Minor subsidence problem

[paulbandler]

Can anyone comment on a subsidence issue without actually seeing it? Here are the facts...?

A bay window in a 1920's property has evidence of movement. I believe it is due to subsidence as adjacent there is a concrete path that also appears to have subsided and cracked, neatly mirroring the direction that the bay appears to have moved.

The largest evidence of movement is an opening in a stone sill of about 7mm. But there has been an attempt to address the problem previously as I can see where cracks have been filled but they have reopened again. It's difficult to know how fast it has moved since the repair attempt but judging by the age of the decoration that was covering one old repair internally and the additional cracks my guess is that is has moved a further 2-3 mm since it was repaired 10 or more years ago.

There is mature tree 2.5 meters from the corner of the bay in the direction that the bay has moved. I can't identify the tree - might be a form of plumb tree, but its trunk is about 25cm wide and its canapy has been trained to form an 4m wide a kind of hedge atop an 2m wall.

I dug out the earth around the bay to look to for any other cracking around the footings/founds. However I _think_ I may have found evidence of previous underpinning. I'm not sure what underpinning would look like, but this is a rough concrete block area under the bricks whereas elsewhere where I have dug out for drainage I have found what I understand to be a 'footing' brick course, or is it called a 'spreader' brick course protruding from the brick face.

There is no other evidence of subsidence - i.e. there is a second symetrical bay on the other side of the house where there are no trees closeby and there is movement there. No mention was made of this movement in a recent survey (simple mortgage survey).

My research into subsidence seems to indicate that 80% of all subsidence problems are caused by trees too close to a building and extracting moisture from the ground causing shrinkage and that without removing such trees underpinning will not always cure a problem.

So, my question really is - is the above enough evidence to condem the tree and simply remove it, or should I engage the various experts (aborial consultants, structural engineers etc) who may want to monitor it for the next 6 months before I can make a decision and move forward...

In any case, at this stage (whether or not the tree is removed), would it be prudent to strap the bay with helibars?

Thanks in advance.

Paul

 

[Mark]

any roots down there when you were digging ?

Is the bay original to the building or a later add-on?

 

[Penners]

Mark's question about whether the bay is original is important. If it was added after the original structure was built, this could explain a concrete footing, rather than a brick one. So underpinning may not have taken place.

I think it would be very unwise just to cut down the tree and assume that this is going to solve the problem. Tree removal can cause as many problems as trees themselves do, because their removal can cause a change in the water content of the subsoil. This in turn can change the behaviour of the soil.

I really think you need to get a professional opinion on this. Diagnosis "at a distance" is virtually impossible, and incorrect diagnosis could even exacerbate your problem.

I would call in a structural engineer.

 

[paulbandler]

"any roots down there when you were digging ? "

Nope, but I may not have gone down far enough...

"Is the bay original to the building or a later add-on?"

I've no reason to believe the bay's are not original. It occurs to me I could dig out the other bay that hasn't moved and look at what it looks like as a way of answering that ...

 

[biffvernon]

might be a form of plumb tree,

You can always tell a plumb tree - they grow very upright.

 

[thiambal]

I am no expert, but is there any evidence of movement in any other part of the house?

I think I am right in saying that bays were often not very well tied into the main structure of the house and can be prone to movement. I guess it would be useful to know what history there is of any remedial work on the bay - any chance of contacting the previous owners or talking to neighbours.

 

[Lime]

might be a form of plumb tree,

You can always tell a plumb tree - they grow very upright.

:lol: :lol: :lol: :lol:

What type of soil is the house built on?

 

[middi]

My guess is that the tree has nothing to do with the subsidence.

It appears, as already pointed out, that if the tree was the root cause of the problem it would affect the house and not be seletive to just pick on the bay.

It is possible that the bay is a later addition ,or , more likely original built on very weak footings.

The underpinning already carried out may not be good enough and monitoring the situation for a while would confirm if more drastic action will be required.

 

[NT]

might be a form of plumb tree,

You can always tell a plumb tree - they grow very upright.

And they have a dull grey appearance.

This thread is spelling chaos, whatever have those spelling bees been fed? Fermenting fallen plumbs perhaps?


NT

 

[Lime]

Mark's question about whether the bay is original is important. If it was added after the original structure was built, this could explain a concrete footing, rather than a brick one. So underpinning may not have taken place.

The bay window may be original and yet still built as an add-on.
It was not uncommon, even in those days, to offer a choice of frontage design.
For instance whether or not to have a bay window, various designs and a choice of materials used such as stone or brick.
Also for front door surrounds.
All were often "tacked on" with very little tie-in to the house wall.

 

[Penners]

A Fair point, Lime, but if the bay was built concurrently with the main building - as an "optional extra" - wouldn't the foundations have been the same?

 

[Mark]

Depends

If the original purchaser had a wife like mine the spec might have said one thing until the day before moving in. Whereupon a totally different reality would impose itself and the bay windows would have suddenly been required three months earlier and she *did* inform you of that fact in between twittering about plans for a coffee morning or something.

Cue quick chat with builders.

'No problem mate, we'll knock up the bays in a jiffy'

 

[biffvernon]

I did some window frames for a bay window of a Victorian house a little while ago and was intrigued to find that the bay was not really tied into the house at all.

 

[paulbandler]

Earlier I wrote:
"However I _think_ I may have found evidence of previous underpinning. I'm not sure what underpinning would look like"


But today I dig a little digging around the other bay that shows no sign of movement - and found exactly the same tyle of rough concrete founds just about a foot below the soil surface. So I guess this that the bay with the movement has probably not already been underpinned.

Mark's question about whether the bay is original is important.

I really think it is.

Tree removal can cause as many problems as trees themselves do, because their removal can cause a change in the water content of the subsoil. This in turn can change the behaviour of the soil.

I am a bit wary of this, but my net-based research to-date seems to indicate that heave (gound swelling due to removal of moisture consumer such as tree) is uncommon.

It appears, as already pointed out, that if the tree was the root cause of the problem it would affect the house and not be seletive to just pick on the bay.

There is also a crack in the first floor wall above the way, running at a diagonal to a window. Plus in the middle of the house there are a couple of cracks that look of long standing.

It's really only the bay that worries me - particularly as it has clearly moved since a repair attempt. It looks very clear that the ground adjacent the bay has subsided slightly and that this seems very consistent with the bay movement. But I think what I need to determine is whether the tree growth extracting moisture from the ground has caused it and whether it is likely that it will continue to subside. It sounds like I need to understand the tree's likely future need for water or shell out the £340 for a visit from an aborial consultant...

 

[Schoolmarm]

Please speak to Treeman on http://www.gardenlaw.co.uk. He is "the man".


If your soil is sandy then the tree is not affecting your house.

 

[Lime]

If your soil is sandy then the tree is not affecting your house.

That is one of the reasons I asked what type of soil the house was built on many posts above.