Chances of heave if willow is felled?

[boris360]

Hi all,

 

I'm looking for advice on whether I need to get an arborist's report before a willow is felled.

 

The willow is at the bottom of my garden.

The distance from my garden retaining wall is 21 metres.

The tree is downhill from the house on a slope of approx 7º.

It's on clayey, loamy soil (I think).

The tree is mature.

The tree was there before the house was built.

The house is 14 years old.

 

I've spoken to a structural engineer, the council tree advisor and also a tree surgeon all of whom say the tree is too far away to cause heave or any other issues. I thought I'd ask here because I expect there are more knowledgable people on this type of issue.

 

I've read about heave on these forums and also about how bad willows can be hence my caution but I'm not sure how far away a willow can be before it's no longer a problem.

 

Any advice would be much appreciated. Thanks.

[kevinjohnsonmbe]

 

An interested read about it for tea break.

 

http://www.geplus.co.uk/download?ac=1423914

 

That's a lot of tea!!

[Gary Prentice]

Um not sure what your getting at. If you reduce a tree over a period of time (you can reduce the roots aswell vat that's a whole different ball game) then you are allowing water to re-wet the soil under the tree slowley and minimise the effect of heave.

 

Sent from my C6903 using Tapatalk

 

Giles Biddle has a photo of a property where heave is continuing after 30+ years (after tree removal)

 

A complex subject

[benedmonds]

. The sure fire way of covering your back if your really worried is to reduce it back over 5-7 years. That is what structural engineers will say as that is what they have advised many of my clients ...

I know this is often the advice given, but I am pretty sure that it's nonsense..

 

If there is a persistent moisture deficit caused by the tree that has caused the ground to shrink, removing the tree over time will just prolong the length of time it takes for the heave to take place. The same change will occur, and so theoretically the same damage. Hence Adams comment..

[Wood-be]

I know this is often the advice given, but I am pretty sure that it's nonsense..

 

If there is a persistent moisture deficit caused by the tree that has caused the ground to shrink, removing the tree over time will just prolong the length of time it takes for the heave to take place. The same change will occur, and so theoretically the same damage. Hence Adams comment..

This is what the rhs says about it. I'm sure they can't be wrong aswell

 

Our tree has been identified as contributing to subsidence in a neighbouring property. We would prefer not to lose the tree. Will pruning be an effective solution?

 

When a specific tree has been found to be the main cause of subsidence having it felled is usually the most permanent solution. However,*tree reduction*can reduce the amplitude of movement so could be considered as a way to manage the problem. To be effective, pruning needs to reduce the crown volume of the tree by at least 70 percent and be repeated on a regular basis such as every three years. Crown thinning (as opposed to crown reduction) has been found to be ineffective at reducing transpiration rates. Also it should be noted that in cases of permanent deficit (i.e. London clay soils where low permeability leads to deeper permanent drying), felling the tree poses a risk of progressive heave. If there is a risk of significant heave, the only effective solution may be to have the property underpinned

 

 

 

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[benedmonds]

This is what the rhs says about it. I'm sure they can't be wrong aswell

 

Our tree has been identified as contributing to subsidence in a neighbouring property. We would prefer not to lose the tree. Will pruning be an effective solution?

 

When a specific tree has been found to be the main cause of subsidence having it felled is usually the most permanent solution. However,*tree reduction*can reduce the amplitude of movement so could be considered as a way to manage the problem. To be effective, pruning needs to reduce the crown volume of the tree by at least 70 percent and be repeated on a regular basis such as every three years. Crown thinning (as opposed to crown reduction) has been found to be ineffective at reducing transpiration rates.

 

That is saying the amplitude of movement from subsidence can be managed by pruning. Nothing to do with heave caused from the swelling of permanently dried soils after the removal of a tree.. Althouth IIRC reductions need to have 70% of foliage removed and it is only effective for 3 years...

 

Also it should be noted that in cases of permanent deficit (i.e. London clay soils where low permeability leads to deeper permanent drying), felling the tree poses a risk of progressive heave. If there is a risk of significant heave, the only effective solution may be to have the property underpinned

 

This bit is relevent to the original post and there is no mention of staged felling....

Edited May 19, 2016 by benedmonds

[Chris at eden]

Um not sure what your getting at. If you reduce a tree over a period of time (you can reduce the roots aswell vat that's a whole different ball game) then you are allowing water to re-wet the soil under the tree slowley and minimise the effect of heave.

 

Sent from my C6903 using Tapatalk

 

I agree with Adam. I know there is a BRE publication that suggests staged reduction but it doesn't stack up for me as the soil will still ultimately recover to the same position and so cause the same damage just at a slower pace. I asked Giles Biddle this question at a seminar and he said the only option is structural re-enforcement using anti-heave precautions.

[Chris at eden]

This is what the rhs says about it. I'm sure they can't be wrong aswell

 

Our tree has been identified as contributing to subsidence in a neighbouring property. We would prefer not to lose the tree. Will pruning be an effective solution?

 

When a specific tree has been found to be the main cause of subsidence having it felled is usually the most permanent solution. However,*tree reduction*can reduce the amplitude of movement so could be considered as a way to manage the problem. To be effective, pruning needs to reduce the crown volume of the tree by at least 70 percent and be repeated on a regular basis such as every three years. Crown thinning (as opposed to crown reduction) has been found to be ineffective at reducing transpiration rates. Also it should be noted that in cases of permanent deficit (i.e. London clay soils where low permeability leads to deeper permanent drying), felling the tree poses a risk of progressive heave. If there is a risk of significant heave, the only effective solution may be to have the property underpinned

 

 

 

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According to Dealga the impact on water uptake is only realistically 12 months, I think this was the conclusion of the hortlink project also. Dealga is adamant that reductions are not a suitable way to manage subsidence. He's a contentious chap for sure but he knows what he is talking about when it comes to subs.

[Chris at eden]

This is a complex issue and not one that an arboricultural consultant should have much to say on.....unless they have experience of heave and know what procedures to follow.....typically with the guidance of a structural engineer.....and it's easier just to ask the SE to do it!

 

So what does it take to assess the risk? It requires an assessment of whether there is a persistent moisture deficit in the soil over the whole soil profile (to at least 4 metres depth) and that cannot be done by looking at a tree! It also requires some complex assumptions/calculations. Once done it requires an assessment of whether the structure can withstand the measure of soil expansion calculated.....another thing that an arboricultural consultant should not be doing....so a light structure (e.g. a wall) is much more likely to be affected than a heavier one (e.g. a house).

 

It is also useful to know the type of foundations that have been used; if the house is relatively recent and the tree was present when it was built we might hope that the foundations would have utilised anti-heave measures i.e. if heave was a possibility then it could have occurred when the foundations were installed .......so the house has a built in mechanism against soil expansion.

 

In conclusion, heave is not a common problem and most houses can put up with a bit of soil expansion; just make sure your house insurance covers heave and forget the risk.....there is not a lot you can do about it - after all the tree may die so what are you going to do then?

 

Best response by a long way.

[Chris at eden]

It is well away to not cause heave. If it was an nhbc build the tree is well outside the zone of influence with regards to potential damage to foundations/heave.

 

You should be careful relying so fully on the NHBC ZOI. Willow has been recorded as causing damage at a distance of 40m which is also well outside of the ZOI. Clearly the trees aren't reading the NHBC guidance!

 

I had a tree officer tell me a few years ago that he had rejected an insurance claim as the tree was not within the ZOI. Additional evidence he had was crack monitoring showing seasonal movement, soil analysis showing it was shrinkable, and root ID from under the foundations. I never heard what happened next but I would say on the balance of probability the ZOI would be a pretty flimsy argument!

[Jon Heuch]

Excellent answer Jon. Can you elaborate on the anti-heave measures or suggest some further reading, please?

 

Claymaster Ground Heave Protection Solutions for Clay | Cordek | Cordek

 

http://www.sedgemoor.gov.uk/BCservices/CHttpHandler.ashx?id=8993 (p27 concerns floor slabs, not foundations

 

Cellcore HX S - Under Slab Ground Heave Solutions | Cordek

 

.......this is well away from arboriculture.......!

 

and remember this needs to be put in at the time of building construction, not once you have discovered you have a large tree needing removal!