Hamilton's bulk water main fix cost grows to $4.6m

Further erosion has scoured away soil around the Eastern Bulk Water Main, just below Hamilton Gardens, since it first ...
HAMILTON CITY COUNCIL

Further erosion has scoured away soil around the Eastern Bulk Water Main, just below Hamilton Gardens, since it first slipped away in February.

It started when a huge chunk of Waikato's riverbank slipped away and could end with a $4.6 million bill.

Thousand of tonnes of soil fell from the bank below Hamilton Gardens in early February, leaving a major water main exposed to the river rushing by.

Since then, Hamilton City Council has been battling to stabilise the bank and protect the Easter Bulk Water Main, which supplies water to about 27,000 homes.

Soil nailing - steel rods through a shotcrete layer - are planned to stabilise the bank. This photo was taken when the ...
MARK TAYLOR FAIRFAX NZ

Soil nailing - steel rods through a shotcrete layer - are planned to stabilise the bank. This photo was taken when the massive slip at the centre of the problems happened on February 2.

Councillors have approved a $1.9m budget for works, but are now being asked to up that to $4.64m. Council will discuss it on Tuesday.

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It was lucky the slip didn't take all the infrastructure with it, city waters unit manager Maire Porter said.

"I guess that's testament to the construction and installation ... back in the '70s."

"[The slip] was a big enough area in the beginning, but you can see that it's quite substantially larger now and a lot of that is from subsequent erosion ...

"If we don't address this, it will undermine what we do [around the infrastructure]."

Staff's best estimate is that it will cost $4.64m to stabilise the bank, as opposed to the previously cited $1.8m.

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"It's emergency work, so basically there is no funding that's been planned for this," Porter said.

Council's insurance may be able to soften the blow to ratepayers, but an investigation to determine the cause of the original slip must finish before that can be determined.

Emergency work started straight away, but bad weather wrought by cyclones has affected progress.

High river levels put the site underwater at least three times, Porter said, reaching almost its historical maximum levels on several occasions.

The $1.8m estimate for repair costs was based on a rock-filling method that council has now found is unsuitable.

Rock filling is "basically replacing the volume that's gone and providing some hard, heavy material to increase stabilisation", Porter said.

That would have had to extend too far into the river to get the stability needed, so staff have recommended soil-nailing instead.

That involves drilling 100 to 150 rods through the shotcrete layer currently protecting the bank from being further scoured away.

"These are steel rods that are basically drilled through that surface and anchored back into the more stable ground," she said.

"We're talking metres and metres here, so it's quite a big project."

It's a difficult site, where most work will have to be done by crane and over water, Porter said.

That water flows by at a rate of about 468 cubic metres a second.

The weather and river levels will also have a big impact on how fast work can progress, she said.

Council has hired Aecom to investigate what caused the February slip and is expecting a report on the matter in late June 2017.

The cause will determine which of council's insurance policies is relevant, if any, Porter said.

But council has a good relationship with its insurers and is keeping them informed.

Councillors will discuss the staff recommendation to up the repair works budget at Tuesday's finance committee meeting.

 - Stuff

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