The levellers - making Christchurch buildings even again

By Amanda Cropp

The Statistics NZ building in Madras Street which is being re-levelled with new "screw piles". Photo / Amanda Cropp
The Statistics NZ building in Madras Street which is being re-levelled with new "screw piles". Photo / Amanda Cropp

Companies specialising in re-levelling buildings are gearing up to cash in on the massive job of remediating earthquake-damaged land in Christchurch.

An estimated 30,000 houses need major repairs and Christchurch City Council environmental policy and approvals manager, Steve McCarthy, says as many as 10,000 are built on concrete slabs that may require re-levelling.

Industry sources suggest a lot of damaged commercial buildings could also potentially be saved by ground-remediation if insurance companies come to the party.

Screw piles, low mobility grout (a mix of sand and cement) and expanding resin are among the methods being touted as fixes for buildings which have sunk due to liquefaction.

Using a technique likened to keyhole surgery, Uretek Ground Engineering (NZ) Ltd is injecting expanding resin under buildings to lift concrete slabs.

Projects to date include lifting the ground floor of a four story office block which had slumped by up to 100 mm.

The resin technology, in use here since 2001 mainly to deal with subsidence issues, has also been used to repair earthquake damage in Turkey, Japan and Australia.

Uretek's South Island manager Phil Johnston says the non-toxic resin is injected through drill holes in floors and through tubes inserted into the ground around buildings. It expands to fill voids and exerts pressure of up to 40 tonnes per square metre to lift slabs. Once levelling is done, cracks often close up and stuck doors and windows open and close properly.

Johnston says the system is less disruptive than other methods, and they have worked on a fully occupied office block. "We went in at 6 pm and finished at two o'clock in the morning each day. We didn't even raise the carpet because they were going to replace it. We cut small holes in the carpet less than the diameter of a 10 cent piece and did it through that. The floor was out by 60 mm in the worst places and it's all levelled up again."

Uretek has worked on about 40 properties in Christchurch, including 15 homes. Quotes for levelling residential properties have ranged from $15,000 right up to $250,000 for a large 500 square metre house.

Johnston says Uretek has made a major investment in the ground remediation market by importing American equipment designed to pump low mobility grout. "We won't get much change out of $1 million."

The grout is a sand/cement mix that packs down to fill cavities. It is a more economical way of stabilising ground under larger commercial buildings and can be used in conjunction with Uretek resin.

Johnston says the grout pumping rig should be ready for use by early December. "We'll have about six people operating the first machine, but if it does take off, we would have up to 12 people working on that one rig and we'll buy more rigs as and when we need them."

Relevel, a joint venture between Keller Ground Engineering and Fletcher Construction-owned Brian Perry Civil, is also using low mobility grout.

Relevel manager Dean Quickenden says the company was formed after the September 2010 quake and has so far re-levelled one substantial commercial building and about a dozen houses.

He says the potential for re-levelling residential properties depends on the standard of concrete slabs they are built on. "There's a lot of difference in the type of foundation slabs to the point where some of them are not repairable."

Piletech, another Fletcher subsidiary, specialises in screw piles which general manager Tony Pike says can be retrofitted to damaged buildings. The piles are screwed into the ground "then we put jacking gear on top... level the building off and lock it in place."

Pike says screw piles are also suitable for residential housing and although up until now residential work has accounted for only 20 per cent of Piletech's work load, that is expected to rise to more than 50 per cent and the company plans to increase its Christchurch workforce 10 fold so it is ready when the rebuild "hits its straps."

But right now wrangles between insurance companies and building owners are slowing progress.

EQC does not cover commercial property, so building owners usually foot the bill for any ground stabilisation, but Johnston hopes insurance companies will come to the party when they realise remediation could save them paying out for a new building.

"Whilst (insurance companies) are not liable for mitigating the ground, if we can fix (the building and the ground) at a good price, it will save them half the price of the building, or something like that, and it becomes attractive all round."

Pike says owners are having difficulty getting insurance to cover the act of re-levelling commercial buildings and there is also the issue of indemnity insurance for designers. "We're thinking that if the industry collectively can get two or three (buildings) re-levelled, then people will see it doesn't need to be such a risky operation, and it can be done in a controlled fashion."

However Pike says the other problem is that once a building is re-levelled, owners then face the cost of bringing the rest of the structure up new building code requirements. "That's what pushes it over the line and people say 'it's too hard, I'll take the insurance, bowl it and build a new one."

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