Progress slow but steady stabilise slip at Lemon's Hill on State Highway 11

Fulton Hogan project manager Ross Harper at the site of the slip on State Highway 11 at Lemons Hill, between Kawakawa and Paihia. Photo / John Stone

A series of giant steps carved into the hillside by diggers will stabilise a massive slip which has closed Northland's main route to the Bay of Islands for two months.

A team of about 20 workers at the site have been hampered by wet weather since heavy rain from ex-tropical Cyclone Fehi brought down the slip between Kawakawa and Paihia on February 13.

The cost of the project is expected to reach $1.25 million.

Between bouts of inclement weather including ex-tropical Cyclone Gita and Cyclone Hola, the roading crew using four diggers and three heavy duty tractor and trailer units has managed to move 15,000 cubic square metres of unstable and excess material from the top section of the slip.

There is about another 15,000 cubic square metres yet to be moved which includes about 2000 cubic square metres of unstable material.

The slip face is about 85m and at the base where it meets the road it spans 80m. The top of the site is 52 metres high from the road.

NZTA's Northland system manager, Jacqui Hori-Hoult, said the road surface had not been damaged and on the other side of SH11 the ground appeared stable and unaffected.

A steep track has been cut near the top of the slip to give the work crews access and when it becomes wet the track is unable to be used and that stops work on the site.

"It's not a quick job, and stabilising the site is crucial so that we ensure there is a permanent fix," Ms Hori-Holt said.

The project was more complex than first thought and loose material was being scrapped away until stable greywacke was reached. Giant steps — or benches as the roading experts call them — were being cut into the hill.

"We are aware of the pressure not having the road is putting on the community and the local businesses," Ms Hori-Holt said.

"We are constantly reviewing our progress and we should have reached a point in stabilising the hillside over the next couple of weeks where we can safely open the road to traffic in the morning and evening for a limited time under traffic control."

Using dynamite on the site was also considered but abandoned after crews were able to remove the unstable material successfully.

"We had favourable weather and the team didn't stop. We got to a stage where the material had been removed and we didn't have to go down the route of blasting."

Ms Hori-Holt said regular examinations at sites on Northland's highway network were carried out when "frittering" or crumbling of soil and banks along the road occurred.

Frittering was noticed on Lemon's Hill last September but there was no indication at that time a major slip would happen.