LA Course #GCE-6-1602
About Sidewalk and Tree Roots Conflict
Edited by Len Phillips and Richard W Gibney RLA/ISA, updated January 2023
Sections Go directly to the Section by clicking on the title below
About Sidewalk and Tree Roots Conflict
Edited by Len Phillips and Richard W Gibney RLA/ISA, updated January 2023
Sections Go directly to the Section by clicking on the title below
- Evaluate the Tree and Sidewalk
- New Sidewalk Planning & Construction
- Alternative Sidewalks Materials - Hard Pavements
- Alternative Sidewalks Materials - Flexible Pavements
- Alternative Sidewalks Materials - Structural Soils
- Alternative Sidewalks Materials - Soil Cells
- Alternative Sidewalks Materials - Other Pavements
- Alternative Sidewalks Materials - Temporary Paths
- Alternative Sidewalks Materials - Soils
- Alternative Designs
- Manipulation of Existing Sidewalks
- Manipulation of Existing Trees
- Advantages and Disadvantages
Note: Click on green text in each section for more information and photos.
The trees in the urban forest provide important infrastructure services through stormwater retention, air and water pollution reduction, climate change mitigation, and reduction of heat island effect. Urban heat islands collect and radiate more heat than nearby rural areas and this is a difference that trees can help reduce. In addition, the urban forest is often home to diverse wildlife and the trees provide food, shelter, and nesting opportunities that are essential to supporting this wildlife. The presence or absence of trees can define a neighborhood, and studies have shown that people enjoy trees and are more comfortable in the presence of trees than they are without them.
Sidewalks also provide an important infrastructure service. They allow residents to walk safely from one location to another in the city. While usually used for a specific purpose such as going to a neighborhood store or to meet friends, they are often used just for taking a relaxing stroll to enjoy the out-of-doors. The sidewalk does have a major disadvantage because it is part of the city's urban heat islands. Sidewalks absorb heat from the sun during the day and raise the temperature throughout the city.
There is another problem common in most cities since the beginning of paved sidewalks. For years we have not been giving any consideration to the amount of quality soil that street-side trees need to properly grow into a mature specimen. Consequently, tree roots must find and use water and oxygen wherever they can find it. In time a small gap develops between the top of the compacted soil and the underside of the sidewalk where groundwater collects from the condensation of soil moisture that has migrated up toward the surface of the soil through capillary action. Opportunistic tree roots find and use this water and oxygen to meet the tree's requirements for growth. As the roots grow they increase in diameter and raise the sidewalks and nearby curbs just as roots grow and increase in diameter in areas where there is no sidewalk. The horizontal roots that are causing the problems are the same roots that make a tree stable against storms and wind. Eventually the sidewalk will have to be replaced because it has been lifted and broken by the root growth. This article has been prepared to address this concern by providing ways to fix the sidewalk as well as properly grow the tree.
To reduce the city's liability of trip and fall incidents from broken sidewalks (and to maintain ADA compliance), the walks have to be replaced or the trees and at least the major roots, have to be removed to perform the necessary repairs. Removing roots will usually result in a tree losing its stability and this often results in the tree weakening and falling down very unexpectedly.
New innovations in repairing or replacing sidewalks can reduce the damage to trees by accommodating rather than ignoring tree roots. Sidewalks and trees in the city are both essential elements of the city's infrastructure and the well-being of the residents. Efforts must be made to preserve the tree's health and preserve a safe sidewalk. This Topic will provide a number of these innovative alternatives that have been somewhat successful in providing safe trees and safe sidewalks.
However, before any of these options are undertaken, a detailed cost benefit analysis for the site should be made that takes into consideration life cycle costing, environmental data, real estate values, pollutant and sediment removals vs statutory requirements, optimal ratio of the tree pit area to catchment for tree species, etc.
The trees in the urban forest provide important infrastructure services through stormwater retention, air and water pollution reduction, climate change mitigation, and reduction of heat island effect. Urban heat islands collect and radiate more heat than nearby rural areas and this is a difference that trees can help reduce. In addition, the urban forest is often home to diverse wildlife and the trees provide food, shelter, and nesting opportunities that are essential to supporting this wildlife. The presence or absence of trees can define a neighborhood, and studies have shown that people enjoy trees and are more comfortable in the presence of trees than they are without them.
Sidewalks also provide an important infrastructure service. They allow residents to walk safely from one location to another in the city. While usually used for a specific purpose such as going to a neighborhood store or to meet friends, they are often used just for taking a relaxing stroll to enjoy the out-of-doors. The sidewalk does have a major disadvantage because it is part of the city's urban heat islands. Sidewalks absorb heat from the sun during the day and raise the temperature throughout the city.
There is another problem common in most cities since the beginning of paved sidewalks. For years we have not been giving any consideration to the amount of quality soil that street-side trees need to properly grow into a mature specimen. Consequently, tree roots must find and use water and oxygen wherever they can find it. In time a small gap develops between the top of the compacted soil and the underside of the sidewalk where groundwater collects from the condensation of soil moisture that has migrated up toward the surface of the soil through capillary action. Opportunistic tree roots find and use this water and oxygen to meet the tree's requirements for growth. As the roots grow they increase in diameter and raise the sidewalks and nearby curbs just as roots grow and increase in diameter in areas where there is no sidewalk. The horizontal roots that are causing the problems are the same roots that make a tree stable against storms and wind. Eventually the sidewalk will have to be replaced because it has been lifted and broken by the root growth. This article has been prepared to address this concern by providing ways to fix the sidewalk as well as properly grow the tree.
To reduce the city's liability of trip and fall incidents from broken sidewalks (and to maintain ADA compliance), the walks have to be replaced or the trees and at least the major roots, have to be removed to perform the necessary repairs. Removing roots will usually result in a tree losing its stability and this often results in the tree weakening and falling down very unexpectedly.
New innovations in repairing or replacing sidewalks can reduce the damage to trees by accommodating rather than ignoring tree roots. Sidewalks and trees in the city are both essential elements of the city's infrastructure and the well-being of the residents. Efforts must be made to preserve the tree's health and preserve a safe sidewalk. This Topic will provide a number of these innovative alternatives that have been somewhat successful in providing safe trees and safe sidewalks.
However, before any of these options are undertaken, a detailed cost benefit analysis for the site should be made that takes into consideration life cycle costing, environmental data, real estate values, pollutant and sediment removals vs statutory requirements, optimal ratio of the tree pit area to catchment for tree species, etc.
Evaluate the Sidewalk and the Tree
The best solution to the problem of heaving walks occurs when the trees are planted. When a tree root has grown near the surface and has lifted the sidewalk, the city administrators must begin to collect information about the tree and the sidewalk. For example:
The initial assessment should require a landscape architect, engineer or an arborist to visit the site and assess the tree and sidewalk conditions together. This will allow for better coordination between both departments as the remedial action moves forward. The professional and arborist will review the information collected and identify the best actions to deal with each tree and sidewalk conflict.
If the tree is in good condition and should be kept, a determination must be made to fix or replace the sidewalk. If the sidewalk is in good condition and should be kept, a determination must be made to replace the tree. If neither the tree nor sidewalk are in good condition the determination must focus on replacing both with the best option available at a reasonable cost.
If there are limitations during the initial assessment and it is not the appropriate time for an evaluation of the pavement, soils, or the tree’s root system, future actions must be scheduled so that the appropriate action and funding for the project can be scheduled and not delayed or overlooked.
The city must accept that trees and sidewalks are important public infrastructure assets and strive to keep healthy trees and have accessible sidewalks. If the city's sidewalks and trees standard cannot be met at a particular site, the city must then take the time and resources to evaluate alternative approaches such as sidewalk width changes, alternative pavement materials and/or adjustments to the tree pit or tree root spaces that can be used to retain a tree and provide an accessible sidewalk.
Further Evaluation
In a situation where action on replacing a tree or a sidewalk requires further study, additional technical information should be collected about the trees and the sidewalks. The city should consider the following:
Evaluate Tree Removal
Most city administrators believe the best solution to a raised sidewalk is to remove and replace the entire tree that is the source of the problem. Sometimes it works to remove the sidewalk and raise it above the problem roots. This is more common and better than the alternative of root pruning. When tree replacement or root pruning is done it should be done in accordance with the municipality's street tree policy and by a certified arborist. Proper tree selection near a sidewalk is critical to the tree's long-term success. The amount of space available should determine the size of the tree being planted.
However, if there is an option to replace the sidewalk or replace the tree, the better option is to replace the sidewalk. The tree has invested many years of growth in harsh conditions to attain its size and absorb a considerable amount of carbon. Its' benefit to the city is approaching the maximum value.
If the tree must be removed, the city should provide options to replace the tree. Ideally, the tree would be replaced at the same location with modifications to improve the longevity of the replacement tree. If this is not possible due to space constraints or other safety concerns, the replacement tree should be planted somewhere else along the same street first. If the street does not offer new tree installation opportunities, the tree should be planted on adjacent residential streets.
Years ago it was popular to plant streets with one tree species along an entire block. These identical tree-lined streets were aesthetically appealing and provided a uniform view down the street. During the initial assessment, the city or the arborist today, may see the need to diversify the street by planting new, younger trees and/or a mix of different species along the street. Another consideration for tree replacement could be the opportunity to manage the trees with corrective actions to provide clearance from the sidewalk, traffic control devices, and vehicles and to provide a maximum beneficial tree canopy.
Public Outreach
When a tree is being considered for replacement, the city should post a public notice about the replacement project. This notice would announce the date and time for a public hearing on the trees. This should occur at the beginning of the project. It is important that the initial public contact be early so the community understands the purpose and scope of the proposed project. This is the opportunity to let the community know that an initial assessment is occurring and to obtain their comments on the proposed action.
A second opportunity for public outreach is following the completion of the initial assessment. It may be desired to share the results of the assessment completed by the engineer and arborist, to discuss potential solutions, and to consider any public feedback in the final decision.
The best solution to the problem of heaving walks occurs when the trees are planted. When a tree root has grown near the surface and has lifted the sidewalk, the city administrators must begin to collect information about the tree and the sidewalk. For example:
- examine the tree's quality or health, and is it worth preserving,
- if the tree exhibits poor health or vigor, can it be trimmed to save it or must it be removed,
- are the roots of the tree in good condition and is the tree worth preserving,
- are the roots large structural roots that can lift the sidewalk or small fibrous roots that seldom cause sidewalk damage,
- examine the sidewalk to determine the cause of failure because tree roots may not be the problem,
- is the soil causing the sidewalk failure,
- should the tree or the sidewalk be replaced?
The initial assessment should require a landscape architect, engineer or an arborist to visit the site and assess the tree and sidewalk conditions together. This will allow for better coordination between both departments as the remedial action moves forward. The professional and arborist will review the information collected and identify the best actions to deal with each tree and sidewalk conflict.
If the tree is in good condition and should be kept, a determination must be made to fix or replace the sidewalk. If the sidewalk is in good condition and should be kept, a determination must be made to replace the tree. If neither the tree nor sidewalk are in good condition the determination must focus on replacing both with the best option available at a reasonable cost.
If there are limitations during the initial assessment and it is not the appropriate time for an evaluation of the pavement, soils, or the tree’s root system, future actions must be scheduled so that the appropriate action and funding for the project can be scheduled and not delayed or overlooked.
The city must accept that trees and sidewalks are important public infrastructure assets and strive to keep healthy trees and have accessible sidewalks. If the city's sidewalks and trees standard cannot be met at a particular site, the city must then take the time and resources to evaluate alternative approaches such as sidewalk width changes, alternative pavement materials and/or adjustments to the tree pit or tree root spaces that can be used to retain a tree and provide an accessible sidewalk.
Further Evaluation
In a situation where action on replacing a tree or a sidewalk requires further study, additional technical information should be collected about the trees and the sidewalks. The city should consider the following:
- the level of impact if the tree is removed or retained,
- any risks for the city or the public as the sidewalk or tree replacement moves forward,
- the cost and benefit of keeping the tree versus maintaining the sidewalk based on public safety, tree species, and budget projections,
- the anticipated maintenance of the sidewalk if the tree is to be kept,
- the public and environmental benefit the tree is providing in terms of shade, view screening, stormwater interception, etc. and how long it will take for those benefits to be replaced with new trees,
- the community values placed on the sidewalk and the tree,
- any policy guidance from a neighborhood plan, urban design requirement, or other guiding document that exists for the project area,
- does the tree provide or contribute to defining character of the neighborhood and/or a sense of place for the neighborhood where it is growing,
- the appearance of public spaces near the site, that might be impacted by the tree.
Evaluate Tree Removal
Most city administrators believe the best solution to a raised sidewalk is to remove and replace the entire tree that is the source of the problem. Sometimes it works to remove the sidewalk and raise it above the problem roots. This is more common and better than the alternative of root pruning. When tree replacement or root pruning is done it should be done in accordance with the municipality's street tree policy and by a certified arborist. Proper tree selection near a sidewalk is critical to the tree's long-term success. The amount of space available should determine the size of the tree being planted.
However, if there is an option to replace the sidewalk or replace the tree, the better option is to replace the sidewalk. The tree has invested many years of growth in harsh conditions to attain its size and absorb a considerable amount of carbon. Its' benefit to the city is approaching the maximum value.
If the tree must be removed, the city should provide options to replace the tree. Ideally, the tree would be replaced at the same location with modifications to improve the longevity of the replacement tree. If this is not possible due to space constraints or other safety concerns, the replacement tree should be planted somewhere else along the same street first. If the street does not offer new tree installation opportunities, the tree should be planted on adjacent residential streets.
Years ago it was popular to plant streets with one tree species along an entire block. These identical tree-lined streets were aesthetically appealing and provided a uniform view down the street. During the initial assessment, the city or the arborist today, may see the need to diversify the street by planting new, younger trees and/or a mix of different species along the street. Another consideration for tree replacement could be the opportunity to manage the trees with corrective actions to provide clearance from the sidewalk, traffic control devices, and vehicles and to provide a maximum beneficial tree canopy.
Public Outreach
When a tree is being considered for replacement, the city should post a public notice about the replacement project. This notice would announce the date and time for a public hearing on the trees. This should occur at the beginning of the project. It is important that the initial public contact be early so the community understands the purpose and scope of the proposed project. This is the opportunity to let the community know that an initial assessment is occurring and to obtain their comments on the proposed action.
A second opportunity for public outreach is following the completion of the initial assessment. It may be desired to share the results of the assessment completed by the engineer and arborist, to discuss potential solutions, and to consider any public feedback in the final decision.
New Sidewalk Planning & Construction
Problems with roots from municipal trees begin when shallow-rooted trees are forced to grow in that ribbon of grass between the street and the sidewalk (usually called a tree lawn). Problems also occur when large trees grow toward the curb or sidewalk and the tree's flare lifts the curb or sidewalk. Here are some ways to deal with these issues.
Sidewalk Design
When the municipality decides it is time to correct the sidewalk problems, a whole new series of problems with the trees and the sidewalks can result. Although the buttress roots supporting the tree cause the worst damage to sidewalks, it is not advisable to prune any of these roots in the interest of saving the walk. The horizontal roots that are causing the problems are the same roots that make a tree stable against wind and storms.
Sidewalks along public streets and curb ramps at street crossings must be accessible, continuous, and unobstructed for use by all people, including people with disabilities. The Americans with Disabilities Act (ADA) and the US Access Board Public Right-of-Way Accessibility Guidelines (PROWAG) provide guidance and regulation for sidewalks in the public right-of-way. Sidewalks shall provide a minimum width of no less than 39 inches (1m) wide clear zone along the path of travel. Whether a sidewalk is brand new or being repaired, the new sidewalk must meet all ADA requirements.
Planning
Solving the problem of heaving walks is best done before the trees are planted. The city should have the best solutions built into construction specifications and subdivision regulations. They should also have the best solutions stated in the tree department regulations or guidelines.
Locations where the existing planting strip or tree pit is too small to accommodate the tree, may still have a tree planted in the tree pit. In many cases this has caused adjacent sidewalks to heave and break, creating potential risk for pedestrians. This condition typically occurs because the tree species that has been planted needs a larger volume of soil to achieve its mature canopy size or the tree has an aggressive root system or a trunk character that spreads at the base but is constrained by its planting area. Regardless of the reason, upheaval cause by roots may occur because the soils are severely compacted or dense and do not allow root penetration. To deal with this issue, many cities have developed an approved street tree list to clarify which species are appropriate in certain locations as well as standards for locating trees near other infrastructure.
Soil Volume
All street trees should have an adequate volume of soil of a type and depth that promotes healthy tree and root growth. Many tree and sidewalk conflicts arise due to tree roots growing directly under sidewalks, with compacted fill and other poor soils below. Providing adequate volume and depth of appropriate soils will help grow healthier trees and reduce any tree and sidewalk conflicts. Soil volume requirements vary by tree species and location, but general guidelines as listed below will provide some guidance.
Generally the following volumes should be provided so trees may reach their mature size:
Planting Site Width
There should at least 1 foot (30 cm) of soil between the curb and the tree trunk at maturity. If this space will not be available, move the tree to a better site, or select a smaller stature tree with roots that are not as invasive. In other words, plant the right tree in the right place.
Proper tree selection near a sidewalk is critical to long-term success. The amount of space available should determine the size of the tree being planted. Here are some general guidelines:
Tree Lawn Width Mature Tree Height
less than 3 feet (0.9 m) No trees
3-5 feet (1 - 1.5 m) 10-30 feet (3 - 9 m)
5-7 feet (1.5 - 2 m) 30-40 feet (9 - 12 m)
7-9 feet (2 - 3 m) 40-50 feet (12 - 15 m)
9 feet or wider (3 m+) 50 feet or taller (15 m+)
Sidewalk construction requires soil that will be loose enough to deflect tree roots downward, yet firm enough to support the sidewalk. A general rule to consider requires an adequate tree pit size and/or provide soil under adjacent pavement for new tree plantings, but not if adding soil volume would require cutting or damaging critical roots on an existing tree. The planting soil available to existing trees should be augmented without adversely impacting the roots.
Increase tree pit size and provide soils that promote healthy root growth to extent possible when repairing sidewalks around existing trees. Planting soils under or at the back of sidewalk may count towards soil volume if appropriate soils are provided for tree root growth and there is an opportunity provided for the tree roots to pass under the paved area where they can grow at a normal rate. The actual soil volumes needed for optimum tree health will vary with location, tree species, and other conditions.
This street view link is a good example of the effect of soil volume on urban tree growth. These hornbeams (Carpinus) were planted at the same time in both rows. The right-side row are growing in conventional tree pits while the left side row has a strip of lawn adjacent to the trees that provides additional soil volume.
Problems with roots from municipal trees begin when shallow-rooted trees are forced to grow in that ribbon of grass between the street and the sidewalk (usually called a tree lawn). Problems also occur when large trees grow toward the curb or sidewalk and the tree's flare lifts the curb or sidewalk. Here are some ways to deal with these issues.
Sidewalk Design
When the municipality decides it is time to correct the sidewalk problems, a whole new series of problems with the trees and the sidewalks can result. Although the buttress roots supporting the tree cause the worst damage to sidewalks, it is not advisable to prune any of these roots in the interest of saving the walk. The horizontal roots that are causing the problems are the same roots that make a tree stable against wind and storms.
Sidewalks along public streets and curb ramps at street crossings must be accessible, continuous, and unobstructed for use by all people, including people with disabilities. The Americans with Disabilities Act (ADA) and the US Access Board Public Right-of-Way Accessibility Guidelines (PROWAG) provide guidance and regulation for sidewalks in the public right-of-way. Sidewalks shall provide a minimum width of no less than 39 inches (1m) wide clear zone along the path of travel. Whether a sidewalk is brand new or being repaired, the new sidewalk must meet all ADA requirements.
Planning
Solving the problem of heaving walks is best done before the trees are planted. The city should have the best solutions built into construction specifications and subdivision regulations. They should also have the best solutions stated in the tree department regulations or guidelines.
Locations where the existing planting strip or tree pit is too small to accommodate the tree, may still have a tree planted in the tree pit. In many cases this has caused adjacent sidewalks to heave and break, creating potential risk for pedestrians. This condition typically occurs because the tree species that has been planted needs a larger volume of soil to achieve its mature canopy size or the tree has an aggressive root system or a trunk character that spreads at the base but is constrained by its planting area. Regardless of the reason, upheaval cause by roots may occur because the soils are severely compacted or dense and do not allow root penetration. To deal with this issue, many cities have developed an approved street tree list to clarify which species are appropriate in certain locations as well as standards for locating trees near other infrastructure.
Soil Volume
All street trees should have an adequate volume of soil of a type and depth that promotes healthy tree and root growth. Many tree and sidewalk conflicts arise due to tree roots growing directly under sidewalks, with compacted fill and other poor soils below. Providing adequate volume and depth of appropriate soils will help grow healthier trees and reduce any tree and sidewalk conflicts. Soil volume requirements vary by tree species and location, but general guidelines as listed below will provide some guidance.
Generally the following volumes should be provided so trees may reach their mature size:
- Trees larger than 50 ft. (15 m.) tall or spread need 2,700 cu. ft. (80 cu. m.) of soil
- Trees that grow 30 to 50 ft. (9 to 15 m.) tall or spread need 1,200 cu. ft. (34 cu. m.) of soil
- Trees that grow less than 30 ft. (9 m.) tall need 600 cu. ft. (17 cu. m.) of soil
- Trees that are to grow in soil cells, structural soils, or suspended pavement need 1,500 cu. ft. (40 cu. m.) of soil
- If soil volumes cannot be met in a sidewalk location, then all trees should be planted in a minimum 8 ft. (2.4 m.) wide by 3 ft. (1 m.) deep trench parallel to the sidewalk or street so that soil volumes are shared among all the trees.
Planting Site Width
There should at least 1 foot (30 cm) of soil between the curb and the tree trunk at maturity. If this space will not be available, move the tree to a better site, or select a smaller stature tree with roots that are not as invasive. In other words, plant the right tree in the right place.
Proper tree selection near a sidewalk is critical to long-term success. The amount of space available should determine the size of the tree being planted. Here are some general guidelines:
Tree Lawn Width Mature Tree Height
less than 3 feet (0.9 m) No trees
3-5 feet (1 - 1.5 m) 10-30 feet (3 - 9 m)
5-7 feet (1.5 - 2 m) 30-40 feet (9 - 12 m)
7-9 feet (2 - 3 m) 40-50 feet (12 - 15 m)
9 feet or wider (3 m+) 50 feet or taller (15 m+)
Sidewalk construction requires soil that will be loose enough to deflect tree roots downward, yet firm enough to support the sidewalk. A general rule to consider requires an adequate tree pit size and/or provide soil under adjacent pavement for new tree plantings, but not if adding soil volume would require cutting or damaging critical roots on an existing tree. The planting soil available to existing trees should be augmented without adversely impacting the roots.
Increase tree pit size and provide soils that promote healthy root growth to extent possible when repairing sidewalks around existing trees. Planting soils under or at the back of sidewalk may count towards soil volume if appropriate soils are provided for tree root growth and there is an opportunity provided for the tree roots to pass under the paved area where they can grow at a normal rate. The actual soil volumes needed for optimum tree health will vary with location, tree species, and other conditions.
This street view link is a good example of the effect of soil volume on urban tree growth. These hornbeams (Carpinus) were planted at the same time in both rows. The right-side row are growing in conventional tree pits while the left side row has a strip of lawn adjacent to the trees that provides additional soil volume.
Alternative Sidewalks Materials – Hard Pavements
Long-term growing trees have to be planted in a setting where the permanent hard surfaces, infrastructure, and utilities are far enough away that they can be repaired without removing the trees. New innovations in repairing or replacing sidewalks can reduce the damage to trees by accommodating rather than ignoring or removing tree roots. However, concrete does not have to be the only material used to create usable sidewalks.
There are many other products that can form a modular sidewalk that will be a sufficient and effective surface for walking and driving lower weight vehicles, and not require trees to be removed to modify the site as the tree grows. All of these products can be placed over growing roots and perform as well as concrete.
Pavement Treatments
New asphalt and concrete sidewalks can be installed with options. Both products can be used with new colored and stamped products. They can also use easements to run the sidewalk on private property and increase the tree lawn width. These solutions also allow curved sidewalks or curbs to go around trees.
Standard Concrete
Standard concrete walks are usually 3-1/2” (9 cm) thick. This is by far, the most common material used in sidewalk construction. The materials are fairly inexpensive to purchase and the labor to mix and install the concrete is reasonable. Maintenance of concrete sidewalks is minimal throughout most of its anticipated 50 year life span. However, if poor soil conditions or tree roots are a factor, the life span of the concrete slab is considerably shorter. Compared to other options, this thinner pavement will better accommodate existing tree roots. However, vehicular or other anticipated loads on the sidewalk may cause damage to this pavement.
Thicker Pavement
In some cases, thicker pavement may minimize future root damage by providing greater strength and resistance against root pressure. The thicker concrete is 6 inches to 8 inches (15 – 20 cm) thick. The use of thicker pavement sidewalk is similar to the design of driveways, which employ a thicker slab to support vehicular traffic. Thicker concrete will require additional excavation to accommodate the thicker pavement and this might cause damage to the existing tree roots or infrastructure.
Reinforced Concrete
A reinforced concrete sidewalk can be used to help resist uplift by tree roots. Reinforcing may include the use of steel re-bar or wire mesh. Reinforced concrete works well if very little future root growth is anticipated and existing roots can be pruned to accommodate the sidewalk installation. If the tree growth is vigorous, do not use this option. This also requires that there is an adequate soil volume in the areas where the roots are intended to grow. In some cases, the thinner (reinforced) pavement can provide more space for existing tree roots.
Reinforced pavement can resist cracking from soil settlement, soil heaving, and root expansion underneath. As roots grow under the slab they are less able to lift the slab because it is very heavy. If the slab weighs enough roots will deform instead of lifting the slab.
Pervious Concrete
Pervious concrete allows air and water to pass through the concrete to soil layers below. Pervious concrete is made using large aggregates with little to no fine aggregates. The concrete paste coats the aggregates and allows water to pass through the concrete slab. If designed and installed properly, it may deter shallow root growth and reduce root damage to the sidewalk by allowing water and air to infiltrate more deeply into the soil profile and the roots will stay deeper in the ground. Pervious concrete does require an adequate depth for installation of the pervious concrete and necessary sub-base layers without creating an excessive impact to existing roots. Pervious concrete should not be used if adjacent properties are below the surface grade of sidewalk and there are unsafe drops to the ground on either side of the concrete walk. Unfortunately, pervious concrete does require more maintenance than a standard concrete sidewalk. There are several new pervious products coming out that could be considered for walks as well as treatments around the tree. See "Paving Around Street Trees" for more information.
Decomposed Granite
Decomposed granite materials are used where a firm pathway is required for pedestrian or vehicular traffic, but a natural look is preferred. Decomposed granite pathways are available in three variations: standard, stabilized, and wax polymer. These products are an affordable and aesthetically pleasing alternative to concrete, asphalt, pavers and other hardscape surface materials. They are naturally occurring decomposed granite and crushed stone that has been screened to less than a 1/4” size stone, allowing for a firm surface that compacts well, while retaining permeability. The wax polymer mix combines a natural crushed stone or decomposed granite with a combination of engineered polymers in a totally water-less process to create a unique paving alternative. It provides a dustless, sealed, non-permeable natural looking surface for pathways, patios, driveways, and plazas.
Monolithic Sidewalk
A monolithic sidewalk is where the roadway, curb, and sidewalk are constructed as one continuous concrete installation as opposed to two separate installations with an expansion joint separating curb and sidewalk. As one continuous installation there is more concrete weight mass to resist the uplift of tree roots. The elimination of the expansion joint at the back of curb also eliminates a potential future surface weakness in the paving infrastructure. This option is most useful if the tree is at its mature size and future root growth is not anticipated. The existing roots may be pruned to accommodate the installation as needed. This option also requires that there is adequate soil volume in the areas where roots are intended to grow.
Monolithic sidewalks can be used along new installations where the sidewalk is located adjacent to the street to prevent future root uplift. This option will correct uplift of the sidewalk and provide resistance from future uplift after corrective actions have been taken and root integrity can be maintained.
Asphalt
Asphalt is commonly used as a standard sidewalk material for an inexpensive sidewalk. It is also used for sidewalk repairs and replacements. It has less initial cost, is more flexible, and can more easily be repaired than a concrete sidewalk. However, asphalt has a much shorter expected useful life and it requires a higher level of ongoing maintenance than concrete. The useful life of asphalt pavement can vary greatly with site conditions. Asphalt may be used for new sidewalks in areas where concrete sidewalks are not feasible.
Porous Asphalt
Porous asphalt is similar to regular asphalt but will allow water to pass through the pores in the pavement to the soil layers below. Porous asphalt is made using large aggregates with little to no fine aggregates. A tar-like asphalt coats the aggregates and binds the stones together, with pores occurring between them. It may be appropriate to use this material in cases where infiltration in the sidewalk pavement area is desirable. These pavements are used mostly for parking lots because they allow water to drain through the pavement surface into a stone recharge bed and infiltrate into the soils below the pavement. Porous asphalt pavements are being used successfully throughout the United States, in every type of climate and geography.
They may be used for new sidewalks in areas where infiltration is desirable such as an area adjacent to a bioretention facility. One important problem with this option occurs from manufacturing constraints because it cannot be produced in very small quantities. Porous asphalt should only be used for long sidewalk segments such as multiple blocks, unless cost is not a concern.
Landscape Pavers
Landscape pavers and bricks can be used to replace a damaged sidewalk, even if the existing sidewalk is a different material such as concrete. As roots begin to expand, the individual pavers can easily be adjusted without much damage to the entire sidewalk.
Interlocking Pavers
Interlocking pavers are modular units of various shapes and colors set in sand over a firm base. They are porous and flexible. They require a continuous edge on all sides to hold the pavers together. These edgings can be concrete, steel, plastic, or wood. The pavers can be shaped in any form and cut to fit the desired form. They are thinner than concrete. They are fairly easy to remove, and replacement pavers can be used to make repairs.
Bricks and Stone Pavers
Bricks and decorative stone pavers can provide attractive sidewalks. They are usually thinner than concrete, and set in sand. They require an edge border to hold the pavers in place. They can be picked up to perform maintenance and replaced for continuing use. They are commonly used in historic districts, residential landscapes and occasionally found on municipal sidewalks. However, due to their high cost for material, installation labor, and maintenance, they are not installed very often.
Long-term growing trees have to be planted in a setting where the permanent hard surfaces, infrastructure, and utilities are far enough away that they can be repaired without removing the trees. New innovations in repairing or replacing sidewalks can reduce the damage to trees by accommodating rather than ignoring or removing tree roots. However, concrete does not have to be the only material used to create usable sidewalks.
There are many other products that can form a modular sidewalk that will be a sufficient and effective surface for walking and driving lower weight vehicles, and not require trees to be removed to modify the site as the tree grows. All of these products can be placed over growing roots and perform as well as concrete.
Pavement Treatments
New asphalt and concrete sidewalks can be installed with options. Both products can be used with new colored and stamped products. They can also use easements to run the sidewalk on private property and increase the tree lawn width. These solutions also allow curved sidewalks or curbs to go around trees.
Standard Concrete
Standard concrete walks are usually 3-1/2” (9 cm) thick. This is by far, the most common material used in sidewalk construction. The materials are fairly inexpensive to purchase and the labor to mix and install the concrete is reasonable. Maintenance of concrete sidewalks is minimal throughout most of its anticipated 50 year life span. However, if poor soil conditions or tree roots are a factor, the life span of the concrete slab is considerably shorter. Compared to other options, this thinner pavement will better accommodate existing tree roots. However, vehicular or other anticipated loads on the sidewalk may cause damage to this pavement.
Thicker Pavement
In some cases, thicker pavement may minimize future root damage by providing greater strength and resistance against root pressure. The thicker concrete is 6 inches to 8 inches (15 – 20 cm) thick. The use of thicker pavement sidewalk is similar to the design of driveways, which employ a thicker slab to support vehicular traffic. Thicker concrete will require additional excavation to accommodate the thicker pavement and this might cause damage to the existing tree roots or infrastructure.
Reinforced Concrete
A reinforced concrete sidewalk can be used to help resist uplift by tree roots. Reinforcing may include the use of steel re-bar or wire mesh. Reinforced concrete works well if very little future root growth is anticipated and existing roots can be pruned to accommodate the sidewalk installation. If the tree growth is vigorous, do not use this option. This also requires that there is an adequate soil volume in the areas where the roots are intended to grow. In some cases, the thinner (reinforced) pavement can provide more space for existing tree roots.
Reinforced pavement can resist cracking from soil settlement, soil heaving, and root expansion underneath. As roots grow under the slab they are less able to lift the slab because it is very heavy. If the slab weighs enough roots will deform instead of lifting the slab.
Pervious Concrete
Pervious concrete allows air and water to pass through the concrete to soil layers below. Pervious concrete is made using large aggregates with little to no fine aggregates. The concrete paste coats the aggregates and allows water to pass through the concrete slab. If designed and installed properly, it may deter shallow root growth and reduce root damage to the sidewalk by allowing water and air to infiltrate more deeply into the soil profile and the roots will stay deeper in the ground. Pervious concrete does require an adequate depth for installation of the pervious concrete and necessary sub-base layers without creating an excessive impact to existing roots. Pervious concrete should not be used if adjacent properties are below the surface grade of sidewalk and there are unsafe drops to the ground on either side of the concrete walk. Unfortunately, pervious concrete does require more maintenance than a standard concrete sidewalk. There are several new pervious products coming out that could be considered for walks as well as treatments around the tree. See "Paving Around Street Trees" for more information.
Decomposed Granite
Decomposed granite materials are used where a firm pathway is required for pedestrian or vehicular traffic, but a natural look is preferred. Decomposed granite pathways are available in three variations: standard, stabilized, and wax polymer. These products are an affordable and aesthetically pleasing alternative to concrete, asphalt, pavers and other hardscape surface materials. They are naturally occurring decomposed granite and crushed stone that has been screened to less than a 1/4” size stone, allowing for a firm surface that compacts well, while retaining permeability. The wax polymer mix combines a natural crushed stone or decomposed granite with a combination of engineered polymers in a totally water-less process to create a unique paving alternative. It provides a dustless, sealed, non-permeable natural looking surface for pathways, patios, driveways, and plazas.
Monolithic Sidewalk
A monolithic sidewalk is where the roadway, curb, and sidewalk are constructed as one continuous concrete installation as opposed to two separate installations with an expansion joint separating curb and sidewalk. As one continuous installation there is more concrete weight mass to resist the uplift of tree roots. The elimination of the expansion joint at the back of curb also eliminates a potential future surface weakness in the paving infrastructure. This option is most useful if the tree is at its mature size and future root growth is not anticipated. The existing roots may be pruned to accommodate the installation as needed. This option also requires that there is adequate soil volume in the areas where roots are intended to grow.
Monolithic sidewalks can be used along new installations where the sidewalk is located adjacent to the street to prevent future root uplift. This option will correct uplift of the sidewalk and provide resistance from future uplift after corrective actions have been taken and root integrity can be maintained.
Asphalt
Asphalt is commonly used as a standard sidewalk material for an inexpensive sidewalk. It is also used for sidewalk repairs and replacements. It has less initial cost, is more flexible, and can more easily be repaired than a concrete sidewalk. However, asphalt has a much shorter expected useful life and it requires a higher level of ongoing maintenance than concrete. The useful life of asphalt pavement can vary greatly with site conditions. Asphalt may be used for new sidewalks in areas where concrete sidewalks are not feasible.
Porous Asphalt
Porous asphalt is similar to regular asphalt but will allow water to pass through the pores in the pavement to the soil layers below. Porous asphalt is made using large aggregates with little to no fine aggregates. A tar-like asphalt coats the aggregates and binds the stones together, with pores occurring between them. It may be appropriate to use this material in cases where infiltration in the sidewalk pavement area is desirable. These pavements are used mostly for parking lots because they allow water to drain through the pavement surface into a stone recharge bed and infiltrate into the soils below the pavement. Porous asphalt pavements are being used successfully throughout the United States, in every type of climate and geography.
They may be used for new sidewalks in areas where infiltration is desirable such as an area adjacent to a bioretention facility. One important problem with this option occurs from manufacturing constraints because it cannot be produced in very small quantities. Porous asphalt should only be used for long sidewalk segments such as multiple blocks, unless cost is not a concern.
Landscape Pavers
Landscape pavers and bricks can be used to replace a damaged sidewalk, even if the existing sidewalk is a different material such as concrete. As roots begin to expand, the individual pavers can easily be adjusted without much damage to the entire sidewalk.
Interlocking Pavers
Interlocking pavers are modular units of various shapes and colors set in sand over a firm base. They are porous and flexible. They require a continuous edge on all sides to hold the pavers together. These edgings can be concrete, steel, plastic, or wood. The pavers can be shaped in any form and cut to fit the desired form. They are thinner than concrete. They are fairly easy to remove, and replacement pavers can be used to make repairs.
Bricks and Stone Pavers
Bricks and decorative stone pavers can provide attractive sidewalks. They are usually thinner than concrete, and set in sand. They require an edge border to hold the pavers in place. They can be picked up to perform maintenance and replaced for continuing use. They are commonly used in historic districts, residential landscapes and occasionally found on municipal sidewalks. However, due to their high cost for material, installation labor, and maintenance, they are not installed very often.
Alternative Sidewalks Materials – Flexible Pavements
There are a few types of flexible pavements available, including sidewalks made from materials such as rubber or composite plastics. When properly installed and maintained, these pavements can provide accessible walking surfaces that are more flexible than concrete while also providing room for root growth under the sidewalk. Many urban design projects recommend these alternative paving surfaces for their aesthetic appeal. These pavements can be installed on sand. The sand is spread as a sub-base without cutting roots and then the pavement is installed on the surface. As roots expand in diameter and raise the sidewalk, sections of the walk can be lifted and reset after adding sand to cover the enlarged roots.
Flexible pavements are not recommended if there are a large number of utility structures, such as water valves or manhole covers, in the walkway because the pavements would have to be cut around these structures. The depth of the installation varies greatly depending on the pavement type and material. As a result, some pavements will not work at locations where the existing trees have shallow root systems. Maintenance needs and durability will vary by type of material.
Flexible pavements are also used when a small section of sidewalk needs to be replaced or when a temporary solution is needed until a larger section of sidewalk can be reconstructed.
Rubber Sidewalks
An alternative to the concrete sidewalk is the use of recycled rubber modular sidewalk panels. This product uses recycled tire crumbs to form a molded sidewalk panel. The panels are affixed to each other with epoxy or dowels and held in place with a solid edge band along the outside edge of the sidewalk. The panels are 2 by 2.5 feet (60 by 75 cm) so they can be installed in multiple widths starting at 4 feet or 5 feet (120 or 150 cm). They are two inches (5 cm) thick. The base is ¾ inch (2 cm) aggregate which can be placed around roots and allows for water movement and storage. Geotextile fabric can be used to provide additional support and improve stability under the sidewalk while minimizing the compaction of the soil.
If a panel has to be removed for root pruning or regrading, it can be released and lifted, then anchored back into place. The panels do not expand or shrink in weather and they are flame resistant. They can be hosed off when they get dirty and are resistant to most compounds except sulfuric acid or long-time exposure to oil based products. The modular design minimizes the effect of aggressive root growth and it is durable enough to last for many years longer than concrete. Tests show that after eight years, there is no displacement of the rubber sidewalk by roots. The rubber's porosity allows water and air to pass through the product. This feature creates an environment where the tree roots develop as fibrous roots instead of large diameter roots that would lift the sidewalk. The advantage of rubber panels is the thin cross section and they can be placed on top of a root if necessary to delay removal or root pruning. Rubber panels have other advantages.
Poured-in-place Rubber
Poured-in-place rubber is an alternative to modular rubber panels. It is rubber mixed with other materials and aggregate that can be poured in any shape over the roots. It is installed like concrete and comes in a variety of colors. It is often found on many track courses for athletic events. Many mixes are permeable. This material is often found on school track facilities. The layer can be thinner than concrete and placed over a porous base. The disadvantages are that the rubber cannot be reused and that it will not do well with vehicular traffic. Plus it starts to fail after two years and is not firm or safe. This rubber product is destroyed by tree roots and cannot be easily maintained.
Terrewalks
Terrewalks are made from waste plastic molded into sidewalk panels that can be placed over tree roots. They create an interlocking modular paving system of permeable plastic panels. Like rubber sidewalks, the panels are 2 by 2.5 feet (60 by 75 cm) so they can be installed in multiple widths starting at 4 or 5 feet (120 or 150 cm). They are also two inches (5 cm) thick. They interlock in the middle and are individually staked along the ends with space between panels so there is more flexibility between panels as roots grow and put pressure on the sidewalk. They have a concrete-like appearance. They are light colored and have a matte finish that is textured like stone.
All the components are inert solids with no volatile organic compounds and no latex content. They are easy to remove and perform maintenance work on the tree roots and replace for further use, with no waste. The base is ¾ inch (2 cm) aggregate with no additives or fillers and can be placed around the tree roots to allow for water movement and water storage. The thinner panel creates an environment where the tree roots develop as fibrous roots instead of large diameter roots that would lift the sidewalk. Geotextile can be used to provide additional support and improve stability under the sidewalk while minimizing the compaction of the soil. The channel design system on the underside of each panel provides strength, facilitates water drainage, and accommodates tree root growth in a fibrous form.
Terrewalks can be compared to Trex, a popular plastic/sawdust wood deck material. Environmentally, they are walkable and an ADA accessible pedestrian surface because of a high coefficient of friction for non-skid in both dry and wet conditions. They are cost-effective and a LEED accredited alternative to concrete sidewalks and pathways. In addition, they are non-fading and non-cracking in any climatic condition. Maintenance of Terrewalks consist of a simple sweep, hose down, mop, or steam clean. Chewing gum is easily removed and leaves no mark.
Environmental Benefits of Terrewalks
Cost Savings of Terrewalks
There are several cost advantages of the Terrewalks panel system:
Sandwich System
The sandwich system creates rootable soil volume for trees, underneath the hardscapes in urban areas. This sandwich system, also known as a suspended pavement system consists of a rigid ‘floating’ geocellular raft system which spreads vehicular and pedestrian weight loads over a large area, preventing compaction of soil in the tree’s root zone. At the same time, the open structure of the raft system prevents roots from causing any damage to the pavement and serves as a distribution and delivery mechanism for air and water for the entire rootable soil volume. With this system, healthy trees can grow in the most dense urban areas, underneath pedestrian, bike, light weight traffic, and parking lots without reducing the available above ground urban space functionality.
There are a few types of flexible pavements available, including sidewalks made from materials such as rubber or composite plastics. When properly installed and maintained, these pavements can provide accessible walking surfaces that are more flexible than concrete while also providing room for root growth under the sidewalk. Many urban design projects recommend these alternative paving surfaces for their aesthetic appeal. These pavements can be installed on sand. The sand is spread as a sub-base without cutting roots and then the pavement is installed on the surface. As roots expand in diameter and raise the sidewalk, sections of the walk can be lifted and reset after adding sand to cover the enlarged roots.
Flexible pavements are not recommended if there are a large number of utility structures, such as water valves or manhole covers, in the walkway because the pavements would have to be cut around these structures. The depth of the installation varies greatly depending on the pavement type and material. As a result, some pavements will not work at locations where the existing trees have shallow root systems. Maintenance needs and durability will vary by type of material.
Flexible pavements are also used when a small section of sidewalk needs to be replaced or when a temporary solution is needed until a larger section of sidewalk can be reconstructed.
Rubber Sidewalks
An alternative to the concrete sidewalk is the use of recycled rubber modular sidewalk panels. This product uses recycled tire crumbs to form a molded sidewalk panel. The panels are affixed to each other with epoxy or dowels and held in place with a solid edge band along the outside edge of the sidewalk. The panels are 2 by 2.5 feet (60 by 75 cm) so they can be installed in multiple widths starting at 4 feet or 5 feet (120 or 150 cm). They are two inches (5 cm) thick. The base is ¾ inch (2 cm) aggregate which can be placed around roots and allows for water movement and storage. Geotextile fabric can be used to provide additional support and improve stability under the sidewalk while minimizing the compaction of the soil.
If a panel has to be removed for root pruning or regrading, it can be released and lifted, then anchored back into place. The panels do not expand or shrink in weather and they are flame resistant. They can be hosed off when they get dirty and are resistant to most compounds except sulfuric acid or long-time exposure to oil based products. The modular design minimizes the effect of aggressive root growth and it is durable enough to last for many years longer than concrete. Tests show that after eight years, there is no displacement of the rubber sidewalk by roots. The rubber's porosity allows water and air to pass through the product. This feature creates an environment where the tree roots develop as fibrous roots instead of large diameter roots that would lift the sidewalk. The advantage of rubber panels is the thin cross section and they can be placed on top of a root if necessary to delay removal or root pruning. Rubber panels have other advantages.
- they can be recycled.
- they require little maintenance.
- there is less trip-hazard liability.
- they reduce lawsuits caused by pedestrians injuring themselves on concrete.
- they are economical, sustainable, and aesthetically pleasing.
Poured-in-place Rubber
Poured-in-place rubber is an alternative to modular rubber panels. It is rubber mixed with other materials and aggregate that can be poured in any shape over the roots. It is installed like concrete and comes in a variety of colors. It is often found on many track courses for athletic events. Many mixes are permeable. This material is often found on school track facilities. The layer can be thinner than concrete and placed over a porous base. The disadvantages are that the rubber cannot be reused and that it will not do well with vehicular traffic. Plus it starts to fail after two years and is not firm or safe. This rubber product is destroyed by tree roots and cannot be easily maintained.
Terrewalks
Terrewalks are made from waste plastic molded into sidewalk panels that can be placed over tree roots. They create an interlocking modular paving system of permeable plastic panels. Like rubber sidewalks, the panels are 2 by 2.5 feet (60 by 75 cm) so they can be installed in multiple widths starting at 4 or 5 feet (120 or 150 cm). They are also two inches (5 cm) thick. They interlock in the middle and are individually staked along the ends with space between panels so there is more flexibility between panels as roots grow and put pressure on the sidewalk. They have a concrete-like appearance. They are light colored and have a matte finish that is textured like stone.
All the components are inert solids with no volatile organic compounds and no latex content. They are easy to remove and perform maintenance work on the tree roots and replace for further use, with no waste. The base is ¾ inch (2 cm) aggregate with no additives or fillers and can be placed around the tree roots to allow for water movement and water storage. The thinner panel creates an environment where the tree roots develop as fibrous roots instead of large diameter roots that would lift the sidewalk. Geotextile can be used to provide additional support and improve stability under the sidewalk while minimizing the compaction of the soil. The channel design system on the underside of each panel provides strength, facilitates water drainage, and accommodates tree root growth in a fibrous form.
Terrewalks can be compared to Trex, a popular plastic/sawdust wood deck material. Environmentally, they are walkable and an ADA accessible pedestrian surface because of a high coefficient of friction for non-skid in both dry and wet conditions. They are cost-effective and a LEED accredited alternative to concrete sidewalks and pathways. In addition, they are non-fading and non-cracking in any climatic condition. Maintenance of Terrewalks consist of a simple sweep, hose down, mop, or steam clean. Chewing gum is easily removed and leaves no mark.
Environmental Benefits of Terrewalks
- panels save the urban forest by reducing the need for tree removal,
- panels allow water infiltration, reducing storm water run-off into the storm drain,
- one-square-foot (0.1 sq. m.) of the sidewalk panel diverts 35 pounds (15.9 kg) of waste plastic from landfills,
- panels are 100% recycled and recyclable,
- patented low energy manufacturing technology results in a low-carbon footprint,
- no scrap is generated in the manufacturing process,
- panels can be fabricated with a Solar Reflectance Index of 44,
- the base can be laid over and around roots resulting in less root pruning compared to concrete construction.
Cost Savings of Terrewalks
There are several cost advantages of the Terrewalks panel system:
- interlocking tab design reduces installation time and cost, and provides uniform seams,
- the panels are durable and easily maintained,
- the panels reduce likelihood of injury and costly claims for trip and fall accidents,
- the panels are designed for use in permanent and temporary sites,
- the panels allow periodic tree root inspection, access to utilities, or site maintenance over time at minimal cost compared to concrete.
Sandwich System
The sandwich system creates rootable soil volume for trees, underneath the hardscapes in urban areas. This sandwich system, also known as a suspended pavement system consists of a rigid ‘floating’ geocellular raft system which spreads vehicular and pedestrian weight loads over a large area, preventing compaction of soil in the tree’s root zone. At the same time, the open structure of the raft system prevents roots from causing any damage to the pavement and serves as a distribution and delivery mechanism for air and water for the entire rootable soil volume. With this system, healthy trees can grow in the most dense urban areas, underneath pedestrian, bike, light weight traffic, and parking lots without reducing the available above ground urban space functionality.
Alternative Sidewalks Materials – Structural Soils
Structural soils are soils that are specially designed with stone mixed into soils that are modified to provide nutrients, pore space, and porosity to accommodate root growth while also allowing for compaction to support pavement and traffic on the surface, without settling. There are proprietary structural soil mixes available as well as various non-proprietary mixes that have been used in many municipalities. They may also be used as fill material around existing roots in areas where a sidewalk will be replaced, if adequate structural soil depth can be obtained. Structural soils are packed on a firm base 2 – 3 feet (60-90 cm) below the surface and they allow root growth in the pores between the stones. The surface pavement can be any of the products mentioned in this article.
CU-Structural Soil®
CU-Structural Soil® is built with uniformly sized crushed stone coated with clay loam soil and a hydrogel tackifier stabilizing agent. The soil and hydrogel are mixed to coat the stone, which is then dumped into the planting pit. The stone is placed under the sidewalk, curb, and/or street. This process was patented by Cornell University's Research Foundation. CU-Structural Soil® can safely support pavements and sidewalks and is designed to provide ample rooting area for street trees, thereby decreasing tree mortality and sidewalk failure. The large pores between each stone encourage root growth deep under the sidewalk while the rock provides a stable base for the sidewalk.
It is marketed under the name CU-Structural Soil® or CU-Soil. Properly manufactured and installed, CU-Structural Soil® provides an excellent rooting environment with ample air and water movement, nutrient exchange and beneficial biological activity to promote a vigorous life for street trees. The installation process is patented by Cornell University and administered by Amereq. CU-Soil is produced by companies across the US, Canada, and the UK who are licensed by Amereq to use tested and approved local material which meets the Cornell University specifications. Using local producers and materials helps to keep the costs down and reduces the carbon footprint.
Also, while CU-Structural Soil® will not only aid in filtration, due to its porous nature, it also can act as a stormwater reservoir, slowing the flow of water into the storm drainage system. CU-Structural Soil® is sustainable because it is rarely shipped over 100 miles (160 k.) from the planting site, thereby reducing the carbon dioxide (CO2) emissions released during material delivery and the installation and tree planting process.
CU-Structural soils are only recommended for new sidewalk and tree planting installations.
Stockholm Structural Soil
Stockholm structural soil is similar to CU-Soil. A deep trench or pit is dug 30 – 40 in. (0.8-1 m.) deep and uniformly sized rocks are placed at the bottom 24 inches (60 cm) deep and when the first layer of stone is compacted, soil is lightly sprinkled on top of the rocks. The soil is then washed into the voids between each stone with high pressure water. A second layer of stone is added on top of the previous layer and soil with humus is washed in to this layer of stone. The process is repeated until all the voids in the rock are filled with soil. A slow-release fertilizer is sprinkled over the stone/soil mix of each layer.
On the top of the compacted stone, corner pads are placed for the installation of a square precast concrete box for the tree. The bottom and holes on the side of the box are open to the compacted stone/soil mix. When the project is completed, the trees are planted in the box so the roots may grow down and into the stone/soil planting area. If the tree needs replacement, the old tree is easily removed from the concrete box and a new tree planted in the same box.
When the structural soil has reached the finished grade, a layer of finely crushed stone is laid on the top to level off the top of the stone. Geotextile fabric is laid over the entire planting pit followed by a base material for the pavement and the surface layer for the specific paving of the road, sidewalk, or bike path to be installed. The fabric prevents the pavement base material from migrating down into the planting soil mix and prevents roots from growing up into the base material.
Structural soils are soils that are specially designed with stone mixed into soils that are modified to provide nutrients, pore space, and porosity to accommodate root growth while also allowing for compaction to support pavement and traffic on the surface, without settling. There are proprietary structural soil mixes available as well as various non-proprietary mixes that have been used in many municipalities. They may also be used as fill material around existing roots in areas where a sidewalk will be replaced, if adequate structural soil depth can be obtained. Structural soils are packed on a firm base 2 – 3 feet (60-90 cm) below the surface and they allow root growth in the pores between the stones. The surface pavement can be any of the products mentioned in this article.
CU-Structural Soil®
CU-Structural Soil® is built with uniformly sized crushed stone coated with clay loam soil and a hydrogel tackifier stabilizing agent. The soil and hydrogel are mixed to coat the stone, which is then dumped into the planting pit. The stone is placed under the sidewalk, curb, and/or street. This process was patented by Cornell University's Research Foundation. CU-Structural Soil® can safely support pavements and sidewalks and is designed to provide ample rooting area for street trees, thereby decreasing tree mortality and sidewalk failure. The large pores between each stone encourage root growth deep under the sidewalk while the rock provides a stable base for the sidewalk.
It is marketed under the name CU-Structural Soil® or CU-Soil. Properly manufactured and installed, CU-Structural Soil® provides an excellent rooting environment with ample air and water movement, nutrient exchange and beneficial biological activity to promote a vigorous life for street trees. The installation process is patented by Cornell University and administered by Amereq. CU-Soil is produced by companies across the US, Canada, and the UK who are licensed by Amereq to use tested and approved local material which meets the Cornell University specifications. Using local producers and materials helps to keep the costs down and reduces the carbon footprint.
Also, while CU-Structural Soil® will not only aid in filtration, due to its porous nature, it also can act as a stormwater reservoir, slowing the flow of water into the storm drainage system. CU-Structural Soil® is sustainable because it is rarely shipped over 100 miles (160 k.) from the planting site, thereby reducing the carbon dioxide (CO2) emissions released during material delivery and the installation and tree planting process.
CU-Structural soils are only recommended for new sidewalk and tree planting installations.
Stockholm Structural Soil
Stockholm structural soil is similar to CU-Soil. A deep trench or pit is dug 30 – 40 in. (0.8-1 m.) deep and uniformly sized rocks are placed at the bottom 24 inches (60 cm) deep and when the first layer of stone is compacted, soil is lightly sprinkled on top of the rocks. The soil is then washed into the voids between each stone with high pressure water. A second layer of stone is added on top of the previous layer and soil with humus is washed in to this layer of stone. The process is repeated until all the voids in the rock are filled with soil. A slow-release fertilizer is sprinkled over the stone/soil mix of each layer.
On the top of the compacted stone, corner pads are placed for the installation of a square precast concrete box for the tree. The bottom and holes on the side of the box are open to the compacted stone/soil mix. When the project is completed, the trees are planted in the box so the roots may grow down and into the stone/soil planting area. If the tree needs replacement, the old tree is easily removed from the concrete box and a new tree planted in the same box.
When the structural soil has reached the finished grade, a layer of finely crushed stone is laid on the top to level off the top of the stone. Geotextile fabric is laid over the entire planting pit followed by a base material for the pavement and the surface layer for the specific paving of the road, sidewalk, or bike path to be installed. The fabric prevents the pavement base material from migrating down into the planting soil mix and prevents roots from growing up into the base material.
Alternative Sidewalks Materials – Soil Cells
The soil cells are placed on a compacted surface at the bottom of the large planting vault. The weight of the pavement and any surface loading is transferred downward through the cells to the compacted surface at the bottom of the planting vault while the soil within the cells remains loose and suitable for a tree's root growth. The optimum amount of high-quality soil for tree root growth also provides a stormwater reservoir function. Excess water infiltration into the vault escapes through a drain at the bottom of the vault. During the installation of the soil cells, they should be arranged to avoid the existing or proposed utilities in case there is a need to dig the utilities up in an emergency or for routine maintenance.
There are several manufacturers of soil cells that are currently available, each with their own positive and negative features. Soil cells tend to be somewhat expensive when compared to the other options discussed in this article. Despite the cost however, they are also excellent options for producing fast growing, healthy trees in the middle of the city. The most well-known soil cell products available in the U.S. and Canada are Silva Cells, StrataCells, StrataVaults and WaterBlock. Other brands and designs are coming on to the market every year.
1. Silva Cells – which are a type of soil cell that look like industrial shelving and can be stacked to various
heights before they are topped with a deck. The ideal soil is deposited on the open shelves. Each stack of
Silva Cells is completely open along both vertical and horizontal faces and the stacks stand independently,
making future excavation and repairs simple and avoid any impact to the rest of the system. They can be
spread laterally as wide as necessary. They are plastic units manufactured by Deep Root.
2. StrataCells – made by Citygreen, are made from 100% polypropylene resin that can be stacked to various
heights and cut and shaped as required during construction. They are one component modules that
resemble large flower containers and clip together to form a skeletal matrix which is then filled and
surrounded with the ideal soil. This makes for a quick and easy installation compared to multiple component
systems. They can also be spread laterally as wide as necessary including under ligt vehicular traffic areas
with minimal surface coverage. StrataCells provide 94% of the total volume filled with soil that is available for
root growth.
3. StrataVault system - employs advanced design geometry and polypropylene resin to produce a very strong
skeletal matrix. Test results have been verified by consulting engineers to provide adequate support for heavy
pavement loads. StrataVault is more advanced than StrataCells and are also made by Citygreen.
4. WaterBlock – soil cell consists of a domed shell at the top of long poles that extend to the bottom of the
planting pit that is filled with ideal topsoil containing lots of organic matter. An option has the domed shells
placed on large square pads on top of the topsoil. A concrete surface is poured over the top of the domes
which will support various uses and weights depending on the design options. The load of traffic is taken up
by the concrete construction and not by the soil in which the tree's roots are growing. The companies that
offer this option are limited to Europe and recently in Western Canada.
5. Tree Box High Performance - consists of a large concrete block in the shape of an "X", stacked vertically, on
which a pre-cast concrete cap or slab is placed that will carry very heavy traffic loads. The area between the
X's is filled with high quality loam for root growth. The non-compacted soil can be refilled as necessary. This
company supplies their products only in England and the Netherlands.
Each soil cell supports the hardscape and can meet AASHTO H-20 loading requirements with proper engineered surfaces above, as reviewed by a qualified engineer. Irrigation and aeration systems as well as local utilities can be integrated into the soil cell layout. Water can also enter the system through pervious paving, drains, catch basins, and the opening around the tree trunk.
All of these planting techniques are more expensive than conventional urban street tree planting sites in existing soil. However, research has demonstrated that they are much better in terms of supporting tree growth. While they are not practical for infrastructure replacement projects around existing trees, they should be considered for every new sidewalk and street tree construction project that has the space for creating at least 100 square feet (9.3 square meters) of soil space or can potentially connect tree planting sites with linear soil volumes. For more information on cost comparison, see LA Course 'About Urban Tree Planting'.
The use of soil cells should provide more soil volume, more available soil space, and improved tree growth. The research also supports that less compaction improves tree growth. The only things to watch are the sidewalk openings and trunk flare expansion.
Continuous Trenches
Continuous trenches may be used to provide extra soil volume for root growth along a street by allowing trees to share soil space. A trench is constructed to connect with several tree pits. The trench is built in the tree lawn and not under the sidewalk. The soil is usually sufficient for good growth.
The difficulty with this option is the need for adequate structural support for pavement above the trench. The pavement must be supported by bridging to appropriate supports on either side of the trench or by the inclusion of structural support elements (such as soil cells or structural soil) that can accommodate planting soil and root growth within the trench. This option does provide adequate soil volume to improve or repair soil conditions without extensive damage to existing trees.
Root paths
Root paths are narrow trenches, roughly 4” (10 cm) wide by 1’ (30 cm) deep, installed in a compacted subgrade before the gravel base for pavement is added. A commercially available strip drain material could be added to the trench to support drainage. Root paths should be installed for new plantings during construction, at the time of subgrade preparation and before the paved surface is installed. Root paths extend radially from the tree pit and may connect to adjacent tree pits, and/or other nearby planting areas such as native soil, lawns, or open space on the opposite side of the sidewalk from the street.
Root paths may be most applicable in urban areas where tree roots need to be directed around utilities and planting space is limited.
The soil cells are placed on a compacted surface at the bottom of the large planting vault. The weight of the pavement and any surface loading is transferred downward through the cells to the compacted surface at the bottom of the planting vault while the soil within the cells remains loose and suitable for a tree's root growth. The optimum amount of high-quality soil for tree root growth also provides a stormwater reservoir function. Excess water infiltration into the vault escapes through a drain at the bottom of the vault. During the installation of the soil cells, they should be arranged to avoid the existing or proposed utilities in case there is a need to dig the utilities up in an emergency or for routine maintenance.
There are several manufacturers of soil cells that are currently available, each with their own positive and negative features. Soil cells tend to be somewhat expensive when compared to the other options discussed in this article. Despite the cost however, they are also excellent options for producing fast growing, healthy trees in the middle of the city. The most well-known soil cell products available in the U.S. and Canada are Silva Cells, StrataCells, StrataVaults and WaterBlock. Other brands and designs are coming on to the market every year.
1. Silva Cells – which are a type of soil cell that look like industrial shelving and can be stacked to various
heights before they are topped with a deck. The ideal soil is deposited on the open shelves. Each stack of
Silva Cells is completely open along both vertical and horizontal faces and the stacks stand independently,
making future excavation and repairs simple and avoid any impact to the rest of the system. They can be
spread laterally as wide as necessary. They are plastic units manufactured by Deep Root.
2. StrataCells – made by Citygreen, are made from 100% polypropylene resin that can be stacked to various
heights and cut and shaped as required during construction. They are one component modules that
resemble large flower containers and clip together to form a skeletal matrix which is then filled and
surrounded with the ideal soil. This makes for a quick and easy installation compared to multiple component
systems. They can also be spread laterally as wide as necessary including under ligt vehicular traffic areas
with minimal surface coverage. StrataCells provide 94% of the total volume filled with soil that is available for
root growth.
3. StrataVault system - employs advanced design geometry and polypropylene resin to produce a very strong
skeletal matrix. Test results have been verified by consulting engineers to provide adequate support for heavy
pavement loads. StrataVault is more advanced than StrataCells and are also made by Citygreen.
4. WaterBlock – soil cell consists of a domed shell at the top of long poles that extend to the bottom of the
planting pit that is filled with ideal topsoil containing lots of organic matter. An option has the domed shells
placed on large square pads on top of the topsoil. A concrete surface is poured over the top of the domes
which will support various uses and weights depending on the design options. The load of traffic is taken up
by the concrete construction and not by the soil in which the tree's roots are growing. The companies that
offer this option are limited to Europe and recently in Western Canada.
5. Tree Box High Performance - consists of a large concrete block in the shape of an "X", stacked vertically, on
which a pre-cast concrete cap or slab is placed that will carry very heavy traffic loads. The area between the
X's is filled with high quality loam for root growth. The non-compacted soil can be refilled as necessary. This
company supplies their products only in England and the Netherlands.
Each soil cell supports the hardscape and can meet AASHTO H-20 loading requirements with proper engineered surfaces above, as reviewed by a qualified engineer. Irrigation and aeration systems as well as local utilities can be integrated into the soil cell layout. Water can also enter the system through pervious paving, drains, catch basins, and the opening around the tree trunk.
All of these planting techniques are more expensive than conventional urban street tree planting sites in existing soil. However, research has demonstrated that they are much better in terms of supporting tree growth. While they are not practical for infrastructure replacement projects around existing trees, they should be considered for every new sidewalk and street tree construction project that has the space for creating at least 100 square feet (9.3 square meters) of soil space or can potentially connect tree planting sites with linear soil volumes. For more information on cost comparison, see LA Course 'About Urban Tree Planting'.
The use of soil cells should provide more soil volume, more available soil space, and improved tree growth. The research also supports that less compaction improves tree growth. The only things to watch are the sidewalk openings and trunk flare expansion.
Continuous Trenches
Continuous trenches may be used to provide extra soil volume for root growth along a street by allowing trees to share soil space. A trench is constructed to connect with several tree pits. The trench is built in the tree lawn and not under the sidewalk. The soil is usually sufficient for good growth.
The difficulty with this option is the need for adequate structural support for pavement above the trench. The pavement must be supported by bridging to appropriate supports on either side of the trench or by the inclusion of structural support elements (such as soil cells or structural soil) that can accommodate planting soil and root growth within the trench. This option does provide adequate soil volume to improve or repair soil conditions without extensive damage to existing trees.
Root paths
Root paths are narrow trenches, roughly 4” (10 cm) wide by 1’ (30 cm) deep, installed in a compacted subgrade before the gravel base for pavement is added. A commercially available strip drain material could be added to the trench to support drainage. Root paths should be installed for new plantings during construction, at the time of subgrade preparation and before the paved surface is installed. Root paths extend radially from the tree pit and may connect to adjacent tree pits, and/or other nearby planting areas such as native soil, lawns, or open space on the opposite side of the sidewalk from the street.
Root paths may be most applicable in urban areas where tree roots need to be directed around utilities and planting space is limited.
Alternative Sidewalks Materials – Other Pavements
Foam Layer
A layer of rigid foam is added between existing roots and new pavement to support the pavement and help prevent movement or damage by roots and poor soil components. The foam layer is used below the sidewalk pavement to prevent future root damage and may be more effective when combined with other tools, such as root barriers. As a root increases in diameter, the root compresses the foam to some degree before the growth affects the pavement slab. This option can be used at the time of sidewalk repair, replacement or new construction. It is best used on mature trees where the root growth is slow. Foam boards are commonly available from 1/2 to 2 inches (1.25 to 5 cm) thick. Two 2 inch (5 cm) thick boards can be glued together to provide a 4 inch (10 cm) base for the greatest protection from root growth damage.
Suspended Pavement
Suspended pavement consists of a system of four long, metal screws (similar to helical piles) that are screwed into the soil at the four corners of a concrete sidewalk section. The screws are connected together by a metal frame and the concrete sidewalk section is set on the metal frame. The soil underneath the sidewalk is uncompacted and excellent soil for growing trees. At this time, it is not known if this system, common in Europe, will meet current US engineering standards such as AASHTO H-20.
Suspended pavement systems may be used in new tree plantings where there is not an adequate volume of soil available for tree root growth. These systems provide structural support for pavement while allowing the use of planting soil as fill, which provides space for roots to grow, promoting healthy trees and preventing pavement damage by roots near the surface. This system also allows a tree to borrow root space from adjoining trees as well as the planting strip. Suspended pavement should be used for new tree plantings, particularly in urban conditions with limited planting area within the streetscape.
Bridging
Similar to suspended pavements is a system called bridging. Bridging can provide grade separation between a sidewalk and the root zone of a tree. Various bridging techniques exist, including pier and beam bridges, cantilevered sections, and boardwalks. Bridging techniques are used to provide space for tree roots to grow in soil without lifting or otherwise damaging the adjacent sidewalk. The “bridge” section of the sidewalk supports itself, from the ends, on piers, without the need for compacted subgrade below it. Various deck materials may be used, including concrete, or steel panels with an appropriate non-slip finish.
Bridges are used to preserve high-value trees and also meet sidewalk accessibility requirements. They may also be used to replace a damaged sidewalk if other measures would not allow for a more basic sidewalk repair and continued root damage would be likely.
Bridges are generally a short distance above the soil however, if the drop to the adjacent grade is greater than 18 inches (50 cm), then the bridge would require a handrail. Roots that have raised and cracked the sidewalk can be bridged with metal, wood decking, or other materials. The trees remain healthy following this treatment because the roots do not have to be cut.
Foam Layer
A layer of rigid foam is added between existing roots and new pavement to support the pavement and help prevent movement or damage by roots and poor soil components. The foam layer is used below the sidewalk pavement to prevent future root damage and may be more effective when combined with other tools, such as root barriers. As a root increases in diameter, the root compresses the foam to some degree before the growth affects the pavement slab. This option can be used at the time of sidewalk repair, replacement or new construction. It is best used on mature trees where the root growth is slow. Foam boards are commonly available from 1/2 to 2 inches (1.25 to 5 cm) thick. Two 2 inch (5 cm) thick boards can be glued together to provide a 4 inch (10 cm) base for the greatest protection from root growth damage.
Suspended Pavement
Suspended pavement consists of a system of four long, metal screws (similar to helical piles) that are screwed into the soil at the four corners of a concrete sidewalk section. The screws are connected together by a metal frame and the concrete sidewalk section is set on the metal frame. The soil underneath the sidewalk is uncompacted and excellent soil for growing trees. At this time, it is not known if this system, common in Europe, will meet current US engineering standards such as AASHTO H-20.
Suspended pavement systems may be used in new tree plantings where there is not an adequate volume of soil available for tree root growth. These systems provide structural support for pavement while allowing the use of planting soil as fill, which provides space for roots to grow, promoting healthy trees and preventing pavement damage by roots near the surface. This system also allows a tree to borrow root space from adjoining trees as well as the planting strip. Suspended pavement should be used for new tree plantings, particularly in urban conditions with limited planting area within the streetscape.
Bridging
Similar to suspended pavements is a system called bridging. Bridging can provide grade separation between a sidewalk and the root zone of a tree. Various bridging techniques exist, including pier and beam bridges, cantilevered sections, and boardwalks. Bridging techniques are used to provide space for tree roots to grow in soil without lifting or otherwise damaging the adjacent sidewalk. The “bridge” section of the sidewalk supports itself, from the ends, on piers, without the need for compacted subgrade below it. Various deck materials may be used, including concrete, or steel panels with an appropriate non-slip finish.
Bridges are used to preserve high-value trees and also meet sidewalk accessibility requirements. They may also be used to replace a damaged sidewalk if other measures would not allow for a more basic sidewalk repair and continued root damage would be likely.
Bridges are generally a short distance above the soil however, if the drop to the adjacent grade is greater than 18 inches (50 cm), then the bridge would require a handrail. Roots that have raised and cracked the sidewalk can be bridged with metal, wood decking, or other materials. The trees remain healthy following this treatment because the roots do not have to be cut.
Alternative Sidewalks Materials – Temporary Paths
Problems with roots from municipal trees begin when shallow-rooted trees are forced to grow in that ribbon of grass between the street and the sidewalk (called a tree lawn in this article). Problems also occur when large trees grow toward the curb or sidewalk and the tree's flare lifts the curb or sidewalk. However, sidewalk damage is often caused by the soil in its expansion and contraction features, than by the tree roots. Regardless of the reason, tree roots are often the blame even when soils are at fault. When the problem is reported to the city and action is necessary, it is usually a good idea to accept public involvement opportunities before tree and sidewalk work occurs. Since the public is going to call for the tree to be removed, they must be given the options to consider soil alternatives. During the time it takes for decisions to be made, a temporary path may be necessary. Here are some options.
Crushed Gravel
Decomposed granite, or small crushed gravel, may be used as a path or sidewalk surface in quiet residential areas. It may also be used as a finished surface or an inorganic mulch on top of the planting soil where a tree is growing in areas of low pedestrian traffic. Crushed gravel may be used in higher-traffic areas as a means of providing a walkable but flexible surface in the tree pit area. It is not suitable for areas of heavy pedestrian traffic or as an ADA-compliant path. This product is common in the Western states.
Gravel paths are also suitable for new pedestrian routes, as a temporary sidewalk until a pathway is clearly defined and a permanent sidewalk can be installed. However, gravel paths will require more regular maintenance than asphalt or concrete pavement materials.
Modified Gravel Layer
An open-graded gravel base course may be applied and compacted under the sidewalk pavement to discourage root growth directly under the pavement and reduce likelihood of sidewalk damage. This requires extra depth in the pavement profile to include at least a 4” (10 cm) layer of gravel. The extra depth of excavation to install the gravel layer might damage critical existing roots. The thickness of the gravel layer can be adjusted around existing tree roots.
Washed Gravel
Researchers have found that a washed gravel layer at least 4 inches (10 cm) thick under the sidewalk pavement may reduce damage by tree roots. Washed gravel is washed to remove any dirt or dust and then graded to 1/2 inch - 3/4 inch (1–2 cm) size stone. The gravel does not hold water, and it lacks soil and nutrients in the voids, thereby discouraging root growth while supporting the pavement after compaction.
Hoggin
Hoggin is used to create a soft-looking but hard-wearing path of gravel that is often found in the UK. It is usually known as either as "dug gravel" or "pit-run gravel". Hoggin contains a mix of sand and gravel and at least 30% clay or fine dirt that acts as a binder. Hoggin is used as a road base or for surface work on low traffic ways. This material, when compacted makes a very stable surface and seldom needs raking. Hoggin should generally contain no particles larger than 1-1/2 in. (40mm) with all the different sizes of particles evenly mixed.
Hoggin is usually applied as a permeable surface layer, over compacted gravel or crushed stone. It should be spread evenly over a suitable base and compacted with a 9 ton (8 metric tonne) vibrating roller to the required thickness. Water must be added at the time of rolling to bring the clay particles to the surface to seal all the stone products. If the thickness has to be greater than 3 inches (80mm) it should be laid and compacted in successive layers not exceeding 3 inches. Usually a depth of around 3-4 inches (80/100mm) after compaction will be suitable for pedestrian use. It is important that the Hoggin does not sit in water. Edge restraints are essential to keep the Hoggin from migrating.
Similar to Hoggin is PolyPavement. This is a proprietary blend of Acrylic Polymers that are 100% safe for human exposure. The product is sprayed onto a fine graded and compacted natural soil path and the soil turns as hard as a pavement. PolyPavement incorporates natural, indigenous soil, and has a much higher SRI (Solar Reflective Index) than black asphalt, and can be up to 100% recycled, it is favored by many green builders and architects. The vapors are not harmful and no protective clothing or respirators are required when PolyPavement is being spray-applied.
Mulch
Mulch is sometimes suitable for a temporary sidewalk. It has a longevity of no more than two years. Mulch can be made from wood chips, shredded bark, ground corn cobs, and other ground-up organic matter. Mulch may be used at the surface to promote tree health, suppress the growth of weeds and grasses that compete with a tree for moisture, and encourage root growth in appropriate areas. Mulch should be used over any soil left exposed in the planting area or areas that would otherwise not be planted. New tree plantings should be mulched with a product appropriate to the location. For example, gravel mulch is typically used in tree pits in neighborhood commercial areas and concrete sidewalks.
Wood Chip Mulch
Wood chip mulch provides a natural appearing path for nature walks, and is sometimes suitable for a temporary sidewalk. Wood chip mulch helps prevent soil compaction and allows water to infiltrate into soils in planting areas. Wood chips or other mulches containing compost can contribute beneficial humic acid to the tree’s root zone. Mulch should be applied over bare soil and to an existing tree's root zone where the soil has settled or the mulch layer has become depleted and there is exposed bare soil. Keep all mulch 4 inches (10 cm) away from tree trunks.
Problems with roots from municipal trees begin when shallow-rooted trees are forced to grow in that ribbon of grass between the street and the sidewalk (called a tree lawn in this article). Problems also occur when large trees grow toward the curb or sidewalk and the tree's flare lifts the curb or sidewalk. However, sidewalk damage is often caused by the soil in its expansion and contraction features, than by the tree roots. Regardless of the reason, tree roots are often the blame even when soils are at fault. When the problem is reported to the city and action is necessary, it is usually a good idea to accept public involvement opportunities before tree and sidewalk work occurs. Since the public is going to call for the tree to be removed, they must be given the options to consider soil alternatives. During the time it takes for decisions to be made, a temporary path may be necessary. Here are some options.
Crushed Gravel
Decomposed granite, or small crushed gravel, may be used as a path or sidewalk surface in quiet residential areas. It may also be used as a finished surface or an inorganic mulch on top of the planting soil where a tree is growing in areas of low pedestrian traffic. Crushed gravel may be used in higher-traffic areas as a means of providing a walkable but flexible surface in the tree pit area. It is not suitable for areas of heavy pedestrian traffic or as an ADA-compliant path. This product is common in the Western states.
Gravel paths are also suitable for new pedestrian routes, as a temporary sidewalk until a pathway is clearly defined and a permanent sidewalk can be installed. However, gravel paths will require more regular maintenance than asphalt or concrete pavement materials.
Modified Gravel Layer
An open-graded gravel base course may be applied and compacted under the sidewalk pavement to discourage root growth directly under the pavement and reduce likelihood of sidewalk damage. This requires extra depth in the pavement profile to include at least a 4” (10 cm) layer of gravel. The extra depth of excavation to install the gravel layer might damage critical existing roots. The thickness of the gravel layer can be adjusted around existing tree roots.
Washed Gravel
Researchers have found that a washed gravel layer at least 4 inches (10 cm) thick under the sidewalk pavement may reduce damage by tree roots. Washed gravel is washed to remove any dirt or dust and then graded to 1/2 inch - 3/4 inch (1–2 cm) size stone. The gravel does not hold water, and it lacks soil and nutrients in the voids, thereby discouraging root growth while supporting the pavement after compaction.
Hoggin
Hoggin is used to create a soft-looking but hard-wearing path of gravel that is often found in the UK. It is usually known as either as "dug gravel" or "pit-run gravel". Hoggin contains a mix of sand and gravel and at least 30% clay or fine dirt that acts as a binder. Hoggin is used as a road base or for surface work on low traffic ways. This material, when compacted makes a very stable surface and seldom needs raking. Hoggin should generally contain no particles larger than 1-1/2 in. (40mm) with all the different sizes of particles evenly mixed.
Hoggin is usually applied as a permeable surface layer, over compacted gravel or crushed stone. It should be spread evenly over a suitable base and compacted with a 9 ton (8 metric tonne) vibrating roller to the required thickness. Water must be added at the time of rolling to bring the clay particles to the surface to seal all the stone products. If the thickness has to be greater than 3 inches (80mm) it should be laid and compacted in successive layers not exceeding 3 inches. Usually a depth of around 3-4 inches (80/100mm) after compaction will be suitable for pedestrian use. It is important that the Hoggin does not sit in water. Edge restraints are essential to keep the Hoggin from migrating.
Similar to Hoggin is PolyPavement. This is a proprietary blend of Acrylic Polymers that are 100% safe for human exposure. The product is sprayed onto a fine graded and compacted natural soil path and the soil turns as hard as a pavement. PolyPavement incorporates natural, indigenous soil, and has a much higher SRI (Solar Reflective Index) than black asphalt, and can be up to 100% recycled, it is favored by many green builders and architects. The vapors are not harmful and no protective clothing or respirators are required when PolyPavement is being spray-applied.
Mulch
Mulch is sometimes suitable for a temporary sidewalk. It has a longevity of no more than two years. Mulch can be made from wood chips, shredded bark, ground corn cobs, and other ground-up organic matter. Mulch may be used at the surface to promote tree health, suppress the growth of weeds and grasses that compete with a tree for moisture, and encourage root growth in appropriate areas. Mulch should be used over any soil left exposed in the planting area or areas that would otherwise not be planted. New tree plantings should be mulched with a product appropriate to the location. For example, gravel mulch is typically used in tree pits in neighborhood commercial areas and concrete sidewalks.
Wood Chip Mulch
Wood chip mulch provides a natural appearing path for nature walks, and is sometimes suitable for a temporary sidewalk. Wood chip mulch helps prevent soil compaction and allows water to infiltrate into soils in planting areas. Wood chips or other mulches containing compost can contribute beneficial humic acid to the tree’s root zone. Mulch should be applied over bare soil and to an existing tree's root zone where the soil has settled or the mulch layer has become depleted and there is exposed bare soil. Keep all mulch 4 inches (10 cm) away from tree trunks.
Alternative Sidewalks Materials – Soils
Soil modification
Soil modification includes improvements and amendments to soils in the tree lawn near trees to encourage root growth in planted areas rather than under the pavement. The use of specific beneficial soils to replace the existing soils or to improve conditions for tree root growth should be encouraged.
Organic Soil Treatment
One recommended amendment is humic acid, an organic soil treatment that can loosen tightly packed soils to improve water infiltration and help foster root growth deeper into the soil. The addition of a high-quality, biologically-active and pathogen-free compost in soil areas where root growth is desirable, is also recommended.
Soil Profile Rebuilding
Another modification is called the Soil Profile Rebuilding Method. This method, developed by Susan Day and her team at Virginia Tech uses a backhoe with a tined bucket to physically break up the compacted soil and incorporate compost to a depth of two feet (60 cm). Then four inches (12 cm) of topsoil are applied and rototilled to a depth of six to eight inches (15 to 20 cm), followed by planting trees whose roots help the soil continue to develop the biological activity. Using the soil profile rebuilding method, trees grew more rapidly and the soil depth at 6 to 12 inches (15–30 cm) below the surface, had a reduced soil bulk density. The trunk cross-sectional area and canopy area of these trees matched or surpassed similar trees planted in undisturbed agricultural land. The method reduced compaction and set the stage for long-term improvement of soil quality. This method improved the soil's potential as a tool for stormwater mitigation. This method also accelerates establishment and growth of urban trees planted in urban soils illustrating that the below-ground environment should be a key component in policy and decision making.
Holes for Roots
At suburban locations where better soils exist on the opposite side of the sidewalk, efforts can be made to create root corridors deep below the pavement. Once the roots grow through the corridor, they can expand into the better soil and the tree will thrive for many years. A very effective way to encourage the roots to grow toward the opposite side of the sidewalk is to bore some holes with a high pressure water hose or a very long drill under the sidewalk at the time of installation. The one to two inch (3-5 cm) diameter holes start from the bottom of the planting pit and go through the compacted soil under the sidewalk. One alternative is to make the series of holes with a steel bar driven under the sidewalk with a hammer and pulled back out. At the end of the hole, the root will expand into the better, less compacted soil. The hole should not be filled with anything so the tree root grows through the hole within one growing season.
Soil modification
Soil modification includes improvements and amendments to soils in the tree lawn near trees to encourage root growth in planted areas rather than under the pavement. The use of specific beneficial soils to replace the existing soils or to improve conditions for tree root growth should be encouraged.
Organic Soil Treatment
One recommended amendment is humic acid, an organic soil treatment that can loosen tightly packed soils to improve water infiltration and help foster root growth deeper into the soil. The addition of a high-quality, biologically-active and pathogen-free compost in soil areas where root growth is desirable, is also recommended.
Soil Profile Rebuilding
Another modification is called the Soil Profile Rebuilding Method. This method, developed by Susan Day and her team at Virginia Tech uses a backhoe with a tined bucket to physically break up the compacted soil and incorporate compost to a depth of two feet (60 cm). Then four inches (12 cm) of topsoil are applied and rototilled to a depth of six to eight inches (15 to 20 cm), followed by planting trees whose roots help the soil continue to develop the biological activity. Using the soil profile rebuilding method, trees grew more rapidly and the soil depth at 6 to 12 inches (15–30 cm) below the surface, had a reduced soil bulk density. The trunk cross-sectional area and canopy area of these trees matched or surpassed similar trees planted in undisturbed agricultural land. The method reduced compaction and set the stage for long-term improvement of soil quality. This method improved the soil's potential as a tool for stormwater mitigation. This method also accelerates establishment and growth of urban trees planted in urban soils illustrating that the below-ground environment should be a key component in policy and decision making.
Holes for Roots
At suburban locations where better soils exist on the opposite side of the sidewalk, efforts can be made to create root corridors deep below the pavement. Once the roots grow through the corridor, they can expand into the better soil and the tree will thrive for many years. A very effective way to encourage the roots to grow toward the opposite side of the sidewalk is to bore some holes with a high pressure water hose or a very long drill under the sidewalk at the time of installation. The one to two inch (3-5 cm) diameter holes start from the bottom of the planting pit and go through the compacted soil under the sidewalk. One alternative is to make the series of holes with a steel bar driven under the sidewalk with a hammer and pulled back out. At the end of the hole, the root will expand into the better, less compacted soil. The hole should not be filled with anything so the tree root grows through the hole within one growing season.
Alternative Designs
Curb Bulb
A curb bulb, also called sidewalk bulge, is a large sidewalk bulge at downtown street intersections. It is designed to provide a shorter walking distance for pedestrians walking across the street and encourage drivers to slow down because of the narrower intersecting street. The sidewalk bulge has become very popular because of the Complete Streets program. In these situations, trees may be planted within the bulb area to provide a larger area of soils suitable for a tree's root growth. Soil cells are highly recommended in these situations because of the lack of soil volume. Designs that include trees and landscaping must ensure that proper sight lines are maintained. Trees that normally have ascending branches are encouraged while trees with low hanging branches are prohibited from these areas.
Relocating the curb will not work if there are drainage and other infrastructure problems. Traffic concerns may also prohibit the use of bulbs. Other street uses may be planned for the existing roadway width such as bicycle and pedestrian facilities.
Curb Realignment
Curb realignment involves shifting the curb location for a significant distance such as along an entire block in order to widen the planting strip and provide more space for trees. This requires that there is space in the right-of-way to create additional width in the planting strip, while taking space from the traffic and the street.
Efforts like this are usually done as part of a large-scale street repair or reconstruction effort such as a capital improvement project or a Complete Streets project to provide additional space for new trees, provide a comfortable setting for pedestrians and improve the traffic flow. Curb realignment will require traffic studies and engineering because of the other impacts to parking, transit, and other facilities.
Curving or Offset Sidewalks
Curbs, streets, and sidewalks can be designed to go around existing trees with the goal to increase the distance from the tree to the edge of the pavement. This can be done by either narrowing the sidewalk or rerouting it around the tree. This technique slows traffic down and keeps the tree healthy because roots do not have to be cut. These locations also provide an opportunity for enhancing the landscape with additional trees, shrubs, and flowers. This effort also protects the existing tree especially if the tree has high value.
The offset can potentially be combined with an easement to locate the sidewalk on private property adjacent to the right-of-way. The easement may provide more space for existing or new trees. The width of the easement is site specific. This is useful if adequate planting space is not available in the right-of-way.
Curb Replacement
Curbs are often replaced during road resurfacing, especially if they are damaged or insufficient in height. The alternatives below will reduce the amount of root damage during the replacement process:
Setback Planting
Setback planting is a favorite of many communities, in suburban locations, and involves reversing the sidewalk and grass strip locations, when practical, to reduce the need to root prune and to provide more space for new trees and where there is better, unlimited soil area. If the tree lawn is less than 3 feet (1 m) wide, use this option to plant trees on or near private property at least 3 feet (1 m) beyond the sidewalk. While cities may require public trees to grow on public property, some cities will allow planting on private property so they will be maintained by the property owner and not at city expense.
With the trees near or on private property, the roots have plenty of space to develop in lawn areas. The trees are healthier and they grow faster than the tree next to the street. Research has shown that a tree set behind the sidewalk will achieve a street canopy equal to or faster than the same tree planted beside the street in a narrow tree lawn, and it will live for many years more than the tree near the street.
This setback planting eliminates trees that might suffer from exhaust fumes, damaged trunks and vehicular damage. In addition, salt and snowplow damage are eliminated. The roots will not have to grow in poor quality, compacted soil in a very limited space. Furthermore, there are fewer conflicts with overhead and underground utilities. These factors all result in better tree growth, a longer life span and lower maintenance costs to the city.
This option is not feasible in the downtown or heavily developed areas where there is no existing soil suitable for tree root growth. Also, some states prohibit this type of tree planting on the basis that it puts publicly funded trees on private property. The municipal attorney should be consulted as necessary.
Varied Elevation
Elevation changes may be used to provide spatial separation between the finished grade of the tree planting pit and the surrounding sidewalk or other pavement. This approach will prevent compaction of soil around the tree pit.
When a tree is growing at a raised or lowered site attention to water issues becomes very important. Too much water from the root flare being below the street, curb or sidewalk can be a concern along with these sites tending to accumulate trash and debris and may be more difficult to access. Tree grates or other materials may provide a walkable surface level with adjacent grades over the lowered tree pit area. Planting techniques and details may be similar to tree planting in bioretention planters so that stormwater may flow into them. The design must provide drainage in lowered planting areas to avoid prolonged soil saturation.
When a tree is growing at a raised site, the lack of water may become an issue. Raised trees may need to be surrounded with curbing to prevent soil erosion and a means of providing supplemental water.
Curb Bulb
A curb bulb, also called sidewalk bulge, is a large sidewalk bulge at downtown street intersections. It is designed to provide a shorter walking distance for pedestrians walking across the street and encourage drivers to slow down because of the narrower intersecting street. The sidewalk bulge has become very popular because of the Complete Streets program. In these situations, trees may be planted within the bulb area to provide a larger area of soils suitable for a tree's root growth. Soil cells are highly recommended in these situations because of the lack of soil volume. Designs that include trees and landscaping must ensure that proper sight lines are maintained. Trees that normally have ascending branches are encouraged while trees with low hanging branches are prohibited from these areas.
Relocating the curb will not work if there are drainage and other infrastructure problems. Traffic concerns may also prohibit the use of bulbs. Other street uses may be planned for the existing roadway width such as bicycle and pedestrian facilities.
Curb Realignment
Curb realignment involves shifting the curb location for a significant distance such as along an entire block in order to widen the planting strip and provide more space for trees. This requires that there is space in the right-of-way to create additional width in the planting strip, while taking space from the traffic and the street.
Efforts like this are usually done as part of a large-scale street repair or reconstruction effort such as a capital improvement project or a Complete Streets project to provide additional space for new trees, provide a comfortable setting for pedestrians and improve the traffic flow. Curb realignment will require traffic studies and engineering because of the other impacts to parking, transit, and other facilities.
Curving or Offset Sidewalks
Curbs, streets, and sidewalks can be designed to go around existing trees with the goal to increase the distance from the tree to the edge of the pavement. This can be done by either narrowing the sidewalk or rerouting it around the tree. This technique slows traffic down and keeps the tree healthy because roots do not have to be cut. These locations also provide an opportunity for enhancing the landscape with additional trees, shrubs, and flowers. This effort also protects the existing tree especially if the tree has high value.
The offset can potentially be combined with an easement to locate the sidewalk on private property adjacent to the right-of-way. The easement may provide more space for existing or new trees. The width of the easement is site specific. This is useful if adequate planting space is not available in the right-of-way.
Curb Replacement
Curbs are often replaced during road resurfacing, especially if they are damaged or insufficient in height. The alternatives below will reduce the amount of root damage during the replacement process:
- The excavation bucket should be only 12 inches (30 cm) wide, for space to set forms, stone, or run a curb machine.
- The curb can be removed and replaced at a better height or location.
- The pavement surface can be ground down with a milling machine so a new layer of pavement can be installed without touching the curb and the curb remains as it was.
Setback Planting
Setback planting is a favorite of many communities, in suburban locations, and involves reversing the sidewalk and grass strip locations, when practical, to reduce the need to root prune and to provide more space for new trees and where there is better, unlimited soil area. If the tree lawn is less than 3 feet (1 m) wide, use this option to plant trees on or near private property at least 3 feet (1 m) beyond the sidewalk. While cities may require public trees to grow on public property, some cities will allow planting on private property so they will be maintained by the property owner and not at city expense.
With the trees near or on private property, the roots have plenty of space to develop in lawn areas. The trees are healthier and they grow faster than the tree next to the street. Research has shown that a tree set behind the sidewalk will achieve a street canopy equal to or faster than the same tree planted beside the street in a narrow tree lawn, and it will live for many years more than the tree near the street.
This setback planting eliminates trees that might suffer from exhaust fumes, damaged trunks and vehicular damage. In addition, salt and snowplow damage are eliminated. The roots will not have to grow in poor quality, compacted soil in a very limited space. Furthermore, there are fewer conflicts with overhead and underground utilities. These factors all result in better tree growth, a longer life span and lower maintenance costs to the city.
This option is not feasible in the downtown or heavily developed areas where there is no existing soil suitable for tree root growth. Also, some states prohibit this type of tree planting on the basis that it puts publicly funded trees on private property. The municipal attorney should be consulted as necessary.
Varied Elevation
Elevation changes may be used to provide spatial separation between the finished grade of the tree planting pit and the surrounding sidewalk or other pavement. This approach will prevent compaction of soil around the tree pit.
When a tree is growing at a raised or lowered site attention to water issues becomes very important. Too much water from the root flare being below the street, curb or sidewalk can be a concern along with these sites tending to accumulate trash and debris and may be more difficult to access. Tree grates or other materials may provide a walkable surface level with adjacent grades over the lowered tree pit area. Planting techniques and details may be similar to tree planting in bioretention planters so that stormwater may flow into them. The design must provide drainage in lowered planting areas to avoid prolonged soil saturation.
When a tree is growing at a raised site, the lack of water may become an issue. Raised trees may need to be surrounded with curbing to prevent soil erosion and a means of providing supplemental water.
Manipulation of Existing Sidewalks
Most of these manipulations are short-term solutions and when the sidewalk is more than 1 inch (3 cm) out of alignment with the adjoining panel, plans should be made to fix the problem. There is a limit to how much manipulation can be done at each point on a sidewalk based on pavement thickness and severity of the misalignment. The longevity of the fix will depend on how rapidly additional uplift or subsidence of the pavement occurs. Such repairs rarely correct the problem for the long term, but they do reduce the trip hazard to pedestrians until a permanent solution can be made.
Ramping
If one slab has settled, a ramp can be installed on the adjoining lower slab. The ramp must meet the upper slab as smoothly as possible to avoid other pedestrian safety problems. Temporary fixes consist of using wood, asphalt, or concrete ramps to bridge over the lower slab to match the height of the higher slab created by a problem root. If the sidewalk is made of asphalt, which is flexible, root expansion would occur only in the area of the root itself.
The opposite process of building a ramp is the beveling of the raised slab.
Beveling
Beveling, shaving, or grinding involves cutting down the raised edge of a concrete panel to make a smoother transition and reduce the tripping risk. Beveling the concrete is done with a specialized grinding machine. While this fix is less expensive in terms of labor hours, it is generally limited to slabs with less than 1 inch (2.54 cm) of displacement.
Raising the Slab
If the sidewalk has shifted and one slab is higher than the next, another option available to fix the problem is to raise the lower slab. By drilling a hole in the lower slab, concrete grout can be pumped under the slab to raise it level with the adjoining slab. The grout is a cement and soil mixture that is pumped under pressure below the existing concrete panel. The practice may also be performed using a foam fill material. The mixture fills the void beneath the surface and adds additional support under the sidewalk panel. This option works well if there is just a small section of sidewalk that needs to be replaced and the sidewalk panels remain in good condition. Special equipment is necessary to drill the hole in the sidewalk and another machine to do the pumping of the grout.
This option should not be used if there is an existing tree whose roots would be severely damaged by this process. It is also not cost-effective to do just one sidewalk slab at a time, unless the city already owns the equipment.
Shims or Wedges
Shims or wedges are temporary measures to treat cracked or settling sidewalks in order to reduce tripping hazards and improve accessibility. They are similar to ramping which is for a longer term. Asphalt is typically used to construct a shim. Shims are used in response to an issue that must be immediately addressed. Shims are considered a temporary measure and will require more frequent attention and repair or replacement than a fully-repaired sidewalk
Expansion and Control joints
Expansion joints are transverse joints used to allow movement of concrete sidewalks due to temperature and subgrade moisture variations. The standard interval for expansion joints in sidewalks is a maximum of 28 or 30 feet (8 to 9 m).
When the sidewalk is being installed, use permanent expansion joint material in the concrete so when expansion and contraction occur, any sidewalk failure will be absorbed by the expansion material. When a broken section is replaced, it is easy to correct the problem. Expansion joints may also be used at the ends of replacement slabs, following the removal of damaging roots or the application of other subgrade solutions. This process may confine future damage from new roots to a smaller area.
Control joints are shallow grooves in the pavement designed to control the location of cracking. If a crack occurs it will follow the control joint and not break up the entire slab. The control joint is the weakest part of the sidewalk. In locations where there is poor soil, a wide range of temperature fluctuations, and expansion and contraction of the sidewalk material, a much closer frequency of control joints may be necessary. For example a 5 feet (1.5 m) wide sidewalk should have control joints 5 feet apart. When a break does occur, efforts should be made to prune the existing roots if they caused the break.
Most of these manipulations are short-term solutions and when the sidewalk is more than 1 inch (3 cm) out of alignment with the adjoining panel, plans should be made to fix the problem. There is a limit to how much manipulation can be done at each point on a sidewalk based on pavement thickness and severity of the misalignment. The longevity of the fix will depend on how rapidly additional uplift or subsidence of the pavement occurs. Such repairs rarely correct the problem for the long term, but they do reduce the trip hazard to pedestrians until a permanent solution can be made.
Ramping
If one slab has settled, a ramp can be installed on the adjoining lower slab. The ramp must meet the upper slab as smoothly as possible to avoid other pedestrian safety problems. Temporary fixes consist of using wood, asphalt, or concrete ramps to bridge over the lower slab to match the height of the higher slab created by a problem root. If the sidewalk is made of asphalt, which is flexible, root expansion would occur only in the area of the root itself.
The opposite process of building a ramp is the beveling of the raised slab.
Beveling
Beveling, shaving, or grinding involves cutting down the raised edge of a concrete panel to make a smoother transition and reduce the tripping risk. Beveling the concrete is done with a specialized grinding machine. While this fix is less expensive in terms of labor hours, it is generally limited to slabs with less than 1 inch (2.54 cm) of displacement.
Raising the Slab
If the sidewalk has shifted and one slab is higher than the next, another option available to fix the problem is to raise the lower slab. By drilling a hole in the lower slab, concrete grout can be pumped under the slab to raise it level with the adjoining slab. The grout is a cement and soil mixture that is pumped under pressure below the existing concrete panel. The practice may also be performed using a foam fill material. The mixture fills the void beneath the surface and adds additional support under the sidewalk panel. This option works well if there is just a small section of sidewalk that needs to be replaced and the sidewalk panels remain in good condition. Special equipment is necessary to drill the hole in the sidewalk and another machine to do the pumping of the grout.
This option should not be used if there is an existing tree whose roots would be severely damaged by this process. It is also not cost-effective to do just one sidewalk slab at a time, unless the city already owns the equipment.
Shims or Wedges
Shims or wedges are temporary measures to treat cracked or settling sidewalks in order to reduce tripping hazards and improve accessibility. They are similar to ramping which is for a longer term. Asphalt is typically used to construct a shim. Shims are used in response to an issue that must be immediately addressed. Shims are considered a temporary measure and will require more frequent attention and repair or replacement than a fully-repaired sidewalk
Expansion and Control joints
Expansion joints are transverse joints used to allow movement of concrete sidewalks due to temperature and subgrade moisture variations. The standard interval for expansion joints in sidewalks is a maximum of 28 or 30 feet (8 to 9 m).
When the sidewalk is being installed, use permanent expansion joint material in the concrete so when expansion and contraction occur, any sidewalk failure will be absorbed by the expansion material. When a broken section is replaced, it is easy to correct the problem. Expansion joints may also be used at the ends of replacement slabs, following the removal of damaging roots or the application of other subgrade solutions. This process may confine future damage from new roots to a smaller area.
Control joints are shallow grooves in the pavement designed to control the location of cracking. If a crack occurs it will follow the control joint and not break up the entire slab. The control joint is the weakest part of the sidewalk. In locations where there is poor soil, a wide range of temperature fluctuations, and expansion and contraction of the sidewalk material, a much closer frequency of control joints may be necessary. For example a 5 feet (1.5 m) wide sidewalk should have control joints 5 feet apart. When a break does occur, efforts should be made to prune the existing roots if they caused the break.
Manipulation of Existing Trees
Root Barriers
Root barriers are made of plastic or fiberglass sheeting or they are interlocking metal panels installed from the surface level to a depth of at least 18 inches (45 cm) or more at the interface between a root zone and adjacent paving or other infrastructure. The top must be above the soil and the mulch surface to be effective.
The intent of using root barriers is to physically deflect the roots downward along solid panels, to a depth where they cause no harm to the sidewalk. After the new roots are directed down and under the pavement they would then grow back to the surface, but usually on the other side of the sidewalk. The intent is to install the barriers before the roots become a problem. However, most of the time the barriers are installed after the roots become a problem and the barriers are installed along with a severe root pruning. The problem with the barriers is that the trees are less stable when the roots are pruned and the barriers are installed, than when alternative methods are used to deal with roots before they become problems. Typical placement barriers are vertical, although horizontal root barriers also exist.
Root barriers are made of plastic or fiberglass sheeting or they are interlocking metal panels installed from the surface level to a depth of at least 18 inches (45 cm) or more at the interface between a root zone and adjacent paving or other infrastructure. The top must be above the soil and the mulch surface to be effective.
Root barriers should be installed when a new tree is being planted and there is pavement nearby that may be damaged by future root growth. They should be used in areas where there is adequate soil volumes for the roots to grow.
Chemical Treatments
Chemically treating the sidewalks to retard root invasion is very effective but is an option seldom used any longer because of the potential harm to the environment. An option to this treatment is to use “biobarriers” which are a very popular fabric containing time release capsules of root retardant chemicals attached to the fabric. The fabric is placed in a vertical manner similar to the use of root barriers. However, even the fabric treatments are losing their popularity due to environmental harm from the slow release of the herbicidal chemicals.
Root Pruning
Root pruning is the least desirable solution because it requires the roots to be cut and removed, followed by a sidewalk re-installation. However, since pruned roots will quickly regrow, the problem will come back unless a barrier is used to deflect roots down and away from the sidewalk. Root pruning can also result in root decay that is difficult to monitor and inspect. It is also a very expensive alternative in terms of labor hours.
A thorough tree structural analysis should be conducted before roots are removed. For trees 12 inches in trunk diameter (30 cm) or less, excavation work or root cutting should not occur closer than 3 feet (1 m) from the trunk of the tree. Trees greater than 12 inches (30 cm) in trunk diameter should not be cut closer than the distance measured by the circumference of the trunk or 6 feet (2 m) from the trunk, whichever is more. If roots need to be severed closer than this, tree removal should be considered since it is very likely that tree stability will be affected. Soil excavation work is permitted closer if all the excavation of soil is done by hand or water spade or by a pneumatic soil-excavating tool and no roots greater than 2 inches (5 cm) in diameter are cut. A trained arborist should do all of the root pruning. All root-pruned trees should receive an annual inspection to check their stability and recovery.
If root pruning on large roots is done, the crown of the tree may be reduced to cut back on the wind sail and make the tree less likely to fail. In this case, the pruning should be thinning cuts, and not necessarily on the side of the tree that has had the root severance. Some trees appear to respond with limb shedding on the same side as the root cutting and some trees on the opposite side of the root cutting. Since it may be difficult to predict which branches the tree will shed in response to root cutting, it is better to avoid pruning until necessary and return later to remove any dieback. Ideally, trees should have their crown pruned one or two years prior to the root pruning if this pruning is going to become necessary.
Corrective Pruning
Corrective pruning involves above-ground pruning to establish good structural form and to remove dead or diseased material, weakly attached limbs, and provide clearance for surrounding conditions such as street traffic, bicyclists, pedestrians, overhead utilities, or adjacent buildings. Trees must be in good health and vigorous if they are to be worthy of preservation. Structural pruning must be used to establish good structural form and proactively address potential future limb failure issues. Trees will typically achieve their best form if they are structurally pruned two years after transplanting and every 3 – 6 years after that. For more information on pruning see LA Course 'About Tree Pruning'.
All pruning maintenance performed on street trees should be in accordance with current tree industry standards and supervised by an ISA-certified arborist or an ISA-certified tree worker.
Avoid unnecessary root loss. Sidewalks and curbs are often raised up if large trees are planted in small spaces. Cutting roots so walks and curbs can be reset into their original location causes severe health problems for the trees and can result in the tree falling over long before it becomes mature.
The best way to avoid root loss is to consider using a tree growth regulator to allow the tree root systems to grow slowly and without excessive growth above ground which places an unrealistic demand for water to sustain the tree.
Root Pinning
Root pinning requires water spading or using a pneumatic soil-excavating tool around a tree's roots that are growing toward or under a sidewalk. With the soil under the roots blown away, the roots can be lowered, redirected toward an open landscape, or relocated as necessary to avoid any damage to the sidewalk. The soil is then replaced with CU-Soil or equivalent. The stone/soil mix will keep the roots pinned down and away from the sidewalk. This technique will avoid pruning any roots.
Root Shaving
If a root has caused the sidewalk to rise and the sidewalk must be reinstalled flat and in the same location from where it was removed, the height of the offending roots will need to be lowered. For mature trees, it is best not to cut roots greater than 1 1/2 or 2 inches (5 cm) in diameter even though these are the roots that cause sidewalk problems. It is better to remove the top surface of a root than the entire root. This is done by shaving the root using a chainsaw or debarking tool. Shaved roots do tend to callus quickly, so a layer of rigid foam or pipe insulation foam should be installed between the root surface and the new concrete. The tree should be inspected annually for health and stability.
When reinstalling the sidewalk next to a root cut tree, allow space for callus and trunk diameter growth. Installation of a root barrier or root excluding base (foam board or washed gravel) should also be considered under the new sidewalk to avoid a reoccurrence of the problem.
Aeration Piping
Aeration and perforated water piping may be installed directly under a new sidewalk to help encourage deeper root growth. The perforated pipe provides some air and water into deep layers of soil, particularly where there is a sidewalk at the surface. The pipe may also serve as a means of adding stormwater and drainage into the soil. The addition of water within the perforated piping can further aid in desirable root growth. The pipe may also be placed in structural soil or other fill at least 12” (30 cm) below the finished grade of the sidewalk.
This aeration piping and subsurface irrigation must be installed during subgrade preparation for the sidewalk and tree planting effort. If the pavement is to be replaced or added adjacent to existing trees the addition of subsurface aeration and water piping may help maintain adequate growing conditions for the existing roots. One caution is that the piping may fail due to root intrusion within 5 to 10 years. However, this may be acceptable if the tree has become established during the piping lifespan and the drainage function is no longer essential for stormwater control.
Root Barriers
Root barriers are made of plastic or fiberglass sheeting or they are interlocking metal panels installed from the surface level to a depth of at least 18 inches (45 cm) or more at the interface between a root zone and adjacent paving or other infrastructure. The top must be above the soil and the mulch surface to be effective.
The intent of using root barriers is to physically deflect the roots downward along solid panels, to a depth where they cause no harm to the sidewalk. After the new roots are directed down and under the pavement they would then grow back to the surface, but usually on the other side of the sidewalk. The intent is to install the barriers before the roots become a problem. However, most of the time the barriers are installed after the roots become a problem and the barriers are installed along with a severe root pruning. The problem with the barriers is that the trees are less stable when the roots are pruned and the barriers are installed, than when alternative methods are used to deal with roots before they become problems. Typical placement barriers are vertical, although horizontal root barriers also exist.
Root barriers are made of plastic or fiberglass sheeting or they are interlocking metal panels installed from the surface level to a depth of at least 18 inches (45 cm) or more at the interface between a root zone and adjacent paving or other infrastructure. The top must be above the soil and the mulch surface to be effective.
Root barriers should be installed when a new tree is being planted and there is pavement nearby that may be damaged by future root growth. They should be used in areas where there is adequate soil volumes for the roots to grow.
Chemical Treatments
Chemically treating the sidewalks to retard root invasion is very effective but is an option seldom used any longer because of the potential harm to the environment. An option to this treatment is to use “biobarriers” which are a very popular fabric containing time release capsules of root retardant chemicals attached to the fabric. The fabric is placed in a vertical manner similar to the use of root barriers. However, even the fabric treatments are losing their popularity due to environmental harm from the slow release of the herbicidal chemicals.
Root Pruning
Root pruning is the least desirable solution because it requires the roots to be cut and removed, followed by a sidewalk re-installation. However, since pruned roots will quickly regrow, the problem will come back unless a barrier is used to deflect roots down and away from the sidewalk. Root pruning can also result in root decay that is difficult to monitor and inspect. It is also a very expensive alternative in terms of labor hours.
A thorough tree structural analysis should be conducted before roots are removed. For trees 12 inches in trunk diameter (30 cm) or less, excavation work or root cutting should not occur closer than 3 feet (1 m) from the trunk of the tree. Trees greater than 12 inches (30 cm) in trunk diameter should not be cut closer than the distance measured by the circumference of the trunk or 6 feet (2 m) from the trunk, whichever is more. If roots need to be severed closer than this, tree removal should be considered since it is very likely that tree stability will be affected. Soil excavation work is permitted closer if all the excavation of soil is done by hand or water spade or by a pneumatic soil-excavating tool and no roots greater than 2 inches (5 cm) in diameter are cut. A trained arborist should do all of the root pruning. All root-pruned trees should receive an annual inspection to check their stability and recovery.
If root pruning on large roots is done, the crown of the tree may be reduced to cut back on the wind sail and make the tree less likely to fail. In this case, the pruning should be thinning cuts, and not necessarily on the side of the tree that has had the root severance. Some trees appear to respond with limb shedding on the same side as the root cutting and some trees on the opposite side of the root cutting. Since it may be difficult to predict which branches the tree will shed in response to root cutting, it is better to avoid pruning until necessary and return later to remove any dieback. Ideally, trees should have their crown pruned one or two years prior to the root pruning if this pruning is going to become necessary.
Corrective Pruning
Corrective pruning involves above-ground pruning to establish good structural form and to remove dead or diseased material, weakly attached limbs, and provide clearance for surrounding conditions such as street traffic, bicyclists, pedestrians, overhead utilities, or adjacent buildings. Trees must be in good health and vigorous if they are to be worthy of preservation. Structural pruning must be used to establish good structural form and proactively address potential future limb failure issues. Trees will typically achieve their best form if they are structurally pruned two years after transplanting and every 3 – 6 years after that. For more information on pruning see LA Course 'About Tree Pruning'.
All pruning maintenance performed on street trees should be in accordance with current tree industry standards and supervised by an ISA-certified arborist or an ISA-certified tree worker.
Avoid unnecessary root loss. Sidewalks and curbs are often raised up if large trees are planted in small spaces. Cutting roots so walks and curbs can be reset into their original location causes severe health problems for the trees and can result in the tree falling over long before it becomes mature.
The best way to avoid root loss is to consider using a tree growth regulator to allow the tree root systems to grow slowly and without excessive growth above ground which places an unrealistic demand for water to sustain the tree.
Root Pinning
Root pinning requires water spading or using a pneumatic soil-excavating tool around a tree's roots that are growing toward or under a sidewalk. With the soil under the roots blown away, the roots can be lowered, redirected toward an open landscape, or relocated as necessary to avoid any damage to the sidewalk. The soil is then replaced with CU-Soil or equivalent. The stone/soil mix will keep the roots pinned down and away from the sidewalk. This technique will avoid pruning any roots.
Root Shaving
If a root has caused the sidewalk to rise and the sidewalk must be reinstalled flat and in the same location from where it was removed, the height of the offending roots will need to be lowered. For mature trees, it is best not to cut roots greater than 1 1/2 or 2 inches (5 cm) in diameter even though these are the roots that cause sidewalk problems. It is better to remove the top surface of a root than the entire root. This is done by shaving the root using a chainsaw or debarking tool. Shaved roots do tend to callus quickly, so a layer of rigid foam or pipe insulation foam should be installed between the root surface and the new concrete. The tree should be inspected annually for health and stability.
When reinstalling the sidewalk next to a root cut tree, allow space for callus and trunk diameter growth. Installation of a root barrier or root excluding base (foam board or washed gravel) should also be considered under the new sidewalk to avoid a reoccurrence of the problem.
Aeration Piping
Aeration and perforated water piping may be installed directly under a new sidewalk to help encourage deeper root growth. The perforated pipe provides some air and water into deep layers of soil, particularly where there is a sidewalk at the surface. The pipe may also serve as a means of adding stormwater and drainage into the soil. The addition of water within the perforated piping can further aid in desirable root growth. The pipe may also be placed in structural soil or other fill at least 12” (30 cm) below the finished grade of the sidewalk.
This aeration piping and subsurface irrigation must be installed during subgrade preparation for the sidewalk and tree planting effort. If the pavement is to be replaced or added adjacent to existing trees the addition of subsurface aeration and water piping may help maintain adequate growing conditions for the existing roots. One caution is that the piping may fail due to root intrusion within 5 to 10 years. However, this may be acceptable if the tree has become established during the piping lifespan and the drainage function is no longer essential for stormwater control.
Advantages and Disadvantages
All the products mentioned above have been designed to have a similar friction and skid resistance factor as the surface of concrete. Every product, like concrete, has some problems, or one product would have already replaced concrete.
Terrewalks have to be laid in a straight line. Cutting the material to shape or narrow around a tree reduces the effectiveness of the product. If the offset can be aligned with concrete, the Terrewalks can be laid straight alongside the tree.
Rubber sidewalks have one of the least desirable appearances and because they need an edge band, when the edge band is lifted by roots, the whole sidewalk can be cantilevered. In some climates, the rubber edges may curl.
Poured in place rubber and rubber mixes are not easy to remove and re-use when root maintenance under the sidewalk is needed. Some rubber materials are not dense, and may squish with walking, and can grab skate board and roller skate wheels.
Polymers with aggregate are very firm materials and depending on the size and connectivity but may not be easy to remove and re-use after maintenance under the sidewalk is performed. They have to be cut with a concrete saw to enlarge the opening around the tree trunk.
Bricks and decorative stones that are not interlocking, may offset and settle over time. The edge border can be lifted by roots and cantilever away from the walking surface. Interlocking pavers are a thicker section (although thinner than concrete) and need to be installed on a firm base.
Geotextiles can provide support and reduce soil compaction. The edge border is usually 1 inch (2.5 cm) deeper than the paver thickness and it is necessary to hold the sand in place on which the pavers are set. When the deeper edge border is lifted by a root, it cantilevers the edge.
Cost
While the initial cost of alternative materials may be higher, the better question should be how much do they save? The second time a concrete sidewalk is re-damaged after a repair, the alternative materials become less expensive.
Durability
Most materials are as durable as concrete. There may need to be some monitoring to make sure they retain a relatively level surface. When alternative materials are installed, there are no opportunities for scoring or scribing initials and words that sometimes occurs in freshly poured concrete. Most paint and ink graffiti are treated the same as on concrete. As mentioned above, concrete has an inherent property to crack over time. When concrete is placed over growing roots, which cause a less stable base, concrete is more likely to crack.
Climates
Most alternative materials show no expansion in hot temperatures such as in Palm Springs, CA, where summer daytime temperatures often exceed 100°F (38°C). In general, like concrete, most alternative materials warm up in sunlight, but reach a lower temperature than concrete and retain less heat. Because the alternative materials are thinner and the base can be porous, the propensity for root growth under the materials is less than concrete. Alternative materials in subzero climates generally retain their stability, coefficient of friction, and resistance to snowplow damage. The alternative materials also resist ice melting chemical stains and there is no freeze/thaw concerns and no snowplow damage.
One of the major purposes for replacing concrete sidewalks with alternative materials in areas where tree roots are causing chronic buckling is to reduce lawsuits caused by pedestrians injuring themselves on offset concrete sidewalk panels. The alternative materials are more flexible, so even if they are slightly lifted, the separation between panels or pavers is less than concrete. The same materials can be re-used during the repair which does not have to cure at the after the replacement.
The strategic use of alternative materials is a real and long-range solution for cities. While preserving trees, they have lower risk, more long-term economical benefits, and they are aesthetically more appealing than short-term asphalt patching. The most attractive part the alternative material phenomenon is the idea that it keeps trees in our urban areas from being cut down. The trees are harder to replace in-kind than the sidewalks. Our cities need to grow as many larger trees as possible and keep them where people live, work, and play.
Root Pruning Solution
One solution that does work is to plant trees that have been grown with fibrous roots. These roots are small and will hardly ever cause a sidewalk to lift or buckle. Fibrous root systems are also better for urban trees to obtain nutrients, moisture, and air from the compacted and poor-quality soil often found in our cities.
Conclusion
One of the biggest challenges in using alternative materials is getting them approved for use by agency engineers where concrete is the standard material. Unfortunately, the use of concrete adjacent to trees has already cost millions of dollars in repairs. The future liability will also require millions of dollars in repairs. If the concrete is replaced and the tree retained, the likelihood of repeat failure is very high on most sites. If the concrete is replaced and the tree requires too much root pruning the tree may decline somewhat. If the tree will need to be removed, pre-mature removal of trees causes a huge loss in benefits and services that the trees will produce over time. Trees usually need to be growing for at least 15 years to really start producing higher benefits.
Sources
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All the products mentioned above have been designed to have a similar friction and skid resistance factor as the surface of concrete. Every product, like concrete, has some problems, or one product would have already replaced concrete.
Terrewalks have to be laid in a straight line. Cutting the material to shape or narrow around a tree reduces the effectiveness of the product. If the offset can be aligned with concrete, the Terrewalks can be laid straight alongside the tree.
Rubber sidewalks have one of the least desirable appearances and because they need an edge band, when the edge band is lifted by roots, the whole sidewalk can be cantilevered. In some climates, the rubber edges may curl.
Poured in place rubber and rubber mixes are not easy to remove and re-use when root maintenance under the sidewalk is needed. Some rubber materials are not dense, and may squish with walking, and can grab skate board and roller skate wheels.
Polymers with aggregate are very firm materials and depending on the size and connectivity but may not be easy to remove and re-use after maintenance under the sidewalk is performed. They have to be cut with a concrete saw to enlarge the opening around the tree trunk.
Bricks and decorative stones that are not interlocking, may offset and settle over time. The edge border can be lifted by roots and cantilever away from the walking surface. Interlocking pavers are a thicker section (although thinner than concrete) and need to be installed on a firm base.
Geotextiles can provide support and reduce soil compaction. The edge border is usually 1 inch (2.5 cm) deeper than the paver thickness and it is necessary to hold the sand in place on which the pavers are set. When the deeper edge border is lifted by a root, it cantilevers the edge.
Cost
While the initial cost of alternative materials may be higher, the better question should be how much do they save? The second time a concrete sidewalk is re-damaged after a repair, the alternative materials become less expensive.
Durability
Most materials are as durable as concrete. There may need to be some monitoring to make sure they retain a relatively level surface. When alternative materials are installed, there are no opportunities for scoring or scribing initials and words that sometimes occurs in freshly poured concrete. Most paint and ink graffiti are treated the same as on concrete. As mentioned above, concrete has an inherent property to crack over time. When concrete is placed over growing roots, which cause a less stable base, concrete is more likely to crack.
Climates
Most alternative materials show no expansion in hot temperatures such as in Palm Springs, CA, where summer daytime temperatures often exceed 100°F (38°C). In general, like concrete, most alternative materials warm up in sunlight, but reach a lower temperature than concrete and retain less heat. Because the alternative materials are thinner and the base can be porous, the propensity for root growth under the materials is less than concrete. Alternative materials in subzero climates generally retain their stability, coefficient of friction, and resistance to snowplow damage. The alternative materials also resist ice melting chemical stains and there is no freeze/thaw concerns and no snowplow damage.
One of the major purposes for replacing concrete sidewalks with alternative materials in areas where tree roots are causing chronic buckling is to reduce lawsuits caused by pedestrians injuring themselves on offset concrete sidewalk panels. The alternative materials are more flexible, so even if they are slightly lifted, the separation between panels or pavers is less than concrete. The same materials can be re-used during the repair which does not have to cure at the after the replacement.
The strategic use of alternative materials is a real and long-range solution for cities. While preserving trees, they have lower risk, more long-term economical benefits, and they are aesthetically more appealing than short-term asphalt patching. The most attractive part the alternative material phenomenon is the idea that it keeps trees in our urban areas from being cut down. The trees are harder to replace in-kind than the sidewalks. Our cities need to grow as many larger trees as possible and keep them where people live, work, and play.
Root Pruning Solution
One solution that does work is to plant trees that have been grown with fibrous roots. These roots are small and will hardly ever cause a sidewalk to lift or buckle. Fibrous root systems are also better for urban trees to obtain nutrients, moisture, and air from the compacted and poor-quality soil often found in our cities.
Conclusion
One of the biggest challenges in using alternative materials is getting them approved for use by agency engineers where concrete is the standard material. Unfortunately, the use of concrete adjacent to trees has already cost millions of dollars in repairs. The future liability will also require millions of dollars in repairs. If the concrete is replaced and the tree retained, the likelihood of repeat failure is very high on most sites. If the concrete is replaced and the tree requires too much root pruning the tree may decline somewhat. If the tree will need to be removed, pre-mature removal of trees causes a huge loss in benefits and services that the trees will produce over time. Trees usually need to be growing for at least 15 years to really start producing higher benefits.
Sources
- Comments from several members of the LinkedIn Urban Forestry discussion group.
- Phillips, Leonard, Editor, “Rubber Sidewalks”, City Trees, Vol. 38, No 6: November/December 2002.
- Phillips, Leonard, Editor, “Topic #5“, Online Seminars for Municipal Arborists, 2015.
- Seattle Department of Transportation, “Trees and Sidewalks Operation Plan”, February 2015.
- 'Sidewalk and hardscape solutions', University of Florida, 2015.
- Smiley, Thomas, PhD, “Sidewalk Repair near Trees”, Bartlett Research Laboratories Technical Report.
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