trees on clay

[dexterdo]

We wish to build an extension but we have 2 cypress trees 5-8m from the closest part of the extension on weald clay. We will need 2.5m foundations or piling if the trees stay but our neighbour has offered to remove them to help out. The building control officer has seen the trees and knows they are there! If these closest trees are removed, then the closest trees are leylandii at 13m away, which I understand are too far away to be a problem. So if the 5m away cyrpesses are removed, do you think the BC officer will accept 1m foundations or still make us put in piling because of the risk of heave due to water swelling the clay once the roots have gone? The trees were about 30` tall but were lopped to half height 2 years ago and are now about 20` high. Would a soil analyst be worthwhile ie would he/she be able to answer this question to the satisfaction of the BC officer?
Another question too-we have an attached garage that is on its last legs-if we rebuild it on the same footprint do we need building regs approval?
Many thanks-site is fantastic

 

[theoldun]

When we answered your last post on this subject, we only gave a cost comparison for traditional founds as against deep dig. We did not bother to check the depths etc as previously given as the information given was a bit sketchy.
This post, as more information is available, we have checked everything out and will try to put a smile on your face.
The following information is all taken from NHBC, Building Near Trees. Every Local Authority that we know follows these guide lines.
Cypres. High Water Demand.
Lawsons, mature height 18m.
Leyland mature height 20m
Montery mature height 20m
You state that before the tops were cut out, the trees were 30 foot tall, which is approx 9m.
To calculate foundation depths when a tree is removed you are allowed to take 50% of the mature height, therefore in your case the calculation for the height will be 10m.
The distance is from the centre of the trunk to the nearest edge of foundation.
High water demand. High shrinkage. Distance 5m. Depth 1.45
Ditto 6m 1.00
High water demand. Medium shrinkage. Distance 5m. Depth 1.25
Ditto 6m 1.00
High water demand. Low shrinkage. Distance 5m Depth 1.05
Ditto 6m 1.00.
As you live in Guildford, any of the above depths over 1m then you can deduct 50mm for climate variations.
Have checked above figures and they are correct.
With regard to the Leylandii 13m away, even if they are full height, they are outside the zone of influence.
You could have a test done for plasticity index, your choice, your money. It may prove that you are low shrinkage
Would suggest block and beam floor with a 225mm void below to allow for any possible heave.
Suggest you take a copy of this and ask your local authorities approval.
Suggest you put last part of your post ref garage on Building Regs and Planning Permission
Any questions come back.
Regards oldun.

 

[dexterdo]

So if we take the trees out we can go down 1.45m-50mm=1.4m and bobs your uncle everything will be ok , no need for piling? Is that what you`re saying? No need for soil testing, just pay for tree removal? If so,thats brilliant news-and such a relief!!!.
Do I need to have put in a building regs application to get the BC officer to agree this before we start work or can i just get him to agree it tomorrow? Thank you so much for your thorough answers

 

[dexterdo]

So if we take the trees out we can go down 1.45m-50mm=1.4m and bobs your uncle everything will be ok , no need for piling? Is that what you`re saying? No need for soil testing, just pay for tree removal? If so,thats brilliant news-and such a relief!!!.
Do I need to have put in a building regs application to get the BC officer to agree this before we start work or can i just get him to agree it tomorrow? Thank you so much for your thorough answers

*Sorry, forgot to say we have a 40 foot silver birch at 13m away too-does that have any effect-it wont be removed *

 

[Static]

Birch are low water demand so dont influence that far..

To add to what theoldun has already written.. new nhbc guildance states 300mm depth below suspended floors for high PI soils..

Also keep them trees capped at 30ft max or you may be back into piling depths even with them being removed.. get all this agreed (in writting) before you remove the trees, also take a stack of photos of them..

edit additional: Also ensure you go at least 300mm down below any roots you find in the foundation trenches.

 

[dexterdo]

thanks

 

[Richard C]

Can’t add anything more the excellent & detailed advice already given but to reinforce the bit about getting tentative agreement from the BCO before you start. It's best to clear the site before LABC get involved as once they are aware there were trees there, you may still be asked to sink foundations/piles half way to Australia if roots are apparent in the exposed trench work.

Another question too-we have an attached garage that is on its last legs-if we rebuild it on the same footprint do we need building regs approval?

If the garage is attached & is not a car port (open on at least 2 sides & less than 30 sq/m) it does come under Building Regulations (as will any electrics & glazing). Problems you may have is that the existing base may not meet current regs. Rebuilding is pushing the “maintenance & repairs angle” a little far & as your having other inspections, the BI may take the view it’s a new garage rather than a refurbished one which may also raise planning issues even though there was a garage there before. A look here may give you some guidance;
http://www.planningportal.gov.uk/england/public/buildingwork/projects/workcommonoutbuildings/

http://www.planningportal.gov.uk/en...kcommonoutbuildings/workmorebregsoutbuilding/

It may be wise to discuss with your LABC as there is no real way of knowing how they will interpret it.

 

[theoldun]

So if we take the trees out we can go down 1.45m-50mm=1.4m and bobs your uncle everything will be ok , no need for piling? Is that what you`re saying?

That is what we are saying based on NHBC recommended practice for working within the zone of influence of trees, however until you have got a few spits out, you never know your ground conditions unless you drop a trial hole down first
You should apply for Building Regs either by Notice or full plans. We would suggest full plans. Once that is done, we would go about it like this. Rattle trees over and grind out stumps. Get your neighbours garden back into pristine condition. Your plans will show a metre dig. We would rip the founds out metre deep and ask for inspection. Say nothing about the trees unless mentioned. If they do come into the conversation then you may have to have another 400mm out. It is a gamble what ever way you go about it. A lot depends on BCO.
Word of warning 1400 deep is a tadge deep for safety. Do not put any spoil on top of banks, if it rains, go up pub, when bottoming up one eye on shovel other on banks.
Luck. oldun

 

[dexterdo]

BCO still wants struc engineers report to approve trench foundations when trees removed. He would like soil analysis done

 

[Richard C]

BCO still wants struc engineers report to approve trench foundations when trees removed. He would like soil analysis done

As I said, it always pays get rid of the trees & any evidence of them first

 

[abuildingsite]

So if we take the trees out we can go down 1.45m-50mm=1.4m and bobs your uncle everything will be ok , no need for piling? Is that what you`re saying?

That is what we are saying based on NHBC recommended practice for working within the zone of influence of trees, however until you have got a few spits out, you never know your ground conditions unless you drop a trial hole down first
You should apply for Building Regs either by Notice or full plans. We would suggest full plans. Once that is done, we would go about it like this. Rattle trees over and grind out stumps. Get your neighbours garden back into pristine condition. Your plans will show a metre dig. We would rip the founds out metre deep and ask for inspection. Say nothing about the trees unless mentioned. If they do come into the conversation then you may have to have another 400mm out. It is a gamble what ever way you go about it. A lot depends on BCO.
Word of warning 1400 deep is a tadge deep for safety. Do not put any spoil on top of banks, if it rains, go up pub, when bottoming up one eye on shovel other on banks.
Luck. oldun

You do risk heave by ripping out trees and ignoring them completely. For me, it's not worth it for the sake of getting one over on the BCO, especially if it's your own house.

 

[Richard C]

You do risk heave by ripping out trees and ignoring them completely. For me, it's not worth it for the sake of getting one over on the BCO, especially if it's your own house.

Agreed but in my view & that of others I know who have far more experienced than me, the current interpretation of regulations can be totally OTT. I have a developer friend been forced to dig 2m deep footings FFS!

 

[abuildingsite]

You do risk heave by ripping out trees and ignoring them completely. For me, it's not worth it for the sake of getting one over on the BCO, especially if it's your own house.

Agreed but in my view & that of others I know who have far more experienced than me, the current interpretation of regulations can be totally OTT. I have a developer friend been forced to dig 2m deep footings FFS!

It wouldn't surprise me on heavy clays with thirsty trees nearby. That being said, any BCO that asks for an engineer's justification beyond NHBC guidelines is more than likely just covering their back.

 

[theoldun]

Agreed but in my view & that of others I know who have far more experienced than me, the current interpretation of regulations can be totally OTT. I have a developer friend been forced to dig 2m deep footings FFS!


Richard, you have hit the nail firmly on the head. Not only are some BCO interpreting the regs completely OTT, but they are not interpreting them in accordance with the regs.
In my opinion this is because they have not got the knowledge, experience and are frightened to make a decision without covering their own backs. We have noticed this more and more just lately.
It would be very interesting to ask this BCO why he needs both a Modified Plasticity Index and a SE report.
If you assume in your calculations that you have a high volume change potential of 40% or greater what is the point of having an Atterberg Limits test carried out, when you have already assumed the worst scenario in the calculations.
Dexterdo, we would be in to BC and argue the toss, but as you have no knowledge of this subject, that will not help you.
Possibly Static can throw some light on this.
Dexterdo. Looks like we have not been of much help to you, but in fairness we would have gone about it in a different way. As said we would have ripped founds out metre deep, asked for inspection and took it from there.
The only advice that we can now offer is to consult SE for a report and guidance.
oldun

 

[Static]

Right..

Ive had a quick look into the basic geology of the area, and yes there is a small band of Gault Clays running between the Upper Greensands and Chalks in Guildford.. as well as possible london clay and weald clay regions..

I dont think you have answered the question on how you know the soil is Gault Clay.. would be nice to know where you base this info..

Have you dug any trial holes? You could be on chalk.. so dig a 1m deep hole where you plan to put the foundation..
If soil conditions are unknown then they are assumed to be high PI and thus need the deepest depth footings the nhbc tables provide.. and as stated by others these guildlines tend to get worse and worse every year.. their latest change has increased the void depth requirements for suspended floors..

I think you have probably progressed it as far as a forum discussion can go so have to pick a path to follow:
1/ Get the footings properly designed by an SE which may require some soils testing and trial pits (unless they have done similar local projects).. and let them deal with BC on getting things approved..
2/ Propose the tree removal and shallower footings based on high plasticity clay and get written acceptance before trees are removed.. footings deeper than 1.5m will need some heave protection..