Root Protection Area questions

[Treecreeper1961]

I am currently working on a couple of BS5837 surveys and as usual issues arise.

Namely, not enough space to accommodate tree RPAs and other things.

 

It would help me if any comments on the following could be posted (nothing rude please, i'm sensitive and like trees):

 

1. If the RPA of a tree inhibits a development the options are:

a)no development

b)remove tree

c)engineering solutions

d)depending on the value of the tree and it's location, breach the RPA and hope it survives?????(This is ill advised of course, but in some circumstances better than removal?)

 

2. The BS5837 indicates it is acceptable to undertake certain work within an RPA, such as hand digging, thrust boring, pile foundations and radial trenching. These would inevitably lead to a limited loss of rooting environment an damage to some roots. In a similar context, could removal of a shallow layer of soil(250m) within the outer area of an RPA be an acceptable intrusion.

Assuming the roots are mainly within the top 600mm of soil horizon removing 250mm of soil, pruning any damaged/exposed roots, replacing 250mm of soil with cellular material and clean granular infill, area of RPA impinged upon 2m of 4.5m radius.

 

3. Does the species and height of trees allow for reduced RPAs ( if compromise is needed)? For example, a mature Hawthorn may command a fairly large RPA. I think they are worthy of as much protection as any other tree but in reality the mechanical implications of lost roots are not as severe compared to loftier species.

 

Thanks....

[Paul Jenks]

You have to be true to your principles. If you are conducting a BS5837 it has to be objective. If you don't know the design of proposed development then caveat with 'subject to future use/design etc' when making recommendations for works. If you do know the design layout you can say whether the footprint impinges too far into an RPA and make a professional decision as to whether you feel the tree is sustainable or not. (Include shading, future growth potential and nuisance).

Often the design will have been completed on a computor using a package prior to the architect knowing the site trees. (I understand that the new draft of the BS5837 will push designers to consider trees prior to designing and that later manifestations of the BS will require them to so do.)

We have found that it helps the case if a tree is to be retained its life chances should be demonstrably improved as a result of development;terraventing, porous surfaces, cellweb etc.

Alas, most developers have a culture of maximising the return per hectare and if a tree means 2-3 less houses.................

It is generally a good idea to suggest different foundation systems to minimize the impact on the tree and the trees' future impact on the building. Also endeavour to include in your method statement arboricultural supervision for any works within the RPA and for the initial installation of the fencing. It may get you involved throughout, therefore more £££. If they ignore your recommendations, as happened to us recently, you have the high moral and legal ground. Doesn't help the trees but may serve as a lesson to the developers if the tre officer decides to take it to court.

Consideration has to be given to the category of the tree. Some trees will have a limited life left and one has to take a pragmatic view. Whilst we would like to see trees left to live, grow, die and decay undisturbed sometimes this is not possible if a piece of land is needed for development.

(this is not an exhausive review, just some thoughts based on how we do it. I have to tuck the kids in now, ( and I sense some of you may be bored))

[Treecreeper1961]

My kids put themselves to bed nowadays and Your reply doesn't bore me. Thanks.

[Paul Barton]

I'll stick in my tuppence worth then:

 

Question 1: I think you have covered the options well with your 4 suggestions. All depends on the amenity value and long term contribution that the tree can make to the development.

 

I try to think forward as much as possible, bearing in mind what I have estimated as the remaining contributions for each tree. To my mind it is better to be pragmatic and remove trees with a low life expectancy and recommend some really good re-stocking of the site that will make a long term contribution. However, all too often I find myself recommending some engineered solution so that the client can squeeze the proposal as close as possible to the tree(s). No matter how much I personally feel it is inappropriate, the client's wishes are what drives the survey and the planning application so they must be 'respected'.

 

Question 2: working in the RPA. I try to avoid any removal of top soil at all now, instead suggesting levelling the ground by infilling divots with sharp sand and then overlaying the geotextile and cellular confinement system. This does upset architects though because it messes with their levels!

 

A pile and voided raft is always preferable to a ground beam - I am astounded how many arbs and architects think groundbeams are adequate near tree roots. They are usually installed to a depth of 500mm!

 

Question 3: absolutely the species and habit (not to mention soil conditions) should affect the anticipated RPA. Remember the BS is only guidance - if in your opinion the RPA can be sensibly argued to be smaller or larger then so be it. If the LPA don't like it you will just have to be prepared to prove the point to a planning inspector at appeal!

 

Personally I am bored of these reports now...an arb in the private sector doing BS5837 reports is really just part of the development machine that is shoe-horning buildings too near to trees and I am fed up with it. I fundamentally disagree with the constant infilling and intensification of urban areas - we are running out of green space in our cities!

 

Anyway, I could rant on...

[Treecreeper1961]

Thanks Paul, useful food for thought.

 

If these surveys are now boring you what work are you enthused by? Tree condition and health care?

 

Cheers.

[Paul Barton]

Thanks Paul, useful food for thought.

 

If these surveys are now boring you what work are you enthused by? Tree condition and health care?

 

Cheers.

 

To a degree yes. I am more interested in urban forest management now...it's the realm of tree officers really unless you have a contract for entire tree management for an LA (which appears to be becoming more common).

[treeseer]

"1. If the RPA of a tree inhibits a development the options are:

a)no development

b)remove tree

c)engineering solutions

d)depending on the value of the tree and it's location, breach the RPA and hope it survives?????(This is ill advised of course, but in some circumstances better than removal?)"

 

Other options include helping the tree survive some breaches and intrusions

e) Enlarge the RPA beyond that circle, where conditions favor.

f) Improve rooting conditions in the RPA--aerate, inoculate, fertilise, mulch...

g) Reduce tree to reduce demands on roots (yes obviously this can be overdone and backfire, so it is at the bottom of the list)

 

"2. The BS5837 indicates it is acceptable to undertake certain work within an RPA, such as hand digging, thrust boring, pile foundations and radial trenching. These would inevitably lead to a limited loss of rooting environment an damage to some roots. In a similar context, could removal of a shallow layer of soil(250m) within the outer area of an RPA be an acceptable intrusion."

 

NO excavation, on most sites: as Paul notes, find another way. Grading down is not in the same context as specified vertical intrusions. Have you done a soil survey with long probes, to see where the roots are? If they run deep on that site, that would be a factor in the decision to allow excavation--of soil, not roots.

 

"Assuming the roots are mainly within the top 600mm of soil horizon removing 250mm of soil, pruning any damaged/exposed roots, replacing 250mm of soil with cellular material and clean granular infill, area of RPA impinged upon 2m of 4.5m radius."

 

That's a huge impingement (?)

 

"3. Does the species and height of trees allow for reduced RPAs ( if compromise is needed)? For example, a mature Hawthorn may command a fairly large RPA. I think they are worthy of as much protection as any other tree but in reality the mechanical implications of lost roots are not as severe compared to loftier species."

 

Yes, of course. If BS5837 does not have an adjustment for species and condition it is far too rigid. Our scale slides down to .5 for tolerant species, and up to 1.5 for old vets.

 

I like working with infill development, because I do not like urban sprawl.

Edited February 15, 2012 by treeseer

[Paul Barton]

Hi Treeseer,

 

By point (g), do you mean reduce the crown to reduce the structural demands on roots, or to curb root development?

[treeseer]

Hi Treeseer,

 

By point (g), do you mean reduce the crown to reduce the structural demands on roots, or to curb root development?

 

Paul, it depends on the demands. Mainly I had in mind the biological/hydraulic demands, hence the caveat and caution. 9% max. live crown off is a common spec, and I have written 4% too. Reduction is more often targeted to improve symmetry/balance/alignment with structural roots than overall. Crown cleaning is typically done in advance as well.

[Treecreeper1961]

Yes, of course. If BS5837 does not have an adjustment for species and condition it is far too rigid. Our scale slides down to .5 for tolerant species, and up to 1.5 for old vets.

 

I like working with infill development, because I do not like urban sprawl.

 

Treeseer,

As far as I am aware there is no clear guidance regarding varying RPA for different species.

 

Paul, how do you work out a smaller RPA for a more shrubby species?