Greening the Site: Design Options for Sustainability

A discussion of current trends in site sustainability techniques to improve the health of urban forests and stormwater quality, and provide design solutions if there is no room for trees
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Sponsored by Deep Root Partners, L.P., greenscreen®, IRONSMITH, SOIL RETENTION
Angela D. Dye, FASLA, LEED AP

Given this void space, these types of soil amendments also show potential to be used for stormwater retention. According to a study prepared by the Kestrel Design Group, a single layer of suspended pavement (41 cells in all) with 410 cubic feet of soil volume will hold 75 cubic feet of water or a watershed area of 953 square feet in a one-inch 24-hour storm. (This assumes a 100 percent impervious surface.)

Rendering of suspended pavement

Image: Deep Root Partners, L.P.

In September 2009, the State of Washington Department of Ecology issued this system a General Use Designation, finding that a suspended pavement system can be considered the functional equivalent of a rain garden, meeting filtration requirements for bioretention. Likewise, according to the Cornell Urban Horticulture Institute, a gravel/stone matrix has water-holding capability due to its rapid infiltration rate and high porosity.

A framing system has been shown to meet H-20 loading standards, as observed by a test case in the City of Calgary, Alberta, Canada by Public Works in 2007. Structured soils are designed to achieve 100 percent dry density for load-bearing pavements. With this type of loading capability, trees can thrive in streetscapes, plazas and parking areas without diminishing space for either function. In situations where trees are adjacent to but separated from parks or turf areas by pavement, "break-out zones" can be created to lead tree roots to open area where they can spread unimpeded.

To install a structured soil system, a stone matrix is mixed with either a hydrogel, or with a sand/mulch mix, then installed in trenches or an area at a specified volume based on the anticipated mature tree crown diameter at a rate of 2 cubic feet per square foot of crown diameter. A geogrid textile separates the pavement from the soil matrix and wraps part-way down the side of the plant pit to keep native soil from infiltrating the structured soil mixture. Some manufacturers require the installer or landscape contractor obtain a license to mix and install their product for quality control. Irrigation lines can be placed within the matrix, often in sleeves to prevent crushing by the stone matrix. A separate and deeper trench or area can be dug for stormwater retention below porous pavement.

To install a suspended pavement system, a trench or area is excavated 8 to 12 inches beyond the area the units will occupy. Each unit is set on a compacted subbase aggregate, allowing for rootball openings.

 

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Originally published in GreenSource
Originally published in November 2010

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