Liquefaction: lessons from the 2011 Christchurch earthquakes

Misko Cubrinovski is interested how the ground and the structures on - and in - it behave during an earthquake.

Misko Cubrinovski has spent his professional career studying liquefaction caused by earthquakes, but even he was surprised by how widespread and extensive the effects of liquefaction were following the 2011 Christchurch earthquake.

"Probably it is the largest urban liquefaction on record in the world."

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On 22 February 2011, Christchurch experienced a severe earthquake resulting in much loss of lives, destruction ... and liquefaction.

Misko Cubrinovski, a University of Canterbury geotechnical earthquake engineer, explains that liquefaction is the result of a solid medium - soil - turning into a fluid medium due to violent shaking.

Soils that are susceptible to liquefaction are generally sandy, silty or gravelly, and have to be saturated with water.

These loose, wet soils densify as a result of earthquake shaking, which creates pressure in the groundwater. This high pressure water forces soil particles apart and the soil loses its structure, says Misko, becoming a viscous fluid.

Liquefaction develops very quickly during strong earthquakes.

"During the February 2011 earthquake," says Misko, "it took two, three, four seconds to liquefy the loose soil in the eastern suburbs of Christchurch."

"Then there is a period of a minute or two when that effect of liquefaction remains very strong during which large deformations and effects occur. And then it takes hours or even days for the soil to get back to its equilibrium."

Misko says that Christchurch is still experiencing the significant consequences of liquefaction ten years after it occurred.

While liquefaction can extend 15-20 metres deep, it's most damaging consequences happen close to the ground surface, in the top five metres, says Misko.

Misko helped map and study liquefaction in Christchurch after the 2011 earthquake. He says one of the most significant discoveries from the research was that "liquefaction at large depths can prevent manifestation of liquefaction effects on the ground surface."

A deep layer of liquefaction acts as an isolation mechanism, preventing earthquake waves from reaching shallower soils.

He says this finding explained why some areas of the city that contained liquefiable soils sustained much less damage than expected.

Misko is interested in how buildings and other structures such as pipes respond to shaking and liquefaction.

Misko is part of the Quakecore Centre of Research Excellence…

Go to this episode on rnz.co.nz for more details