Structural Soil

[Kari Biddix]

Please help!  I have a project that is a mixed-use development in the middle of town that is currently under design.  There are lots of 5′ x 5′ tree wells throughout the project which will use structural soil.  The architect will install tree grates in the high traffic areas but would like to have the option to use liriope or decomposed granite with binder in the other tree wells over the top of the structural soil.

I am having a difficult time finding projects that have used anything other than bark mulch in these areas.  Has anyone had experience using structural soil and what material did you use at the surface of the wells?  How does it look after some time?  What would you have done differently?  How are the trees doing?  Any information would be very helpful.  I need to give some design direction but I have nothing to back up my preference.

I don’t think decomposed granite with binder would be permeable enough and DG without binder would be a huge mess without the proper maintenance.

[Tyson Carroll]

A few questions. 

what type of trees are you using?

if you are looking at creating an impermeable surface on top of the rootball and root flare how will the tree have access to water?

what climatic zone are you in?

Realistically with a tree well that small your options are somewhat limited on what species you can and cannot use. Whether or not the tree will affect the surrounding pavement has much to do with species, genetics, and cultural habits. Even with the use of root barriers there is no guarantee that there will not be any affect on the pavement. 

Have you looked into using Silva cells?

We have used tree well with dg (no binder), 3/4-1″ base rock, and mulch at projects in much of California. Personally we have never used structural soil but LA’s back east have spoke very poorly of it and do not recommend it due to cost, lack of proper installation, and overall negative impact on the tree. 

You are wrestling with a very difficult issue  whose simplest solution is to give the tree more space. 

Good luck

[toby]

DG with a binder is both permeable and not; water slowly seeps in but will otherwise stand/puddle or runoff.

The surface will erode slowly with foot traffic.  Some maintenance is required.  DG is a good paving material on top of roots.  It has none of the caustic properties that concrete does.

If you are installing treewell irrigation (RB RWS or similar), then it should be flush with the top of the DG to not create tripping hazards.

Trees and shrubs do fine with DG around the base.  Keep the DG thin right around the crown and then make it thicker as you go out – up to 3″.

And what ever you do, DO NOT dig the tree hole any deeper than the root ball or put any amended soil below the root ball !  When it decays to nothing after a year, the tree will sink and you’ll have another problem to deal with.  And once it does sink, you can fill in the hole with DG only so high on a root crown before it begins to kill the tree.  Eventually, filling in that hole surrounding the tree will be impossible except with things like cigarette butts.

I’ve seen more problems with tree grates that never get cut larger as the tree grows.  I’ve seen them lifted as the crown grew to create tripping hazards.  Maintenance is required to cut the hole larger to prevent this and girdling.

I’ve done a rubber sidewalk paver project around Sycamore street trees (~78 locations), and they are slowly being lifted too.  Maintenance is required for this too.

What trees are you considering for your project ?

[Jason T. Radice]

I’d skip the structural soil altogether and redesign the structure of the sidewalk to span a tree trench or use something like Silva-Cells where you can use proper soil.

[Rob Halpern]

This might be helpful:

http://urbanforestry.frec.vt.edu/stormwater/Resources/TreesAndStruc…

and this:

http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2952&Type=2

[Goustan BODIN]

If you’re on Linked-In, I’ve found this group of certified arborists most resourceful :

http://www.linkedin.com/groups?home=&gid=3107134

[Dennis J. Jarrard, PLA, CLARB]

The City of Chicago requires structural soil for plant pits in narrow planting areas.  The projects that I have worked on the trees were in similar 5’x5′ plant pits with tree grates.  My last project was in the downtown area and it required lava rock to be used in lieu of mulch.  You might also google Cornell Structural Soil or Amsterdam Tree Sand for additional information that might be helpful to you.

The goal is to make sure that structural soil extends beneath the walkway it is supporting and not just in the plant pit for long term success.

By the way the trees are still alive and it has been 6 years since they were installed.

[Tyson Carroll]

Dennis, 

I was told by some local Chicago LA’s that the city was moving away from structural soil because of the reasons I mentioned. You would know that better than I through. That being said the issues arising from the use of structural soil have become more apparent in years 10-15 but it also depends on the species you selected, genetics, and site characteristics as to the extend of potential damage a tree may or may not cause. 

[Jason T. Radice]

Read this:

http://www.deeproot.com/blog/pdfs/Comparing_Silva_Cells_and_Structural_Soil.pdf

I’ve heard the same issues with the long term performance of structural soils. It does not work as well as other methods and the trees don’t get nearly the size they do with trenching or mechanical support methods.

[Author]

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