Foundation near tree line - raft? Systems available?

[Pabbles]

Hi all.

 

you know that feeling when you’re one bit of info, and a subsequent Google search, away from realising you’ve been woefully naive? Well…. that.

 

We’re building two very large outbuildings along the back edge of our property. Architect drew the outline plans, we’ve picked a private building control firm and architect is now adding building control details to his plans for review.

 

i’d assumed strip foundations of an unknown depth, say 1m, and then a cavity construction above DPC.

 

building 1 is 12m x 9.5m and is a garage. Interestingly on this one, I would be specifying a high spec floor (250mm of C30 reinforced concrete) due to the manufacturer requirements of a twin post car lift that would be installed. Imagine this building as being a very pretty, but functionally industrial work space.

 

building 2 is a 8.5m x 4.5m garden office and day room.

So, had a chap out for an early quote on groundworks - lovely man and reasonably priced. He points to the massive row of trees that run along the property boundary (combination of hedgerows and a number of mature trees including tall old oaks). Asks how we’re getting around the foundation requirements for them. They’re around 5m from the proposed building edges.

 

Quick google, realise how big an issue this is.

 

The LABC calculator says that strip foundation depth at best would be 2.5m, if they’re even possible.

 

I haven’t budgeted for foundations that deep - I’d assumed 1m. I have enough saved to build the buildings but this issue could kill the projects if the foundation cost becomes crazy.

 

obviously, the first physical (private) building control visit will confirm how be a concern this is, but aim fearing the worst.

 

there is one glimmer of hope: raft foundations. I’ve been reading about them and how they’re recomended for some soil types and some problem ground. But they don’t mention trees and roots: am I right in assuming that they would be a potential solution? If so, do I dare think that this could be a bit of a silver lining situation as a  structural raft foundation would also meet or exceed the requirements of the 2-post vehicle lift too - in other words a simpler, quicker combined foundation and floor slab?

 

 

 

Edited June 4, 2023 by Pabbles

[Marvin]

Oh dear.

 

This was a long time ago...

 

We had to do 2.5m trench filled concrete with suspended beam and block floor. The roots travelled about 18m from a mature oak tree. The extension was about 10 meters from the tree so it is not enough just to do the side facing the tree in my opinion.

This may help:

 

https://nhbc-standards.co.uk/4-foundations/4-2-building-near-trees/

 

Good luck

 

Marvin.

 

 

[nod]

3 hours ago, Pabbles said:

Hi all.

 

you know that feeling when you’re one bit of info, and a subsequent Google search, away from realising you’ve been woefully naive? Well…. that.

 

We’re building two very large outbuildings along the back edge of our property. Architect drew the outline plans, we’ve picked a private building control firm and architect is now adding building control details to his plans for review.

 

i’d assumed strip foundations of an unknown depth, say 1m, and then a cavity construction above DPC.

 

building 1 is 12m x 9.5m and is a garage. Interestingly on this one, I would be specifying a high spec floor (250mm of C30 reinforced concrete) due to the manufacturer requirements of a twin post car lift that would be installed. Imagine this building as being a very pretty, but functionally industrial work space.

 

building 2 is a 8.5m x 4.5m garden office and day room.

So, had a chap out for an early quote on groundworks - lovely man and reasonably priced. He points to the massive row of trees that run along the property boundary (combination of hedgerows and a number of mature trees including tall old oaks). Asks how we’re getting around the foundation requirements for them. They’re around 5m from the proposed building edges.

 

Quick google, realise how big an issue this is.

 

The LABC calculator says that strip foundation depth at best would be 2.5m, if they’re even possible.

 

I haven’t budgeted for foundations that deep - I’d assumed 1m. I have enough saved to build the buildings but this issue could kill the projects if the foundation cost becomes crazy.

 

obviously, the first physical (private) building control visit will confirm how be a concern this is, but aim fearing the worst.

 

there is one glimmer of hope: raft foundations. I’ve been reading about them and how they’re recomended for some soil types and some problem ground. But they don’t mention trees and roots: am I right in assuming that they would be a potential solution? If so, do I dare think that this could be a bit of a silver lining situation as a  structural raft foundation would also meet or exceed the requirements of the 2-post vehicle lift too - in other words a simpler, quicker combined foundation and floor slab?

 

 

 

We are surrounded by trees on our previous build and had to go down to 1800 on the main house and even the garages 

 

Private building control are less convenient and nearly always more expensive than local Authority 

[Jilly]

You might need piles. Get different quotes tho’ because they can differ wildly. Maybe be certain of your planning permission, presumed permitted development rights before you go too far? 

[twice round the block]

Abbey Pynford trees are foundations could work for you.

 

www.abbeypynford.co.uk

 

[George]

You need to know what the soil type is. If it is clay then potentially a problem, but if it is low plasticity clay then the impact is reduced. 

 

It is a slight gamble but a ground investigation up front (which would be needed in any case if you are piling) would give you a lot more information and more certainty on the issue. 

 

Piling I would tend to dis-favour for an outbuilding where the consequences of failure (which for foundation problems are slight movement and cracking) are less than for a house. In addition to the GI, a combination of a root barrier, vegetation management and heave protection I would consider sufficient.

 

In terms of a raft, the rule of thumb is that you need to dig out and replace soils with volume change potential half what you would for a traditional foundation. So, a equivalent of 2.5m strip is a stone mat 1.25m deep. This depth reduces as you move away from the trees but some sort of stone layer does need to be below the entire footprint of the structure. 

 

I would get in contact with a structural engineer as they should be able to save you some money compared to following the building control charts. 

[saveasteading]

5 minutes ago, George said:

 I would get in contact with a structural engineer

Agreed. You will have to at some stage so best now.

 

As you will have a thick slab anyway, a raft should be the answer.

Then the whole building can rise and fall seasonally.

They can also test the ground, look at the topography and ' take a view'.

As nobody will be sleeping in the buildings the risk to life is negligible.

[Gus Potter]

On 04/06/2023 at 04:24, Pabbles said:

We’re building two very large outbuildings along the back edge of our property.

I would spend a bit of money on a ground report and identification of the trees and vegitation. That really is your starting point for me.

 

Next I would look at the weight of the super structure.. how does it work and does what you have work best for the ground conditions in the round, budget wise? Would a portal frame work instead?

 

The slab thickness you mention.. 250mm.. that is a chunky slab..that is getting close to/ is a very heavy industrial slab.. It looks too thick. You mention a twin post car lift. It's not acutually a big load.. have designed plenty MOT stations with double lifts.. worst case needs some extra rebar or local thickening. If you use a 250mm slab as a structural member then you will need a lot of rebar to comply with the codes. The material cost will escalate, as will the construction cost.. the aim here is to keep it as simple as you can so your guy can build it with little complexity.. and have to worry less about concrete curing and so on.

 

If you want a bit more out of BH what can you tell us about the ground and type of trees?

 

You do have a bit of a challenge on your hands.. but if you can let us know a bit more about you'll get some good pointers on BH about how to tackle this engineering and cost wise.

 

Looking ahead. How do you want to finish the floor? Are you going to paint it and for the garden office.. assume you are going to insulate? Two distinctly different designs are require if so.

 

 

 

 

 

 

 

 

 

 

 

 

[Barny]

On 04/06/2023 at 12:42, twice round the block said:

Abbey Pynford trees are foundations could work for you.

 

www.abbeypynford.co.uk

 

Does anyone know of anyone else like Abbey Pynford for tree foundations? I am looking for a firm to do the engineer and supply of foundations like AP so I can benchmark their fees.

[Alan Ambrose]

V. interesting thread...

[Alan Ambrose]

V. interesting thread....as I want to build within 9m of some fully grown Ash trees. I hope this moves the discussion along on the original poster's question rather than hijack the thread...

 

Reading the NHBC guidance I see the calcs - which are completely new to me. On first blush, I'm getting about 1.4m foundation depth assuming I'm on a 'moderate' volume change clay. I'm looking at these charts:

 

https://nhbc-standards.co.uk/4-foundations/4-2-building-near-trees/4-2-12-foundation-depth-charts/ 

 

Which begs the question - can I get a quick and dirty plasticity index reading by sending some soil in to, say, here: https://k4soils.com/ based in Watford.

 

This is more for peace of mind than anything as I'll get a proper ground investigation done when we have a SE on board.

 

If so, what depth should the sample(s) come from?

[saveasteading]

3 hours ago, Alan Ambrose said:

what depth should the sample(s) come from

1m or 1.2 would be practical, because that is where your best scenario foundation depth would be. 

Have you dug any holes elsewhere? If so , describe in non technical terms.

[Dave Jones]

cart before the horse.

 

mature oak > 12m in height if your on clay means engineered footing.

 

Talk to your SE he will have a soil test supplier, ours wanted to witness the trial pit and took the samples.

 

Once he has the results he will provide you with a spec which you can then get priced.

 

Not really possible to DIY/cheap this step.

[Big Jimbo]

@Alan Ambrose I have recently used K4 in Watford. I'm local, so i actually delivered my samples to them.  Nice to know that they actually do the testing in-house unlike my engineers company who send the samples off to Poland. I did samples at 1.4 and 2.1 meters. They need about 1.25 kilo of material, and if you ask they will do the modified plasticity index calc for you. Nice bunch of guys. I paid an extra £18 to get my results in 5 days.

[Big Jimbo]

My engineer was happy to work with the results i received, and my Two samples cost less than £100 to get tested.

 

[Big Jimbo]

Your engineer might want to take his own samples however. You can understand that, because you might have taken the sample from better ground, rather than the correct place. It is there PI insurance at the end of the day. But £100 quid for an early indication has to be a good thing.

[Alan Ambrose]

@Big Jimbo- thanks for your very useful info, BH is great!

 

>>> Have you dug any holes elsewhere? If so , describe in non technical terms.

 

Only by hand to see the invert level of a culvert pipe to use for potential drainage. It was last year, I don't remember much about it except that it was xxxx hot and the ground was hard .

 

We do have some services to run and a drive to dig though so I'll get some samples at a good depth while the digger man is on site.

[Big Jimbo]

I dug my samples a few meters outside of the new house footprint. My engineer said not to disturb the ground where the new property was actually going to be.