Moving the coast road further inland from just north of the Kakanui village, where two slips threaten it, is the most cost-effective way of keeping it open, according to a report commissioned by the Waitaki District Council.
In an area previously considered stable, a slip just north of Kakanui encroached on the road in May, and a single lane had to be built next to it to keep the road open.
Another slip in early last month did not reach the road, but safety netting has been put up so vehicles cannot pull over and stop on the shoulder.
As a result, the council asked Dunedin consultant GeoSolve to investigate the stability of the limestone cliffs between the road and the sea.
Engineers abseiled down the face of one of the slips.
The report, released to the Otago Daily Times yesterday, said while the southern slip had been evolving since 2006, the second in September was a more recent failure.
Wave erosion of limestone at sea level had caused rock overhangs that, combined with pre-existing defects in the rocks and saturated soils from heavy rain, had caused the slips.
The possibility of the September slip regressing further inland was considered high, particularly if soils became saturated or there was an earthquake.
Groundwater and surface drainage going inland from the cliffs' crests should be watched during heavy or sustained rain and measures should be implemented to eliminate, reduce of divert flows where practical.
One key consideration was that further retreat should tend to be substantially less in the future, because the cliffs' base was more resistant.
A detailed study with comprehensive subsurface investigations and conservative design would be required to ensure a robust solution.
Cost, including investigations, design and construction, could be between $5000 and $10,000 a metre of road protected.
''For these reasons, it is recommended that the relocation option should be explored first,'' the report said.
The council is already in discussions with the neighbouring landowner at the May slip site.
The council is working with the New Zealand Transport Agency to develop a business case to explore and evaluate the options available before any final decisions.
