Can we fix it?

JOHN MCCRONE

Last updated 11:17 16/04/2011

 

 

Whether the land is worth rebuilding on or not is the big question hanging over quake-hit Christchurch suburbs. John McCrone investigates.

Stick or flick? The first time round, after the September 4 earthquake, they were going to repair almost all the land. Only eight houses scattered around Canterbury were going to have to be knocked down and the owners moved on.

Earthquake Recovery Minister Gerry Brownlee saw this as a massive result. Homes would stay. The Government was going to protect people's investment in their greatest asset.

But this time round, after February 22, there are whispers that thousands of properties will be condemned. Whole streets and suburbs may be grassed over, abandoned. The lower Avon could revert to a wildlife park.

And now there are also damaged hillsides to be considered. More land where it might be impractical to rebuild.

So how is that going to work in terms of compensation? Should a government be responsible for all the costs of a disaster, or just some portion? And how much say will residents be allowed to have over whether they must go, then where they will go? Especially if the Government is providing the new housing plots.

Talk about big decisions. And in an election year. Meanwhile, until those decisions are made, people's lives are going to continue to be on hold.

In Avondale and Dallington, Kaiapoi and Sumner, they will be camping in broken neighbourhoods through a chilly winter, or forking out for rental accommodation, awaiting the tick of some distant bureaucrat's pen, waiting to discover whether their property file will be marked stick or flick.

The verdict on the land is going to be a two-step process. As Christchurch East MP Lianne Dalziel says: "The geotechnical engineers will tell us what is possible, and the Government will tell us what we can afford."

So first must come the science. A research effort of a scale probably unique in New Zealand's history reckon those involved, like Kate Williams, senior engineering geologist for consultants Tonkin and Taylor.

As the Earthquake Commission's (EQC) lead engineer, Tonkin and Taylor has become a familiar name. After the September earthquake, it had already delivered the first two stages of its geotechnical report - the good news that almost all the land could be fixed with a mix of stone column "earthdams" along river edges, good drainage, and machine compaction of liquefied soils.

A third and final report was within weeks of being made public when the February 22 earthquake struck. Now, it is back to the drawing board, start all over again, says Williams.

There is an advantage in that the firm has built up its numbers in Christchurch. It employs some 280 engineers doing EQC property assessments alone. "Following the second quake, all the systems had been developed, a process had been ingrained in our teams, so people could be sent out straight away," says Williams.

However, the scale of the damage is many times greater now. And more agencies have had to get involved as some completely new issues have arisen.

Bodies are being thrown at the work, says Williams. Science that would normally take months, if not years, is being crammed into weeks because everyone realises the land remediation question is what is going to hold up the whole recovery process. It is pointless fixing sewers or sending in builders until it is known whether the houses can stay.

Williams says the Government is expecting its first report from Tonkin and Taylor by mid-May. So far, only 30 per cent of the necessary information is in. Yet, she believes the report can be complete enough to allow some larger decisions to be made by June.

"We're not going to go to 100 per cent by then - though you know, as scientists, we'd love to be before we give any recommendations. But there has to be a point in time where we say we have enough information that is robust.

"So by around mid-May, we may be able to inform people, not on individual properties, but probably on a suburb or street-by- street basis, about what may happen in their neighbourhoods."

Williams says the damage story is complex and cannot be prejudged. Prime Minister John Key has already indicated bad news. At a post-Cabinet briefing in early March, he said there will be some areas that won't be rebuilt on: "There will be parts of Christchurch where the option is simply to take a cheque or say 'here is a subdivision, you can choose a site and a building plan'."

But Williams says no-one wants to alarm people with partial information. And the numbers that may have to move is still very unclear.

One of the reasons for uncertainty is a new factor to be considered after February 22. That of flood risk. The shaking and liquefaction was so bad, some suburbs may have sunk by a good metre.

All that grey silt which boiled up out of the ground and got carted away - Fulton Hogan shifting 32,000 truck-loads in a fortnight - has left the land lower.

And then whole slabs of Christchurch may have been dropped or tilted because of the way the fault disturbed the ground.

An early survey by the government research institute, GNS Science, found that the Port Hills had been lifted 40 centimetres higher by the quake, while the Avon-Heathcote estuary had been shunted sideways by some tens of centimetres and may have fallen by a similar amount as well.

Williams says Land Information New Zealand (Linz) is responsible for mapping the new level of the city. Linz researchers have already flown over Christchurch, bouncing a laser off the ground to take accurate readings. But crunching the terabytes of data is another one of those normally multi-year projects being squeezed into a matter of months, she says.

Yet the information is crucial because suburbs along the lower Avon especially, like Bexley, New Brighton, Avondale and Horseshoe Lake, could now be far more prone to spring tides and storm surges.

The council had to sandbag danger areas ahead of the spring tides last month. "A metre makes a big difference," says Williams.

In assessing the flood risk, there is the further complication of Variation 48, a recent rule change to the council's city plan brought in to deal with climate change and expected sea-level rises.

The variation means all new buildings in low-lying parts of town have to raise their floors to a level high enough to cope with once-in-200-year floods. So when it comes to any post-quake rebuilding in these areas, this becomes another expense to be added to the land repair balance sheet.

Williams says there is always an engineering solution to any problem. Homes can be put on stilts, land can be filled, dykes or stopbanks can be constructed to keep the sea and floods out. But then there is the cost. At some point, it makes more sense to shift instead.

Williams would not be drawn, but others are looking at suburbs like Bexley and wondering if there is any choice besides an orderly retreat before the tides eventually reclaim such areas as part of the estuary wetlands.

The big question, of course, is still the issue of liquefaction and lateral spreading.

Civil engineers have long warned that much of eastern Christchurch is built on poor soils. A lot of it is old swamp which will bubble up with silt or jellify and slither sideways with a good shaking. And even if truckloads of silt have now been taken away, there are still metres more below the surface.

Canterbury University liquefaction expert Misko Cubrinovski says whole parts of town would never have been built on if the risks had been known a hundred years ago. "Silt has no cohesion. There is no binding force to glue the grains."

Cubrinovski was out with a student immediately after February 22, methodically going down every street in the stricken areas of town to map the evidence of liquefaction before it could be cleared away.

His map shows how a corridor of land all down the Avon River is a problem as might be expected. Richmond, Avonside, Dallington, Shirley, Avondale, Burwood and Bexley. All the places which might have been dank lagoons or oxbow lakes at some stage in the river's history.

The poor soils also have tongues poking up into Parklands and coiling round into Papanui. To the south of the city, there is another large band running from Charleston up past AMI stadium to nearly Cashel St where the Hotel Grand Chancellor stands precariously balanced.

Cubrinovski says the Heathcote River margins look to have fared much better. Fendalton and Merivale have spotty distributions of unstable ground, probably connected to their many small waterways.

And then there is a striking strip of liquefaction cutting through the central city, roughly bordered by Salisbury and Gloucester streets, and stretching into North Hagley Park.

"In some parts of the city, the line of liquefaction was so sharp, that I could 'geo-target' it. I could walk along it to mark it."

- The Press