Liquefaction map could save building costs

Thousands of dollars in building costs could be saved with the release of a report showing damaging liquefaction is unlikely in parts of Canterbury.

The report maps areas where deep ground examinations for building projects are unlikely to be required and other areas where testing, in most cases, will be necessary.

Geological reports can cost up to $10,000 a site.

Environment Canterbury and the Natural Hazards Research Platform's report shows where large areas of Canterbury, outside Christchurch City, will generally not require in-depth liquefaction testing.

However, geotechnical investigations for other hazards, such as susceptibility to flooding or land subsidence, may be required on some of those sites.

The report is referred to in the Ministry of Business, Innovation and Employments' revised building and housing guidelines to be released today.

ECan geological hazards analyst Marion Irwin said there were too many variables to say definitively where liquefaction would or would not happen in earthquakes, but she was confident the "conservative" map had correctly identified the areas more susceptible to it.

"In the grey area [not prone to liquefaction], we are saying you don't need to worry too much. On the other side [liquefaction-prone area], we don't know,'' she said.

"It could be a lot or a little, so you need to get a geotechnical assessment done."

Large areas of the map are covered by Canterbury Earthquake Recovery Authority technical categories - TC1, TC2 and TC3 - so are not included in the report.

In general, western Christchurch and Banks Peninsula are seen as not prone to liquefaction, while north and south of Christchurch appear prone to the geological process.

The report is based on 20 years of studies completed in Christchurch City and the Selwyn and Waimakariri districts, as well as data from the Canterbury quakes starting in September 2010.

Irwin said it would be used by local authorities to make decisions on planning and consenting.

Kelvin Berryman, of GNS Science, said the report cost $700,000 and involved several organisations, including Cera, GNS Science and Lincoln University.

"It's mainly a tool for the territorial authorities to use with ongoing planning decisions."

He said it gave authorities discretion when making decisions on land, rather than making them use a prescribed formula.

The area assessed in the research was selected because it was the most likely to be affected by liquefaction.

The Press