Options for Sidewalk Repair Around Existing Trees
Tree root-sidewalk conflicts are very common in developments. And the most commonly planted swale tree in many parts of Florida is the live oak (Quercus virginiana), which is a wonderful long-lived, sturdy tree, that simply requires more root and trunk space than the 6 foot swale designated by the designers. And as many homeowners and property managers have learned, the problem seems to escalate after the first 10 years as the trees begin to mature and their root systems expand and try to grow out of the much-too-small space allotted between the street curb and the sidewalk.
Obviously, it would be better to plant trees in larger spaces such as the front yard away from sidewalks and streets. But it is too late for many people who buy a home in a development that has already planted large-maturing trees in a narrow swale between the sidewalk and the street. This is a landscape design flaw, in my opinion, that is creating significant maintenance costs for homeowners and property managers.
Because removal of the trees by sidewalks is not practical, sometimes not allowed by local ordinance, and would remove an important environmental amenity from streetscape, I will primarily discuss options to accommodate roots on existing large trees, infrastructure-based strategies, not options for recently planted young trees by sidewalks that we refer to as root-zone based strategies. The goal is to adapt the infrastructure to accommodate the tree to reduce the sidewalk damage and increase the interval between sidewalk repairs.
Types of damage
Over the years we have observed swale trees (mostly live oaks in this part of Florida) lifting sidewalks usually at the seam or expansion joint. The lifting can occur anywhere between the slab next to the trunk to as far away from the tree as the third concrete slab which is about 15 feet from the trunk. Once the slab lifts over ½ inch, there is a serious liability issue — a trip and fall hazard.
In addition to the lifting pressure caused by roots, the tree root flare of trees will slowly expand outward and even upward putting sideways and upward pressure on the sidewalk. Often the root flare begins to become a problem with nearby infrastructure when trees reach a diameter of around 20 inches. The root flare of a 30 to 36 inch DBH live oak can extend almost the entire width of the 6-foot swale.
We have found that cutting roots is only a temporary solution. Severed roots regrow under a sidewalk within three years after being cut. In San Francisco and other cities, the interval between root pruning and renewed sidewalk lifting is about five years. This relatively short repair interval can create an escalating and compounding effect of needed repairs as the trees continue to grow. An additional problem with root pruning is the loss of tree stability. Trees have stability against wind throw because of the lateral roots. Tap roots are rare and quite small in most broadleaf trees and provide virtually no support. When the important lateral roots are pruned, tree stability can be reduced. Research at the Bartlett Tree Research Laboratories has demonstrated that cutting large lateral roots within the root plate, which is a distance from the trunk of three times the trunk diameter, can destabilize a tree. For example a 20-inch diameter tree should not have roots cut closer than 60 inches from the tree (3 X 20 = 60). In a 6-foot wide swale with a growing tree, that does not allow space to root prune at the edge of the sidewalk. So root pruning should only be done as a last resort when all other methods have been judged as not feasible. And those ordering cutting of major lateral roots should be aware of the liability for tree failure that root cutting creates. There are consequences to cutting roots such as reduced tree health and destabilization.
There are several options to consider when repairing lifted sidewalks. These include options that sometimes can be combined with others. All of the techniques except for #1 Shaving and #2 Slapjacking are used when removing and replacing sidewalk slabs.
Shaving lifted concrete: Once a slab is lifted by roots, it can be shaved down several times until the thickness of the sidewalk is too small to support the weight of pedestrians and golf carts using the sidewalk. This relatively inexpensive technique should be used at first to even out lifted sidewalks. Another version of shaving is the use of asphalt to level the sidewalk slabs where they have lifted. Asphalt on a concrete sidewalk is not a very esthetic alternative.
Slabjacking: This is a process where concrete is injected under high pressure beneath the low side of a lifted slab causing the slab to lift upward to match the lifted section of the adjacent slab. This process has been around for several years but I have yet to speak with anyone who has had the slapjacking done.
Meandering sidewalk: Move the repaired sidewalk out away from the tree by several feet preferably outside the root plate, which is the distance from the tree that is three times the tree diameter.
Excavate beneath offending roots: Instead of cutting the offending roots that have lifted the sidewalk, leave the root intact and use an air excavation tool like an Air Spade or Air Knife to remove soil beneath the root. This void can be left open beneath the root or filled with clean pea gravel that will move out of the way as the root expands downward. Because the concrete sidewalk is being replaced above the root, the root should expand downward filling the void. This technique should be combined with reinforced sidewalks and possibly thicker sidewalks.
Bridge over roots: Sometimes the roots are too large to cut and have expanded above the natural grade. In these cases the root(s) must be bridged possibly creating a slight rise in the sidewalk where it passes over the root.
Use reinforced concrete: Use rebar or wire mesh when repouring concrete sidewalk slabs. This will make the sidewalk stronger. It is important to connect the slabs together with rebar to avoid the lifting of a single slab. In that way the root will be pushing against two or three slabs rather than a single slab.
Use thicker concrete: Increasing the concrete thickness from 4 inches to 6 inches will make the sidewalk less likely to break or lift. This technique should be used with the reinforced concrete technique above for best results. There is currently research being done to determine the best concrete thickness to use. But no data is available at this time.
Place sidewalk over a geogrid and gravel base: Recent research into roots and sidewalks has demonstrated that a geogrid mesh base placed on top of roots will spread the force of the upward pressure of the roots over a wide area. Then if we cover the geogrid with clean #57 stone for at least a depth of 3 to 4 inches, the concrete sidewalk can be poured on top of the stone. The stone should be covered with a geotextile fabric to help keep the sand and soil from filtering into the stones, which provide a partially flexible buffer to diffuse the force of lifting roots on the concrete sidewalk slabs above. This arrangement may cause the sidewalk to be higher than the original sidewalk.
Place clean gravel beneath sidewalk: Recent research has demonstrated that simply placing clean gravel beneath a sidewalk slab will cause roots to grow below the gravel not directly below the concrete slab. The large air spaces in the gravel cause any roots to grow below both the concrete sidewalk and the gravel layer. Consequently, the sidewalk is less likely to get pushed upward by the roots.
Rubber Sidewalks and Pavers: Rubber sidewalks, other flexible materials and brick pavers allow for reduced repair costs to lift out a root-damaged sidewalk section and replace it. However, often the repair involves cutting the offending root that is lifting the sidewalk. These materials, do not solve the problem of what to do with an existing large root that is increasing in diameter. Other methods may be needed to accommodate the large roots rather than cutting them.
Other options that may not have merit
https://shadetreeexpert.com/the-use-of-root-barriers-to-protect-infrastructure-from-roots/Pervious pavement: This type of pavement will allow moisture to percolate through the concrete directly to the soil and roots below. But pervious concrete is not as strong as regular concrete so a thicker layer of pervious concrete may be needed. And allowing moisture to percolate through the concrete may increase root growth beneath the concrete. Pervious concrete is a good product to use in a parking lot or area where root growth is to be encouraged and stimulated. Stimulating root growth under a sidewalk is not helping the situation, in my opinion.
Root Barriers: Root barriers are sometimes useful on well-drained soils on new plantings if the root barriers are properly installed and at least 2 or 3 feet deep. The top of the root barrier must be slightly above grade or roots will grow over the top of the barrier and this root barrier protruding edge can be unsightly. Proper installation is very important. Root barriers are usually not appropriate where trees are already established. For more on root barriers and recent root barrier research click here.
Tree Growth Regulator (TGR) Applications: Generally arborists use TGR chemicals to slow top growth of trees. Electrical utilities have been using TGR’s for over 30 years to control top growth of trees near power lines and extend the pruning cycle. Subsequently some arborists have been selling the relatively expensive TGR’s as a way to reduce pruning and slow tree growth in developments where tree root-sidewalk conflicts are beginning to be a problem. Ironically, the benefits of the most widely used TGR, paclobutrazol, is to divert energy from top growth into root growth. So TGR’s may make the tree root-sidewalk conflict worse by increasing root growth even though top growth is being reduced. Until there is research indicating the TGR’s are in reducing sidewalk damage or increasing the sidewalk repair interval, I do not recommend TGR’s as a viable solution.
Heavy crown pruning to slow tree growth: The extra expense of heavy crown pruning to keep a large-growing tree short and stunted is not really practical. Large maturing trees will quickly grow to their genetically predisposed size once the excessive and expensive pruning stops. There are many other reasons, which I will not go into here, why heavy pruning will not work and is bad for the health of the trees.