Christchurch earthquake: Collapsed buildings on soft soil - QV

Pyne Gould Corporation building in central Christchurch. Photo / Mark Mitchell

The two buildings that claimed the greatest number of lives in the earthquake were built on soft soil prone to a "large increase in shaking" and with high susceptibility to liquefaction.

The risks are outlined in Quotable Value property-hazard reports which reveal common features of concern for the sites of the Canterbury Television and Pyne Gould Corporation buildings, in which more than 100 people were killed.

The Government has ordered an inquiry into the safety of affected buildings. Yesterday, Prime Minister John Key said as many as one-third of buildings in central Christchurch might have to be demolished.

The hazards identified in the QV reports - obtained by the Herald - are likely to also apply to other buildings in Christchurch because of the ground conditions.

Owners of the CTV and PGC buildings say they were given clearance after the last big quake in September, and engineering reports had been obtained.

But concerns have been raised about the safety of the CTV building.

Lionel Hunter, sole director of CTV building owner Madras Equities, told the Herald he lost a good friend in the building's collapse.

"If we had known anything was wrong with the building, I would have pushed it over myself," he said.

CTV chairman Nick Smith, who lost many of his staff, said he had no qualms about trusting the judgment of the building owner and the engineering reports ordered.

"I'm not an engineer. You accept the expert advice, and you hope it's correct," he said.

The property-hazard reports for both buildings, compiled from a variety of official sources, say they are on ground classified as "very soft soil".

"In a strong close-by earthquake these materials could cause ... a moderate increase in shaking for high-rise buildings.

"In a strong distant earthquake these materials are likely to cause a large increase in shaking."

The reports also say the buildings are "in an area where the ground is
classified as having a susceptibility to liquefaction that is very high".

GeoNet project director Ken Gledhill said liquefaction probably occurred in central Christchurch as well as in the suburbs, but large buildings were expected to have substantial foundations to combat it.

Madras Equities spokesman Ken Jones said the Christchurch City Council cleared the CTV building after September's 7.1-magnitude quake.

A detailed structural engineers' report commissioned by the building's manager had raised no issues about the structural integrity of the building.

"The report recommended internal and external work to repair superficial damage to the building fabric sustained in the earthquake and its immediate aftershocks," he said.

This work was being done at the time of the building's collapse.

"The February 22 earthquake appears to have generated unusual forces that relatively modern buildings built to recent seismic standards were not able to withstand," Mr Jones said.

A written statement from the company that owns the PGC building said: "The building was green-stickered after the September quake. The owner had four assessments and reports prepared by structural engineers after the September earthquake and then again after the Boxing Day earthquake".

The director of the company, Stephen Collins, could not be reached for comment.

Mr Key said yesterday that a third of the buildings in central Christchurch might have to be demolished.

Before they could be replaced, geotechnical work would have to be done on the land, and deep piles could be needed to reach solid ground.

"We are talking two years before you see much," he told NewstalkZB. "In five years you will see quite a bit, in 10 years you will see a lot."

Rebuilding Christchurch would be "a 15-year job".

Buildings erected under modern building codes could survive without damage, and that was shown by the Inland Revenue Department building across the road from the CTV site.

"It is a normal glass building," Mr Key said. "Not a pane of glass broken, it is in perfect condition."

He suggested that because of the number of central-city buildings under threat of demolition, Christchurch might develop as a city with large satellite business districts.