Civil contractors Dempsey Woodinstalled Stonebloc to divide the terraced super lots at Ara Hills, a brand new, 84ha subdivision on the steep hills 3km above Orewa town centre, Auckland.
When owners AV Jennings Properties Ltd began work on Stage 1 of the development, Envirocon’s Stonebloc retaining system was specified to retain and divide the super lots. Terracing was an obvious way to maximise the available land and prepare these lots for sale, and Stonebloc was a convincing choice for the job.
This was a big job - engineers had called for a combination of 400mm and 800mm-thick blocks to be used, across different three types of wall. One wall type was 800mm tall (550mm above ground), another 1200mm (950mm above ground), and the third 1600mm (1350mm above ground). Spine walls were 105m long, and the longest walls coming off the spine were 30m. Thousands of Stonebloc blocks were required. Dempsey Wood were the civil contractors, and it was the first time Project Manager Ryan Lunn had used this system. “I was told it would be like putting LEGO together,” he says.
On the relatively steep grades it had been anticipated that construction would be a bit more complicated than that, but once foundations were in place and installation and back filling was completed, the job was absolutely straightforward, says Ryan. “There were lots of complex issues to this job, including our need to use specialised polystyrene fill at some points to reduce weight on the sandy soils. “There was a lot of weight, especially on the upper lot for each wall. It was important to design and install specialised EPS foam as backfill material to reduce weight while still meeting structural requirements. And of course aggregate/scoria was required to allow drainage to occur.”
But once everything was set up properly, Stonebloc was really easy to install. “The walls themselves were all built on flat ground (one percent) slope,” explains Ryan. “The slopes of the surrounding roads and lots were up to six percent. All the walls were built on 200mm concrete foundations due to the soft ground underneath, and our building them in winter. “The thousands of Stonebloc blocks were easy to install and work with – they’re a good option for retaining walls. The product looks nice at the end, and it’s easy to adjust onsite.” An independent University of Auckland report provides conclusive evidence that correctly-installed blocks can withstand <35.6 kN lateral loadings from, for example, active earth pressure.
The report also showed evidence of the out of plane behaviour and unfactored restoring moment capacity of this type of wall, giving engineers and contractors additional confidence in Stonebloc’s performance over many years – making it ideal for the Ara Hills project.
Envirocon were not surprised by the results. “Our design approach is guided by the rule of simplicity, because simple products are usually the best – they’re also fast for people to learn and are easy to use,” says Envirocon’s Sales and Marketing Director Jack Bright. “Stonebloc is a unique system that not only delivers a strong performance in situ, but it’s good looking as well.”
Stonebloc is usually seen as a relatively low-standing (up to two-metres tall) and cost effective option for residential retaining walls. It can be installed right on a boundary, and its architectural fascia designs provide visual interest beyond the standard concrete or timber faces most often seen.
