability of the hillside. Safety remains our priority."

He said the big rig will drill holes up to 20m deep, with the crane then lifting steel casing into the hole. A reinforcing cage will be inserted into the casing, which is then filled with concrete. The slip is 40m wide but the completed wall will be 75m long to future-proof other unstable sections of road.

Oldfield said in the new year the road will be closed again so the works team can cut into the hillside to realign the road and restore it to two lanes. In designing the work, the team will be looking for opportunities to have the road open to traffic.

He said the area, also known as Maungataniwha, has great significance to many iwi/hapū in the north. A hui was held last week to provide an update on the works and explore areas for collaboration, such as harvesting seedlings from removed vegetation and ensuring the permanent repair design does not take away the natural beauty of the ngahere (forest).

The Mangamuka Gorge has been closed since the July storm brought down eight slips in the gorge. Apart from the remaining big slip, the road has been cleared to at least one lane and maintenance crews have worked to repair the road surface, replace roadside guard rails, improve drainage and generally tidy up the roadside.

"Waka Kotahi acknowledges the impact of the road closure on local communities and the extra time and cost of travelling on the recommended detour route - SH10 - which can add 20-30 minutes to the journey," Oldfield said.