We use the term “suspended pavement” a lot on this blog and in our presentations, but it occurred to me recently that we’ve never properly defined this term. Here is my attempt to do that by walking through suspended pavement’s history, uses, composition, and case studies.
Suspended pavement is a general term for any technology that supports the weight of paving, thereby creating a void space underneath. The void space is then filled with soil – either the soil that was excavated from that site or a specified mix – and the system is paved over as it would be normally. In this respect, suspended pavements are essentially soil-delivery systems, allowing lightly compacted, high-quality soils to be made available for tree roots in cities and other heavily paved environments. In addition to aiding urban tree growth, the soil is also used for on-site stormwater management, maintaining pre-development hydrology, minimizing non-point source pollution and flooding, and recharging watersheds.
The first use of the term “suspended pavement” that we are aware of is from a study published in Arboriculture & Urban Forestry in July 2006 by E. Thomas Smiley, Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley titled“Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement.” In that study, the authors set up a plot of trees – a mix of Snowgoose cherry (Prunus serrulata) and Bosque lacebark elm (Ulmus parvifolia) – growing in 5.4m3 (189 ft3) of five different soil treatments: compacted soil, gravel/soil mixture, Stalite, Stalite/soil mixture, and suspended pavement.
After 14 months, the suspended pavement over uncompacted soil treatment outperformed all the others in terms of trunk diameter growth, twig growth, chlorophyll rating, and root growth (in the Elms only). The authors concluded that “suspended pavement over noncompacted soil provided the greatest amount of tree growth and health and should be considered when designing urban planting sites for trees.” Furthermore, they wrote, “The trees in the noncompacted/suspended pavement treatment were larger, faster growing, had better color, and more root growth than most other treatments.”
This study is discussed in more detail in the “Where Is It Being Used?” section further down. You can also view photos of the study over the last eight years here.
HOW IS SUSPENDED PAVEMENT USED?
Trees keep us safe, clean our air and water, calm people and traffic, and make our environment more livable in a number of ways. They are an essential part of healthy cities, yet we largely plant them in conditions that make it extremely difficult to survive. Access to usable soil is the most limiting factor for urban tree growth, but few street trees have access to soil beyond what is provided in their opening.
Suspended pavement systems address this problem by creating underground rooting areas beneath paving. With access to adequate amounts of high quality soil, growing mature, healthy, long-lived trees in heavily paved environments becomes possible. Properly designed, this technology enables us to grow forest size and quality trees even in urban areas.
Suspended pavements are ideal low-impact development design solutions for streets, parking lots/lanes, roofs, promenades, plazas, green walls, and light-rail platforms.
Rain water can be directed in to suspended pavement systems in order to keep it on-site. There are many ways for doing this, including pervious paves, curb cuts, catch basins, slot drains, and more.
WHAT IS SUSPENDED PAVEMENT MADE OF?
Suspended pavement can be completely customized on a site-by-site basis, or can be constructed using modular systems. In the United States, custom systems are usually constructed using pre-cast or cast-in-place concrete. The earliest suspended pavement projects in North America that we are aware of were all constructed that way.
The Silva Cell, on the other hand, is an example of a modular system. The 24” wide x 48” long x 16” high frame can be stacked to meet the depth of any excavation area and spread laterally as wide as necessary. Utilities can be easily accommodated using a number of tested methods. The Silva Cell is made of a lightweight polypropylene blend, with a galvanized steel tube in the deck for additional strength. Like the earliest concrete suspended pavements, the open framework for the Silva Cell allows almost all of the space to be available for rooting, but is much less expensive than a custom system. Stacks of Silva Cells are attached vertically, but not horizontally, making future excavation, repairs easy to isolate and maintenance simple.
WHERE IS SUSPENDED PAVEMENT BEING USED?
Bartlett Urban Tree Plaza Study (Charlotte, NC)
This independent study (also discussed above) was set up by Dr. Tom Smiley at the Bartlett Tree Laboratory to test how well trees fare in different growing media. The trees are monitored and tracked from year to year to measure their leaf extensions, height, diameter at breast height (DBH), and more.
The research plots were set up so that 12 trees planted grown in a suspended pavement system (which is identical in performance to the Silva Cell), 12 trees were planted in a stalite/soil mix, 12 trees were planted in compacted soil, 12 trees in pure stalite, and 12 trees in a gravel/soil mix. All trees had access to 189 ft3 of growing medium.
By all measurements, the trees in the suspended pavement treatment are outperforming all the others in size, color, and root growth. This study is now entering its ninth year, and we hope Dr. Smiley will publish an updated paper sharing the results soon.
Christian Science Center (Boston, MA)
The Christian Science Center, on Huntington Ave between Mass Ave and Belvidere Street in Boston, is the oldest suspended pavement installation that we know of in the United States. These trees were planted in 1968, making them 45 years old.
Sasaki & Associates designed this site to include the soil trenches, which are made up of structural slabs resting on grade beams over a garage roof. Each tree has access to around 800 cubic feet (22.6 cubic meters) of soil. In 2011 this plaza was designated with Landmark status by the Boston Landmark Commission.
Downtown Charlotte – Tryon & Trade (Charlotte, NC)
In 1985 Charlotte, NC City Arborist Don McSween conceived of a major renovation project along 10 blocks of Tryon Street and two blocks of Trade Street, two of the major thoroughfares in downtown Charlotte.
McSween wanted each new tree to have 1,000 cubic feet (28 cubic meters) of good, usable soil. In order to supply the trees with that quantity the City needed to create a custom suspended system using precast concrete pavement supported by concrete piers. The whole system was topped by pavers (not pervious). Almost 170 trees were planted.
The trees flourished. In 2009, while investigating some trees that weren’t doing so well, Tom Smiley with the Bartlett Tree Lab discovered that the contractor had actually only installed about 700 cubic feet (20 cubic meters) of soil per tree. However, since the tree pits connected along the length of a block with multiple trees in each pit, they had the benefit of sharing soil. Today, the Willow Oaks (Quercus phellos) have an average DBH of 16 inches (40.5 cm) and an average height of 44 feet (13.4 meters).
Lincoln Center Barclay Capital Grove (New York, NY)
This is a Silva Cell project that was installed in 2009 in the heart of New York City. This grove of trees actually rests on an underground parking garage, making this a (secret) green roof!
In conclusion, there is ample evidence that suspended pavement is the best available tool for growing trees in paved environments. In Dr. Smiley’s words from his 2006 article, “The differences in tree growth among treatments was dramatic; trees growing in the noncompacted soil suspended pavement treatment are visually healthier in appearance and provide more shade more quickly than any of the other treatments.”
The trees from all of these projects – and this is only a handful – are a powerful example of how much soil it takes to grow a mature tree, and a reminder that with planning and commitment we can have trees like this in all of our cities.
Craul, P.J. 1992. Urban Soil in Landscape Design. John Wiley and Sons, New York, NY.
Smiley, E. Thomas, et al. July. 1996. Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement. Arboriculture & Urban Forestry.
Interviews with James Urban, FASLA, and Peter MacDonagh, RLA, IS, ASLA.