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   
In a me=
ssage dated 2008/03/11, ian@........... writes:



underneath is clay.  It's=20=
very tough stuff, digging it out was quite hard.



Hi Ian,



       Thiscan be expected to expand quite a l=
ot when it absorbs moisture.



       Could you have an air pocket underneath=
 the slab? That could respond to air pressure changes.



       Regards,



       Chris