Wellington, Oct 21 NZPA - Taxpayers are likely to have to pay up to $200 million to build a grid of reinforcing around soils susceptible to liquefaction and ground-spreading in the Canterbury suburbs hardest-hit by damage in last month's earthquake.
The grid of timber piles, stone and cement columns was today described by Earthquake Recovery Minister Gerry Brownlee as a "massive dam" around eight of the areas damaged in the 7.1 magnitude quake on September 4.
A geotechnical report from Earthquake Commission (EQC) engineering consultants Tonkin and Taylor -- released today -- said the worst land damage was near river bends and river deltas, particularly from lateral spreading with some land moving as much as a metre.
Mr Brownlee said strengthening the ground in these areas would minimise the impact of future large quakes, with the reinforcing installed underground on vacant public land that borders rivers and other waterways.
The reinforcement of a perimeter of land between 10-20 metre wide and 5-10m deep around the worst-affected suburbs would limit the effects of lateral spreading in future quakes. In some cases grouting would have to be injected into the ground, or the soils compacted with massive weights capable of driving water out of the sub-soil as deep as 6m.
Tonkin and Taylor proposed that inside those areas, homeowners should re-level the houses "where practical" and that any new structures should be put on piles or "robust foundations".
Of the two main types of homes, concrete block, brick, and timber frame homes on a shallow unreinforced strip footings, with concrete unreinforced floors not tied into the foundations performed poorly where there was land deformation, and would be difficult and more costly to repair. These buildings were typical in newer subdivision at Kaiapoi, Bexley and Brooklands.
Older-style timber frame homes on shallow concrete piles performed better.
The consultants presented a range of nine options for fixing areas of liquefaction and ground-spreading, but said in their report they had been "instructed" to focus on the treatment of perimeter areas. This was lowest-cost option with the least social disruption.
The consultants were also working on a second option of providing no ground treatment, but re-levelling existing buildings.
This option would involve a "building restriction zone" for very severely-damaged properties, which would require strengthened foundations and engineering of any new houses built at these sites.
Some owners would need to decide whether to spend more money than their insurance payout -- after EQC has paid for repairs to the land -- if they wanted to rebuild, and the process might take years.
"The decision as to whether or not to rebuild on improved foundation systems is therefore a matter for the owners," the report said.
Tonkin and Taylor assessed more than 16,500 properties in the 17 suburbs which had the most severely affected reported land damage. About 60 percent of those properties had not experienced any land damage.
"Our best estimate is that the actual process of repairing land and rebuilding houses affected by the quake will take up to two years," EQC said.
"It is unlikely any physical remediation work will get underway before early in the New Year."
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