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Proximity of Trees to Carriageway and Tree Root Intrusion into Pavement for Urban Roads

  • 1.  Proximity of Trees to Carriageway and Tree Root Intrusion into Pavement for Urban Roads

    Posted 12 July 2020 23:10

    A recent enquiry to a local council about tree root intrusion, kerb breakup and proximity of the trunk to the carriageway resulted in a reply that in effect gave greater priority to the tree rather than safety to the road user and structural/surface damage to  the kerb and pavement. For a cyclist on a local 50kmhr road the surface breakup was sufficient to require deviation away from the kerb and reduce passing width for a car. My query is do any Councils have policies that relate to these problems and the interaction with vulnerable road users. Additionally can I get a copy of those policies.

    Cheers

    Tony Pyke

    Pro Cert 2


  • 2.  RE: Proximity of Trees to Carriageway and Tree Root Intrusion into Pavement for Urban Roads

    Posted 16 days ago
    Hi Tony,

    This is a very good question. I myself would be interested to see answers to this as I have to deal with these issues weekly. There are a number of issues though. From my experience the tree root problem on gutters and pavements take longer to solve due to the very high importance placed on maintaining the urban forest and minimising removal of mature trees. Below I'll outline just practical aspects of the issue rather than policy. Council policies can be obtained from all Council websites.

    1. Generally in our LGA you cannot cut a root within 3m of the trunk (structural root zone) without a tree officer approving this. They cannot approve it unless they can see the root which means you have to dig up the area and expose the root to get an answer.

    Consider this - what if the answer is "no you cannot cut the root". You have just dug up the whole place and now you are told you cannot cut the root without risking the health of the tree OR creating another safety issue that the tree might fall over in wind and kill or seriously injure someone or damage property or utilities.

    2. Yes there are safety issues and pavement requirements for cyclists and motorists. These need to be weighed up in relation to all other issues (point 1) which includes financial capacity for the Council to solve the problem or problems if there are many of these instances. Our LGA is 900km of roads with many tree root versus gutter issues.

    3. Council policies or the way Council thinks about problem solving issues fundamentally rests on the Civil Liabilities Act and its capacity to solve the myriad of issues with a finite budget. You are advised to start with that.

    4. The Council I work for doesn't have a specific policy on tree roots versus pavement risks, however we would look at any complaint and weigh the risk or risks versus budget capacity.

    I am regularly reconstructing deformed pavements and removing tree roots from the pavements due to it being considered an unnacceptable risk to road users. I currently have two jobs out to contractors for this now. One is in a road and one in a carpark.

    It just depends on the situation and each matter needs to be addressed on a case by case basis.

    ------------------------------
    Matthew Holt
    Construction & Maintenance Engineer
    Northern Beaches Council (NSW)
    t 02 9942 2843 m 0466 926 193

    northernbeaches.nsw.gov.au
    ------------------------------

    Pro Cert 2


  • 3.  RE: Proximity of Trees to Carriageway and Tree Root Intrusion into Pavement for Urban Roads

    Posted 15 days ago
    Hi Tony

    Trees in proximity to roads is a classic asset management dilemma in cost, risk and service. I've worked in organisations that are safety driven and/or environment drive trying to resolve this issue including at a state government policy level.  I hope this longish answer helps.

    I don't have a specific solution but I'll offer some insights on how to frame the discussion to get a useful outcome.

    When you have a tree close to a road edge you have several risks.  Risk of vehicle collision, risk of branches obstructing vision or carriageways, risks of falling branches or trees. You also have identifiable costs such as pavement damage from root intrusion, vegetation care, keeping drains clear and so forth. You also have beneficial effects such as soft green environment, people friendly streetscape, micro and macro environmental benefits (shade, CO2 reduction) and social/heritage value. You further have negative effects (such as pavement roughness and reduction in safe travel speed).

    The dilemma is how you manage these competing demands.  This dilemma is made worse because of different human (social/political) interests and inflexibility of approach by different stakeholders.  This is all compounded because you have a Boolean choice to make – a) damage or remove the tree or b) accept costs and risks that are undesirable.

    The choice is clear where you have one dominant concern.  For example on a high speed highway the traffic safety concern is dominant so you remove trees in impact zones.  Where you have a threatened ecological community or high environment value you need to engineer a solution (crash barriers, root barriers) and accept additional costs.

    It is the in between areas like neighbourhood streets and low volume rural roads that become topical and difficult.  Exactly the problem situation you are looking at.

    Somehow you need to be able to objectively assess and compare the risk, the service impact (benefit and reduction) and the cost across your road network to inform a valid policy. 

    For risk you can develop a risk assessment using your corporate risk approach that is difficult to argue with (as an approach although people will argue the details). That puts you on solid ground with respect to peoples safety 

    Cost you can presumably develop an estimate of the cost across the City of root damage based on proximity to the road although this would be a significant piece of work and require data.  This would include costs of disruption to traffic and other services to repair damage.

    Value of the tree is far more difficult to gauge because it's primary values are social and environmental.  However what you can do is offer informed stakeholders a choice about how much risk and cost they are prepared to pay for having a tree.  I know this is not particularly robust science but you'll almost always end for residential streets with a "we'll pay as a much as it costs and the risks are not relevant" response in residential areas. That's an answer that justifies a Council budget.

    Your problem then becomes (a) when does a residential tree become a significant risk (shedding limbs, age, sight lines),(b)when do you have to intervene as a professional organisation; and (c) what is the additional cost to keep trees.

    For you as an engineer I would suggest you adopt the OSH approach (modified for cost and asset damage).  What is the real cost and risk of the tree?  Then in order consider Eliminate (remove), Substitute (plant replacement trees away from the pavement), Engineering (crash barrier, pavement and drainage structures to divert roots), etc), Administer (warning signs, routine checks and pavement repairs) and Protection (crash and root barriers).

    Whatever you do recognise that you are dealing with a political/social decision as much as an engineering/cost/risk decision. Likely you will need to propose the compromise solution and accept everyone will dislike it a little bit .  You need to retain the ability to keep people safe but beyond that everything is a compromise in service, cost and risk.

    Hope this is useful.



    ------------------------------
    Graham Lantzke
    Principal Asset Engineer
    WSP Pty Ltd
    graham.lantzke@wsp.com
    PERTH WA
    ------------------------------

    Pro Cert 2


  • 4.  RE: Proximity of Trees to Carriageway and Tree Root Intrusion into Pavement for Urban Roads

    Posted 14 days ago
    Matt and Graham

    Thank you for your time and detailed reply on this subject. Though I am not connected directly with this subject at the moment it enabled me understand the issues faced by Asset Managers .

    Having supplied crash barriers to protect trees I used to wonder about the expenses being incurred to protect trees and how the LGA or Asset owner justifies it!

    Your replies in detail made me understand the subject better. Thank you.

    Regards

    Swami Nathan B.E MIE(Aust) PG DIT
    International Consultant
    LB AUSTRALIA PTY LTD
    PO Box 94, Wentworthville
    NSW 2145, Australia
    Phone: +61 2 9631 8833
    Fax: +61 2 9688 4503
    Mobile:0411 559371
    e-mail:swami.nathan@lbaustralia.com.au




    Pro Cert 2


  • 5.  RE: Proximity of Trees to Carriageway and Tree Root Intrusion into Pavement for Urban Roads

    Posted 14 days ago
    Hi Anthony,

    I echo previous comments in that it is a complex issue and i wouldn't expect any council's to have a specific written policy focused on tree roots vs road pavements.
    The only thing i would add is that a lot of council's have policies whereby every tree is assigned an "amenity value" (expressed in dollar terms, based on size, species and remaining life) which must be paid if the tree is removed. In my case (road designer for council) if the arborist determines a tree is no longer viable due to significant works in the TPZ/SRZ and must be removed, we would need to pay the amenity fee for the trees replacement, the fees are significant and put a lot of pressure on designers to seek alternative construction options.
    For example in one recent case where the road pavement was severely affected by root intrusion the council has opted to extend pavement life in line with the remaining life of the trees by asphalt overlay and isolated kerb replacement (pluck out existing kerb and form new kerb without any destructive excavation). In this case the trees are expected to be near to end of life in another 5+ years whereby the road could be fully reconstructed.



    ------------------------------
    liam collins
    Wyndham City Council
    Project Engineer
    WERRIBEE
    Australia
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    Pro Cert 2


  • 6.  RE: Proximity of Trees to Carriageway and Tree Root Intrusion into Pavement for Urban Roads

    Posted 10 days ago
    Well put Graham. Thanks for answer.

    ------------------------------
    Matthew Holt
    Construction & Maintenance Engineer
    Northern Beaches Council (NSW)
    t 02 9942 2843 m 0466 926 193

    northernbeaches.nsw.gov.au
    ------------------------------

    Pro Cert 2