New Zealand prime minister John Key said this week that 10,000 houses in Christchurch face demolition because of earthquake damage.
He said some parts of the city will have to be abandoned altogether.
The magnitude 6.3 quake struck on 22 February, shattering homes, heritage buildings and office blocks (News last week). Officials said the confirmed death toll was now 166 and that this was expected to rise to more than 200.
Key also told a press conference on Sunday that “there are some parts of Christchurch that can’t be rebuilt on” because of liquefaction and other land damage.
Ground liquefaction causes soil to lose its shear stiffness and start to flow, causing structures it supports to rotate or sink. Sand and silty soils and a high water table prevail in Christchurch which is on an
alluvial plane and as such prone to liquefaction. The Christchurch area was also affected by liquefaction following the magnitude 7.1 earthquake last September.
“The liquefaction damage from the earthquake is so great and the land damage is so significant we can’t remediate it”
In Christchurch, 260,000t of silt has been scraped away following the February quake.
“The liquefaction damage from the earthquake is so great and the land damage is so significant we can’t remediate it,” said Key.
Key said that 3,300 of the 10,000 houses facing demolition had been damaged by last September’s quake.
The latest numbers from New Zealand’s Ministry of Civil Defence and Emergency Management show that 1,359 commercial buildings and 1,368 residential buildings have been red stickered as unsafe to enter and are likely to be demolished.
Many thousands more homes have been yellow stickered which means that access is restricted or cannot be entered until repairs are complete. Some of these may also have to be demolished.
Eastern suburbs ‘most likely to be abandoned’
Christchurch’s eastern suburbs were most badly hit by liquefaction and so are the most likely to be abandoned.
“There are some areas of Christchurch which will need to be abandoned and we will have to provide other alternatives for people to live in because the land has been so badly damaged, we can’t fix it − certainly not in a reasonable time frame,” said Key.
He said modular houses would be brought in to provide temporary accomodation for some of the thousands of displaced homeowners.
Decisions about whether the city’s older building stock will be repaired will be the toughest.
New Zealand has some of the most advanced seismic codes in the world but they were only introduced in the mid-1980s and are not retrospective, so older buildings are most at risk.
Local authorities had been retrofitting older buildings so that they met at least 30% of the requirements of design code standards, but this policy is now under review.
Institution of Professional Engineers New Zealand (IPENZ) chief executive Andrew Cleland said it would be hard to decide whether to upgrade some structures or demolish them instead.
Heritage sites at risk of demolition
“An owner of a heritage building will have to fund upgrades or else they will not be able to obtain insurance. However, if they do not have the funds, they then face the possibility having to demolish a historical building so you reach an impasse.”
Work crews are still clearing rubble from the quake, which badly hit central Christchurch and cut water and power supplies across the city. Almost all electricity supplies have been restored, but residents are being told to boil tap water because of the ongoing contamination risk.
Engineers are focusing on restoring the city’s sewerage system. Half of the city is still cut off from the sewer network and sewers were severely damaged by the quake.
Latest official numbers show that 68 of the city’s 95 pumping stations are fully operational, with another eight ready to come back on line and 19 damaged beyond repair.
Fears for Wellington
Huge amounts of liquefaction caused by last month’s earthquake in Christchurch has prompted New Zealand officials to look at vulnerable areas in its largest city, Wellington.
Wellington is New Zealand’s capital and political centre, with a population of 180,000 in the city and 440,000 in the region.
GNS Science, New Zealand’s equivalent of the British Geological Survey, said it was examining the ground response caused by the 6.3 magnitude earthquake.
GNS Science seismic hazards lead scientist Mark Stirling said new information will help officials plan for an earthquake in Wellington.
It is expected that a magnitude 8.0 quake will strike Wellington within the next 30 to 100 years.
“It’s important to understand how the ground has responded to the shockwaves,” said Stirling. “Wellington has areas of soft ground, similar to Christchurch, so we need to gather as much information as possible.”
Stirling said GNS Science is monitoring the fault about 20km below Wellington where tectonic plates are “squeezing together”. At some point this is expected to produce a huge earthquake and possibly a tsunami.
GNS has also revealed further details about the Christchurch fault. It believes that the earthquake resulted from the rupture of an 8km by 8km fault running east-northeast at a depth of 1km to 2km beneath the southern edge of the Avon-Heathcote Estuary.
It dips southwards at an angle of about 65˚ from the horizontal beneath the Port Hills − between the epicentre near Lyttelton and Christchurch.