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Tuesday, 10 January 2012

The REAL Dirt on Liquefaction

Yesterday's post showed how earthquake damage to a small town could be carefully, and usefully, described. Knowing what happened is important for many reasons, and provides a context for all that follows.

That, in itself, is not sufficient to ensure that towns or cities are as safe as possible. While it is often heard that the liquefaction risk in Christchurch was well known, it seems to have been well known to only a small number of people. In addition to this limitation, hindsight also suggests there was little desire to commit to gaining more information, and ensuring it was widely available. The Earthquake Royal Commission will establish the extent to which this was true for the inner city, but no credible investigation seems likely on the same issues for urban areas.

Once again this is an issue better handled overseas. In 2001 ABAG (Association of Bay Area Governments, in the San Francisco Bay Area) published a document The REAL Dirt on Liquefaction: A Guide to the Liquefaction Hazard in Future Earthquakes Affecting the San Francisco Bay Area. Written for Bay Area residents, it sets out the hazards that arise from liquefaction, earthquake scenarios, the conditions under which liquefaction is expected to occur, the areas most likely to experience liquefaction, the damage to be expected, and what preparations can be made.

For those in a TC3 zone, Table 5 on pages 23 and 24 is of interest. It discusses techniques for liquefaction hazard mitigation, when they work best, and when the technique is probably inappropriate. A useful little checklist for whatever methods are proposed by EQC or the Department of Building and Housing.

The ABAG website is here and The REAL Dirt on Liquefaction here.

Parts of the publication don't tell Cantabrians what we don't already know. What it does do is put in one place a detailed and intelligible explanation of what has been learnt, and what can be anticipated. This is important as there is a risk that, in the name of business as usual, what has been experienced and discovered will be allowed to quietly disappear to avoid the city being seen in a bad light.

While some in the east will have long memories, many of the thousands of newcomers to Christchurch will not understand, and be oblivious to the hazards they may face if they choose to live in some parts of greater Christchurch.

Careful documentation of earthquake hazards is another area where the Council, ECan, and EQC need to be encouraged to emulate those who have done better to inform their citizens.

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