Liquefaction in Christchurch

Understanding liquefaction is important to decision making on the future of Christchurch. Last night a public lecture  on liquefaction was given by Canterbury University associate professor of civil and natural resources engineering Misko Cubrinovski.

He says "one of Christchurch's hardest-hit suburbs might be too expensive to rebuild to an acceptable standard."

The Government announced a bailout package in June for 5100 earthquake-damaged properties in the city's east, northeast and Waimakariri District beach areas that would be bought by the Crown.

Bob McKerrow examining liquefaction in a business in Chrischurch. The dry mix is from February's earthquake and the darker mix is from the June earthquake. The water saturated particles dry out and get into everything. Photo: Ruia McKerrow

Canterbury University associate professor of civil and natural resources engineering Misko Cubrinovski, who gave a public lecture on liquefaction last night, told The Press there was a good indication that the entire "residential red zone" had weak soils.

"Robust" measures, including better foundations and strengthened or compacted soils, would be needed if the worst-affected areas were to be rebuilt, he said.

However, in some areas, reaching an acceptable standard and performance would be too expensive, he said.

"They [the Government] should be looking at alternative ways of using that land," he said.

"The soils are weak and highly susceptible to liquefaction and underwent very large deformation ... so rebuilding in that area would certainly require [significant] improvements in their performance.

"It's not acceptable that if this is going to be repeated in 50 or 100 years from now, that again you lose 5000 houses."

Liquefaction occurs when loose and water-saturated soils are shaken in a quake, separating the particles. It causes ground settlement and forces water to the surface. Liquefied soils lose their ability to support heavy buildings.

The process, which ejects water, sand and silt, has affected roughly half of Christchurch since September, with some areas inundated four times.

Cubrinovski was the major author of the New Zealand Geotechnical Society's guidelines for the identification, assessment and mitigation of liquefaction hazards, published in July last year.

More detailed guidelines should be prepared, he said.

"The seismic hazard of Christchurch is higher than anticipated before these earthquakes, and that means we will have to provide more robust design against liquefaction and be more rigorous in the assessment of liquefaction, especially because this higher seismic hazard may remain for some time," he said.

"The fact that the hazard proved to be much higher than expected is, in my view, the key reason why we are in the situation we are."

Last night's lecture was the second of six free lectures run by the university to explain the science behind the recent earthquakes.   (Thanks to stuff.co.nz for permission to run this article)