Land Remediation after the Earthquake: What is “Perimeter Treatment”?

One good place for information is the Tonkin and Taylor Stage 2 Report done for EQC:

http://canterbury.eqc.govt.nz/publications/2010/11/stage2?page=0,0

Perimeter Treatment is the work that will be done in areas adjacent to rivers, such as the Avon in Christchurch and the Kaiapoi and Courtenay in Waimakariri.  The diagram on the left gives the general idea.

During the 7.1 Earthquake on 4 September, some of the worst damage occurred when soils liquefied and then moved sideways toward the rivers.  Big cracks opened up and houses started to fall apart.  Many were held together only by their roofs.  This process is known as lateral spreading and occurred near rivers because there was nothing to stop the liquified soil moving sideways. (The diagram comes from the Tonkin and Taylor Stage 2 Report.)

The perimeter treatment is designed to reduce this lateral spread in the event of a future earthquake.  Remember that the Kaiapoi area experienced liquefaction in the 1901 Cheviot earthquake.  A future quake could occur somewhere else in Canterbury other than Darfield.

The perimeter treatment has been described as “barriers” or  “walls”.  There are actually different ways that they will be constructed.  A common method will be to insert a vibrating probe into the ground to a point below the liquefaction layer.  This vibration consolidates the soils and makes them less prone to liquefaction. Gravel is then poured into the hole and also consolidated.  It takes about 30 minutes for this work and the then another one is done in close proximity.  They are totally underground.  Once done, the ground surface can be built on, have a road put over it or grassed back into parkland.

There will be something like 22 thousand of these columns inserted in the Kaiapoi area.  That’s a lot of half-hours!

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2 Responses to “Land Remediation after the Earthquake: What is “Perimeter Treatment”?”

  1. Andrew Mitchell Says:

    I think this is possible wit the Kaiapoi River, but can’t see how they’ll do this for the Avon River – there is too much trees and other fauna.

    What would save tens of thousands is building houses on raft foundations like they do in Australia. You cannot protect against the forces of an unpredictable earthquake, and raft foundations don’r “key” the house to the earth, mitigating the transmission of earthquake forces to the house structure.

    Of course, this would not be supported by the construction/building community, and so will never be implemented.

    • David Ayers Says:

      Raft-type construction has certainly been talked about and might occur in some places, I guess. I suspect the real driver won’t be the construction industry so much as the insurance industry. Where any house is being rebuilt, there will be an insurer involved – EQC’s liability stops at $100,000+GST, which, of course, won’t build a new house.

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