Thursday, 24 February 2011

Hard to believe but Soil Liguefies in NZ Earthquake

I'm sure everyone (especially those who have visited NZ) feel for those people in Christchurch who suffered the devastating earthquake on the 22nd Feb 2011. We are all concerned for those people who lost family members & those who are injured & suffered massive damage to their houses & farms.

I've just returned from NZ & we (a Breton Discussion Group from France ) stayed in the centre of Christchurch for a week. To see the destruction in the CBD is frightening & very sad. Outsiders will be unaware that since the Sept earthquake there have been more than 4000 after shocks. Nerves are strained as everyone is not sure whats happening next. Very scarey!
Our group was hosted by staff of Ravensdown, Cropmark, Lincoln University, Farmwise, Consulting Officers & numerous very generous dairy farmers.....we sincerely hope that you & your families & friends are all safe.
The damage to city buildings is obvious in most TV & YouTube clips.

What is not so obvious is whats happening to the soil. This is of huge importance to farmers & to every householder.
While visiting Darfield we were shown the extensive damage caused by the Sept 4th earthquake where there was sideway movement of soil by up to 4metres & upward lift of 1.5metres in places.....earthquakes are violent & have massive forces. See the photos at the top of the blog showing the massive sideway movement of soil/road & irrigation channels.
What is very scarey in big earthquakes is that the soil turns to liquid. The forces are so great that ground water & soil are mixed & pushed to the surface often causing flooding. The concept that soil could be liquefied will sound almost unbelievable to most farmers. Christchurch is built on an alluvial plain over a soft sedimentary bed so it is prone to liquefaction. It is liquefaction that probably explains why so many buildings have been destroyed despite the very tough building code aimed at earthquake protection. After the water drains away there are tonnes of structureless sand or silt left on the surface. These videos show the liquefaction of soil in Christchurch within minutes or hours of the earthquake.

Here is a technical explanation of soil liquefaction

Richard Christie from Ravensdown has sent me two photos of soil liquefaction that occured in central Christchurch within 2 hours of the big shake this week.The first shows the Avon River (normally only say 30cm deep with clear water) now in flood with liquified soil. The second is of the liquified soil being forced up thru new cracks in Tuam street outside the Ravensdown building in central Christchurch.

A number of Youtube video clips visually show how horrific it has been in Christchurch this week.
Everyone expects many aftershocks.....we sincerely hope this is the last big earthquake Christchurch & Canterbury suffers. To our Canterbury friends, fellow dairyfarmers & Agricultural support industry we are thinking of you!

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