Foundation problems

[paul865]

Four years ago i had an rear extension built to my detached property. The house is built on a raft foundation however the extension was built on a piled foundation, as all extensions in the area have been done. Unfortunately the house has gradually moved over the four years and now is showing approximately 8mm gap at top joint with extension reducing down to the bottom.
The result of this is that the roof of the extension, which is tied via wall plate to the original house, is being slowly pulled off the extension with cracking at the ceiling /wall joints internally.
I am confident that it is the house moving as i have had it monitored over the time by a surveyor.
My builder and i parted company after having a number of issues and i am sure there will be no helpful advice coming from that quarter.
I would be grateful to hear if anybody has experienced similar problems, or if anybody has any advice.

 

[KenGMac]

paul865, good evening.

Could be an insurance claim? but there is a £ 1,000 Excess?

The only way to be sure what is moving where, either the house or the Extension is to have a Level Survey undertaken, OK not cheap but definitive answers, what monitoring did your Surveyor instruct?

One issue to think about is poor workmanship by your one time builder???

Ken

 

[paul865]

hi Ken

My surveyor son in-law has been monitoring it for me using GPS and assures me it is the house moving.
I had a structural engineer look at it and he agreed with the findings and suggested contacting my insurance company, i did and they sent a surveyor out who did not accept the findings saying that GPS was not the way to monitor it (my son in-law is a professional land surveyor) and that the builder was responsible for up to 10 years.
He also pointed out in his report that there was a tree in next doors garden within 5mtr, ( yes to the extension but not the house) and another at the front of the property, again yes but i have recently pulled that BUSH out with two hands roots and all !!!

My dilemma is, and nobody can tell me is should the extension have been built on piled foundations or should it have been on an extension to the existing raft ???

I have spoken to the local authority building control department and because we used a private company for building control they blanked me.

Don't know who to ask next ?????

 

[KenGMac]

paul865, good evening again.

Were your Insurers using a Loss Adjusting Company called In-Front or the Innovation group ?

As for Piling or raft, I think if you asked three Design Engineers what way to go you will get 4 or 5 differing answers, in effect the decision would come down to costing of each system.

As an aside? was there a soils investigation survey undertaken during the original work?

Back to the "monitoring" can your son-in-law undertake a level monitoring survey? [this form of monitoring is the accepted Insurance method]

As for the adjacent tree??? you would need to get root samples analysed to prove that the [supposed tree roots] are indeed affecting the property but it would have to be one hell of a tree to affect a raft?

Ken.

 

[paul865]

Ken

The insurers sent :
Subsidence Management Services
Innovation Property (UK) Ltd

I was lead to believe, from various sources including the architect, that the local authority did not allow raft foundations now. However having spoken to them since they would neither confirm or deny it !

I am not aware of any soil samples being taken however there are peat seams running through the area at various levels depending where you are. All extensions and new builds in the area are piled and I'm not aware of anybody having the same problem, and i have asked around.

I will ask my son-in-law if he can do a level monitoring survey.

I am wondering if i was to have piles fitted to the front of the property crossbraced to the raft whether that may solve the problem.

 

[^woody^]

A raft foundation is designed to move. Piles are not, and the two are incompatible.

Movement of a house on a raft is not a defect, but perfectly normal.

 

[paul865]

Thanks woody that is my understanding and therefore i would suggest that the original structural engineer should have spotted this and is at fault.
However there are 20/30 extensions built this way in our road alone and countless more in neighbouring roads in the area, have i just been unlucky ??

and Ken i have just spoken to my son-in-law and apparently it was a topographical survey based on a local grid and accurate to .5mm and not GPS, my mistake.

 

[^woody^]

What should have happened, is the designer incorporated flexible movement joints into the connection between the extension and house, and allow for potential future movement in his design.

So not the SE's fault per-se, but the extension designers if he is a different person.

 

[garyo]

What should have happened, is the designer incorporated flexible movement joints into the connection between the extension and house, and allow for potential future movement in his design.

As a lay man, my question is why isn't it best practice to match the foundation types... So in this case, build the extension on a raft so that they all float together. Seems like a recipe for trouble doing anything else?

 

[acurachris]

The house is built on a raft foundation however the extension was built on a piled foundation, as all extensions in the area have been done

This is how they appear to built around me too, including my extension that's under way!

As a lay man, my question is why isn't it best practice to match the foundation types... So in this case, build the extension on a raft so that they all float together.

I suppose from a logic point of view, a raft foundation will be designed in a way to only hold the building it was designed for, so even if you attach a raft extension, it'll never be a part of the original house (never behave like one building) and will likely still move independently from the existing house.

Piled foundations are probably recommended in the hope that the existing 30+ years house has fully settled. Piled foundations are not supposed to move (or much), so it's hoped the two can sit harmoniously next to each other with only slight movement.

That's my view on it, possibly wrong, but it makes sense to me!

When i was discussing foundation types (spoke to a few structural engineers and the building inspector) all but one SE were hesitant to recommend a raft for the extension. The building inspector didn't want one either. The structural engineer that recommended me to have a raft extension said it would still settle downwards by 10-15mm from the original house.

 

[paul865]

What should have happened, is the designer incorporated flexible movement joints into the connection between the extension and house, and allow for potential future movement in his design.

This is all well and good for a flexible joint between the brickwork, standard practice but how do you cater for the roof joint to the original house wall.?

 

[paul865]

Piled foundations are probably recommended in the hope that the existing 30+ years house has fully settled. Piled foundations are not supposed to move (or much), so it's hoped the two can sit harmoniously next to each other with only slight movement.

This is what my independent se said and was surprised that there was so much movement. Also stated that it could also move back.
I am wondering whether the piling has disturbed the raft.

It is becoming apparent that there is no hard and fast rule for this dilemma and is most probably driven by cost as piling is now a cheaper option as the equipment is more readily available, more transportable and cheaper to throw a few more men on the job than pay for expensive ready-mix.

 

[acurachris]

I am wondering whether the piling has disturbed the raft

I don’t know for sure of course, but surely this would’ve shown up sooner if it was the piling.

We had an incredibly dry summer by British standards, could this have caused the ground shrinkage and consequently movement of the raft? Your piled extension would’ve been much deeper into the ground and unaffected by the ground shrinkage.

 

[paul865]

I don’t know for sure of course, but surely this would’ve shown up sooner if it was the piling.

The movement first became apparent within six months and has gradually increased over the last 4 years.

 

[^woody^]

This is all well and good for a flexible joint between the brickwork, standard practice but how do you cater for the roof joint to the original house wall.?

The roof structure can be fixed, but the cover flashings designed to accommodate movement while maintaining cover. Covers at mono gables to. Or an exotic steel bracket that slides, could hold the roof structure.

The design can only accommodate any normal and expected movement, which would typically be about 25mm or so.