This looks like a case of "oh it will be alright" when planning the building or someone not bothering to do a proper ground survey!
Not too sure on the requirement for building on clay as never had to do it but I know building on "settled" made up ground such as a old tip or slag heap requires building on a raft so that the structure will "float" as one instead of tearing itself apart
I would guess that something similar should have been specified here but wasn't
As an aside an ex friend of mine lives on a a former slag heap where all the posh houses are built on a raft for the above reason, when he had his extension built (garage and bedroom etc above) I happened to walk past and saw normal footings with no steel and no tie in to the existing raft
I mentioned this to him in the pub that evening and he got quite shirty as his architect knew what he was doing with the spec, and I was just a builder
With an attitude like that i just let him get on with it and said no more!
Five years later you could put your hand almost through the garage wall the cracks were so big ! the whole lot had to come down and be done properly
Me being me I took the piss mercilessly hence him being an ex friend and its no loss
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