25 March 2014

Liquefaction report for Timaru released

Environment Canterbury today released a review of liquefaction information for Timaru District, indicating that only small amounts of liquefaction may happen in a large earthquake.

The review report, commissioned by Environment Canterbury and the Timaru District Council and produced by Geotech Consulting Ltd, summarises and updates liquefaction studies from the last 15 years.

Environment Canterbury Commissioner Donald Couch says the review targets areas that have been identified in the past as potentially liquefiable in a big earthquake and offers guidance about appropriate levels of geotechnical investigation at the time of subdivision or development.

“This comprehensive piece of work gives the community better information for those considering building,” Mr Couch said. “It essentially confirms what was already known – there is very limited liquefiable ground in Timaru District, meaning only small pockets of liquefaction are likely.

“It does not tell individual landowners whether there is a risk of liquefaction on their particular property, but there are ways to get information about their properties and it will show them when liquefaction hazard investigation should be included in consent applications. It will also avoid their being asked for liquefaction hazard investigations in areas where potential for damage is low.”

The report includes the results of new geotechnical testing in specific areas of Timaru and Geraldine. It shows that ground that is more susceptible to liquefaction is in low-lying areas close to the coast, and along rivers and floodplains.

“The most susceptible areas are valley bottoms close to the coast in Timaru City, between Washdyke Lagoon and Saltwater Creek. The area subject to river engineering works at Washdyke Creek is not susceptible, however,” Mr Couch said.

“Timaru Port is an area of low susceptibility. Much of Redruth, including the area currently occupied by the landfill, may need further investigation to make sure the liquefaction hazard is appropriately managed.”

The main purpose of the report is to provide Timaru District Council and the community with general guidance on where geotechnical investigation and engineering assessment of liquefaction potential may or may not be required for plan changes, and for subdivision and building consents. It may also be useful for lifeline utility and emergency management planning.

“The report divides the district into four areas of relative liquefaction susceptibility ranging from ‘nil to extremely low’ potential to ‘moderate’ potential,” Mr Couch said. “Liquefaction assessment is needed in two of these areas. Nowhere has been identified where development may lead to unacceptable economic risk.”

The report identifies potential liquefaction hazards in respect of land use planning for general residential and light commercial development. It does not provide information for major infrastructural and critical facilities.

Mr Couch advises potential purchasers in Timaru District and elsewhere always to get a LIM. “You might also need an investigation for hazards other than liquefaction – for example, for susceptibility to land subsidence,” he said.

“This report will help ensure that costly liquefaction investigations will not be required in areas where they are not needed and will be focused on areas where they can ensure appropriate building solutions.

“This is the way it should be towards making sure our buildings are as safe as possible for the future of Canterbury,” Mr Couch concluded.

For more on the report and related information go to www.ecan.govt.nz/liquefaction

Background

The occurrence of liquefaction depends on whether underlying soils include liquefiable sediments, which have very distinctive characteristics, and whether these sediments are saturated by water (under the water table).

In the report:

• Zone 1 land (“moderate potential”), which covers a very small part of the district, has the highest liquefaction susceptibility and all new development should have appropriate site investigations and liquefaction assessment by a suitably qualified geotechnical engineer or engineering geologist.
• Zone 2 land (“low potential”) may contain areas of liquefiable ground, and liquefaction assessments should be carried out for new subdivisions at consent stage.
• The geological nature of the ground in zones 3 and 4 (“very low” and “nil to extremely low” potential) is such that future earthquakes are unlikely to cause land damage from liquefaction.

ENDS

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