What slides beneath?

There's a sinking feeling under many Aucklanders at the moment
Carl Pynenburg has taken the long route to and from work for two weeks after a section of Candia Rd in Swanson started falling away.
"There was a big split across the road and the run-off was going down the hill," says Mr Pynenburg. The damage, on the corner of Sturgess and Candia roads, has since been patched with gravel and barriers, but insurers are still adding up the cost of recent storms across the region.
A number of slips in Waitakere have blocked streams, roads, and one house in Swanson has been evacuated. More properties on the North Shore have been affected by slips.
Residents of 14 houses in Torbay and two in Glenfield are staying in motels while the damage to their homes is assessed. The trouble, though, is not because of the buildings. The Auckland area lies on a group of soils called the Waitemata Series, that are notoriously difficult to build on and difficult to predict in certain weathers.
Waitemata Series soils predominantly cover areas between Manukau and Orewa, although the consistency varies from place to place. Through winter, the soil, which contains large quantities of clay, often becomes boggy and saturated, while during summer it dries and cracks.
"These soils, once saturated, can slump if they are on steep slopes without vegetation," says Mark Kirykos, manager of geotechnical engineering at Harrison Grierson Consultants.
"The characteristic of the soil is difficult for laying foundations, but there is no general assessment as to whether a particular area is unstable." Because of Auckland's varying landscape, it is difficult to detect which areas have an increased risk of slips. Waitakere City Council's public affairs manager, Wally Thomas, says many houses could be at risk because of the area's rugged environment.
"We've got the ranges which are slip-sensitive because they're mountainous, and there are hundreds of houses in slip-sensitive areas, such as Titirangi, Huia and Laingholm."
Councils try to take particular care when developing land to avoid any slips, says Chris Glanfield, land development regulatory manager at North Shore City Council.
"Slips are natural. You can engineer everything to get it exactly right, but Mother Nature can still overcome."
Mr Glanfield says clay-based soils are difficult to build on but deep foundations are usually enough to halt any likely damage from heavy rain.
"But if something is built on a bank or a cliff, the more water that gets in the soil, the less stable it can become."
This was the problem at the Lingham Cres property in Torbay and the house is now expected to be demolished. Auckland City Council continues to take precautionary measures along Cliff Rd in St Heliers, where the natural erosion of the cliff face has caused some concern, but otherwise does not monitor areas to see if there are risks of erosion or slips.
Mr Glanfield says if people are concerned about the stability of their home, they should contact their council to see what can be done.