Is Christchurch the world’s worst city for liq­ue­fac­tion?

The Press - - News - Paul Gor­man

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.

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