Liquefaction. Suddenly we are all talking about it. Bob Parker even suggested today it might mean not building again in some parts of Christchurch, that tough decisions might have to be made.
I’m no geologist but a look at ECAN’s very helpful website suggests that in fact most of Christchurch is built on land that could be at some risk from liquefaction. I suspect this applies to large swathes of our other inhabited areas too, especially those built around rivers. Hamilton, Wanganui, the Hutt Valley, Invercargill rapidly come to mind.
http://ecan.govt.nz/publications/General/solid-facts-christchurch-liquefaction.pdf
This is a fascinating and readable summary, done pre-quake. It defines liquefaction precisely as we are seeing the results in places like Avonside, Bexley and Kaiapoi.
When the ground shakes during an earthquake the soil particles are rearranged and the soil mass compacts and decreases in volume. This decrease in volume causes water to be ejected to the ground surface.Sand volcanoes or sand boils, water fountains and associated ground surface cracking are evidence that liquefaction has occurred
It records that: The liquefaction potential in any part of Christchurch is very dependent on the height of the groundwater table at the time of the earthquake. So the fact that Saturday’s quake struck at the end of a very wet winter increased the liquefaction damage.
And the ECAN study reports that the liquefaction has been known about for a century.This from The Press reporting the Cheviot Earthquake in November 1901.
“At Kaiapoi, when the shock had passed…his land was apparently flooding from springs having been opened. It was then discovered that across his land…fissures from 1in to 3in [25 to 75mm] in width, and several chains [40m] in length had opened…From these earthquake openings the water was freely issuing…a liberal supply of sand from some grey quicksand layer below the level of the river, and this was deposited…in the shape of round and oval porridge pots and little hills. “The fissures remained open, and could be probed to a depth of six feet [2m].
Where does this leave us? That liquefaction is nothing new. That it might affect much of Christchurch, not just particular suburbs. That it might manifest itself once in a century or so, and perhaps once in several centuries, it might cause the sort of widespread damage that occurred on Saturday. (Note only Kaiapoi damaged mentioned in 1901)
This is not a reason to abandon suburbs. We have not lost any lives even in houses. often built cheaply and with no thought of earthquake-proofing up to a century or more ago. Cracks in the earth can be filled. Houses can be repaired. New commercial buildings can be put on deeper and more robust foundations.
Today I drove around the perimeter of my electorate. In Papanui, there were a few shops on Papanui Road closed and some chimneys down but not widespread signs of major structural damage. Yes, it’s a lighter risk area but not out of the woods. Only the Port Hills are out of some liquefaction “risk.”
Even so, this photo shows the damage caused at the huge Foodstuffs (New World + Pak n Safe) warehouse in Papanui. Today’s 5.1 aftershock caused more damage here and at supermarkets as well.
Meanwhile, in the run-down part of Colombo St between Moorhouse Ave and Brougham St, many old two-storey brick buildings are facing demolition.
Were they are damaged because of liquefaction or because they are century old unreinforced brick? Remember the Haiti quake was of similar intensity but left 200,000+ dead. Would you rather have Haiti or New Zealand’s building code.
Obviously we will need to assess the risks of liquefaction and that is underway.
There may need to be stronger building codes in some parts of Christchurch than others. Some areas, notably sub-divisions built on fill or in acutely high risk areas may not be viable.
But given that ECAN map showing some liquefaction risk across much of our city – and the fact that this is likely to have been a once in a part-millenium event, it seems to me that we will in most cases need to fill in the holes and rebuild.
First though, we need to get through the continuing swarm of quakes. No less than 300 since Saturday.
If you haven’t seen it, this extraordinary graphic maps the intensity and time of each of those tremors. http://www.christchurchquakemap.co.nz/
Here’s to a good night’s sleep.
Kia kaha Christchurch
Here’s to robust buildings and a successful clean up
Well, they all talking about it because it trendy.
While it is an issue I think the truth is the earthquake was that strong it was going to tear down things regardless.
Its a freak event! It wouldn’t of matter where some buildings where built. They would still fall down, especially if it was older brick building.
Also a lot of towns/cities in NZ are at risk of liquefaction. Built near/beside rivers,swamps,coast etc…
Physical factors are important Red. Maximum loads, strains, stresses, and more…I wouldn’t discount them too much.
In china some modern buildings collapsed during relatively minor earthquakes. When authorities investigated they found that concrete foundations had been adulterated with sawdust, etc.
In a decent earthquake gimme a well built single story wooden house atop bedrock any day…
Physical factors are important, but most modern building built here tend to be built to fairly tight regulations. I just think people (media) need to remember that it was a violent earthquake and I’m surprised the damage wasn’t worse.
Just as well ECAN are all publicly elected officials who are accountable to their public . . . oh, hang on . . .
Good post Brendon – I was reading about it on the internet last night as I have constituents who are worried about whether or not they can rebuild. Well done.
It seems that in many instances of liquefaction on properties, the gardens, driveways are most effected. The house may be largely untouched. I dont think liquefaction is a reason for not continuing to live at a site where its a well constructed one or two storey building.
Perhaps some residents ‘hopes’ of getting a new house are higher than what is realistically possible.
Best go under the house and check the foundations before making that call.
Yes you are right. However wooden houses on pile foundations can be ‘jacked up’ where any piles may have settled.
For cost reasons there has been a shift to concrete slab floors recently. Maybe this will mean the slabs will become more costly and the shallow piles will be more suitable again.
From memory they have this problem in Beijing, the solution there was deeper foundations (the foundations are on the Airport Maglev are 80 metres deep!) so I guess it is a matter of if people are willing to pay for really deep foundations in those areas of Christchurch with issues…
While our leaders are talking about fast tracking the rebuilding process to get the city back to normal, there is no excuse for not keeping to the very best building regulations and if that takes time to ensure accurate knowledge of ground conditions, let’s take the time.
The big question is: why did the Labour party not bring the facts up to the general public when they were in office prior to the National party taking office.
I would say Christchurch will not be rebuilt, at least not in its present location. I feel Labour let us, the general public down.
Check out googles earthquake map:
http://www.christchurchquakemap.co.nz/
The Christchurch Quake Map allows you to play time-lapse visualisations of the 4,870 quakes and aftershocks that have struck in the Christchurch area since September 2010. It is also possible to view visualisations of the last seven days of quakes or the 47 that have hit today. The Google Maps plot earthquake data from GeoNet.
Hello Todd… thanks for the info, but have had this map info for some time. An interesting place to view is the Christchurch Cathedral, you can see why there is so much damage to this stout building and the reason for my remarks. On this mornings’ news I see others asking re liquefaction.