We have a crack that runs down the side of our house (I've posted about it on here before) and I just wondered, can a structural engineer tell if it's subsidence just by looking at and around the house, or does the ground need to be dug up to check and the cracks monitored? Just wondering how they can determine for definite that it's subsidence or not.
Subsidence - how is it confirmed?
- Thread starter skm1981
- Start date
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skm1981, good evening.
In a lot of cases a wall crack internally or externally is not by definition Subsidence, experience will assist greatly, but is fallible.
A crack following the line of a Chimney Flue in a Sandstone property is probably Sulphate attack.
Internally cracks in straight lines on [especially] ceilings are probably down to Plasterboard sheets.
Testing for Subsidence, there are several ways to find the geology of the ground under the property, one fast way is the App for the I=Pad from the BGS.
If as happens up here a lot the property may be sited in an area of Old mines, Leaking drains are a killer but silent.
When an Insurance claim for Subs comes in it is a process of elimination if you want to be sure as to what you are dealing with.
1/. Instruct a CCTV survey of all the drains within a 10 / 15.m radius of the subs area.
2/. Monitor, some people like to use studs stuck on to the wall and take measurements over a 6 month period, this to eliminate the cold and warm months thus eliminating thermal Movement
3/. The Acid Test?? instruct a "Level Monitor" survey, this takes the form of permanent studs fixed to the wall over the area of concern, take readings over the same time scale as above. I say "Acid Test" because, the definition of Subs in insurance terms is the vertical downward movement of the foundations,
OK the above is the basics, but will give a good idea of what is going on as far as Subs investigations are concerned.
As for excavations? OK But difficult to absolutely confirm with no creditable evidence ?
Ken.
In a lot of cases a wall crack internally or externally is not by definition Subsidence, experience will assist greatly, but is fallible.
A crack following the line of a Chimney Flue in a Sandstone property is probably Sulphate attack.
Internally cracks in straight lines on [especially] ceilings are probably down to Plasterboard sheets.
Testing for Subsidence, there are several ways to find the geology of the ground under the property, one fast way is the App for the I=Pad from the BGS.
If as happens up here a lot the property may be sited in an area of Old mines, Leaking drains are a killer but silent.
When an Insurance claim for Subs comes in it is a process of elimination if you want to be sure as to what you are dealing with.
1/. Instruct a CCTV survey of all the drains within a 10 / 15.m radius of the subs area.
2/. Monitor, some people like to use studs stuck on to the wall and take measurements over a 6 month period, this to eliminate the cold and warm months thus eliminating thermal Movement
3/. The Acid Test?? instruct a "Level Monitor" survey, this takes the form of permanent studs fixed to the wall over the area of concern, take readings over the same time scale as above. I say "Acid Test" because, the definition of Subs in insurance terms is the vertical downward movement of the foundations,
OK the above is the basics, but will give a good idea of what is going on as far as Subs investigations are concerned.
As for excavations? OK But difficult to absolutely confirm with no creditable evidence ?
Ken.
Most cracks are indicative of the cause by their shape and location, and subsidence can be diagnosed in terms of being most likely above all over potential causes relatively easily just by looking at it and the surroundings. It literally is not rocket science, although some surveyors and engineers try to make it seem so.
The issue with subsidence is whether it's ongoing or not, and that requires monitoring or ground investigations, and that is purely because you can't devise a repair unless you know the cause and more crucially whether the cause is on-going or not.
The issue with subsidence is whether it's ongoing or not, and that requires monitoring or ground investigations, and that is purely because you can't devise a repair unless you know the cause and more crucially whether the cause is on-going or not.
Thanks. The structural engineer has said historic subsidence on the back corner of the house. He can't see any evidence to suggest it's ongoing, but suggested having heli bars put in and then it is likely no further problems will occur. But then someone told me you can't definitely determine subsidence unless they dig into the ground, so it just made me wonder - but this person is not a structural engineer. The structural engineer said this has most likely been caused by consolidation of fill material. We've lived here for 15 years, my survey at the time noted historic movement but no evidence to suggest ongoing, so I'm assuming and hoping that this movement is from way, way back, long before we purchased.
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That raises the question of what the fill material is, (how does the engineer know its fill material without soil samples), has the consolidation been caused or assisted by something else, and then what is to stop it consolidating further.
But then if it's not moved in 15 years, then there's a good chance that movement episode has stopped.
He didn't say what the fill material was, so I've no idea. I know that in the report, it says that the ground the house is built on is able to withstand large weights, so not prone to subsidence, but yeah, didn't say anything about what fill material was.
I felt he was sitting on the fence a bit cos when he spoke to me after, he said that the house had suffered movement, but from what I understand, movement doesn't necessarily mean subsidence. But he did say the bathroom floor slopes down to one side (I can't see this, it doesn't look like it's sloping at all), and he said the same about the kitchen floor (I can see this), but then I wonder if that is because the kitchen flooring doesn't go all the way under the units, like the kitchen units only just sit on the edge of it if that makes sense, and the flooring is only sitting on top of hardboard. He also said he can't measure how much the house has dropped because the kitchen units are in the way and in the bathroom (directly above the kitchen), the bath is in the way so he can' t measure, but the bath panel comes out pretty easy to be fair.
I don't know, I just felt he was on the fence, and when he was talking to me, he said he doesn't think I have anything to worry about (we're selling our house), and that insurance companies tend to take issue with settlement, not subsidence, but from what I've read, it's the other way round.
I felt he was sitting on the fence a bit cos when he spoke to me after, he said that the house had suffered movement, but from what I understand, movement doesn't necessarily mean subsidence. But he did say the bathroom floor slopes down to one side (I can't see this, it doesn't look like it's sloping at all), and he said the same about the kitchen floor (I can see this), but then I wonder if that is because the kitchen flooring doesn't go all the way under the units, like the kitchen units only just sit on the edge of it if that makes sense, and the flooring is only sitting on top of hardboard. He also said he can't measure how much the house has dropped because the kitchen units are in the way and in the bathroom (directly above the kitchen), the bath is in the way so he can' t measure, but the bath panel comes out pretty easy to be fair.
I don't know, I just felt he was on the fence, and when he was talking to me, he said he doesn't think I have anything to worry about (we're selling our house), and that insurance companies tend to take issue with settlement, not subsidence, but from what I've read, it's the other way round.
I'll give you an example. Many years ago I worked on a job that was an old Medieval church that had some serious structural cracking.
At least 3 firms of structural engineers had looked at it over the years and declared it was "historic" although the cracks kept coming back and kept getting wider.
That's when I was called in. The cracking pattern was classic subsidence but I think everyone thought as the church was 12th Century it couldn't be and so discounted it and no one ever dug trial holes. I got a local archaeology firm to dig some trial holes after we got permission from the church authorities and hey presto it was subsidence. It turned out the chancel had been rebuilt in the 19th Century and they had built it slightly larger so the walls were directly on top of the old graves.
Whilst you can get a pretty good idea from the pattern of the cracks you can never be sure unless you dig a hole and examine the foundations and the type and condition of the sub-soil. I must have looked at thousands of foundation trenches and trial holes over the years and it never ceases to amaze me how variable it can be over very small areas like a single house, especially if the land has been built on before, then the scope for old excavations and fill becomes far more prevalent.
At least 3 firms of structural engineers had looked at it over the years and declared it was "historic" although the cracks kept coming back and kept getting wider.
That's when I was called in. The cracking pattern was classic subsidence but I think everyone thought as the church was 12th Century it couldn't be and so discounted it and no one ever dug trial holes. I got a local archaeology firm to dig some trial holes after we got permission from the church authorities and hey presto it was subsidence. It turned out the chancel had been rebuilt in the 19th Century and they had built it slightly larger so the walls were directly on top of the old graves.
Whilst you can get a pretty good idea from the pattern of the cracks you can never be sure unless you dig a hole and examine the foundations and the type and condition of the sub-soil. I must have looked at thousands of foundation trenches and trial holes over the years and it never ceases to amaze me how variable it can be over very small areas like a single house, especially if the land has been built on before, then the scope for old excavations and fill becomes far more prevalent.
I've had customers complain when I refuse to put a fixed price on an orangery job.
When I point out below ground is unknown, I've often had the "dig a trial hole" solution.
A job a couple of years back had a well in one corner, a big concrete pad which seems to be from an old garden feature and a huge lump of granite of unknown purpose. Oh and in the reduce dig we came across some gravestones from a "pet cemetry"
I'm surprised how many jobs I've done with deep top soil - 800 mm or so. Not a problem but it means changing from floor slab to block n beam and thus a delay.
The way I like to play it out is.
1/. Is the wall [whatever] still moving? [all as previous post]
2/. If the wall is actually "Subsiding" IE the vertical downward movement of the Founds, then instruct at least two Soil Investigation bore holes, generally done manually with an auger or similar. At the same time it is important to expose the foundations themselves, and log it all for posterity.
3/. Once the results are in for the above and the problem in indeed a Subsidence issue make a judgment call as to which method of repair to instruct.
Totally agree with the previous posts as regards what a SI will uncover, I have been surprised on a couple of occasions by a layer of Peat, one of which was a peat band 500.mm. thick, the SI was on a property about 100.m. from the Sea.
Ken.
1/. Is the wall [whatever] still moving? [all as previous post]
2/. If the wall is actually "Subsiding" IE the vertical downward movement of the Founds, then instruct at least two Soil Investigation bore holes, generally done manually with an auger or similar. At the same time it is important to expose the foundations themselves, and log it all for posterity.
3/. Once the results are in for the above and the problem in indeed a Subsidence issue make a judgment call as to which method of repair to instruct.
Totally agree with the previous posts as regards what a SI will uncover, I have been surprised on a couple of occasions by a layer of Peat, one of which was a peat band 500.mm. thick, the SI was on a property about 100.m. from the Sea.
Ken.
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