Christchurch earthquake: Homes failed by 'standards oversight'

The Southshore home of John and Cheryl McCombe is slowly sinking on a lean. Photo / Herald on Sunday

The September 4 earthquake left 3000 Christchurch homes uninhabitable. This time the city will come off much worse, and the destruction has experts questioning the nation's building standards.

All houses must be built to the New Zealand building code and relevant standards. But according to engineers, homes could have been better protected if the building code had required better reinforcement and more stable ground conditions.

Ground quality in Christchurch has intensified the debate over reinforcement. For houses built on the swamp-like wetland, liquefaction is an issue.

Liquefaction happens when the soil's sand and water particles are not compact and when shaken water is pushed up to the surface. The building standards make no reference to liquefaction, which could also be a possibility in areas such as Warkworth.

Reports have shown that half of the city experienced liquefaction in the first earthquake. Mayor Bob Parker apologised for supporting re-zoning attempts by development companies, despite knowing land in areas such as Bexley was risky.

University of Auckland researcher Michael Newcombe said Bexley was built on a wetland. Liquefaction after the first earthquake caused sludge to bubble up through roads and houses.

Executive officer of the New Zealand Society of Earthquake Engineering, Win Clark, said the problem started with the first clause of the New Zealand Building Code, which worked on the basis that modern one-storey houses needed to be built on good ground. But there was nothing about liquefaction in the code, Clark said. He described those omissions as "oversights".

"There certainly needs to be a review to identify and highlight these issues."

When Mary and Patrick Harding built their home 25 years ago, an engineer recommended piles to support the concrete slab foundation because the soil was risky. Patrick said he was annoyed at the time because it wasn't in the regulations but would now like to "find the guy and shake him by the hand". The house has survived both earthquakes.

However John McCombe, a photographer in Southshore, said no regulations could account for the level of damage caused by the second quake.

His house was built above and beyond the standards but was damaged heavily in the first quake. The second quake left straight edges twisted like a corkscrew.

Three families are now sheltering in the house, which is slowly sinking on a lean towards the estuary.

Seismic structural engineer Les Megget said in many cases the council had a responsibility to tell people the ground was risky. In Bexley, 1991 documents giving the go-ahead for construction make no mention of such risks, despite staff warnings.

Since the September quake, Warkworth engineering consultant Colin Ashby has been lobbying the Christchurch City Council and the Department of Building and Housing to change the standards.