Consequences of liquefaction

[Winston001]

I think I understand liquefaction of soil. Essentially vibrations energise water molecules in the subsoil which rise to the surface in sand and mud. It can only occur in wet soil and gravel.

But what about the other side of the equation? Water (plus sand and mud) rise to the surface but heavy objects must sink as a result.

Nothing I have heard in the news mentions this. Surely some/many of the homes in eastern Christchurch have now sunk a little. And the surrounding land will have sunk too.

Any engineers out there? How can you re-mediate entire suburbs which are barely above the water-table? This looks like a long-term nightmare for the council and ordinary people who have put their lifetime's savings into their homes.

[onearmedbandit]

I remember with the last earthquake a lot of questions were raised about building consents that had been accepted in the areas that were worst hit by liquefaction, mostly new suburbs. I don't know what came of it though.

[onearmedbandit]

As a side note here is a good visual example of liquefaction. This was filmed by a local resident cleaning out his mothers rose garden.

[scumdog]

yep, subtract the volume of the removed liquifaction soil from the area it has been taken from and in theory that area will have sunk by that much...that much closer to the water-table...

[miSTa]

Not necessarily, I've seen a couple of sewerage pumping stations that have risen out of the ground.

One problem, and quite a significant one, that the vibration has caused is the slumping of the banks of the Avon River (perhaps the Heathcote as well). Just up the road (Avonside Drive) the road has slumped a good metre, the river is noticeably narrower. In the other direction the Dallington bridge and Gayhurst Rd are now at massively different levels, the bank has pushed the bridge supports further into the Avon as well (bridge is closed to buses and trucks). The slumping is also very evident at the rowing clubs where all three buildings are rooted as is the ground, some very long cracks between the buildings.

[Lurch]

Houses sure have sunk, one of my colleagues houses sank by 30cm into the ground and will be a write-off.

[vifferman]

We used to live in St. Martins (18 St. Martins Road). We think we saw our old house on TV in an item about how badly St Martins was hit by liquefaction.
Strange... just a few weeks ago I was thinking wistfully about living in Chch, as of all the places we've lived, we had arguably the best lifestyle there. Now I'm glad we didn't stay.
The vifferbabe spent two days this week helping out at the airport. It was almost a life-changing experience for her; she was very deeply affected by it, and felt privileged to be able to do her small bit to help.

[Winston001]

I remember with the last earthquake a lot of questions were raised about building consents that had been accepted in the areas that were worst hit by liquefaction, mostly new suburbs. I don't know what came of it though.

Ah yes. Hindsight. I believe the Christchurch City Council planners pointed out the soft soil and liquefaction risk at the time the subdivision was applied for. Marshlands has its name for a reason...

But they were ignored because the councillors were persuaded it was low risk, everyone wanted the city to keep growing, and the land was close to the rest of urban Christchurch.

Edit: as a Southland farm boy, I was horrified 5 years ago to see these new subdivisions on land (out near QE II) which was swamp. No farmer ever builds on a swamp. Why anyone else happily would do so is beyond my comprehension - but most NZ cities are on flood plains. Go figure.

Not necessarily, I've seen a couple of sewerage pumping stations that have risen out of the ground.

Yeah that doesn't seem logical but as I understand it, those buildings and pipes actually floated on the liquefied substrata. Thus they are not like the stones in Onearmedbandit's video above: the stones are heavier and sink, the pumping stations are mostly air and float.

[Brian d marge]

Edit: as a Southland farm boy, I was horrified 5 years ago to see these new subdivisions on land (out near QE II) which was swamp. No farmer ever builds on a swamp. Why anyone else happily would do so is beyond my comprehension - but most NZ cities are on flood plains. Go figure.
.

Yeah me too

I often would be walking back from a party somewhere , and especially over marshlands way ,,, Shirley somewhere like that and the ground was wet , I mean it never seemed to dry out , always wet and damp

I remember thinking to myself wouldn't want to buy a house here , it always seems damp

Never in my wildest dreams would I have thought of liquefaction

I was worried about the mud on the boots.......

My mums house I think has sunk a wee bit ..

Stephen

[Shadows]

Been a nightmare for them for a long long time.
Why else would CHCH be the only city on the world where you can buy properties so near to the beach for a song?
Because the risk has been known for ages and land in those areas valued accordingly by successive councils.

[AllanB]

There are plenty of examples of items sinking in CHCH after September. The city has had various survey points remeasured and we have basically dropped. Probably down more now too! Winter will be interesting!
A Geo-tech chap MrsB was talking to said there was a entire house swallowed up by liquefaction overseas!

Evil shit, that is heavy and like a clay when wet but dries up into an extremely fine sand.

[Winston001]

Is "thixotropic" the correct word to describe this subsoil? I know it applies to types of clay - the reason the Kilmog hill north of Dunedin is continually being repaired is because the hill is thixotropic clay.

[Brett]

Not necessarily, I've seen a couple of sewerage pumping stations that have risen out of the ground.

One problem, and quite a significant one, that the vibration has caused is the slumping of the banks of the Avon River (perhaps the Heathcote as well). Just up the road (Avonside Drive) the road has slumped a good metre, the river is noticeably narrower. In the other direction the Dallington bridge and Gayhurst Rd are now at massively different levels, the bank has pushed the bridge supports further into the Avon as well (bridge is closed to buses and trucks). The slumping is also very evident at the rowing clubs where all three buildings are rooted as is the ground, some very long cracks between the buildings.

Pump stations: Liquefaction would have disturbed/consolidated the soil matter around the building, building most likely goes quite a way into the ground and also has a lot of infrastructure (ie pipes etc) that are attached and could have helped keep the sub station up by supporting it. Thus ground level drops, building stays up and looks like it has 'risen' from the ground.

Just my theory...let a civil/geotech engineer correct me if they want.

[NighthawkNZ:Me]

I get liquefied every bike rally I go to... and I am a fat heavy barstooled, and yup I sink with my liquefaction...

so there is some truth to it

[JimO]

i was talking to a Dunedin City Council building inspector who is just back from ChCh he says there is a theory that the ground once liquified and had water/silt come out it should be better packed down as a building site, he wasnt convinced and neither am i, i think ChCh is fucked and spending billions rebuilding on dodgy ground is ludicrous