A geotechnical assessment is under way on one of the major underslips on the Mangamuka Gorge, which is expected to re-open next year. Photo / Waka Kotahi NZTA
A geotechnical assessment is under way on one of the major underslips on the Mangamuka Gorge, which is expected to re-open next year. Photo / Waka Kotahi NZTA
A plan has been formed to deal with the 15 significant slips along State Highway 1 over Mangamuka Gorge, but it will be at least another year before the road is fully open.
It’s expected to cost around $100 million to restore the road over Mangamuka Gorge to its original condition after it was closed again last August.
Waka Kotahi New Zealand Transport Agency said a programme of works to repair the slips on the Mangamukas has been established, following investigations, planning and design work to understand the damage and plan for how to repair the challenging transport corridor.
The road is scheduled to open and be fully operational by May 2024. However, the national roading body is investigating opportunities to provide limited access for light vehicles over the Christmas period.
Waka Kotahi project director Norman Collier acknowledged the impact the sustained closure was having on travel for work, school, business and connectivity, and that the prospect of having the gorge closed for an even longer period of time would be disappointing for the community.
“We know how critical this route is for the community, and we are continuing to investigate if it is possible to make more resources available to accelerate this work,” Collier said.
Northland’s ground conditions, including through the Maungataniwha Ranges which surround SH1 Mangamuka Gorge, have always been fragile, and almost 70 per cent of the geological material which forms rocks in the region is made up of Northland allochthon.
Work is continuing on State Highway 1 at Mangamuka Gorge after severe damage following heavy rain in August 2022 closed the main route to Kaitāia.
This type of material has long been challenging for the construction and maintenance of roads, and meant repairs to slips of this nature needed to be carefully planned, as finding competent material to anchor the road to could be difficult, he said.
“One of the most significant slips on the southern end of the gorge requires the most complex fix and will take approximately eight months to complete. There are 14 other slips which require a similar approach, and will average five to six months of construction time each.
“It has been a complex process to understand firstly, the damage which occurred, and then investigate and plan the best way to fix each slip. We’ll also be making the ground less susceptible to slips by improving the drainage through the gorge, so it’s more suitably equipped to handle the increase in severity and frequency of weather events.”
Crews are now focused on ensuring the road is safe and stable enough to accommodate the large machinery needed to carry out slip repairs. This includes installing almost 700 piles throughout the gorge, stabilising the road and retaining and strengthening areas.
“Our teams are working in a very constrained environment, within an already tight road corridor which is made even tighter with the slips impacting the road, large machinery and multiple worksites,” Collier said.
”We want to ensure we get these slip repairs right while also looking after the health and safety of our teams on the ground and protecting the environment we are working in. We know the community joins us in this sentiment, and we thank them for their understanding.”
