Increased soil volume allows for larger trees in Canberra suburb
by Kristyn Maslog – Levis
The Citygreen® modular Stratacell™ system is starring in Canberra’s new hot residential subdivision of Casey.
Oculus designed the landscape works and opted to use the system after realising the constraints of the area. Sellick Consultants were the chosen engineers for the project. The Casey project, which started in 2012 and was completed only February this year, required the planting of sizable trees on the street in front of the retail centre on Kingsland Parade.
Keith Stead, associate director from Oculus, said that by planting big trees on the street, it connects the surrounding areas to the retail centre. However, they were struggling to do that because of the constraints on the area, such as the lack of available space, the presence of services and street parking on one side.
“Initial investigation was clear that the available areas we had weren’t going to be enough to have large oak trees used in the neighbourhood. We needed to come out with an approach to increase soil volume while still allowing also enough room for the parking to work. The obvious solution was to use the system, which allowed us to take advantage of the ground underneath where the parking was going. We used that to increase soil volumes,” Stead said.
Although Oculus have previously investigated the system for other projects, this is the first time they’ve used the Stratacell system. “The system worked well. They were the right size for the tree down that street. With other solutions, we couldn’t have done that,” Stead said.
Like with everything else, trying out something new for the first time needs a lot of discussion. As the contractors were new to the system, it needed “a bit of working through to get everyone on board”.
“Everytime you try a new system, there’s always a learning curve. It’s just a new way of doing things,” Stead said.
They didn’t have difficulty installing the system but they experienced some issues trying to get the soil into the cells.
“We wanted to make sure it was consolidated enough so that we got soil in there and allowed for settlement without over compacting it,” he said.
[Editor’s note: The Stratacell has a honeycomb-like structure for high strength to support pavement while accommodating structural roots.]
They had to find a solution for the use of permeable pavement over the cells. “It wasn’t enough depth for the paving. We had to have a custom approach and had to place a thin layer of [porous] concrete with reinforcing mesh. The cells had projecting nibs on them and we had to get them cut down because they were sticking out,” Stead said.
He clarified that it’s not a criticism of the system. “If adequate depth had been allowed for the permeable paving in accordance with how Citygreen normally have in their standard detail, it wouldn’t have been an issue.”
Stead said the project received a lot of support from Citygreen, replying to their queries through phone, email and personal appearance on the site. “It was a bit of team effort. The engineers, contractors, Citygreen and myself had a few discussions on the installation.”
There were no major issues with the project, as it’s all very straightforward. But there were some areas where they had to break out rocks to make the services work. “It’s a tough environment, it’s quite windy on the site. Canberra is pretty cold in winter and hot in summer. There are some environmental concerns to work on.”
**photo courtesy of Andrew Easey at Sellick Consultants
Still, Stead is quite certain that they picked the right system for the job. He said that the Stratacell is the only system that gives the additional volume for the space.
“We used structural soil before but in reality a large percentage of structural soil is not actually available for rooting. Whereas with the Stratacell system, something like 95 percent of the volume is available to be filled with soil and available for rooting. The additional volume gives Citygreen the significant edge to other options,” Stead said.
Everything is working well since the completion of the project. They’ve tested the parking bay on top of the site and actually drove a concrete truck on top of it.
“It all appears to be very rigid. The expectation is that there will be no issues, which is obviously a key consideration,” he said. Although the trees were only put in early this year, they appear to be doing well.
“At the moment they’re doing as well as can be expected. The additional soil volume will give them that stability in the long term,” Stead said.
“This is definitely a good system and we’re considering to use it in other projects. I think the landscape industry in general are more aware with issues of street tree health and how small soil volumes given to street trees are not a good thing for their long term health. We have to work harder to give street trees proper soil volume and the Citygreen system provides a good way of doing that,” he added.