The New Zealand Transport Agency says underground water springs, rather than leaking Dunedin City Council pipes, could be to blame for a slip threatening part of State Highway 1 leading north out of Dunedin.
The Otago Daily Times yesterday reported a 100m-long section of SH1 at Pine Hill, near the Bank St intersection, was slipping away at an accelerating rate, threatening "significant" damage to the highway and Town Belt below.
Monitoring of the road had detected a 97mm drop in the past three years, but a 6mm drop in just the last month - equating to 72mm a year or 216mm in three years.
NZTA Coastal Otago area manager Roger Bailey said yesterday investigations in August had focused on whether council pipes were leaking in the area, causing a build-up of moisture saturating the ground.
Investigations three years ago, when the land under the highway was found to be slipping, had uncovered a broken sewer pipe, although it was impossible to say whether the break caused the slip, he said.
CCTV cameras had again been used to check pipes in the area in August, after monitoring found the slip was accelerating.
No broken pipes had been found, and council staff were not aware of any leaking water mains in the area, so NZTA staff were confident pipes could be ruled out as a cause of the slip, he said.
Instead, the variable speed of the slip indicated the cause was more likely ground water, he said.
One Pine Hill homeowner told the ODT his driveway was constantly wet from water seeping out of the ground although he was not sure of the exact source, while another suspected underground springs.
Council water and waste services manager John Mackie was reluctant to comment yesterday, saying he had "no idea at all of anything to do with this".
"It's a state highway responsibility and if they [NZTA] have any issues I would suggest they call me.
"They haven't."
Staff were involved in leak detection work and repairs across the city, and it was unlikely they would not have been working in Pine Hill, but he could not provide details of recent leaks or repair work in the area yesterday.
In May, the ODT reported about 15% of the city's water supply, or 2.4 million cu m, was being lost to leaks and burst pipes each year, despite the council recently spending $64 million upgrading the quality of the city's drinking supply.
Councillors will today consider a request from NZTA staff for an easement over part of the Town Belt to allow work to secure the stretch of SH1 threatened by the slip, including installing 40 drains under the road.
Mr Clarke said the drains would clear moisture from under the road, and could also help confirm the source of the water.
If approved, the work was expected to begin early next year.
The work would take six to eight weeks and cost about $250,000.
