Is Christchurch the world’s worst city for liquefaction?
Canterbury’s aquifers may have made Christchurch the worst place in the world for earthquake liquefaction.
GNS Science Dunedin principal scientist Dr Simon Cox believes the water pressure from aquifers below the city made eastern suburbs even more vulnerable to liquefaction when the big quakes of September, 2010, and February, June and December, 2011, struck.
He is still trying to get academic and engineering recognition for his research into the causes of the choking grey sand and silts that flowed and set across parts of the city after the largest quakes.
The volume of water released, and the height to which it was ejected above ground level, showed more was going on than textbook liquefaction, he told the joint Meteorological Society-Hydrological Society in Christchurch yesterday.
‘‘The standard message you will get about liquefaction is you have a loose soil and when you shake it, you get a settlement and a compaction and densification of that soil and the water comes out at the surface. There are numerous experiments that show this process.
‘‘I’m not denying this process happens – in no way am I denying that. But the question is, if you have got artesian pressure sitting underneath that, what would happen as you add the component of that pressure into that system that’s liquefied?’’
Once part of the 40-metre thick layer of sediment below the eastern suburbs that acted like a lid on the water below – an aquitard – had liquefied in the quakes, there was less ability to contain the groundwater pressure, he said. ‘‘We think a lot of the ejection is occurring because the pressure comes and brings that water out during the earthquake.
‘‘Now it may or may not contribute to the pressure which actually liquefies the sediment, but we can’t prove that.
‘‘So I tried to publish this. And it was basically rejected, and continually in the first instances, because [they said] just because you’ve got this correlation, doesn’t mean it caused it.
‘‘Part of the problem is across Christchurch we have a whole series of gradients – you go from the fluvial sediments into the coastal sediments, so it could be an east-west variation in the amount of material that could liquefy.
‘‘But then of course we had gradients in the acceleration during the earthquake as well in an east-west direction,’’ Cox said.
When the pressure from the underlying deep aquifers was released, it had brought sand and fine materials to the surface.
‘‘Some time afterwards, in some cases hours later, that pressure has dropped off and those cracks started to self-heal.’’
Christchurch’s unique geological situation on top of that artesian pressure exacerbated the hazard, he said.
‘‘We have had leakage. I think it’s very clear.
‘‘Are there other places as bad as Christchurch? Well, there’s not many places with the artesian head [pressure] we have here in Christchurch, so I would say no. I think Christchurch was a worst-case world example, of the worst liquefaction you can possibly have. And it’s because of that artesian pressure that comes from that water flowing all the way down the Canterbury Plains.’’
In most other seismically active parts of the world that had aquifers below, that pressure had been drawn down and the hazard had largely been removed, he said.
Liquefaction piles, like this one in Burwood, were a common sight in Christchurch and Canterbury after the 2010 and 2011 earthquakes.