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Earthquake liquefaction models 'flawed'


A car stuck in a sink hole caused by liquefaction in the 2011 Christchurch earthquake (Source: Simon Baker /Reuters)

Much more complex Overturning a long-held assumption could change our understanding of when and where soil liquefaction will occur in earthquakes, say US researchers.

Their study is published today in the Proceedings of the Royal Society A.

After an earthquake some soils can liquefy causing bojects to sink into the ground, says co-author Dr Daniel Lakeland, who did the research as part of his PhD at the University of Southern California, Los Angeles.

Liquefaction, which was a major source of damage during the 2011 Christchurch earthquake, is generally thought to involve the shake-up and compaction of soil grains in loosely-packed sediment. This leads to a build-up of water pressure underground, causing the soil to act like a liquid.

However, a new mathematical model developed by Lakeland and colleagues questions one of the crucial assumptions in many of the models used to study liquefaction.

No-drainage assumption

Lakeland says many laboratory studies of liquefaction put small samples of soil such as sand under pressure in a piston-like device to model what happens during an earthquake.

He says the "textbook" assumption used in these studies is that there is no release of water pressure through drainage during the seconds that the quake is shaking.

"They've made that assumption in order to make things simpler to analyse," says Lakeland.

However, such models give different results to those from more sophisticated experiments that model the entire soil profile, rather than just a small section of sand.

Lakeland and colleagues' new model helps explain the discrepancy.

"We can't just look at what happens to a very tiny piece of sand in the laboratory," says Lakeland. "It's not that piece of sand that causes liquefaction, it's the properties of the whole deposit."

Water flow

Lakeland's new model shows that water can flow through compacted soil during the seconds of earthquake shaking.

"Squeezing [the sediment] does introduce the water pressure, but at the same time migration of water is moving that water pressure around through the sand," says Lakeland.

"We need to pay attention to this factor that we've been ignoring for 50 years."

While liquefaction is often assumed to occur where the original compaction of sediment occurs, Lakeland says his model suggests something else.

"You squeeze one layer of soil at the bottom and the water pressure builds up and then the water starts to move upward towards a lower pressure region," he says.

"If you have just pure sand, this wave propagates very quickly at 10 metres in a tenth of a second."

But, if the water hits a layer in the soil that has low permeability then pressure may build up and liquefaction can occur at that point.

"We show that changes in the permeability through the whole deposit are important," says Lakeland.

"If the permeability decreases in a certain place then you are going to get a bottling up of the water and you're going to get water pressure build up right at that layer."

Study welcomed

Professor Misko Cubrinovski, an earthquake engineer who specialises in liquefaction at the University of Canterbury, New Zealand, welcomes the paper.

"Anyone who is dealing with liquefaction would agree that groundwater flow is important in the liquefaction phenomenon," he says.

Cubrinovski disagrees with some technical points made by Lakeland and colleagues and emphasises that drainage is included in models more often than the researchers suggest.

But he describes Lakeland and colleagues' new approach to looking at liquefaction, as "a good contribution".

Scientists hope that such research will help them better predict the damage caused to cities such as Christchurch by liquefaction.

"The total economic loss due to the Canterbury earthquakes is estimated at NZ$30 billion and about half of that loss (or NZ$15 billion) can be attributed to liquefaction," says Cubrinovski.

He says 60,000 residential buildings were affected by liquefaction, 20,000 were severely affected, and more than 8000 were abandoned.

Drinking water and waste water pipes, bridges and road networks were also hit hard by liquefaction, says Cubrinovski.

Tags: earthquake, engineering