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Half of Auckland is at "high risk" of being affected by the devastating effects of landslides and it could cost homeowners up to $500,000 in engineering earthworks to make their houses safe.
An Auckland Regional Council map obtained by the Herald on Sunday shows a widespread susceptibility to
landslides throughout the region.
More than half the area is labelled as "high" or "moderate risk", meaning slopes are unstable or may become unstable depending on rain or erosion.
The map follows serious landslides in Bucklands Beach, Kawakawa Bay and the North Shore over the past three months - described as the worst in 40 years by one expert - in which families have been forced to evacuate their condemned homes.
Engineers also warn that Auckland homeowners may be facing hidden costs of up to $500,000 when it comes to safeguarding their properties.
"Councils are being more rigorous now than what they were even 20 years ago," said John Leeves, Tonkin and Taylor senior geotechnical engineer.
The extent of remedial work to stabilise areas depended on the specific site, but Leeves said homeowners could spend "anywhere from $5000 to $500,000".
This winter had been "remarkable" for several larger slips due to the continuous rain that occurred throughout the season, said Leeves.
"This is probably the worst in terms of bigger, deeper slips than anything in the last 40 years."
Ka-Ching Cheung, a civil engineer with Bucklands Beach consultancy Peters & Cheung, said advances in geotechnical engineering in the past two decades had tightened the requirements for building on slip-prone sites compared with the 1970s and 1980s
"The councils have done quite a lot in terms of building consents - every site that might have a risk must have a geotechnical report done on it. And the council look at that a lot more now."
Drilling piles into the earth, or placing reinforced concrete bars into the sides of cliffs, were among methods of site stabilisation which are expensive.
"But the money you spend on that work will be buried in the ground, you won't see anything for it."
The ARC map is only indicative of where slopes may be unstable, according to natural hazards management adviser Ryan Paulie. The level of risk could vary for each property.
Anyone concerned about instability on their individual property should contact their local council for advice, said Paulie.
However, any development in the high-risk areas, which include many coastal suburbs and large areas of northern Auckland, could need geotechnical assessment and subsequent stabilising work.
"If you are building near a clifftop, or inland on top of a steep slope, you would want to think about it fairly carefully," said Michael Pender, geotechnical engineering professor at the University of Auckland.
"It's a matter of identifying the problem beforehand and having enough money to do the work."
Manukau City Council spokeswoman Robyn McCulloch encouraged prospective home buyers to purchase a LIM report and follow through with any necessary assessments.
Meanwhile, the slip at Clovelly Rd, Bucklands Beach, which has condemned the home of former Manukau mayor Sir Barry Curtis, was still "moving significantly" this week. McCulloch hoped that some of the Clovelly Rd residents who were forced out of their homes would be able to return soon.
The Kawakawa Bay slip, which has blocked the main access road for two weeks and forced the evacuation of six homes, is being cleared by the Manukau City Council and should be finished by the start of October.
An Earthquake Commission report on the Torbay slip, which has seen a home demolished on Lingham Crescent and one family still unable to return home, is due out this week.
