There is a lot of opinion over the best spec for a porous tree pit paving specification. We have been installing tree pits for nearly 20 years and worked with a lot of engineers, architects, landscapers, and arborists.
There is no single specification or product mix that solves all scenarios, so we try to work on a job by job basis. Here is a summary of some of the most common considerations:
New urban tree installation
Typically found on pathways, on roadsides, in cities, and shopping areas. Because councils recognise the environmental and social benefits of trees in these spaces, they are found on nearly every city shopping street.
The main issue associated with these trees is that the soil around the tree and structural base mix on top of that is new it has had no time to settle. Compacting is often difficult because of area restriction (these pits are typically about 1.2m x 1.2m) and protection of the new tree.
Our best advice for these areas is to fill the area intended to be paved with porous paving to the top, and allow it to settle naturally over the course of a few months. They should be hand compacted a few times over this period and topped up with loose aggregate to minimise trip hazard.
Once the area has settled fully, the top 30-40mm of loose stone can be removed, and the StoneSet is then simply applied on top.
Of course this is not always possible, so hand compact as much as possible and allow some excess budget to reapply the top layer of paving in the event of any sinkage.
Existing urban tree surround
When we are installing over an established tree, it is usually just a matter of removing 30-40mm of the top layer and replacing with new StoneSet paving. Tree roots can be an issue, so we will allow a small gap when finishing up to a trunk (about 50mm) and tree roots (about 20mm). This is done in consideration of trip hazards and potential root movement or tree growth.
We will always recommend the installation of a 200mm dia. tree collar for new or young trees. The inside of the collar can be back filled with loose stone after installation of the StoneSet porous paving to discourage the accumulation of litter.
We recommend creating a metal ring tree collar, which can be removed or left in place.
Stone or Rubber
Stone offers a strong, solid finish that performs as a pathway in the same way the surrounding concrete, blocks or asphalt will. For longevity it relies on good compaction of base, and minimal root and tree trunk disruption. We generally recommend the incorporation of rubber within the stone mix if roots are prominent or cracking has been an issue previously. See this video to see the flexibility you can achieve with this blend.
Why not just use rubber on its own, you might ask? Well, in our experience rubber has two main flaws as a product for tree surround paving.
Unlike solid stone, rubber granules are susceptible to shrinkage. This can lead to gaps appearing around the edge of the tree pit and ultimately result in lifting of the surface.
The second is a concern regarding porosity. Rubber granules are generally used in a 4mm size, which leaves very small voids for water penetration. It also leaves a finish with greater surface tension, which needs to be broken each time it starts to rain to allow the flow of water. This means the root ball won’t receive as much water as a stone 6 or 10mm finish. The tree will then be more likely to go looking for water, leading to more root movement and the potential of surface movement or cracking.
Rubber granules are a great addition to many scenarios, but need to be used within a mix of stone to create a hardwearing, stable surface and maximise void content.
A tree trench is the term we use to describe a large area that has trees in it, but has areas between the trees that will be trafficable by foot. A nature strip, as pictured, would be a tree trench.
With tree trenches you need to consider the higher than normal volume of traffic using the areas between the trees. All preparation and consideration is the same as a normal tree pit, but with the area away from the tree trunk needing to be treated as a regular footpath. This means gaining compaction to match required specification for use, and if compaction is an issue, the use of a recycled plastic cellular system to give base stability and load bearing capability without risk of sinkage. We use the Atlantis Gravel Cell.
There are two main considerations when deciding on particle size - expected traffic, and porosity required.
Large tree surrounds have a good catchment area so a smaller stone can be used. We would recommend no smaller than 6mm. Small tree pits will generally receive less foot traffic but require a higher void content to maximise water penetration. A 10mm stone is recommended in this case.
StoneSet always looks at each project based on its own merits, and will customise a solution and product mix recommendation depending on the individual site requirements.