[CANUFNET] sidewalk materials suitable for a winter city

[James Urban]

Martha:

It is not just the material but the dynamics and existing root preservation and the interplay between the engineer and the arborist.  

Principle 1 - The new material needs to be placed above the existing root mat.  This will require investigation as to how much you can grade,  and the closer you get to the tree trunk the harder the problem and typically the higher the grade on the sidewalk needs to be.  See photo or how one city solved this issue to the extreme around a live oak.

Principle 2 - Principle 1 suggest that the new material needs to be as thin as possible as you will quickly start running into these grading issues .

Principle 3 - The space around the tree is dynamic with lifting and pushing roots particularly difficult with elm, so you either need a material that is quite strong OR very flexible.  Flexible usually is the best way to proceed.  Going back to Grady's question on poured in place rubber resin bonded pavement, I would think that this material might be your best option but it is expensive.  Apparently you can blend crumb rubber and stone together and then add the resin.  As you make this material mostly rubber, it feels like a playground surface and is likely too soft for a sidewalk.  As you add stone the material stiffens but still stays flexible, but  at some point too much stone and the material is too stiff and cracks.  I also like pavers as they can easily be reset once they start to move, but that may be a problem for you.  Pavers also need thicker subgrades which brings you back to Principle 2.  Note that if you are dealing with mature trees the problem is not going to be settlement.  The roots are making a solid foundation.  most paver sections are designed thicker to prevent settlement.  On the other hand a thicker pavement will also resist lifting better, so it is a compromise.  Good old asphalt has lots of things going for it except its image.  But at Harvard University, a place that is very image conscious and not lacking money, uses asphalt for many important campus walks.

Principle 4 - Salt is an issue but at what levels of application?  I am finding that salt in retail and heavy commercial districts is used at greater levels than in residential districts.  Is the sidewalk in a shady location in winter? The type of salt is also and issue.  Something about porous paving bothers me on the salt issue but porous paving also often needs less salt as the melt water flows thru and does not refreeze on the surface (casual observations).  Porous paving might also diffuse the salt and its spring leaching while a non porous surface may concentrate salts often in the tree space.  I Have no experience with how different materials hold up under salt.  I leave it to the Canadians to do more research on salt.  You have much more to loose than your southern neighbor. 

Principle 5.  This is a compromise situation.  if you design the system to assure that the tree is favored, the sidewalk will not be what the engineers require.  if you design what the engineer wants, you will harm the tree.  In between is the politics of urban forestry.  How much support will the community give for the tree at the risk of some future issue with the sidewalk.

In my book Up By Roots in Part 2 chapter 5 there is a more extensive discussion on this issue.  The above is my most recent musings.

Good luck!

Jim Urban, FASLA / Urban Trees + Soils
915 Creek Drive / Annapolis, Maryland 21403
Cell	410 693 9053     Office 410 263 4838
jimtree123 at gmail.com
http://www.jamesurban.net


On Aug 11, 2015, at 5:01 PM, Barwinsky, Martha <MBarwinsky at winnipeg.ca> wrote:

> We are looking for alternative sidewalk treatments adjacent to mature American elms for a new apartment development in an older neighbourhood near our city center (Osborne Village). The proposed sidewalk is within 2 m of the trees where there has never been a sidewalk previously.
>  
> We are looking for a treatment other than concrete or pavers. Has anyone had experience with rubber sidewalks or other materials / methods in conditions similar to ours – freeze/thaw cycles, clay soils, very active winter road maintenance program (de-icing salt and snow clearing equipment on sidewalks)? The sidewalk would have to withstand sidewalk snow clearing equipment and meet accessibility design standards.
>  
> Any information would be greatly appreciated!
>  
> Thanks,
>  
> Martha Barwinsky
> City Forester
> City of Winnipeg
> 105-1155 Pacific Ave.
> Winnipeg, Manitoba
> R3E 3P1
> Ph: 204-986-3701
>  
> CONFIDENTIALITY NOTICE:   The information contained in this message is intended solely for the person or entity to which it is addressed and may contain confidential and/or privileged information.  Any use, dissemination, distribution, copying or disclosure of this message and attachments, in whole or in part, by anyone other than the intended recipient is strictly prohibited.  If you have received this message in error, please notify the sender and permanently delete the complete message and any attachments.  Thank you.
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