Hi,

thinking about its construction (concrete poured onto sand), it is 
possible that there could be trapped air underneath, which would have an 
obvious effect...

Cheers

Ian

ChrisAtUpw@....... wrote:
> In a message dated 2008/03/11, ian@........... writes:
>
>> underneath is clay.  It's very tough stuff, digging it out was quite 
>> hard.
>
> Hi Ian,
>
>        Thiscan be expected to expand quite a lot when it absorbs moisture.
>
>        Could you have an air pocket underneath the slab? That could 
> respond to air pressure changes.
>
>        Regards,
>
>        Chris 
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