Think of the growing medium specified for green infrastructure as analogous to an iceberg.
“There is a lot that happens below the surface that people don’t see,” said Helle Brodie, landscape architect and president of Brodie & Associates.
Brodie, a speaker on a panel examining what it takes to produce healthy engineered soils for green infrastructure, said success comes when all the players in a green infrastructure project are on the same page.
That includes the owner who must spend “extra money,” the design team, the project manager to make sure everything is built correctly and the contractor responsible for the installation, she told a seminar audience at the Grey to Green Conference in Toronto recently.
Some green infrastructure projects go off the rails because the soils installed by a contractor do not meet the landscape architect’s specifications, she said, citing an example in which a contractor provided sandy loam topsoil as specified but the soil “was filled with salt.”
When she asked the project manager to leach the soil prior to use, she was told, “We don’t have time for that. We are way beyond schedule.”
The result was that the planted native grasses “didn’t establish well,” Brodie said.
In another case vegetation didn’t establish itself in a bioswale — a landscape feature designed to filter out debris and pollution from surface runoff water — because of poor installation. The result was that stormwater caused soil and mulch erosion plus the loss of seeds and some of the plants.
“We had great soils but construction methods weren’t great,” she explained.
Another concern, she said, is when drainage and base preparation specifications are not followed.
The whole world of soil specifications is an evolutionary thing
— Neil McKeown
“We can have great engineered soils…but when they get too compacted (from heavy construction equipment), everything fails.”
Depth of planting soils is another concern, said Brodie, noting an example where some of the granular matter was removed from a large tree’s root ball to fit it into the tree pit, and then covered with “a light fluff of engineered soil that was specified” so no one would notice.
In another case, a contractor requested Brodie to certify its work but the contractor hadn’t removed granular matter as was requested and “we found they used very heavy clay topsoil, instead of the light free-draining soil we specified.”
Kees Govers, technical sales manager at LiveRoof Ontario, which focuses on green roof systems, said coming up with the perfect soil medium can be a challenge. It took LiveRoof three to four years to develop the optimum growing media — “the backbone of every green roof system.”
While the medium has to be within the specifications provided by the landscape architect, he said it also must be physically and chemically stable.
“You can’t have heavy metals leaching out…coming down the downspout.”
It must be lightweight, “but not so light that it goes sailing off the roof,” he told the seminar audience. If that seems straightforward, “just imagine how much wind there is on top of a 60-storey building.”
The soil medium must also be fire-resistant, he said, noting most of the successful roof mediums are composed primarily of water-absorbing mineral aggregates, with some buffering material to balance pH levels.
Clear specifications are paramount but contractors should ask questions. Don’t make assumptions when in doubt, Govers said.
“The whole world of soil specifications is an evolutionary thing,” pointed out Neil McKeown, general manager at Gro-Bark, a large soil blender for such products as sustainable engineered soils. The first soil mixes were developed in the 1970s but engineered soils for green roofs are a recent development.
“There is still debate about how long the organic matter really holds in the soil volumes in rooftops,” said McKeown.
Lessons can be learned about green infrastructure from the principles of China’s sponge city initiative — aimed at stormwater management through permeability, said Dean Young, a project manager with the Sustainable Technologies Evaluation Program of the Toronto and Region Conservation Authority (TRCA).
Young said it is paramount to restore soils post-construction to their permeable state.
“Depending on how you treated the soil (during construction) it could be now acting like an impervious surface,” he stated.
Sheila Boudreau, a senior landscape architect with the TRCA, said she’s been in many meetings with contractors who hadn’t read the drawings/specifications for growing media because it wasn’t their job. That is a mistake on their part, she noted.