Raft foundation on a two story extension

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Dave Johnson

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Hi I'm Dave I don't know if anybody could help me but I have a bit of a problem with my two story extension I had built eight months ago, I live in a soft clay area my original house was built in the late 50s the house sitting on 400mm concrete foundation, when I had the drawings done building Regs say it is a soft soil area and you need to build a raft foundation if your building in that area, so with the drawings I had done for my extension I had to have the calculations done for a raft foundation. The trouble is my builder didn't seem keen on building a raft foundation so he dug 600mm down and mass filled The trench without building a raft foundation and the building instructor passed it, which I think is wrong I don't think the builder showed the inspector All of the drawings so the inspector didn't realise it was a soft soil and clay area now I'm worried something will go wrong with my extension in the near future can anybody give me some advice on what to do, or what I can do. thanks .
 

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That soil looks suspicious to me and I would be surprised if a BCO has passed it off. It looks like made up ground the way the trenches are collapsing in. How deep did those connie footings go down next door?

EDIT:

(Bet it was a private inspector)
 
The Connie footings next door are sitting on 400mm I think, and yes it was a privete inspector.
 
It was raining the whole time they was digging out the footing with a mini digger I think that's why it was collapsing so much.
 
For a small residential extension like this the foundation approval is entirely down to the site inspection. You can put notes on the plan asking for rafts or piles but until you actually dig out the foundation trench and actually inspect the sub soil you really don't know what foundation will be needed.

The term "soft clay" is fairly meaningless. There are numerous different kinds of clay, all with differing qualities so it depends exactly what type of clay was found at the bottom of your trench.When I used to work in a clay area we had 2 types, Oxford and Boulder. The Oxford was great stuff, hard as hell and a dark bluey colour, the guidance from the building inspectors was "go down to the blue". The boulder clay was far more variable ranging from hard as concrete to really quite squidgy so you had to take each foundation on it's merits but it was always just a matter of digging a bit deeper until you found the firmer clay and sometimes a bit of reinforcement to bridge a soft spot. I do not recall ever seeing a raft foundation due to the clay, occasionally piled due to trees but that was all.

As noseall said the collapse of the sides of the trench would suggest poor ground either fill or loose sandy soil near the surface, HOWEVER the bottom of the trench is holding water so I would suspect the builder has gone down into the clay.

I would say that if the building inspector has signed it off you are OK plus the fact that the existing house is not on a raft and that has stood up for over 60 years without too much trouble.
 
I think the original house was built on a raft foundation that's the trouble, I don't know why the inspector didn't want us to put a raft when it stated on the drawings that it needs one, The local council insists you need a raft to build on the area I live in I've asked over five of the people who have had extensions and they said they'll was made to put a raft in. Do you think it's because I went with a private inspector .
 
You need to go back and check.

Is the existing house built on a raft? (Not "I think it is"...find out for sure, I would have expected the edge of the raft to have been exposed when the new foundation trench was dug)
What type of clay is present? (not just "soft")
Why does the council Building Control insist on raft foundations locally? ("soft clay" is not a good enough reason)

If there is a valid reason why a raft should have been used then your builder and the approved inspector have stuffed up big time.
 
That's what I was thinking, but the building as been passed off now and the builder is no where to be found. There's no problems with the building so far but what do you think could go wrong in the future.
 
I missed the fact that the extension was finished 8 months ago.

Make sure you have all the relevant approvals, completion certificate etc. I think you've got 6 or 7 years to go back the the approved inspector and 6 years after the defect is visible to go after the builder if it starts to move. But fingers crossed you wont need to.
 
Tip of the day. When you get someone in to do work on your house, they do what you want them to do not what they want to do.

Anyway, its not a problem until it becomes a problem, then you have a claim and not until then. You have house insurance don't you? Just keep the premiums up to date.

BTW, did you still pay for raft foundations as per the builders quote, or did you reduce the money you paid to the builder because of this? If the latter, your position is weakened, just hope not literally.
 
One thing I was looking at was because of the trench collapsing when digging it made trench about 6ft wide and 2 meters deep it cost me £1500 in concrete to fill which was way over what i needed to dig all most double what was on the drawings, do you know if that would make it as strong has if I reinforced it with a floating raft.
 
No. It's not about one being as strong as another. Strip and rafts perform differently based on their design and the ground conditions
 
Is there any way I can put this right may be digging all the way round the foundation exposing the concrete then building a raft all the way round then Shuttering and fitting it with concrete do you think that will solve the problem.
 
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