Are Your Planting Plans Tripping Us Up?
Are Your Planting Plans Tripping Us Up?
by Laurel Kelly, ASLA
When trees damage paving, the social costs of reduced accessibility for people with disabilities may be even greater the maintenance costs of repair.
Maintaining Accessible Features
Think of the hours and days we spend ensuring that the paving we design — sidewalks, plazas, decks, and pedestrian rights-of-way — are barrier-free, stable, firm, slip resistant, and at accessible grades. Yet, how often have we revisited a completed project years later only to find that our work has failed the test of time. Our project’s paving surfaces are cracked or have settled or heaved and are now a significant barrier for pedestrians rather than a site amenity.
The U.S. Access Board’s publication Accessible Rights-of-Way: A Design Guide offers the following general advice to avert this all-too-common problem. “Knowledgeable design, wise material selection, good construction practices, and regular maintenance procedures can help ensure that differences in level between adjacent [paving] units do not exceed the limits of usability [1/4 inch].” The Board publication also notes that the Department of Justice regulation includes requirements for the maintenance of accessible features.
While there are many factors that contribute to pavement failure—ranging from paving design and construction practices to environmental conditions—this month’s article is primarily focused on the placement and selection of trees to minimize damage to hardscape.
Design Solutions to Accommodate Trees
The secret to resolving tree root-paving conflicts is to internalize the following maxim: Trees must grow larger [in every dimension] each year in order to remain healthy. Dr. Ed Gilman
Not all paving damage is caused by tree roots. In fact, research suggests that tree roots may “preferentially develop under preexisting cracks because of enhanced soil aeration” (Costello & Jones). However, we do know that selecting tree species without regard to their growth characteristics can be responsible for damage to surrounding pavement.
Bay Area trees that are known to develop large trunk flare or root buttresses in relation to their trunk diameter—such as camphor, Washington thorn, and evergreen ash—or those with roots that tend to grow near the soil surface—such as London plane and liquidambar—are responsible for paving and other infrastructure damage, particularly when they are planted in spaces without adequate soil volume or room to grow.
Design: Our designs must provide large areas of non-compacted soil volume to achieve both intact paving and healthy trees. In order to achieve this goal:
- Plant trees in areas with large soil resources such as “tree islands” (planting areas for groups of trees).
- Locate sidewalks closer to curbs and plant trees only on the side of the sidewalk away from the curb. A good rule of thumb is that the soil space should be three times wider than the trunk diameter of the mature tree.
- Provide separation between tree roots and infrastructure such as suspending the paving over the top of the tree root ball so that finished soil grade is 18 to 20 inches below the pavement’s finished grade.
- Avoid using tree grates. Trees growing in compacted soils can eventually lift tree grates enough to create tripping hazards. And tree grates that are not maintained or removed as trunk flares develop can girdle trunks and kill trees.
Materials: Modifying concrete paving material design or using other accessible, flexible paving materials can also reduce tree-related conflicts.
- Reinforce concrete with rebar, mesh, or fiber.
- Place expansion joints close to trees.
- Specify thicker (more than 6 inches) concrete slabs.
- Use pervious concrete.
- Try recycled rubber paving modules (which have been used in several Southern California cities).
- Replace tree grates with flexible sand-set pavers and specify regular maintenance to avoid the creation of tripping hazards if surface roots develop.
Root Guidance Systems:
- Employ root barriers, such as deflectors, inhibitors, or traps. Note that root barriers are more effective in well-drained soils than in areas of compacted soils or locations with high water tables.
- Construct continuous trenches under paving to provide extra soil volume for root growth. Options to consider are root paths (narrow trenches 4 inches wide by 12 inches deep) or root channels (wider trenches or large-diameter pipes filled with soil favorable to root development).
Soil Replacement, Modification, and Maintenance:
- Install alternate sub-base material under concrete slabs (6-inch layer of washed gravel with no fines). The air space between particles prevents roots from growing under paving.
- Increase soil volume to provide a minimum of 2 cubic feet of soil for every square foot of tree crown projection.
- Carefully evaluate the cost-effectiveness and long-term benefits of soil replacement techniques such as structural soil. Research suggests that the longer-term health of trees planted in structural soils may be poorer than trees grown in suspended pavements (discussed above), which allow roots access to more organic matter and greater available soil volumes.
These strategies are not appropriate, cost-effective, or feasible for every situation or every project. But understanding our options and applying these techniques judiciously will certainly keep us all happily strolling and rolling along our tree-shaded hardscapes.
treeScience: above, beneath, and inside the plan view — a regular column on tree-related issues of interest to landscape architects in the Bay Area — is compiled by Laurel Kelly, ASLA, landscape architect at H.T. Harvey & Associates. Laurel is also an ISA-certified arborist and Registered Consulting Arborist® (email: email@example.com).
For more information about designing accessible pedestrian rights of way:
- U.S. Access Board. 1999. Accessible Rights-of-Way: A Design Guide
- U.S. Access Board. 2011. Proposed Accessibility Guidelines for Pedestrian Facilities in the Public Right-of-Way
For more information about finding tree species with a low potential for root-based damage to infrastructure:
- Use the “Attribute Search” at the Urban Forest Ecosystems Institute’s SelecTree: A Tree Selection Guide.
For a comprehensive review of strategies for resolving conflicts between trees and paving as well as Bay Area tree selection guides:
- L.R. Costello and K.S. Jones. 2003. Reducing Infrastructure Damage by Tree Roots: A Compendium of Strategies. Western Chapter of the International Society of Arboriculture, Porterville, CA (866.785.8960)
For those who enjoy webinars:
- Ed Gilman. 2011. ADA, Sidewalks, and Trees—Resolving Conflicts.
For research on tree roots and sidewalks:
- N.E. D’Amato, T.D. Sydnor, M. Knee, R. Hunt, and B. Bishop. 2002. Which comes first, the root or the crack? Journal of Arboriculture 28(6):85-89.
- M. Raza, K.T. Weber, S. Mannel, D.P. Ames, and R. Patillo. 2011. Geospatial Analysis of Tree Root Damage to Sidewalks in Southeastern Idaho. URISA Journal 23(1):29-32
For a discussion of the long-term benefits of structural soils:
- E. T. Smiley, L. Calfee, B.R. Fraedrich, and E. J. Smiley. 2006. Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement. Arboriculture & Urban Forestry 32(4)
- N. Bassuk, R. Pine, J. Urban, and C. Moyles. 2010. The Great Soil Debate Part II: Structural soils under pavement. American Society of Landscape Architects: Annual Meeting Presentation, 11 September 2010.