[CANUFNET] sidewalk materials suitable for a winter city

[Barwinsky, Martha]

Thanks for all of the feedback on email and phone! Much appreciated.

Martha Barwinsky
Ph: 204-986-3701


From: CANUFNET [mailto:canufnet-bounces at list.web.net] On Behalf Of Stacey Auld
Sent: Wednesday, August 12, 2015 10:01 AM
To: canufnet at list.web.net
Subject: Re: [CANUFNET] sidewalk materials suitable for a winter city

Hello Martha,

I am familiar with and sympathetic to this issue, having lived and worked for years in landscape design in Winnipeg.

As the sidewalk is proposed and not final, my first suggestion is to look for an alternate location for the pedestrian access that is comparatively convenient and functional, but which stays further from your elms and thus removes the need for a potentially expensive surface treatment with no guarantee of success. The elms in that area are more valuable than a sidewalk is, both by the City's replacement valuation system and by residents' perception. This approach has great potential to prevent planning and construction headaches, and is an opportunity to build an interesting access that draws attention to the urban forest as well.

If the site does not permit relocation of the access due to spatial constraints, consider a suspended surface, something like the tree vaults that were implemented along Edmonton St along the Manitoba Hydro building. Supports could be run along the outside edges of the proposed walkway (spaced out screw piles perhaps, to minimize root zone disturbance), and a solid or semi-permeable surface supported like a deck across the top, free from the heaving of the clay soils and protecting the root zone from compaction.

For durability of surface materials I have little to add that previous commenters have not already covered. In my experience flexible materials under big trees result in tripping hazards and inaccessible walkways, and more solid solutions are moved despite their best efforts and are expensive to replace. Perhaps a thinner solid surface of solid on top of a "crush zone" of softer material like is used between shifting concrete house foundations would work, but I haven't seen anything like that implemented for walking surfaces.

Good luck! Please post back here with your final solution, I'm interested to find out how you approach this.

Stacey Auld  MSFM RPF(Ab)
E: seauld at gmail.com<mailto:seauld at gmail.com>
Ph: 780-843-9509


Message: 2
Date: Wed, 12 Aug 2015 08:38:19 -0400
From: James Urban <urbantree at toad.net<mailto:urbantree at toad.net>>
To: Canadian Urban Forest Network <canufnet at list.web.net<mailto:canufnet at list.web.net>>
Subject: Re: [CANUFNET] sidewalk materials suitable for a winter city
Message-ID: <8114E3ED-C118-4209-9100-6290D126BDA8 at toad.net<mailto:8114E3ED-C118-4209-9100-6290D126BDA8 at toad.net>>
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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 settle
 ment.  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<mailto:jimtree123 at gmail.com>
http://www.jamesurban.net


On Aug 11, 2015, at 5:01 PM, Barwinsky, Martha <MBarwinsky at winnipeg.ca<mailto: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<tel:204-986-3701>
>
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