Leylandii hedge and Foundation

[jaufaucreiquoda-4633]

I am planning to build extension next to very matured hedge.
Will be removing one tree and New foundation will be less than 50cm from the remaining hedge.
Current height of the hedge is 2.7m.
Just digging the foundation to 2.5m deep is enough?
What are the other things i need to consider in order to keep the hedge?

 

[RonnyRaygun]

You need to know what the soil type is. If it's non-shrinkable (no clay content) then the trees are not an issue.

If it is clay, 2.5m won't necessarily be deep enough if the soil is high or medium volume change. If this is the case you will need to remove more of the trees as you might then be OK.

Better to remove the whole hedge than the alternative (which would be a piled foundation and would work out much more expensive).

 

[Loofah]

What's your soil type?
I'd just remove the hedge

 

[jaufaucreiquoda-4633]

You need to know what the soil type is. If it's non-shrinkable (no clay content) then the trees are not an issue.

If it is clay, 2.5m won't necessarily be deep enough if the soil is high or medium volume change. If this is the case you will need to remove more of the trees as you might then be OK.

Better to remove the whole hedge than the alternative (which would be a piled foundation and would work out much more expensive).

Soil is clay but not sure high or medium volume change.Where/how can i get this information?
Hedge is on the boundary and may cause issues with neighbour to remove it.

Just having deep foundation is only solution?

Thuja plicata is same as Leylandii? Both needs deep foundations as I am not sure which one is ours.

 

[jaufaucreiquoda-4633]

Thuja plicata is same as Leylandii?

 

[Loofah]

No. Cupressus leylandii is a different plant

 

[Loofah]

No. Cupressus leylandii is a different plant

 

[jaufaucreiquoda-4633]

No. Cupressus leylandii is a different plant

Thuja plicata dosn't need deep foundations?

 

[Loofah]

I have no idea but you have clay soil and want to build 50cm from established large trees so I would assume you need very deep foundations. As it is, I would expect issues with the existing roots even if you cut down the hedge.
Ask a pro to be certain.

 

[RonnyRaygun]

Thuja plicata is same as Leylandii?

It’s a type of cypress and grows to a similar height if left so will likely have the same effect on the soil.

If high volume change clay and you choose to keep the trees you will be spending a small fortune on a piled foundation.

You need to get the soils tested, perhaps a sample from 1.5, 2.0, and 2.5m deep. You will need someone with a hand or mechanical auger who can take the samples then send them off to a soil testing lab. Ask them to do Atterberg tests (4 point if possible) and also test for desiccation of the soil.

It shouldn’t be expensive. You don’t need a full site investigation and report (which could cost thousands), you just need to know the volume change potential of the soil and whether it is desiccated at founding depth.

 

[jeds]

Is Thuja Plicata not Western Red Cedar?

Either way, you might get away with a deep foundation if your soil volume change potential is low. If it is medium or worse, your foundation will be more than 2.5m and that means remove the tree or an engineered foundation.

 

[jks7492]

Soil is clay but not sure high or medium volume change.Where/how can i get this information?

You'd need to get a plasticity test done.
If you have a local geotechnical engineering/lab testing company you might be able to get them to do the testing if you drop samples off - someone I know had a local company test samples for the same scenario and charged about £50 each sample (although this was a couple of years ago).

that means remove the tree or an engineered foundation.

Removing the tree doesn't protect against heave, its one of the things most likely to cause clay to heave?

 

[RonnyRaygun]

Removing the tree doesn't protect against heave, its one of the things most likely to cause clay to heave?

If you remove the 2.7m tree then it will never reach its mature height and so a smaller zone of influence can be taken, which means the foundations potentially don't need to be as deep.

 

[jks7492]

If you remove the 2.7m tree then it will never reach its mature height and so a smaller zone of influence can be taken, which means the foundations potentially don't need to be as deep.

True, although at 50cm away from the tree you still won’t be outside of its influence zone if the soil is cohesive.

It was more of a general comment about people assuming that just removing a tree is a fix-all solution for building near trees, which seems to be a fairly common misconception (I’ve no doubt that Jeds knows his stuff and this commented wasn’t directed at him)

 

[jeds]

You'd need to get a plasticity test done.
If you have a local geotechnical engineering/lab testing company you might be able to get them to do the testing if you drop samples off - someone I know had a local company test samples for the same scenario and charged about £50 each sample (although this was a couple of years ago).




Removing the tree doesn't protect against heave, its one of the things most likely to cause clay to heave?

Ronny got there before me. For this species and distance, tree removal makes the difference between an approx. 2.2m foundation and an engineered foundation.