City centre stays despite quake

Buildings in Christchurch's red zone sustained fresh damage in last Monday's earthquakes. Photo / APN

Engineers say it would be too costly to move Christchurch's fractured central business district, despite further damage in this week's aftershocks.

Consulting engineer Jade Kirk said replicating the central city's buildings, roads, sewerage and other infrastructure - much of it largely undamaged - would "cripple" the national economy.

"We'd be looking at a $10 billion to $20 billion writeoff of land to do it, so anyone who seriously entertains it is simply uneducated."

Mayor Bob Parker said there was no doubt the central city would be rebuilt where it is, even though possibly 60 per cent of the buildings within the "red zone" around Cathedral Square would be demolished.

The other 40 per cent of buildings in the zone, and perhaps 80 per cent of buildings in the rest of the business district bordered by the historic "four avenues", were largely intact.

The cordoned-off red zone covers about a fifth of the area within the four avenues. The zone's boundaries run from Kilmore St in the north to St Asaph St in the south, and from Durham St alongside the Avon River in the west to Madras St in the east.

Mr Parker said Christchurch was designed as "a radial city" around Cathedral Square, keeping all suburbs reasonably close to the centre and creating an ideal pattern for public transport.

He said that plan might need amending by creating "broader garden spaces" near the Avon, where liquefaction occurred, and raising earthquake-resistant design standards to about the same level as in Wellington.

"Certainly we'll be bringing in a whole new set of design guidelines and raising the bar very high around issues like sustainability, energy usage, light, all the things we have learned from cities over 50 or 60 years," he said.

"But it will be around that historic grid which still retains a number of historic buildings and gardens and so on, all the things that are part of the fabric of our city."

Mr Kirk, an engineer who worked on the 13-floor HSBC Tower opened in 2008 near the historic arts centre, said modern buildings withstood the quakes with only minor damage. Major HSBC tenants had already moved back and he said the tower would be full again within two weeks.

"The city [centre] is no more at risk than any other part of Christchurch," he said.

"Liquefaction is a risk throughout New Zealand. It just happens that we've had an abrupt earthquake directly under our CBD, which is probably more bad luck than bad management."

He said engineers could design foundations for any ground conditions, including areas liable to liquefaction. It would actually be more economic to build tall buildings in such areas to justify the cost of deep foundations.

"Once you get above six storeys you have enough capital to start doing liquefaction mitigation."

Canterbury University engineer Misko Cubrinovski, who has been leading technical investigations of liquefaction and lateral spreading in the central city, said there was a risk of some liquefaction in perhaps a quarter of the area between the four avenues. But it would only be enough to cause ground deformation in a smaller area.

"If you want to build there, obviously you have to do something special - significant ground improvement or extremely robust foundations and very well-designed superstructure, and in those areas you would want to limit the height of those structures."

$10b to $20b
of land would have to be written off if CBD was moved.

60%
of the buildings within the current "red zone" could be demolished.

40%
of buildings in the zone are still
intact.

80 %
of buildings in the rest of the business district are still largely intact.