The Waitaki District Council is continuing to work with the NZ Transport Agency to develop a long-term solution for a section of coastal road between Kakanui and Oamaru following two major landslips this year.
A report into the instability of Beach Rd, carried out by GeoSolve Ltd, was released by the Waitaki District Council (WDC) yesterday.
The initial slip occurred in May just north of Kakanui and the second one occurred further north in early September.
One landslip appears to have been evolving since prior to 2006, while the other appears to be a more recent failure.
The report suggests the key causes are erosion and groundwater which is undermining the limestone shelf and weakening the road.
WDC roading manager Michael Voss said the report is beneficial in helping the council to work with NZTA to develop a plan and secure funding.
“It’s a major step in the direction of looking at the long-term solution.”
While many of the findings in the report were to be expected, the degree of undercutting in one section was greater than he thought, Mr Voss said.
A large sea cave has developed immediately south of the northern landslip which extends at least 13m under the limestone shelf, however, observation of this was limited due to tide conditions and the general inaccessibility of the location.
The report states that the probability of regression of the northern landslip should be considered high and particular caution is warranted at this location.
It is recommended that the road be regularly monitored with an alarm system installed if the road is to remain open.
Mr Voss said the council would be monitoring the road but would not be installing an alarm system because of the high cost.
They have erected a netting fence along the edge of the road which will act as a visual warning, he said.
The report has recommended that minor road realignment of about 10-20 metres inland should be explored first as a long-term solution because it would be a one-off cost that would last for several decades.
Another option would be to strengthen the road, however, the report and Mr Voss said this type of protection would not provide any guarantees and would potentially be very costly.
The council will be continuing with the proposed realignment of the road around the first slip area and will carefully examine further work with stakeholders, land owners and the community.
By RUBY HARFIELD