Foundations for outbuilding right next to trees?

[indus]

Hi

I'm planning on building an outbuilding under PD rights. I will keep it just under 30m2 so building regs are not actually needed. It will be just under 8mx4m with cavity wall, pitched roof etc.

It would be about 2m from a whole row of huge trees, sorry but I don't know what kind they are. I'm also pretty sure we have clay soil (saw this when we had foundations for a patio made a few years ago)

I need some advice on what sort of foundations I would need for this project. A builder said something like 6 concrete padstones with some steel beams placed across them.

Thanks

 

[cyclonebri1]

I'm no expert but I think you need to do a little more work in 2 areas.

1stly find out what sort of trees they are, it makes a big difference in root structure and water draw. Remember the roots are likely to pread as wide as the height of the trees.

If it was me I'd dig an initial foundation trench along the side nearest the trees, to see the extent of root spread then get advice from there

I have to say 2 metres from "huge trees" sounds tooo close for comfort.

Maybe a piled foundation is required??, but that's a question not an answer.

 

[jeds]

Mature trees at 2m distance in clay will certainly take your foundation beyond a standard strip. The tree calculator only goes to 2.5m and that depth is usually well exceeded for mature trees 5, 6, 7m or more away.

Pads and steel beams makes no sense at all so I'd forget that if I were you.

My advice would be to trial dig along the wall closest to the trees and see what's beneath ground. You need to know where the clay layer starts and check out the density of small roots at depth. i.e. 12-20mm roots?

Then your going to have to take your chance. Piling or 2.5m deep trench fill?

 

[indus]

Mature trees at 2m distance in clay will certainly take your foundation beyond a standard strip. The tree calculator only goes to 2.5m and that depth is usually well exceeded for mature trees 5, 6, 7m or more away.

Pads and steel beams makes no sense at all so I'd forget that if I were you.

My advice would be to trial dig along the wall closest to the trees and see what's beneath ground. You need to know where the clay layer starts and check out the density of small roots at depth. i.e. 12-20mm roots?

Then your going to have to take your chance. Piling or 2.5m deep trench fill?

Thanks

But what does this actually tell me and how do I use the information? It's only worth doing this trial dig if I'm going to make some sort of calculation on the based on what I find.

Lets say I find clay at shallow depth (or deep depth, whichever is worse!) and lots of roots. Then what?

Thanks for your help

 

[stuart45]

The depth of your footing will also depend on the type of trees and what type of clay soil you have. Whether it is shrinkable and high, medium or low. I had to go down 2.2M from the base of the trees in low shrinkable clay. The trees were leylandii within 2M from the footings.
You can get your soil tested for about £40.

 

[indus]

The depth of your footing will also depend on the type of trees and what type of clay soil you have. Whether it is shrinkable and high, medium or low. I had to go down 2.2M from the base of the trees in low shrinkable clay. The trees were leylandii within 2M from the footings.
You can get your soil tested for about £40.

Thanks, I'll try and google somebody to test my soil.

 

[jeds]

I agree with stuarts comments. To clarify my post, it is possible you might have low shrinkable soil and depending on the trees you might get away with a 2m or so foundation - possibly less. But you need soil samples from the clay layer so at some point you're going to have to dig a hole. If the clay type and tree species dictates a foundation deeper than 2.5m then you are going to have to make a decision. You don't need building regs so you could just take a chance on it. But you WILL be taking some risk so you will have to weigh that up. If you find not too much root activity at depth then you might be ok. If there's loads of roots the risk is increased.

 

[indus]

Thanks Jeds.

May I ask why you felt pads and beams are not a good idea in this situation?

The idea was to form between six and eight pads that would be 1m x 1m x 3m deep and then have steel beams across them.


Thanks

 

[joe-90]

I read it as being padstones sitting on the surface like a garden shed rather than piles going down 10 foot. It's all in the terminology.

 

[indus]

I read it as being padstones sitting on the surface like a garden shed rather than piles going down 10 foot. It's all in the terminology.

Yes you are right I used the wrong terminology. When I asked the builder he used the word padstone. When I asked the spec of the padstone he said they would be 3m deep, so not a padstone at all!

Is this what would be called a 'pile' foundation and what do people think of this idea in general?

Thanks

 

[cyclonebri1]

I read it as being padstones sitting on the surface like a garden shed rather than piles going down 10 foot. It's all in the terminology.

Yes you are right I used the wrong terminology. When I asked the builder he used the word padstone. When I asked the spec of the padstone he said they would be 3m deep, so not a padstone at all!

Is this what would be called a 'pile' foundation and what do people think of this idea in general?

Thanks

Yes, that as an engineer rather than a builder is what I call that method and is what I would do.

Are the trees conifers? Are they yours?

 

[jackfrost191]

Trees ~ effect on foundations. Trees up to 30 m distance may have
an effect on foundations, therefore reference to local authority
building control policy should be undertaken before specifying
construction techniques.

Traditional strip foundations are practically unsuited, but at
excavation depths up to 2„5 or 3„0 m, deep strip or trench fill
(preferably reinforced) may be appropriate. Short bored pile
foundations are likely to be more economical and particularly
suited to depths exceeding 3„0 m.

 

[theoldun]

Indus.
Unfortunately, whilst people mean well, in this instance, you are being given some slightly misleading information, so lets give you a few facts.
Nobody, can give you a foundation depth or type of foundation required with out knowing three things.
1. Species of tree.
2. Height of existing tree.
3. Soil classification.
Specie of tree is easy enough to sort out. Once you know that then you know whether it is high, medium or low water demand for broad leaf or high or moderate for conifers. Example a Birch tree 14m high 2m away from building on low shrinkable soil will only need a foundation 1100 deep, whereas an English Oak 2m away from building on shrinkable soil will need an engineered designed foundation at 2500 deep or piles and ring beam.
Soil Classification. The test is called the Atterberg Limits test and determines the Modified Plasticity Limits Index of the soil. It will class the soil as low, medium or high shrinkage. You will need about eight samples from four trial holes to send away to be tested. These want to be taken from a depth of about 900 down and 1800 down This is no problem. Form a pit 600 square and then go down 1200 with some decent post hole shovels. We have not had any samples done for a long time now, as we are ancient enough to be able to blag our way round BC regard the plasticity index when they turn up. We used to run our samples down to Watford, but they have moved now and do not know where to. Been told there is outfit in Devon with branches in Hemel Hempstead and the Midlands. You will have to google or ask Stuart45. We note he said he has had test done recently.
If you wish, post tree specie, approx height and will give you indication of what way to go.
As Jed said ignore comments like this.

A builder said something like 6 concrete padstones with some steel beams placed across them. Complete rubbish, he makes no mention of root growth tween the pads, beam corrosion, clay board and clay shield, nor mention the difficulty of rattling out and bottom up a 3mc pit. You would want a 5 ton 360 to get that depth and she would want 2100 clearance to get her in
Hope we have not spoilt your day.
Regards oldun

 

[indus]

Indus.
Unfortunately, whilst people mean well, in this instance, you are being given some slightly misleading information, so lets give you a few facts.
Nobody, can give you a foundation depth or type of foundation required with out knowing three things.
1. Species of tree.
2. Height of existing tree.
3. Soil classification.
Specie of tree is easy enough to sort out. Once you know that then you know whether it is high, medium or low water demand for broad leaf or high or moderate for conifers. Example a Birch tree 14m high 2m away from building on low shrinkable soil will only need a foundation 1100 deep, whereas an English Oak 2m away from building on shrinkable soil will need an engineered designed foundation at 2500 deep or piles and ring beam.
Soil Classification. The test is called the Atterberg Limits test and determines the Modified Plasticity Limits Index of the soil. It will class the soil as low, medium or high shrinkage. You will need about eight samples from four trial holes to send away to be tested. These want to be taken from a depth of about 900 down and 1800 down This is no problem. Form a pit 600 square and then go down 1200 with some decent post hole shovels. We have not had any samples done for a long time now, as we are ancient enough to be able to blag our way round BC regard the plasticity index when they turn up. We used to run our samples down to Watford, but they have moved now and do not know where to. Been told there is outfit in Devon with branches in Hemel Hempstead and the Midlands. You will have to google or ask Stuart45. We note he said he has had test done recently.
If you wish, post tree specie, approx height and will give you indication of what way to go.
As Jed said ignore comments like this.

A builder said something like 6 concrete padstones with some steel beams placed across them. Complete rubbish, he makes no mention of root growth tween the pads, beam corrosion, clay board and clay shield, nor mention the difficulty of rattling out and bottom up a 3mc pit. You would want a 5 ton 360 to get that depth and she would want 2100 clearance to get her in
Hope we have not spoilt your day.
Regards oldun

Hi Oldun

Thanks for your reply.

There are mixed trees but the main tree is a Sycamore and it is about 50ft tall. I gather Sycamore has medium water demand.

I definitely have clay in the soil, and for the benefit of this exercise please assume I have the worst kind.

I have now moved the proposed position of the building so that it will be 8m away at its closest point. It will be 15.5m away at its furthest point. So the tree is along the rear boundary and the building will be along the side boundary

Thanks for your help

 

[theoldun]

Zone of influence working near trees.

NHBC were the pioneers for the above and would say that 95% of local councils follow their advice.
Two methods can be used. Distance and height in conjunction with series of graphs or mature height and distance with series of charts. In your instance we will use the latter.
Sycamore. Broad leaf. Mature height 22 metres. Moderate water demand Distance from building 8 metres. Soil plasticity. High shrinkage. Therefore Depth of foundation to be 1750 at nearest point of building. London climate variation less 50mm, therefore depth below ground level to be 1700. Half way along the 7000 front and back wall you can lift the depth up to 1450, and the last 1500 of front and back and end flank can be standard depth of 1000.
Above figures all stand up to BR for the worst scenario of ground conditions.
You will need 50mm clay shield (not clay board) to the inside of all trenches deeper than 1500.
As you are possibly on London clay would suggest floor is block and beam with 75 mm screed and fibres or 75mm 10mm aggregate concrete, with 150mm void below or 150mm reinforced concrete slab on clay board. Either of the above will counter any potential heave. Can not give you a budget figure as do not know width of access to rear, parking restrictions, width of access at front for either 8 leg grabs or skips. Trench width, where you require clay shield make 600mm, rest of dig make 450mm.
Approx in the solid 44m3 dig and cart. Decent access, no big deal.
We have read a lot of information by the Forestry Commission with regards to root growth etc, and what we have told you above is correct, but in our opinion, slightly over the top. Not prepared to state how we would do it on Public Forum, but prepared to PM our suggestions to you if you wish.
If you want those drawings PM us your address.
Regards oldun

Mr G.
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