# Formula to compensate TPZ



## Glennak (Feb 12, 2010)

Sorry having trouble loading formula maybe I will try attaching a word doc. Hope that worked.
Hope this is usefull to someone even better if someone can give a more accurate formula. Builders seem to want to give trees the absolute minium area to grow in.


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## pdqdl (Feb 12, 2010)

Quite frankly, the text with the document does not clearly explain what you are trying to measure. We can guess from the drawings, but you really need to explain it like it was a word problem in high school. 

Formulas are nice, but they do not tell you what the problem is. Spell it out for us, please.


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## Glennak (Feb 12, 2010)

Well the new Australian Standard says that the Tree Protection Zone is calculated by multipying the DBH by 12 the same as many USA ones that say 1' radius for every 1" in DBH. Less than Coder who says in a round about way DBH x 15.
The Aus Standard says that if this TPZ is cut into by a trench or what ever it is not allowed to take out more than 10% of the TPZ ( Thus the formula to calculate the area taken out by a straight line.) this area removed must be compensated for by adding to the TPZ in other directions. The second formula is to calculate a new TPZ radius for the remaining semicircle. My formula is slightly incorrect that's why I was asking for a better one but it is well within the accuracy of measuring DBH.
Basically the concept is simple but the maths isn't.


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## treeseer (Feb 12, 2010)

Thanks for clarifying what is clearly an extremely restrictive standard. We allow much more rootzone lost, without losing trees.

Attached is one graphic we use.


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## Boa07 (Feb 13, 2010)

> Thanks for clarifying what is clearly an extremely restrictive standard



Guy, the standard is anything but restrictive IMO, what it is trying to achieve is to ensure that a qualified Arborist is the person determining the degree of the impact on the tree/s in question.

I don't disagree with the implications of the US BMP but the same elements are included within our National Standard.

Our standard AS4970 The protection of trees on development sites, is very closely related to the British Standard.

In terms of the TPZ our standard states:


> Cl 3.3.2 Minor Encroachment:
> If the proposed encroachment is less than 10% of the area of the TPZ (radius calculated as 12 times the DBH) and is outside of the Structural Root Zone (SRZ) then additional detailed root investigations should not be required. The area lost to this encroachment should be compensated for elsewhere and contiguous with the TPZ. Variations must be made by the project Arborist considering relevant factors listed in Clause 3.3.4
> 
> 3.3.3 Major Encroachment:
> ...



There is a lot more but honestly it is probably enough to show that the standard is really just trying to codify what I beleive your BMP is directing.

A lot more than 10% can be permitted provided there is evidence to support the sustainable nature of that loss of root mass and future potential growing space.

In terms of the original question... the maths formula yes it is a little complicated I have a less than perfect excel formula that allows me to input approach distances and provides the % of TPZ area lost (not quite the sequence you are wanting...and not the sequence I was originally trying to produce but it works for me)

A very decent rule of thumb is to say one third of the TPZ radius equates to a 10% loss in area....in other words...
For a 9m TPZ you can encroach on one side by 3m (reducing the distance to the tree centre on that one side to 6m) and be within the 10% specification.


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## treeseer (Feb 13, 2010)

Thanks much, Sean. I hadn't read the ISA BMP since the review drafts, and now find youre AS has some fine guidance that should be worked into ours 3 years from now at revision time.


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## Glennak (Feb 13, 2010)

Boa07 I just calculated your 1/3 radius it is not that accurate actually removes 10.96% almost a 10% error. You would be better off measuring the angle and using the formula I gave in the word doc. I have this in a spread sheet along with the other formula to work out how much larger the radius has to be. Feel free to copy them it saves a lot of effort.

I also use a formula to calculate Mattheck's CRZ it is large and complex but it follows the graph perfectly. Much better than the very inaccurate one in the Aus standard but theirs is correct twice. If you want to try it put this in a spread sheet.
=0.055670882088896+0.010193823895656*A2-0.000015792217071675*A2^2+0.000000013725628719343*A2^3-0.0000000000058289033463574*A2^4+9.5123023115925E-016*A2^5
With the A2 being the diameter above the root base in mm.
I just love measuring the diameters putting them in a spread sheet and you have all the figures you need to write reports with minimal effort. But now I may have to use 
Dsrz=(D x 50)^0.42 x 0.64 as rough as it is seeing it is an Australian Standard.


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## Boa07 (Feb 13, 2010)

Glennak, I was obviously not clear enough I don't use the 1/3rd roughie just meant it is useful to give some idea of when you are needing to be more precise. 

You certainly like playing with the formula which is nice.

No-one is required to be more precise than the actual formula in the standard for SRZ, but I am sure you are right that there might be inaccuracies within that....always remember you have to have the Basal diameter measured to calculate SRZ.

I don't personally think anyone is ever going to pick up any Arb for not calculated the little triangles that are lost using the general grometry calcs for segment areas btw.


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## Glennak (Feb 13, 2010)

Yes the 1/3 radius would be good to get a rough idea of the 10% limit, I will remember that one.
Here is part of my submission to the draft Australian Standard.
DR AS 4970
3.2.2.3.4
I assume your formula SRZ = (D * 50) ^.42* .64
Is an attempted to follow Mattheck’s data, the following formula fits Mattheck’s data far better than can be read from his graph at such a small scale.
F(x)=0.055670882088896+0.010193823895656*X-0.000015792217071675*X^2+1.3725628719343E-08*X^3-5.8289033463574E-12*X^4+9.5123023115925E-16*X^5
It is very long but this is what I use to calculate CRZ in my reports, I calculated it with Math Mechanixs that is also where the graph is from.
I’m sorry I converted your formula to mm because it was much easier to convert than mine. 
As you can see your graph, in red, is not very good at less than 200mm right at 300mm and again at 1m (beyond this Mattheck has little data) but close enough except for less than .2m. You say for trees with diameters less than .15m SRZ = 1.5m. Now in Mattheck’s graph trees with diameters of .2m have CRZ of 1.5m. So maybe the standard should read “for trees with diameters less than .2m SRZ = 1.5m.” This would take the worst of your graph out as well. Another point is trees with diameters of 100mm (.1m) are not going to fall over if you dug 1m all the way around ( Mattheck’s data shows this as well) so why say they need 1.5m?
Perhaps you could use my formula to get very good data and use other software to get a simpler formula then if the lower end was more accurate you could say trees less than .1m in diameter have a SRZ of 1m and use the graph or new formula from there. Or you could use mine if you like.

I assume this .15m or less gives1.5m SRZ was added so there would not be a step in the graph and to reduce the lower end inaccuracies. As you said earlier you can justify smaller areas so this lower end inaccuracy should not be a major problem. Also small trees can just be replaced or moved.
As you may have guessed I have a science background but have worked with trees for 35 years.


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## Glennak (Feb 13, 2010)

my graph didn't upload I'll try again.


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## Ekka (Feb 14, 2010)

Glennak said:


> But now I may have to use
> Dsrz=(D x 50)^0.42 x 0.64 as rough as it is seeing it is an Australian Standard.



Be careful, the red part should read Rsrz (radius) not diameter.


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## Ekka (Feb 16, 2010)

*Error in SRZ graph or formula or my calculator*



Glennak said:


> DR AS 4970
> 3.2.2.3.4
> I assume your formula SRZ = (D * 50) ^.42* .64



Interesting. There's a serious error in the Standard perhaps.

Here's the graph, I made lines so you can get the answer easier.







Now if you look at the graph with a 1.4m stem dia the answer is around 4.5m

But you get your calculator out and it's 3.81m

So I try another number, 0.8m stem dia and the graph reckons around 3.6m but get your calculator out and I get 3.01m

Anyone else want to check this, means the graph is way out in that Standard and extremely misleading or I cant use a calculator.


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## Glennak (Feb 16, 2010)

Wow Ekka you're right they have stuffed up big time.
If you look at the graphs I put up one is my formula that foliows Matthecks graph perfectly and their formula but it looks like their graph is even worse. Will have a play tomorrow have to get some sleep been climbing all day and I'm not used to it and getting old.
Just looked their formula is much better than their graph. I think I will right to them again.


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## sgreanbeans (Feb 16, 2010)

being the brother of one of the Midwest's largest custom home builder/ developer, I can say this with a fact. Builders don't care! Including my brother. I try and give him advice, all he wants to know is, how much, how long and when can ya start. The only thing that matters to these guys when it comes to the trees " I don't care if its gonna die in the next couple years, It wont be my problem then, I want it to stay, It looks better with that 100' Oak RIGHT NEXT TO my $750,000 Condo, leave it alone" They don't care if half the roots were ripped out with a excavator! GET ER DONE. " Were do I want the cement clean out, hmmmmmm, how about right next to that big ol tree over there, that way its out of the way, maybe that could be the base off a paver patio under that tree, yep lets do it, raise that curb appeal, lets excavate about 2 feet of the topsoil away from that tree so the patio has a good firm clay base" 
Trying to educate these guys is like convincing a bear to go on a diet during the salmon run! TPZ is bad word to the landscapers as well, retaining walls and patios, run into root while digging? get the sawzaw!
I know this was kinda off to the side, just reading the comments about builders, just thought I would throw my 2 cents, hope it didn't hit anybody!


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## Boa07 (Feb 16, 2010)

I think perhaps it is true that the graph should have had a notation indicating that it was itself merely indicative of the curve produced by the calculation.

Glennak I have been giving your maths problem some thought, and can really do no better than the equation you provided on your word doc.

I would however point out to anyone using AS4970 that given the reality of root architecture in the ground....please pay attention to ensuring that the area (and therefore volume) being protected is reflective of the actual growing environment surrounding the specific tree, and the discernible pattern of root growth....I would not include the area under the main road 2m off the stem of the tree (unless you can justify the claim that it represents a viable root growing environment!) when calculating TPZ areas....existing constraints are to be identified and accounted for.

Very very few trees I have ever assessed come anywhere near the idealised model represented by perfect circles on a plan.

The standard provides a very simple formula for planners and designers to follow (if they can) to avoid signficant impacts on trees being retained, it is not (IMO) intended to be a short cut for Arborists to avoid carrying out detailed assessments of trees that are proposed for retention in close proximity to developments/buildings.

I know some of us had been incorporating the British standard for a few years before the draft was even a twinkle in the SA committee's eye, but for those who had been applying other methods actually applying AS4970 might be somewhat daunting and confusing.


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## Glennak (Feb 16, 2010)

Thanks Boa07, when I was making my formula I did a rough scetch to see what I was doing and my outside circle was twice the size of the inner circle so the extra triangles were much larger than in realality. With 10% or less they are insignificant


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## Ekka (Feb 16, 2010)

Just received confirmation form Standards Australia, ammendment due out soon.


> Dear Eric
> 
> Many thanks for your email and the concern you have raised.
> 
> ...


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## Corymbia (Feb 27, 2010)

*Where does the formula come from*

I realise that it is from the AS4970 but where did they get it from? Where is the published papers ... perhaps that will explain the error in the graph


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## Ekka (Feb 27, 2010)

It's in the standard


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## Glennak (Feb 27, 2010)

Corymbia said:


> I realise that it is from the AS4970 but where did they get it from? Where is the published papers ... perhaps that will explain the error in the graph



I can't understand what you are asking. Their formula is (I think) a poor attempt to follow Matthecks graph on wind thrown trees. Their graph is probably just an error while printing. If you read the previous posts it should be a bit clearer. I tried to use a formula like the one they used but I got similar misfits so I used a program to get an exact formula seeing I put it in a spreadsheet it doesn't matter how large and complicated it is.


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## Ekka (Feb 27, 2010)

There's an online calculator if you Google it, accurate and to the current standard .... fool proof.


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## Boa07 (Feb 27, 2010)

Stick to the standard or better (and provide compelling evidence that it is better - good luck) and you won't go far wrong.


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## Corymbia (Feb 28, 2010)

*Who wrote the SRZ formula*



Ekka said:


> It's in the standard



I actually said that it was from the Standard but where did they get the formula from? Is it published or did some mathematician at SA just work out a formula that produced a curve close to Mattheck's or what? 

Here we are producing a figure based on a formula that may not be at all related to any published SRZ formula. The author or source of the formula does not appear to be cited in the Standard or did I miss it?

Does anyone know?


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## Glennak (Feb 28, 2010)

Why should a formula be published any where for them to use it? They have published it now. When I asked my son what formula would follow this graph he looked and said Y = (X * A) ^B*C and showed me on his calculator, he was doing year 12 at the time. I tried changing the constants to make it fit but it never fitted properly like Standards Australias formula. I'm sure who ever made up their formula and I were not the only ones who got pissed off trying to read off a tiny graph and wanted a formula. The software Math Mechanixs (which is free) will make a formula to fit any data thats what I used.


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## Corymbia (Feb 28, 2010)

*The formula is based on the graph?*

OK! That makes more sense. So if I understand correctly what we have is a formula that replicates the upper curve for the data for failed trees from page 187 of the Body language of trees. That is fine but it would seem reasonable that the source of the information relating to SRZ's be made known since the size of the SRZ is NOT generally accepted.

Yes, it does make a huge difference from where industry best practice comes. If it comes from a small committee at SA (many of whom lack even graduate studies in arboriculture) then it may not be industry best practice at all ... Surely our profession should determine best practice and the standard then reflect that and not the other way around?

In fact it only takes a few seconds to look at the graph and the data to realise that the size of the SRZ that leads to failure can vary enormously and in fact there were failures that occurred above the curve. With the exception of smaller trees the data largely refers to trees that failed ... that is trees that had defective root plates ... What this data set looks at is the point of failure and not the SRZ

If we look at a tree with a 200mm trunk it needs a TPZ of 2.4 metres a SRZ of 1.26 metres using the formula and yet the data set says that the SRZ could be as large as 2 metres and as small as 0.5 m. If we accept that volume is cubic then there is a potential variation in the mass and volume of a failed root plate of 6400%. It is a good thing that this approach is not used to determine housing interest rates

Lastly, none of the research that I have found considers how partial encroachments into the SRZ affect stability if at all. Ignoring the obvious dilemmas with the data set mentioned above it has to make you wonder what the function of calculating the SRZ is then in terms of Protection of Trees on Construction sites. Perhaps on the odd rare occasion it may be useful to calculate but in most instances it appears to be little more than irrelevant.

Please let me know what I have misunderstood


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## Glennak (Mar 1, 2010)

Corymbia what do you mean the SRZ is not generaly accepted? Yes it would have been good if they gave the source I just assumed it was from Mattheck as their formula is close to the graph.
Why do fallen trees need to have defective root plates? Strong winds push over trees with any sort of root system unless the trunk snaps first.
What else is a Structural root zone but the area needed to hold the tree up ie the point of failure.
Your example of a tree with a 200mm trunk I assume that is diameter. I get SRZ 1.68 and CRZ of 1.56 which is about 8% more than the CRZ. Where does your 1.26 come from?
As for your data set shows from 2m to 0.5m??????????? I assume this is Matthecks data to which you are referring. It is plotted radius not diameter. So your 20cm radius shows trees that failed with root balls of 2.35m to about 0.5m. The SRZ has to be based on the larger the 0.5 could have been caused by anything but it is data and should be shown even if it is not used.
Where do you get "it comes from a small committee at SA" if you look at the standard it states who they consulted. I have been on committees to discuss the prunning standard and training levels in the aboricultural industry it is a lot of unpaid work so don't knock'em. They don't even get a free copy when it is finished. The ISA sent me an email to say the draft standard was out and to read and send comments. I did so and was ignored did you send any suggestions?
Yes the SRZ is usuallly well inside the TPZ and is often not needed but it can be relevent as the standard states that encroachments into the TPZ should not enter the SRZ or something similar.


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## Ekka (Mar 1, 2010)

The minimum SRZ will be 1.5m

The minimum TPZ will be 2.0m

That's in the standards, Corymbia if you used the online calculator you would have got the right answers. 

Why calculate the SRZ's? What about activities inside the TPZ like putting in posts or piers? What about other stumps? If for example a tree has a 10m radius TPZ and 3m Radius SRZ and was a wooded site I could grind stumps right out when I'm say 7m away from the tree however within the SRZ I would only lick them off to the soil surface.


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## Corymbia (Mar 1, 2010)

Ekka said:


> The minimum SRZ will be 1.5m
> 
> The minimum TPZ will be 2.0m
> 
> ...




TPZ radius according to 3.2 will be 2.4 metres (12 times). 

You are right SRZ radius will be 1.68 metres I inputted radius rather than diameter

That makes the situation worse! Most guys in the US would move this size tree with a John 90 or a radius of 1.1 metres ... something seems odd here unless of course the standard says somewhere that this is the maximum SRZ size but it may be smaller and to seek suitably qualified and experienced advice ... I just couldn't find that clause. So if the SRZ is not accurate then why calculate it.

You suggest to limit the location of piers ... but why should an inaccurate figure be used to restrict construction?

You suggest stump grinding and I would agree ... there are grinder operators out there who just don't think but any smart arborist, such as yourself, looking at the image you show should know to operate the grinder that close to another tree with care. If they are not smart enough to figure that out they certainly wont be using the formula or the on line calculator.

If you did need a formula to know how to handle that stump then you shouldn't be using machinery. I have a great sticker on my grinder it says ... "This machine does not have a brain so use yours"


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## Ekka (Mar 1, 2010)

With the numbers I posted it wasn't specific to your 200mm DBH tree but just saying that's the minimum in the standard for all trees.


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## Corymbia (Mar 1, 2010)

Ekka said:


> With the numbers I posted it wasn't specific to your 200mm DBH tree but just saying that's the minimum in the standard for all trees.



Got it ... again I don't pretend to understand that one either ... small or young trees are given additional protection? In fact a 1 inch caliper tree is given 120 square feet of protection yet you can buy a nice one 12 inch B&B ... or less than 1 square foot? I just wonder who pays ... the developer or all the young trees that get removed because of crazy stuff like that!


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## Glennak (Mar 1, 2010)

Yes the standard is hopeless when it comes to small trees but who would report on a tree with a 1" caliper? Anything small can be replaced or moved.


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## Corymbia (Mar 1, 2010)

I guess that same argument could be raised about most trees up to somewhere between 8-12 inches. Unfortunately it is this sized tree that is frequently valued lowest in the design process but the size that adapts to development most readily.

Instead we seem to value the bigger older trees more highly and then try and squeeze and manipulate the design and the tree protection to retain trees that are often less than 50 years older than the smaller trees that are simply trashed.

By comparison if a boat sinks it is the young and the women that get first dibs on the life rafts. We understand the value of youth at that point but not when we design.


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## Ekka (Mar 2, 2010)

Here's a paragraph I recently wrote regarding this issue. Often I see large old half dead trees being saved whilst the vigorous good formed smaller ones get dozed.



> Traditionally arborists, ecologists and green groups gravitate toward “saving” the largest trees .... after all they offer the most benefits.
> 
> The problem is the largest trees:-
> 
> ...


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## Corymbia (Mar 2, 2010)

*Space for growth*

I guess providing for the future needs depends on a lot of factors but we regularly see street trees growing in very confined space with roads on one side and condos on the other. Or worse still planted in the median strips in the middle of the road yet often seem to do very well and there are many youngish trees with 3 foot trunks. 

The bigger problem is damage to the surrounding infrastructure and that is probably more due to poor workmanship, inadequate engineering and a lack of forethought.


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## Glennak (Mar 2, 2010)

Yes with structural soils this is not a problem. We had medium strip street trees in Williamstown (300-400mm diameter spotted gums) a few years ago that were dug all the way around 1m deep some with less than 1m root balls. (the engineer was told this would kill them but he thought he new better than the arborist). These trees got 100km/hr winds on this day and didn't move only one died. The soil they were planted in was structual soil formed accidentally when the road was made in the distant past. Very few roots were cut as they all went down 45deg or steeper. The whole lot were removed as no one would say they were safe and the company that planted new ones dug this structural soil up and put in some top soil to a depth of 1.5 m or so. I thought this would have caused the roots to run along the surface and destroy the road but all seems OK maybe they found the structural soil out side the top soil and have the best of both worlds? The residents lost an avenue of trees and had to pay for replacements at great expence and the idiot kept his job


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## Corymbia (Mar 3, 2010)

*Using grey matter*

Another option is to engineer things so that they are not damaged by roots. My favourite house was built on screw piers. I used the concept to design a footpath that could not be lifted or broken by roots of a fig tree.

This picture shows the underneath of a brick house with concrete floors.

Now basements ... that is another set of problems altogether


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## Ekka (Mar 10, 2010)

Screw piers are very good. Also an engineering option for a clay soil with ground water, screw to spec torque and slab on top


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## Corymbia (Mar 20, 2010)

*Another strange standard*



Ekka said:


> Screw piers are very good. Also an engineering option for a clay soil with ground water, screw to spec torque and slab on top



I agree. New AS draft standard (DR AS 2870) for residential slabs and footings is out for comment and they suggest getting rid of trees (or planting no trees and shrubs on most residential lots) rather than figuring out that at some stage they may get planted ... I say screw piers for all reactive soils ... screw the cost ... screw the footings rather than screwing with the trees.

Unfortunately the draft standard it is too big to attach as a pdf file. I'll see if I can split it into two

Here are some salient points ... no arboricultural representation on the committee.

References as follows



> REFERENCES
> 1 Urban Tree Risk Management–A community guide to a program design
> implementation–1992.
> 2 Key guide to Australian Trees – L. Croninn – 1988.
> ...




That's right ... not a single relevant arboricultural text. Perhaps engineers should go about and do their job properly (screw piers) and let us go about doing ours


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## Corymbia (Mar 20, 2010)

*A delightful little quote*

From DR_AS_2870



> The soil areas around leaky drains or roof gutters will *attract * roots (emphasis mine) because of the availability of water, oxygen and disturbed soil for easier root growth. Neglect of garden watering or water restrictions (which often coincide with droughts) will cause trees to extend their roots into new areas and particularly near well-watered gardens or infrastructure.



To think all the time I thought roots were attracted by mini skirts. So to use some Australian vernacular "the tree root humps its swag and billy and goes looking for water"? For anyone who believes that I own the Sydney Harbour Bridge and have shares for sale


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## Glennak (Mar 20, 2010)

Yes Corymbia those engineers want a document so they can blame trees for even more problems. Many years ago I was told of a court case where a house in Carlton was cracked while digging to repair they discovered the house had no footings, The sewer was cracked under the house behind the crack, and there were a few tree roots there. It was a crack caused by lifting and the tree was trying its hardest to shrink the soil back and repair the problem but it got the blame.
It was a Melaleuca stypheloides which is high on the SA list of trees causing damage so council should have known better. And of cause no tree should be planted within 1.5 times its eventual height from any structure (ie no trees in suburban areas).
The SA list was compiled of a list of trees that caused damage in Adelaide no thought of how many of which type of trees are planted. So this list is a list of the most common trees planted in Adelaide and is used all over Australia as a list of trees that causes damages. Now a reference for this new standard.


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## Boa07 (Mar 20, 2010)

Corymbia said:


> I agree. New AS draft standard (DR AS 2870) for residential slabs and footings is out for comment and they suggest getting rid of trees (or planting no trees and shrubs on most residential lots) rather than figuring out that at some stage they may get planted ... I say screw piers for all reactive soils ... screw the cost ... screw the footings rather than screwing with the trees.
> 
> 
> That's right ... not a single relevant arboricultural text. Perhaps engineers should go about and do their job properly (screw piers) and let us go about doing ours





Thanks for the heads up....this is quite ridiculous I feel a long and unpleasant letter coming on...and not just to the draft committee...if we don't get organised and fast we will find ourselves behind the root barrier on this one and we'll regret it for a VERY long time.


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## Boa07 (Mar 21, 2010)

On closer reading of the draft....yes it does have the absurd bibliography from the 1970's and 80's....however it also states within Appendix CH that 



> This practice note has been written in consultation with senior members of teh arboricultural (ISAA) _[missed a C there I suspect????] _and housing geotechnical industry (FFSA) and provides guidance to reduce the risk of foundation movement and better footing designs in the proximity of trees
> CH5 Alternative Design Mehtods p154



It also makes specific reference to 15yrs of experience from SA supporting the approach taken within the standard, (Design of Buildings for Tree Drying of the Soil p150)

I am going to have to put time aside (which I do not have right now) to carefully read through the document, there appear to be some very odd descriptions of root growth and variations in water demand based on factors including tree height. 

In the mean time I strongly suggest any Aussies reading this advise their own state professional bodies of the draft DR AS2870 and have them review it for comment.


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## Corymbia (Mar 30, 2010)

*Not quite there*

Ignoring all the technical issues, the anthropomorphising etc there is one significant underlying fault to the standard. 

What the standard seems to be saying is that engineers can specify a standard that may result in the footing or slab failing if there is a leaking pipe or if someone plants shrubs or trees to close to the house (too close being one or two house lots away.

One must ask the question "Why shouldn't engineers design the structure to deal with what should be an inevitable event?" (- most people plant a tree on their lot). The issue is not just limited to the owner of the structure but impacts on adjacent property owners and existing trees.

It would be like designing a roof that may leak if we got one inch of rain in an hour. No one would tolerate that yet here we are writing a standard that acknowledges that when subjected to a probable event the standard will produce a product that is likely to fail? Pity help us all.


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