Wall likely to be remodelle

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An Oamaru stone and concrete retaining wall that partially collapsed in Severn St last year may have layers of stonework removed, if the Waitaki District Council approves that.
Several metres of the three-tiered Oamaru stone wall, built by the Oamaru Beautifying Society more than 80 years ago, failed in May last year and stone blocks tumbled on to the busy stretch of State Highway 1.
Only two months earlier, the council spent $45,000 on stabilising the wall, including removing leaning sections, re-establishing drainage behind the wall and building a new stone block wall.
At a meeting of the council’s assets committee last Wednesday, committee members recommended the council approve the removal of the top two layers of stone for the entire length of the wall and reshape the embankment behind the wall and plant it to prevent future scour or erosion, at a total cost
of $150,000.
The recommendation was based on the assessment of the remaining, unreinforced sections of the wall, which are leaning at an angle of up to eight degrees off vertical.
A report prepared by council roading manager Michael Voss said there was risk with the recommended option, as it was “like-for-like”, but lessons learned from the collapse would be put into practice to minimise the risk of it happening again.
Other options considered were to retain the existing wall and not strengthen it, or replace or remove it.
Waitaki Mayor Gary Kircher said the decision was one that had to be made.
“It’s one of those reluctant decisions we have to make. We would all like the wall to be rebuilt, but to build it at retaining wall standard is half a million dollars, which is very hard to justify.
“We have taken the cautious approach … and will see how that looks before we push on and do the whole thing. There is certainly some sections of the wall that are at serious risk of toppling, but we think they’re safe. It’s not likely to bulge out. But we want to deal with it before we get to that stage,
so we’ll see how this works.”
The collapse of the newly built section of the wall was attributed to several factors, according to a council report published in April.
“Overall, the failure … has no single cause and council cannot lay the failure at the wall’s constructor’s feet, as other factors such as not undertaking extensive geotechnical investigations, the differing soil conditions, method of backfilling, effects of old root masses and the unusually intense rain event all
contributed to the failure,” the report said.
The cost of the work would be partially funded through the Oamaru amenity rate (43%) and the NZ Transport Agency (57%) and it would start as soon as possible.