Old slip adds complexity to Mangamuka Gorge road repair

3 MinutesBy NZ Trucking magazineAugust 3, 2020

The NZ Transport Agency says it‘s still too early to say when SH1 through the Mangamuka Gorge can be opened to traffic.

Most of the road has been cleared to at least one lane, but there‘s still no way past the biggest slip on the northern side of the gorge. The hillside under the road fell away during heavy rain nearly two weeks ago, leaving metres of road surface suspended in the air.

NZTA Northland system manager Jacqui Hori-Hoult said safety was a priority as her team assessed the slip and the repair job needed to reopen the road.

“Six to seven thousand cubic metres of material have fallen from under the road to the river below. What‘s left is brittle and unstable and our concern is that more heavy rain could bring down more of the hillside.

“A further complication is that there‘s an historic slip directly above the road and we have to be very careful about disturbing the slope. We need to further investigate the slope to develop our longer-term road repair options.”

Hori-Hoult said there had been only light rain since the slip and the forecast is for better weather this week.

“Our geotech experts are working to understand the soil conditions and locate stable bedrock before deciding on the safest and most effective solution for restoring the road.

“Our plan is to work in stages to open the road initially to a single lane so that light vehicles can get past. Weather permitting, that may take six weeks.”

Elsewhere in the gorge, other slips have been cleared and with safety barriers and other precautions in place, the road is ready for light vehicles.

“The NZTA acknowledges the inconvenience and cost to local communities either side of the gorge of having to use the longer SH10 detour. We‘re working to reopen the road safely and as quickly as we can.”

On average, about 1300 vehicles a day travel on SH1 through the Mangamuka Gorge. It will be some time before trucks and buses can travel the road.

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 to 30 minutes for their journey.