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Geotextiles help save a Toronto park

Features, Landscapes | September 1, 2011 | By:

A plan to rework the Don River Park near downtown Toronto uses geotextiles to remediate soil and drainage problems

A new neighborhood is growing on 24 hectares of former industrial land near downtown Toronto. Called the West Don Lands, it will be home to 6,000 housing units, office and retail buildings, and more than 8.8 hectares of parks and public spaces. This development is possible through the use of geotextile fabric application.

Named for the Little Don River that flanks the site, the West Don Lands is one part of the waterfront renaissance in Canada’s largest city. Waterfront Toronto, a pseudo-public agency funded by three levels of government, but functioning like a private developer and parks agency in one, is in the midst of transforming more than 8km of underappreciated Lake Ontario shoreline into a water-centered swath of parkways, boardwalks, parks, beaches, offices and high-density residential buildings. The area is already known for its WaveDecks—funky, undulating wooden plazas that reach out over the water—and its two beaches: sandy, umbrella-shaded oases near the heart of downtown.

The West Don Lands, slightly inland from the lakefront, used to be home to the worst-of-the-worst in terms of soil contamination: tanneries, a PCB storage facility and underground fuel tanks. And to make matters even more difficult, all those industrial uses had been built on land that used to be Don River floodplain. This was a zone of soppy soil, historically inundated when the river backed up across the land before flowing into Lake Ontario just a short distance downstream. Remediation of the site, therefore, called for some well-planned and well-designed solutions.

That remediation was led by Infrastructure Ontario (IO), an arm of the provincial government. IO’s senior program manager Serge Chukseev described the old river dirt as “pockets of unstable soils, like sponges in the ground.” These pockets were scattered throughout the site and lay anywhere from 3.7m to 18m deep, an indication of the historic dynamism of the Don River system. That soil required dewatering and consolidation before anything could be built, otherwise the new neighborhood would literally slump downward here and there.

IO hired engineering firm AECOM in 2005 to design the major streets for the new neighborhood and to develop a “flood protection landform” (FPL) that would protect the site—as well as much of Toronto’s financial district just to the west—from the Don’s rising waters. The remediation program was designed and managed by global multi-disciplinary consulting firm CH2M HILL. Once Waterfront Toronto became involved as the site’s primary developer, that agency hired landscape architecture firm Michael Van Valkenburgh Associates (MVVA) to design the 7.2 hectare Don River Park (to be completed next year), situated atop the FPL between the neighborhood and the river.

It is IO’s and CH2M HILL’s remediation work that features the geofabrics. The team assessed the site for soil contamination and removed pockets that exceeded standards for redevelopment. Then the entire site was capped with about 1.5m of clean fill. This is a typical remediation effort for chemically contaminated soils. To address the pockets of compressible muck, however, the team needed geotechnical soil consolidation. It installed a system of wick drains to help accomplish this objective. The wicks are soft, perforated, filter fabric-wrapped plastic pipes, which were drilled down into the pockets of soppy soil. The plastic pipes are not unlike those used for underdrains in landscape installations, but they are softer, so they smoosh nearly flat rather than stay rigid and round. They are installed vertically into the ground. Once in place, the 27,000 wicks broke the surface of the site like a man’s five-o’clock-shadow stubble.

Then IO placed a layer of sand to bury the tops of the wicks, followed by a layer of soil to the elevation needed for flood control—about 4m higher than existing grade. Then portions of the site got another layer of soil, sometimes as much as 7m thick, called surcharge soil. That dirt is there for its weight. The surcharge and FPL soils press down on the underground pockets of old marsh. “It’s like taking the sponge and squeezing out the water,” says Chukseev. To continue the analogy, the water is squeezed out through 27,000 straws (the wicks) and forced into the sand layer, where it can dissipate into the surrounding soil. That process took place rather quickly—over eight weeks in 2008—and it made the subterranean soils dry and developable.

Today the wicks are completely buried by the FPL and surcharge soils. The surcharge is still there, too. “Instead of removing those 5.5m to 7m of materials,” explains Chukseev, a landscape architect by training, “we re-graded that into the park.” AECOM worked with park designers at MVVA to sculpt the land and MVVA designed the park on top. Through the innovative use of textile-clad perforated pipe, the swampiest portion of this former industrial site will be home to play areas and trails serving residents in Toronto’s newest urban neighborhood.

Adam Regn Arvidson, ASLA, is a landscape architect specializing in the design of sustainable landscapes and is a frequent contributor to Landscape Architecture and other design journals. His article about the Target Center green roof appeared in the July/August 2010 issue of Fabric Architecture.

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