Dr Michael Naylor, Massey University insurance and finance academic, says botched earthquake repairs are just the "tip of the quakeberg".

Botched earthquake repairs are just the "tip of the quakeberg" and ignoring major foundation damage could prove catastrophic for Christchurch and taxpayers, a leading academic says.

Dr Michael Naylor, a senior lecturer in finance and insurance at Massey University, said the poor repairs to most Christchurch homes with rubble-style foundations left them unsupported in future land movement.

Naylor said recent comments by Earthquake Commission (EQC) Minister Megan Woods indicated her advisers had under-estimated the problem of inadequate repairs. He called on her to "demand" analysis of the "real and looming" issue, which he said could cost billions of dollars if not addressed.

SUPPLIED An estimated 90 per cent of rubble foundations have not been properly assessed and repaired.

"This threatens to be as large as the leaky homes crisis, which has cost at least $6 billion. Treasury should be costing billions into its budgets for future liabilities".



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Research by Christchurch foundations specialist Bevan Craig, of Underfoot Services, indicates up to 65 per cent of Christchurch homes had rubble-style foundations at the time of the quakes. Craig estimated 90 per cent of those should have been rebuilt, but were inadequately repaired instead.

Rubble foundations are ring foundations made of river stones, quarry or brick rubble, sand and cement. They were built in Christchurch until 1968, several decades after most of New Zealand stopped using them.

Craig said his estimate of 30,000 affected homes was conservative, and in reality there could be 60,000.

STACY SQUIRES/STUFF Bevan Craig, of Underfoot Services, says tens of thousands of homes have unrepaired or inadequately repaired foundations.

Christchurch probably had the worst building standards in New Zealand, he said.

"It's a huge issue. When you look at the numbers, it's frightening".

A Ministry of Business, Innovation and Employment survey leaked to Stuff in 2015 found most foundation repairs examined were inadequate.

STACY SQUIRES/STUFF Foundations repaired in Christchurch under the EQC repair programme may not be up to scratch, an academic says.

Naylor said rubble foundations were "useless" once cracked, and would compromise a house's integrity in future quakes "regardless of how strong the top part it".

"The majority of quake-damaged houses are likely to be sitting on foundations that will not support them in any further quakes or land settling."

The problem was exacerbated by the city's base of old riverbeds and sandy and swampy ground, which meant houses settled unevenly, Naylor said.

Current EQC and private insurance figures related only to above ground and "blatantly obvious foundation botch-ups", he said.

SUPPLIED Michael Naylor has studied the insurance industry's response to the Canterbury earthquakes.

Using a conservative estimate of $150,000 to replace an average house's foundations, Naylor estimated the total repair bill would be at least $4.5b. Because quakes were expected to continue for several decades, leaving homes vulnerable would be costly in the long run, he said.

"In every shake, houses will get more and more broken. We will get more and more claims. But because time has passed, the insurance companies can say they are not liable."

Limitations on insurance cover meant any costs would fall on taxpayers and homeowners, he said.

STACY SQUIRES/STUFF Foundation damage in Christchurch is leaving homes at risk, a leading academic says.

Naylor said that with insurers obliged to repair homes to their original state, they could argue against having to upgrade foundations that were always inadequate. However, building codes needed to be followed, he said.

A full repair job could have bankrupted insurers, who, along with Government, chose to patch foundations "and hope Canterbury didn't suffer any further serious quakes".

"The potential liabilities of continuing house damage in Canterbury due to the failure to assess or repair foundations is thus potentially catastrophic for the Government.

"Spending EQC money repairing above-ground cracks or kitchen benches when foundations are shot is throwing money away."