Housing New Zealand is for the first time using a new screw piling system to anchor the floors of houses at its redevelopment of Cedar Heights, Massey.
The steep Auckland site was used to dump the soil from the north-western motorway when it was built, and is probably our most difficult site being redeveloped throughout the country – with up to 2 metres of soft and wet fill on top of the sloping section.
The usual response would be to drive substantial timber piles in using a pile driver. That would be time consuming, and difficult to access for the large machinery needed in a tight and sloping site. The other issue with driven piles is the noise and vibration it creates impacting on surrounding properties.
The builder, Dominion Constructors, conceived the idea of using a screw pile system. This system (pictured below) is used widely in Australia, but is relatively new to New Zealand.
“The steel screw piles are much quicker to install than the usual timber pile driving method and has no impact on the neighbours – as it’s much quieter and causes little vibration,” said Dominion’s Chris Bassett. “They lead to dramatically reduced timeframes and so are less expensive to use overall. Another benefit is they can be screwed up and down to adjust the height of the piles – which is great on such a steep site.”
The piles are made from galvanised steel, and so are well suited to New Zealand’s climatic conditions. It is possible Housing New Zealand will utilise this innovation more in the future.
Cedar Heights once had just five 1970s houses on a 4,512 sqm site. These are being replaced by 12 two-bedroom homes, while retaining and upgrade 2 of the older houses. The redevelopment will likely be finished by the middle of 2018.
While there are some versions of screw piles available in this country, Chris says that the version used at Cedar Heights is preferable because:
- It is a galvanised steel pipe that takes less than 5 minutes to prep and install, in this case a 2.5 metre screw, by a relatively small tracked machine (even as small as a Bobcat) and with no extra soil requiring removal
- Unlike most New Zealand screw piles it is galvanised steel, not plain steel which give it an extended life of at least the life of the building.
The steel version is stronger than timber piles and if a building’s supporting soil slumps, the piles can be jacked up with relative ease by retrospectively lengthening the top screw length.
On a good site, the piles can be installed at the rate of up to 100 a day.