This was originally published on the landscape architecture blog The Field.
When we talk about low-impact development and design today, the term “suspended pavement” comes up a lot, but it occurred to me recently that this term is rarely defined. Here is my attempt to do that by walking through suspended pavement’s history, uses, composition, and case studies.
Suspended pavement (also known as cantilevered sidewalks) is a general term for any technology that supports the weight of paving and creates a subsurface void space that is filled with soil for root growth. The soil that is used to fill the system can either be native, from the excavation area itself (if appropriate), or a specified mix. In this respect, suspended pavements are essentially soil-delivery systems, creating a rooting area composed of lightly compacted, high-quality soils for tree roots in cities and other heavily paved environments. In addition to aiding urban tree growth, the soil can also be used for on-site stormwater management, maintaining pre-development hydrology, minimizing non-point source pollution and flooding, and recharging watersheds.
Suspended pavements are ideal low-impact development design solutions for the long term co-existence of trees and streets, parking lots/lanes, roofs, promenades, plazas, green walls, and light-rail platforms.
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 (root growth outperformance was observed 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.”
Nine years later, the difference in performance between trees planted in suspended pavement and trees planted in the other treatments remained very apparent.
This study is discussed in more detail in the “Where Is Suspended Pavement Being Used?” section further down. You can also view photos of the study over the last nine years here.
Benefits of Trees Growing in Suspended Pavement
Suspended pavements facilitate the growth of healthy trees in urban spaces. Healthy trees have high social and ecological value, including:
- Offering cooling effects via shading and evapotranspiration from the canopy
- Improving air quality by mitigating carbon emissions
- Improving aesthetics and quality of place
- Improving health and well-being
- Encouraging biodiversity due to provision of habitats.
Access to usable soil is the most limiting factor for urban tree growth, and few street trees have access to soil beyond what is provided in their opening. This is a common reason why street trees often do not grow to maturity, or simply fail to thrive. Rooting space becomes limited due to soil compaction that results from paved surfaces/high pedestrian traffic
Suspended pavement addresses this problem by functioning as a soil-delivery system for trees, allowing lightly compacted, high-quality soils to be stored and made available for tree roots in cities and other heavily paved environments.
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.
Healthy, mature trees provide significant stormwater benefits, and suspended pavement systems provide opportunities for integrated stormwater/soil benefits.
If suspended pavement systems are filled with a bioretention soil mix, about 20 percent of their volume can be used for stormwater storage (in 1,000 cubic feet, 200 cubic feet of water can be stored). Rain water can be directed in to suspended pavement systems in order to keep it on-site, making it available for plant growth and preventing it from overflowing sewer systems or polluting nearby watersheds. There are many ways to direct water in to the system, including pervious pavers, curb cuts, catch basins, slot drains, and more. A typical tree in this amount of soil can hold a significant amount of rainwater, preventing overflow into the impervious surface area, not just under the tree canopy but far beyond the canopy’s edge as well.
The three processes that allow trees in lightly compacted soil to provide stormwater quantity and rate control benefits are:
Soil Storage: Soil stores rain water during and after a storm, making it available for plant growth. Stormwater runoff from nearby impervious surfaces can be directed into soil under suspended pavement using a number of different techniques, such as, for example, through pervious pavement installed over the cells, or via a perforated pipe off a trench drain or manhole.
Interception:Interception is the amount of rainfall temporarily held on tree leaves and stem surfaces. This rain then drips from leaf surfaces and flows down the stem surface to the ground or evaporates. Since larger trees have more leaves to intercept rain, they intercept significantly more rain than small trees, with interception increasing at a faster rate than tree age.
Evapotranspiration:Evapotranspiration (ET) is the sum of water evaporated from soil and plant surfaces and the water lost as a result of transpiration, a process in which trees absorb water through their roots and transfer it up to the leaves, where it evaporates into the environment through leaf pore transpiration. Evapotranspiration continues to reduce stormwater volume stored in the soil long after a rainfall event ends.
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 branch extensions, height, diameter at breast height (DBH), leaf count, and more.
The research plots were set up with 12 trees planted in a suspended pavement system, 12 trees planted in a stalite/soil mix, 12 trees planted in compacted soil, 12 trees planted in pure stalite, and 12 trees planted 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 tenth year, and we expect Dr. Smiley will publish an updated paper sharing the results in the coming years.
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 46 years old.
Sasaki & Associates designed this suspended pavement system as a long soil trench 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 19 inches (48.3 cm) and an average height of 71 feet (21.64 meters). Measurements by Tom Smiley, 2013; MacDonagh & Smiley 2013 ISA Proceedings.
Lincoln Center Barclay Capital Grove (New York, NY)
The Barclay Capital Grove at Lincoln Center, installed in 2009, uses Silva Cells (generically known as soil cells), a modular suspended pavement system composed of structural units that form a skeletal matrix. In this project, the cells cover the entire area of the grove where the trees are planted, although in other cases the depth and layout of the soil volume can cover irregular areas and depths. This increases flexibility in terms of creating appropriate tree habitat in constrained areas
These trees are planted in a large, shared, subsurface planter that rests on an underground parking garage. As of 2013, these trees look wonderful, providing a popular refuge from the elements in the heart of New York City.
There is ample evidence that suspended pavement is an excellent tool for growing trees in paved environments. At a time when complete streets and sustainable development appear more necessary and have more popular support than ever before, this approach to growing trees – not just planting them – seems especially important. 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 there are others – 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.
Green Infrastructure for Your Community:
What is suspended pavement?
Trees in hard landscapes
Touch the Soil: healthy soils for healthy trees
Charlotte, NC trees in suspended pavement 25 years later
Suspended pavement at the Bartlett Tree Lab, Year 9
Stormwater Quantity and Rate Control Benefits of Trees
How much soil to grow a big tree?
Clean Air Partnership
Landscape Architecture Resource:
Soil and stormwater top of the list for sustainable design
Toronto ahead of the curve or tree growth and stormwater management
Craul, P.J. 1992. Urban Soil in Landscape Design. John Wiley and Sons, New York, NY.
Smiley, E. Thomas, et al. July. 2006. Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement. Arboriculture & Urban Forestry.
Interviews with Peter MacDonagh, RLA, ISA, FASLA and James Urban, FASLA.