From Tech Transfer Newsletter, Winter 2007 » printer-friendly

Healthy Trees, Smooth Sidewalks

By George Gonzalez
Chief Forester, Bureau of Street Services, City of Los Angeles

Innovative Sidewalk Repair in the City of Los Angeles: A Case Study

The City of Los Angeles has the country's largest roadway and sidewalk system with 6,500 miles of streets and approximately 10,000 miles of sidewalks. Los Angeles also has the largest urban forest with a population of nearly 700,000 street trees. Unfortunately, the city was without a sidewalk repair program for almost thirty years, resulting in approximately 4,600 miles of sidewalk in need of replacement or repair and annual expenditures of approximately $2 million for trip and fall claims.

It was obvious that something had to be done about the condition of the city's sidewalks. The challenge came in: How to replace the sidewalks without destroying or severely impacting the existing street tree population. In Fiscal Year 2000/2001, the City Council provided the Department of Public Works, Bureau of Street Services $8 million to develop a sidewalk repair program.

Since the program's beginning six years ago, the City has repaired more than 400 miles of sidewalks, preserving more than 52,000 trees that would have been removed otherwise. In addition to the trees that were preserved, the City planted more than 30,000 new street trees while removing fewer than 450 trees in the process. This article describes how this new program is preserving street trees while improving sidewalks, and highlights some of the strategies that appear most promising.

Policies, Goals and Standards

Although expanding the City's "green infrastructure" was stated policy in the City's General Plan Framework, implementation was lagging. Too often trees were given last consideration during design and development phases and first consideration for removal when they conflicted with infrastructure. Consequently, policies and goals were developed to expand tree canopy cover while repairing the City's damaged sidewalks.

The overall goals of the program are to:

1. Provide safer and more accessible transit surfaces for pedestrians.
2. Improve street drainage.
3. Maintain a healthy, safe, and sustainable urban forest.

In addition to the overall goals, five tree-related goals were developed:

1. Preserve as many large, healthy trees as possible, while providing for infrastructure stability and public safety on sidewalks and streets.
2. Replant sites where existing trees cannot be safely retained, and proactively plant in nearby, vacant sites.
3. Plant the largest species appropriate for the site and, where feasible, enlarge planting areas.
4. Perform tree removals along the same street in phases to retain an acceptable level of canopy cover.
5. Foster neighborhood involvement in the decision-making process and promote communication regarding infrastructure conflicts.

Evaluation and Implementation

Sidewalk survey. The sidewalk repair program is closely associated with the Americans with Disabilities Act (ADA) handicap ramp program. The ADA created regulations and standards intended to enhance the ability of disabled Americans to access public services. The Bureau surveyed the City's sidewalks to determine the areas of sidewalk damage, the extent of the damage, and whether trees were present. Sidewalk repair sites are selected predicate of ADA requirements, such as condition of transition zones between curb ramps, intended use of the area, severity of sidewalk damage, constituent requests, and frequency of trip and fall claims. The actual locations for repair are then selected after consultation with the respective Council offices.

Site inspections revealed that over 90% of damaged sidewalks occurred where tree roots are found. Restricted growing space is the single most important cause of conflicts between tree roots and hardscapes.

Tree evaluation. After the sidewalk repair sites are selected, City arborists inspect the adjacent trees. The trees are evaluated and placed in one of five grade categories:

• A – Tree is healthy, structurally stable, and has exceptional historical, aesthetic, and/or environmental qualities.
• B – Tree is healthy, structurally stable, and has a useful lifespan of more than five years.
• C – Tree is declining, creating extensive structural damage, and/or is the improper species/size for the site.
• D – Tree is declining, structurally unsound, has a high failure potential, and is senescent.
• F – Tree is dead.

Class "A" trees are some of the most desirable trees in any community. These trees are not root pruned. They are preserved by implementing one or more alternative sidewalk construction options.

Class "B" trees are high-value trees the Bureau wishes to preserve, and alternative sidewalk construction options as well as root pruning may be implemented.

Class "C" trees may or may not be selected for removal. In LA, if the Bureau chooses to remove the tree, it must request permission from the Board of Public Works.

Class "D" and "F" trees will be removed and replaced.

In addition, trees that are deemed a public hazard due to defects, significant amounts of decay, or structural unsoundness are rated using the Tree Hazard Evaluation Form (see resources section at end of article for reference information) and scheduled for removal as soon as possible.

After the trees are evaluated and classified, the Bureau has three options:

1. Preserve the tree, while minimizing the impact on the tree.
2. Retain the tree by root pruning it, or
3. Remove the tree.

Root pruning. Root pruning is the most common mitigation method currently employed. Since root pruning may compromise tree stability, Bureau arborists inspect subject trees by completing a Root Prune Evaluation Form prior to initiating the work.

The evaluation form addresses various critical elements such as the tree's age, size, species, number of roots requiring pruning, size of roots, and proximity of pruning to root flare, to name a few. Information provided on the form, as well as the arborist's experience and a list of species tolerant to root pruning, predicates whether root pruning is appropriate.

Even when performed correctly, root pruning has a potential of destabilizing the tree; therefore, the City performs a series of follow-up inspections on all root-pruned trees.

Within six months of the root pruning, the Bureau monitors root-pruned trees to assess their health, stability, and potential maintenance needs. Follow-up assessments will occur at 18, 40, and 78 months. If the follow-up inspections reveal that the trees are either declining or are unstable due to the root pruning, the trees are then removed.

Fortunately, follow-up inspections have revealed that the overwhelming majority of root-pruned trees are stable and thriving. This can be attributed to the extreme care taken in selecting trees to be root pruned, as well as the precautions taken during the root pruning process.

Tree removal. Root pruning may not be an option at all locations; consequently, some trees must be removed. The City has a longstanding tree removal policy that promotes the retention of healthy, mature trees whenever possible. When it is determined that tree removal is necessary because root pruning is not a viable option, a "Tree Removal Request" is submitted to the Board of Public Works.

In the City of Los Angeles, the Board of Public Works has sole discretion over granting permission to remove healthy, stable trees from the public right-of-way. So, before City forces can remove trees to effectuate sidewalk repairs, the Board's permission must first be obtained. If three or more trees are to be removed, the trees must be posted for a minimum of 30 days. The Bureau then presents its recommendation at a public hearing where residents are allowed to challenge the tree removal request.

The Bureau works closely with the City Council and the public to inform them of the tree removal process. By retaining as many large street trees as possible and minimizing the number of removals, the Bureau is able to reduce the number of tree removal challenges. The Bureau's goal of not removing more than 20% of the trees on a given block also helps reduce the number of challenges to tree removals. Also, the Bureau requires a minimum of a 2-to-1 replacement for all tree removals.

If the tree removal request is granted, City forces will begin the process of removing the trees. Where trees are removed, adequate planting space is created to allow planting of medium- to large-size trees. Creating a larger planting area is accomplished by:

• Enlarging tree wells
• Minimizing the sidewalk width
• Creating a reverse parkway (placing the sidewalk adjacent to the back of the curb)
• Obtaining additional public easement from the property owner
• Creating curb bump-outs

Sidewalk grinding: Sidewalk grinding is a temporary measure that restores the offset or heaved portion of a sidewalk to original grade. The Bureau is grinding sidewalks that are offset a half an inch or less. Sidewalk grinding is performed at approximately 10% of the repair sites. The Bureau is experimenting with grinding offsets greater than ½ inch based on the City of Modesto's reported success rate with grinding up to two inches of off-grade concrete.

Sidewalk cutouts: "Borrowing" space from the adjacent sidewalk creates sidewalk cutouts. This alternative minimizes the sidewalk width for a limited distance adjacent to the tree. The cutout provides a larger grow space for trees and reduces the size of the pruned roots and their proximity to the root flare. Borrowing has limitations, as the room for tree expansion before infringing on the free passage of pedestrians is minimal. Furthermore, the ADA imposes strict regulations as to the amount of free space that shall be provided.

Sidewalk meandering: Meandering—realigning the sidewalk's direction of travel—enables the Bureau to provide more growing space for trees in an aesthetically appealing way. The amount of growing space created can be substantial and, therefore, sidewalk meandering is usually the most feasible way to retain large, mature trees. Also, increased distance from sidewalk edge to lateral roots or trunk flare allows for root pruning, when necessary, to occur further from the trunk, which reduces direct contact between the sidewalk and tree roots or trunk. Sidewalk meandering often requires permission from the abutting property owner to dedicate more of their property to the public right-of-way.

Sidewalk ramping: Sidewalk ramping allows existing roots to remain intact by repouring concrete over the roots to create a gradually sloped ramp. It is used when removal of roots would compromise the stability of a grade "A" tree. Damaged sidewalk slabs are removed and 4-6 inches of topsoil is placed on top of the existing grade. A sand or foam backer is placed adjacent or around the subject roots. A new sidewalk is then installed on top of this new base material. This option enables the sidewalk to be replaced in its original position. Research in the City of Santa Monica has shown that sidewalk ramping does not prevent future damage but can delay it by five years or more.

Flexible paving materials: Flexible paving comes in many forms, which include:

• Interlocking pavers
• Common brick and pavers
• Rubber bricks

Flexible paving is used in conjunction with root pruning when retaining original grade is required and when the level of the paving surface is ramped above or lowered below existing grade. The selected flexible paving material is installed over a compacted sand base. The Bureau is utilizing rubberized, reusable brick in different dimensions that is bonded together with specialized glue. Some of the newer rubberized pavers do not require glue to bind them, but instead use specially designed dowels, which hold the pavers together. Although the use of flexible paving does not prevent future damage, it does provide more time between repairs making repairs easier and less costly.

Tree planting. After the sidewalks are installed, there is one final piece of a successful sidewalk program: tree planting. As stated earlier, Los Angeles has a longstanding policy of replacing every tree removed with a minimum 2 to 1 replacement ratio. The Bureau utilized this opportunity to expand our future canopy coverage by planting every available site within the sidewalk repair area. Where there was no room to replant, vacant planting locations were identified on adjacent streets to plant more than the 2 to 1 replacement ratio.

In the City of Los Angeles, replacement street trees must be 15-gallon size containers or larger. Using larger trees minimizes the occurrences of vandalism and gives the trees a much better chance of surviving.

All trees are planted using root deflection devices, which will help reduce future sidewalk damage. However, the most critical decision for reducing future infrastructure damage is proper species selection. Your municipal arborist will be invaluable in selecting species that are compatible to each site. If your municipality does not have a municipal arborist, consider hiring one or contracting with a consulting arborist that is knowledgeable with the trees in your area.

Planning and Design Strategies

The Bureau's remediation strategies address damage that has already occurred. Eliminating the potential for tree roots and infrastructure conflict is the long-term solution to these conflicts. Although seemingly impossible, it can be achieved with coordinated planning and extensive interagency cooperation.

The key to the success of any tree is providing sufficient space and/or the proper growing medium to fulfill its requirements through maturity. Too frequently, development plans do not include the essential ingredients for a successful and sustainable tree planting. The Bureau is working with the City's Planning Department to ensure that adequate grow space is given to trees from the initial planning stage so trees can coexist with other infrastructure elements.

Clearly, design and development processes that consider trees as integral to the City's infrastructure are essential to avoid future root and hardscape conflicts. The Bureau is working with project designers and planners to ensure that adequate tree space is provided and potential future conflicts are minimized.

Reprinted with permission from the APWA Reporter, April 2006. For more information about APWA, visit www.apwa.net. Author George Gonzalez may be reached at (213) 473-4660 or ggonzale@bss.lacity.org.

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