Footing depth near trees

2 Jun 2005
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West Midlands
United Kingdom
would appreciate a take on this problem where we are up to roof truss level.
For the footing depth the calculator shows 1.7. I have taken that measurement from the origonal site level before we stripped of 900mm of solid, and I mean solid clay.
There was always uncertainty as the deeper we took the foundation dig, because the tree is down the bottom of a slope, the nearer we would be getting to the tree routes.
In conversation with BC and LABC they verbally took a view that I could take the dig depth from the origonal level. They are now saying it should have been from the lower site strip level.
Anyone come across anything similar or who knows a bit about trees at the bottom of a slope
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Technically it should be from the lower level but it is also common practice to draw a line from the base of the tree to ground level adjacent to the existing building and use that sloping line.
I feel you pain as this could seriously cause you problems. It sounds like you have a combination of some bad soil conditions combined with one of the worst tree species.
That being said you wouldnt have been in any better soil if you hadnt stripped the land.
What soil conditions are your footings in now
Jess, thanks for reply but I do not understand your drift. I think you at saying talke a line from the tree base to ground level above the footing but how would I then use the sloping line that would be created.

Chapters, again thanks, the soil is very stable clay with low porosity.

I just cannot get the logic of digging foundations ever nearer to the base of the tree where the roots are......surely the purpose of the depth table when used on a level site is to get the foundation down below and away from the tree roots.
The really annoying thing is that this cannot be unique but I'm dammed if I can find any narrative that covers the situation.
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That sloping line is taken as the mean ground level and would be where you take your measurement from.

The effect of the tree isn't just down to root disturbance, but mostly to do with the moisture uptake of the tree from the ground and how it effects different types of soil for example as a tree consumes water from a clay soil that clay shrinks leaving possible voids beneath your foundations. This is more prevalent nearer the surface where the moisture content is more variable, hence you dig down to where the conditions are more consistent over time and particularly more protected from seasonal change.
As chappers says, it isn't really a case of tree roots causing direct damage it is more about eliminating the possibility of expansion and contraction of clay due to wetting and drying. The main factors are; the species and water demand of the tree, the volume change potential of the soil and foundation depth. (also whether the tree is to be removed or retained) On sloping sites depth has to be adjusted to allow for additional exposure due to the slope. Looking at your sketch it is highly likely you will need an engineered solution.
I went to a house where an extension have been built with 2m deep foundations because the was a line of poplars 50m away!

it looked mad when you stood there.

What you guys are showing is that this is almost a figure in the air judgement. I find it astonishing that it comes down to an engineer's take.....I could ask three different ones and get three different answers. Is there a source of robust information on this subject that you know of. The best my guy can come with is the NHBC regs but they are for the NHBC not for BC or my engineer.
It's only figure in the air because you don't have the information. With the information I listed above the depth can be calculated accurately. If your engineers are coming up with different answers then I would sack them. Find one that knows what information is needed, gather the information and do the calculation. As I said, I'm fairly confident you will need more than a strip foundation but until it is calculated properly you won't know for sure.
I went to a house where an extension have been built with 2m deep foundations because the was a line of poplars 50m away!

it looked mad when you stood there.

They was robbed. Trees 50m away do not influence foundations.
As Jeds says there is a definitive answer to this with the correct information to hand. I'm no expert but from experience I would have thought that an engineered solution might be necessary here.
Jeds will be able to confirm this better, but I was under the impression that low porosity soil was more susceptible to seasonal expansion and shrinkage and was the worst situation combined with a high water demand species of tree.

Jeds just out of interest would that count as a sloping site even though he is building on a plateau or would that depend upon the size of the plateau. if it is defined as a slope would the line between the tree and the house you mentioned previously be the grade of the slope.

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