Paving Around Street Trees - classics

Classics
The following article has been selected because it is deemed very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.

Paving Around Street Trees
                                                                  By Len Phillips and Gordon Mann
Edited by RW Gibney RLA ISA

Municipal arborists around the country are looking at options for improving the aesthetics, reducing maintenance, and discouraging compaction of the soil surrounding downtown trees planted next to the concrete sidewalks.   For many years, metal tree grates were the standard option, but they require routine maintenance, and are rather expensive to purchase and enlarge as the tree grows bigger.

In recent years, some new pavement products have been used to replace these metal tree grates. These products have been installed around sidewalk trees in several downtown cities.   The pavements vary from 100% soil mix, to stone aggregate, and to recycled rubber chips, all in varying proportions of mix. All of these options eliminate the metal tree grate and frame footing.

Flexible Pavement Option - Photo
A flexible surface is created around the tree by spreading rubber chips or small paving stones that have been bonded together with a high-quality clear resin or urethane binding agent, on the soil surface. It results in a flexible, porous surfacing material, that is resistant to cracking. The products are mixed on-site and applied to coat the particles prior to laying the material on the ground. The material is easily cut to enlarge the hole around the trunk as the tree grows. The pavement material is usually not re-usable.

Porous Paving
The first product to be discussed is called porous pavement - see Photo. It is a fully permeable paving product which allows water to freely drain through the surface. The porous pavement's non-skid, 50% rubber content and 50% stone aggregate surface along with a hard urethane binder ensures good traction even when wet, lessening the chance of slip and fall accidents. In addition, the rubber content allows the product to flex with trunk growth, or with any roots that come to the surface. This flexibility resists cracking and heaving commonly found with concrete sidewalks. The porous and flexible rubber content prevents soil compaction but allows air and surface water penetration, which encourages the tree roots to grow deeply. The rubber in these pavements is made with tires diverted from landfills. Porous pavement uses scrap tires to create a 5 x 5 feet (140 cm sq. 5 cm. thick) square of 2″ thick porous pavement. The product comes in several custom colors such as gray-black, tan-black, brown-black, cypress-black, red-black, brown-tan, green-brown and blue-gray. The surface is easily cut to enlarge the hole around the trunk as the tree grows - see Photo.

The most difficult part of the entire installation is the clean-up of the tools. However, if vegetable oil is coated over all the tools and equipment before use, the cleanup with bio-diesel fuel and a stiff bristle brush is much easier. The binder is moisture cured, so if the weather is humid the product will cure more quickly. Generally this product cures within 24 hours after being poured.

Soft Paving
A soft formulation of porous pavement is made with 100% recycled rubber chips, no stone, and held together with a soft binder. Landscape architects and recreation specialists specify this product to provide a more impact-absorbing surface for applications such as playgrounds and pool pavements, while arborists are starting to specify this product to be used for trees surrounded by sidewalks.
·  At 1″ (2.5 cm) thick, the product can be used to overlay existing concrete, asphalt, metal surfaces and wood.
·  At 1½″ (4 cm) thick, the product can be used for bike paths, patios, trails, golf cart paths and any other pedestrian traffic.
·  At 2″ (5 cm) thick, the product can be used for low speed car and truck traffic.

Rubber Panels
Several companies make rubber sidewalk panels made from recycled tires. This product is firm enough to walk on or roller blade and skate board over. However, the panels need a solid edge piece of either plastic or steel angle to hold them inward like interlocking pavers which the concrete sidewalk surrounding the tree planting pit provides. Unfortunately the rubber edges next to the tree trunk may curl up over time. The material has to be cut with a small power saw to enlarge the space around the tree's flare. The product can be pulled up and re-used for other trees.

Plastic Panels
Another sidewalk product is recycled plastic interlocking panels.  These panels are made from recycled tires and plastic. This product is firm enough to walk on or roller blade and skate board over. The panel size is the same as rubber panels of 2’ X 2.5’ (60 X 75 cm) and requires a larger opening to install. The panels are permeable between the seams. The panels are laid over ¾ inch (2 cm) aggregate and staked to the ground around the outside edge of the panels. The panels can be cut with a small power saw to enlarge the trunk opening. This product can also be re-used on other trees.

Resin Bonded Surfacing
This is a product that consists of viscous, high-strength resins that flow around the aggregate pieces and glues them together. The high strength resins are clear, and as they coat decorative aggregate, they bring out the shine and luster of the aggregate, improving the appearance. The site is graded out, the aggregates are mixed with the polymer binder, and spread out over the soil and a geotextile or thin gravel base around the tree pit to create a level area. A spacer or form is needed to create space around the tree trunk.

Polymer Binding Agent
Some resin bonded surfacing products as well as polymer binding agents can be put over other paving materials using a thin layer of resin applied to a solid surface and the aggregate scattered on top. This method creates a surface that is impermeable. The result is a resin bound surface that is more durable and requires less maintenance.   However, these options are not suitable for the base of trees due to the need for a permeable surface.

Hoggin
In the UK, there is a soil process called hoggin that is used to create a soft-looking but hard-wearing path of soil - see Photo. It is also called "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 should generally contain no particles larger than 1-1/2 in.(40 mm) with all the different sizes of particles evenly mixed. Hoggin is used as a road or sidewalk base as well as for a surface material within a tree pit, next to the sidewalk. This material, when compacted makes a very stable surface and seldom needs raking.

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 and tree use. The use of a vibrating roller may prohibit the use of hoggin for trees in small sidewalk openings if the sidewalk is not hoggin. However, in park settings or trees beside hoggin sidewalks, it might work out very well. It is important that the hoggin does not sit in water. Edge restraints are essential to keep the hoggin from moving towards the tree or beyond the edge during installation. The edge restraint can be moved and the product re-compacted as the tree grows.

Interlocking Pavers
Interlocking pavers can be used around the base of trees. A geotextile fabric is placed over the soil, then a layer of rock to be compacted, then geotextile and finally the layer of sand. The pavers are set in the sand and the pavers and rock are permeable. The pavers need an edge restraint which can be the sidewalk, an angle steel material, plastic, or wood form. As the tree grows, the edge restraint is removed, the size enlarged and the pavers can be re-used at other trees.

Benefits
Any of these porous pavements minimize the volume of stormwater run-off compared to a concrete, or bituminous surface. The porous pavements are resistant to damage from oil, chlorine, ozone, UV rays, muratic acid, gasoline, diesel, and transmission fluid, salt water, and many other hostile materials. They all are also resistant to snow plow damage on the surface. All the pavements mentioned here are ADA-compliant surface treatments.

Installation
With all these alternatives, the site has to be prepared by grading, and then installing 2 inches (5 cm) of ½ inch (1.3 cm) stone, lightly compacted in place, on top of the root ball. The pavement material is then placed on top of the stone.

Maintenance
All the materials need to be swept or power washed at least twice a year to avoid the buildup of debris and prevent the growth of moss or algae. However, since most cities mechanically sweep their sidewalks periodically, the tree surfaces can be swept at the same time.

Metal Tree Grates
Metal tree grates have previously been the product of choice for urban area trees. They provide a bridged tree guard and open space around the tree. The challenges that have surfaced with tree grates include:
·  the cost of metal grates is generally higher than any of the options indicated above. Costs do vary         depending on quantity and size of the opening.
· the footing required to support the tree grate frame is usually a thicker section than the adjacent walkway material. The footing is the most likely part of the system to be affected by tree root growth.
· the cost to enlarge the tree grate hole can be expensive, and requires a cutting torch or large steel cutting saw. The torch should be used very carefully around the tree to avoid tissue damage.
· a benefit of tree grates is they can be re-used, and if a community has a large inventory of grates, they can be swapped as the trees grow, and cutting may not be needed.

Case Studies
In its continuing effort to make its downtown more welcoming, and to support its sustainability initiatives, Grand Rapids, MI is completing the replacement of 750 old iron tree grates with a porous pavement.   The first phase of the project was completed in September 2014 when the city replaced 250 cracked and broken grates that presented the most serious tripping hazard in the three square miles of the city's downtown area. An additional 500 grates were replaced in 2015, to finish the project. To leave space around the tree trunk, cardboard tubes were made and cut into 6”- 8” lengths and used as sleeves around the tree trunk prior to adding the pavement. The next day, the material had cured and the tubes were removed. The cost of the product worked out to be about US $400 compared to $800 to $1200 for the previous iron grates.   The cost of maintaining tree grates and cutting the openings larger as the trees grow can range in expense from $100 to $200 per tree. The cutting of the porous pavement should require less effort and time than cutting cast iron grates, and can be done with a saw cutting through the depth of the pavement.

Redwood City, CA used a different product than any mentioned here, and that was poured in place rubber for sidewalks. They tried this at several locations, however, the material was too soft to walk on and being a one-piece section, they were unable to maintain the roots when the roots grew larger under the rubber material. If the City has to tear the material out and replace it, it really isn’t sustainable, unless they can recycle the material for re-use at a reasonable price.

Redwood City also used interlocking pavers around some trees. The pavers required a strong form on the tree side to hold the pavers in place, and the maintenance to keep the pavers from sagging was very costly. They also used tree grates, and the initial grate sizes that were less expensive were failing as roots raised the grate frame footings. The project managers did not want to invest in the more expensive larger grates to
create more space between the tree and the permanent sidewalk.

Boxford, MA used living mulch around its trees. This is very appropriate for a wealthy rural community where aesthetics are very important. More on the subject of living mulches will be coming in the next Seminar.

Specifications
There will need to be clear specifications and a strength analysis of the materials to satisfy most civil engineers before they will include the materials in a walkway repair or walkway design, especially if the potential for vehicle travel over the surface is likely.

Challenges
No system is perfect, and the challenge with poured in place porous pavers is that they are not modular. On any product, the opening around the tree will have to be enlarged or cut to make the space bigger over time as the tree grows. The most practical method to cut the openings larger for each product was mentioned in the product description, and can be performed by in-house crews or contractors. If the opening is too large at planting time, it can be a risk to walkway users. If the planting opening is not large enough for the mature size of the tree, the porous pavement will be lifted similar to typical sidewalk lifting observed when trees are growing too close to concrete or asphalt.