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There’s a whole lot of shaking going on at Prestons

Impacter

Dr Jan Kupec and David Schwartfeger

8 November 2012 - Earth tremors being felt at present around the 203 hectare Prestons subdivision, on the northern outskirts of Christchurch, are of the good variety – they are mechanically made and are helping densify the land, as opposed to the natural ones that have occurred over the last two years.

Ngai Tahu Property, which is undertaking the first stage of development on the northern side of Prestons, is using a giant land stabilisation machine on parts of the development to pack the soil particles more closely together to prevent liquefaction and bring all the land at Prestons to TC1 standard.

David Schwartfeger, Development Manager for Ngai Tahu Property, said that the majority of the development is already classified at TC1 and this remediation was to bring localised areas of TC2 land up to the higher standard.

“We commenced work on the site this week to remediate 25 hectares which should take contractors approximately 20 weeks.”

The three-sided elliptical roller, which weighs approximately 40 tonnes, will go back and forth across the treatment site between 20 and 40 times to pack the soil particles to their densest configuration, which will prevent settlement and any liquefaction from happening.

“It certainly does shake the earth, but the benefits of improving the ground from TC2 to TC1 are far-reaching meaning that standard foundations for concrete slabs or timber floors can be used, thus saving money on foundations, and negating the need for further geotechnical reports,” he said.

Dr Jan Kupec, Technical Director, Ground Engineering, with Aurecon Group, who is overseeing the ground densification, said that the liquefaction susceptibility on the site was marginal anyway.

“Prestons suffered no liquefaction or lateral spreading or any other seismically associated hazards during the entire earthquake series. The reason for this low susceptibility to earthquake damage is because of the presence of dense dune deposits (formed by the wind) in contrast to fluvial deposits around rivers and streams, which are more prone to liquefaction and lateral spread.

"This low susceptibility means we only have to treat selected areas to bring them up to TC1 standard rather than the entire development.  Densification such as this works well on sandy soils such as we have at Prestons and once treated the soil will not settle or spread.

“The machine is used frequently overseas on large infrastructure and mining projects and after an initial trial early this year we found it ideal for treating the site. The earthworks are subject to stringent quality assurance and control testing to confirm that the desired ground improvements have been achieved. The result of the testing will be provided as part of the subdivision certification submitted for approval to the Christchurch City Council,” he said.

The first 200 sections in Stage One, which is known as Korowai, were only released to the market two weeks ago and already 30 have been sold or are under contract. These sections range in size from 450 m2 to 650 m2 and are priced between $180,000 and $245,000. This first stage is being constructed by Ngāi Tahu Property on the northern side of Prestons Road. They hope to release another 200 sections early next year.

Prestons is designed to be a sustainable urban village that will ultimately contain 2500 houses and 8000 residents. It is bounded by Mairehau Road to the south and Lower Styx Road to the north and is bisected by Prestons Road.

 

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