Foundations near tree sand/gravel

[ninro]

We're proposing foundations for an extension about 2.5 meters from a mature Apple tree 4m high. I've been using the labc foundation depth calculator but realise this is designed mostly for cohesive soils i.e. Clay. Our soil is actually Sandy/gravel, quite good soil. so I was wondering what building control will say regarding depth, most especially if they see roots in trenches?

 

[noseall]

We're proposing foundations for an extension about 2.5 meters from a mature Apple tree 4m high. I've been using the labc foundation depth calculator but realise this is designed mostly for cohesive soils i.e. Clay. Our soil is actually Sandy/gravel, quite good soil. so I was wondering what building control will say regarding depth, most especially if they see roots in trenches?

Probably say nowt if its S&G.

 

[jks7492]

The NHBC guidance on building near trees is specifically for building in clay soils which are susceptible to volume change due to changes in moisture content. Sand/gravel doesn’t swell or shrink like a clay, so no heave precautions are required if the soil isn’t cohesive.

 

[^woody^]

If its sand and gravel, will a strip foundation be suitable?

Soils may have some plasticity and may not be pure sand or gravel. There is also the direct action risk, not just indirect.

 

[essex123456]

We had to do this, Tree had to be removed and foundations were supposed to be 3m but we were allowed 2m as no roots showed on digging trench. Block and beam floor with 18" gap to allow for any heave.

 

[noseall]

We had to do this

Quite, but not if you were in a sandy gravel area like the O.P.....

Our soil is actually Sandy/gravel, quite good soil.

 

[ninro]

We had to do this, Tree had to be removed and foundations were supposed to be 3m but we were allowed 2m as no roots showed on digging trench. Block and beam floor with 18" gap to allow for any heave.

Quite, but not if you were in a sandy gravel area like the O.P.....

Yeah, I see Essex soil, mostly clayey, quite different to dorset. Hopefully we may get away with standard depth of 1m ish.

 

[RonnyRaygun]

NHBC tables only apply to cohesive soils with shrink / swell potential. That's not an issue for sand and gravel (although if there is any cohesive content you should still get Atterberg tests done, as you might need to allow for some volume change).

If it's a true non-cohesive soil you just need to ensure you're on undisturbed natural ground below the frost zone (although you should be sure there is no cohesive soil a short distance beneath the sands and gravels as this could still be affected by trees etc).

The other issues with true sands and gravels are the height of the water table and how stable the trenches will be. If the trenches fill with water and/or collapse you've got different problems. I did a similar project recently where they ended up using screw piles and RC ground beams.

 

[ninro]

NHBC tables only apply to cohesive soils with shrink / swell potential. That's not an issue for sand and gravel (although if there is any cohesive content you should still get Atterberg tests done, as you might need to allow for some volume change).

If it's a true non-cohesive soil you just need to ensure you're on undisturbed natural ground below the frost zone (although you should be sure there is no cohesive soil a short distance beneath the sands and gravels as this could still be affected by trees etc).

The other issues with true sands and gravels are the height of the water table and how stable the trenches will be. If the trenches fill with water and/or collapse you've got different problems. I did a similar project recently where they ended up using screw piles and RC ground beams.

Was it building control who insisted on piles and ground beams? I have previously investigated this but the cost seems quite high about 15K for our project.

 

[RonnyRaygun]

No, the reason was that the trenches were unstable and the high water table and fast inflow meant it was very difficult to do traditional trench fill. Another option is PCC rings filled with concrete as pads with beams in between. Much easier to dewater the rings and they also provide the formwork.

 

[^woody^]

Was it building control who insisted on piles and ground beams? I have previously investigated this but the cost seems quite high about 15K for our project.

Building control can't insist on anything.

They are checkers, not designers. So you present them with a proposed design and they approve or reject it. But they are not structural engineers, so if a qualified engineer designs a suitable foundation solution, or proves that something is or is not required, no building inspector can argue with it.

So pay an engineer less than a piling company.