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

Using a Below-Grade Grating System

Elevating Pavement over Soil

Another technique for achieving an important adaptation to help grow trees in harmony with lots of people is to elevate the entire area of pavement over the soil and rootball. This approach reduces the amount of area devoted to grating in favor of disguising it below grade. This may be the more appropriate design option for especially congested areas such as transit centers and urban plazas, where a fully suspended grate system below grade provides for more pedestrian circulation and subsequent shade benefits.

This type of pavement suspension system uses grate framing below grade to suspend a paving system such as pavers or concrete over the tree roots. The pavement can be concrete slabs, individual unit pavers (including permeable), or even decking material. Used in various geographic and site conditions, this system can be retrofitted around existing trees, or specified for new construction. The Pentagon Memorial and the proposed Freedom Plaza at the 9/11 Memorial site feature this type of approach in anticipation of massive crowds and the potential for severe soil compaction.

Paver grate installation in progress

Photo: Carol Brennan - Carol Brennan Associates, Phoenix AZ

Techniques for Urban Forest Health and Stormwater Management

Since healthier roots mean healthier trees, other techniques use engineered soils and suspension systems that aid tree root growth. "We are seeing more and more cities and towns recognizing the significant ecological and financial impact that trees, soil and stormwater can have in their communities," says Graham Ray, Executive Vice President of Deep Root. With proper planning and site conditions, green infrastructure can help clean the air, improve home and business values, reduce urban heat-island effect, and mitigate flooding and pollution. Municipalities are demonstrating their commitment to these green utilities by implementing standards such as mandatory soil requirements and stormwater utility fees that steer designers and developers toward sustainable design solutions."

These approaches prevent compaction, combining additional soil volume with structural qualities to support pavement, potentially aiding in stormwater quality and quantity. At the recent ASLA Annual Meeting in Washington, DC, each approach to this issue was presented as having benefits to helping trees survive in cities. Specifically, suspended systems use structural components below grade to create a framework onto which pavement can be placed, while retaining void space that allows for air exchange (soil aeration) needed for tree root growth, and on-site stormwater retention. Both soil structural amendments and suspension systems are intended to interconnect tree root systems with one another, or even allow them to "break out" to areas that are open soil sites, such as parks or other open spaces. Trees often run out of rooting area when placed in streetscapes or near buildings, bumping into heavily compacted soils near streets on one side and building foundations on the other. Acting as physical barriers, these impediments prevent trees from reaching mature size, stunting their growth and eventually sending them into decline. These types of subsurface soil systems help to provide adequate volumes of noncompacted soil that nourish the tree and allow it to reach maturity, regardless of their urban condition, promoting more stability, longevity, and ecological health.

In an independent 2006 study, "Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement," trees in soil amendments with stone or gravel/soil matrices and suspended pavement were grown and tested over a six-year period. While each approach was better for trees than those in compacted soil, suspended pavement followed by gravel/soil matrix treatment was superior in tree growth and health. In this particular study, suspended pavement promoted growth by as much as 2 to 3 feet more in height.

A suspended pavement framing system creates individual "cells" by building modular units defined by posts and beams. This system uses posts for strength to define a dimensional area and to stack the modular units up to three deep. With the ability to stack one unit on top of another, these frames create a substantial percentage of void space. A platform is applied to the top of each stack of modular units to provide structural support for pavement. The units are snapped into place, and can be arranged to create long trenches or large areas, depending on the site. Since this study was conducted, sand-based structural soil treatments have been developed, relying on sand, mulch and topsoil to sustain void space.

Comparative tree height

Source: E. Thomas Smiley, PhD, Bartlett Tree Research Lab

And creating void space is the key for trees to survive in pavement. While pavement subgrade compaction criteria often has to meet a compaction density of 95 percent density, the maximum compaction density for roots to spread and plants to grow is 80 percent.

 

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Originally published in GreenSource
Originally published in November 2010

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