Just started digging for our single storey, block extension. Now I need to know how far down to go. Once through the topsoil, there's a few rocks, then undisturbed clay from 600mm that is fairly firm but digs OK with a spade. We've got an 8m cherry tree about 4m away at the closest point. I've had a read through the NHBC guidelines, but I need to know the soil change potential/plasticity index. Is there a simple test/rule of thumb? It's central Bristol if that makes a difference. Is it worth speaking to the building inspector at this stage or is he only interested in a finshed hole?
Foundation depth
- Thread starter nickjb
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Clay and nearby trees is not good.
It may mean either digging the trench deeper than standard (1m) or digging it wider to accept clay board.
I would request a site visit from the BCO or consult with your designer/techy.
It may mean either digging the trench deeper than standard (1m) or digging it wider to accept clay board.
I would request a site visit from the BCO or consult with your designer/techy.
You can get a sample tested to see if it's high, medium or low shrinkable.
A quick test is to roll a piece in your hand and see if it breaks up easily which indicates it may be low. The present height of the tree isn't always too important as BC usually assume it will reach it's mature height one day.
A quick test is to roll a piece in your hand and see if it breaks up easily which indicates it may be low. The present height of the tree isn't always too important as BC usually assume it will reach it's mature height one day.
Unless it's less than half its mature height...in which case you can design for its current height...why the mature height isn't a consideration in this case, I don't know.
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Hi
There is a guide line for types of soils and I'm sure its in the NHBC book and it depends on how far you can drive a 50x50 piece of wood. A cherry tree is fairly low risk and according to my charts if you assume the soil is high shrink then you need to go 1.8m
There is a guide line for types of soils and I'm sure its in the NHBC book and it depends on how far you can drive a 50x50 piece of wood. A cherry tree is fairly low risk and according to my charts if you assume the soil is high shrink then you need to go 1.8m
You can get a sample analysed at Structural soils in Princess Street, Bedminster. Costs about £50..ish. Take the sample from the bearing layer - i.e somewhere near the bottom of the foundation - and tell them it's for soil shrinkage/tree calculation. Technically you should collect samples in accordance with British Standards but I've never known it be queried if you provide a test certificate to BC. You also need to identify which Cherry? A wild Cherry is twice the mature height of an ornamantal one, which will make a significant difference in your foundation depth. Then it's a question of working through the calculation.
PS. Or you could just assume the soil is high shrinkage and see how it comes out? All the Cherry's are moderate water demand.
PS. Or you could just assume the soil is high shrinkage and see how it comes out? All the Cherry's are moderate water demand.
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