Peat poses problems in Waikato
More than 20 properties in a western Hamilton suburb are being affected by sinking land - double the number the city council first estimated.
Since homeowners in Thornton Estate went public with their plight two months ago, council staff have commissioned a geo-tech engineer to assess subsidence problems affecting properties in Wimbledon Close and Twickenham Place. One Wimbledon Cl resident estimated the land around his house had sunk by 60 centimetres in seven years.
Council building unit manager Phil Saunders originally estimated up to 10 properties could be affected but this week he confirmed 21 properties had been identified.
"We contacted those owners to give them the opportunity to meet with us to discuss their particular situation.
"As a result, nine property owners responded, seeking further assistance and we have inspected their properties and offered advice," Saunders said.
"We have sought more technical advice on the subsidence that we have seen and will be providing further mitigation information to the affected property owners."
Engineering reports indicated subsidence would be an issue due to peaty soil.
Meanwhile, Waikato University professor and soil scientist Louis Schipper said it was important the region had a discussion about how peat soils were used. About 94,000 hectares of the Waikato plains was covered by peat soils, often more than 8 metres deep.
Schipper said a recently completed region-wide study determined how fast peat subsided. In the areas studied it was discovered that, over the past 10 years, the peat soil had subsided by 2cm each year. Schipper said the study had implications for Waikato's farming sector, which relies on surface peats to be drained to support stock.
Subsidence was likely to continue in areas where peat was drained.
"If you continue to drain it, you can put agriculture on top of it but the surface is going down relative to where the groundwater is," Schipper said.
"You need to keep that water down and away from the surface all the time, and that means you're going to have to dig deeper and deeper drains. If you can easily drain the water down a slope into a river and out to sea then you can do it.
"But if you start getting close to sea level, then that is going to be more difficult and you're going to have to pump it." Schipper said Waikato needed "some consensus" on how property owners managed the continuing peat decline.
"You want to avoid being alarmist and look at what the solutions are and maybe different land uses are possible.
"We need to ask how are we going to use peat soils; 2cm a year is 2cm . . . In 10 years that's 20cm, so it doesn't stop."