By Les Watkins
A revolutionary form of keyhole surgery, vetted and approved by the independent building authority BRANZ, is speeding the restoration of quake-ravaged buildings in Christchurch. It is also saving many buildings from being totally demolished.
The engineering company Mainmark Ground Engineering drills 6-16mm holes in concrete floors or slabs which have buckled or sagged and pumps a structural resin through them.
“These resins are then injected into the ground under minimal pressure,” says Christchurch regional project manager Phil Johnson.
“They react on making contact under buildings, floors or paths and fill cavities by exerting pressure of up to 40 tonnes per square metre. They also compact and stabilise the ground. Laser control ensures extreme precision and ensures minimal disruption for property owners or damage to the environment.”
The system, he adds, has many advantages. No mess is created as there is no excavation, no need for paths or gardens to be dug up, and the resin is non-toxic, inert and environmentally friendly.
One of Mainmark Ground Engineering’s most challenging projects in Christchurch after the February earthquake was at a large refrigerated food store and distribution centre.
Mr Johnson explains: “The ground conditions were among the poorest we have worked on in the South Island. Access to the perimeter of the impacted buildings was extremely limited and that problem was compounded by the ground improvement and re-levelling works having to be undertaken in and around a fully operational refrigerated food storage and distribution facility.”
The complex has a large freezer and separate cool and dry good storage area. It is linked by a drive-through yard to a three-level office block and a multi-level plant room. The strengthening work under the main sunken loading dockyard was a major eight-week task.
Liquefaction added to the immediate effect of the earthquake and in a number of areas that resulted in uneven slumping of perimeter footings and floor slabs by up to 190mm.
The task was made even more difficult by the buildings being steel-framed with envelopes of precast concrete panels 180mm thick and 14 metres high. Another factor was that the walls are dowelled and grounded to perimeter strip footings which, in addition to mostly being 800mm thick, have attached column thickenings.
And yet more complications – the precast panels are lined with insulation panels and the floor slabs have under-the surface heating.
“Yes,” concludes Mr Johnston. “It was extremely challenging.”
Problems of a different nature were faced at a nearby four-storey building.
Liquefaction resulting in the slumping of the reinforced concrete slab created a virtual checker-board effect and the entire ground floor had to be evacuated.
However, the Mainmark Ground Engineering team worked so unobtrusively during the five days needed to re-level the ground floor that for tenants on the upper levels, including the Earthquake Commission, it remained business as usual.
Mainmark Ground Engineering business manager marketing Colleen McMahon says that although the company has had 27 years of successful operation in 78 countries – recently playing a key post-earthquake role, for instance, in Japan and Turkey – the thumbs-up from BRANZ was extremely welcome.
“It provides the reassurance that our product meets performance standards,” she says.
BRANZ technical manager Mike Reed says the appraisal took months to complete, although Uretek has been operating in New Zealand since 2003, as the product was “out of the left field”.
“It is a structural remedial product rather than a building product which meant there was no specific standards to which it must comply,” he says.
A rigorous technical assessment process had shown it to be a good product.
Building work had just been finished on Geert Herders 180sqm three-bedroom house at Wainoni the day before the February earthquake.
The 24-metre back wall slumped 70mm in places and damage included gaps in the brickwork.
“It was gut-wrenching because it seemed that we’d have to demolish it and start all over again,” he says.
“I honestly didn’t think they’d be able to put it back as straight as they did – and in such a short time. It was absolutely brilliant.”
Mr Herders is one of a great number of Christchurch home-owners grateful to Mainmark Ground Engineering.
Many others are reluctant to talk freely because of unresolved issues with builders, insurance companies or the Earthquake Commission.
One in the suburb of Merivale, who asked not to be named, told Engineering News:
“Builders had started four months earlier on what was to be our family’s dream four-bedroom home. We were looking forward to moving in about three months later and my wife and I were there checking progress when the earthquake struck.
“Everything shook violently and one end of the concrete slab sank 70mm or so. It seemed inevitable that the whole place would be a write-off, but the Mainmark Ground Engineering team fixed it beautifully within five days. We were delighted.”
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