In article ,
"Phil L" writes:
| Mike Lyle wrote:
| :: Phil L wrote:
| :: [...]
| ::: That all depends upon the footings...I once worked on some houses
| ::: in Manchester and the entire street's footings were less than a
| ::: foot underground! - four courses of bricks beneath the pavement,
| ::: and they were built up off the sand...they /were/ over a hundred
| ::: yrs old, but surprisingly, there was no subsidence at all.

Only surprising to non-experts :-)

| :: Which puts me in mind of something I read in ?_New Scientist_
| :: several years ago to the effect that, despite what it says in the
| :: Bible, sand is a good thing to build on. I can't remember why; but
| :: wonder if it has anything to do with the trick of pouring half a
| :: pint of water into a pint glass of sand.
| ::
| You're probably correct - we had to build extensions to each house and went
| down over three feet - they were still built on the same sand as the
| houses..

We have an extension that goes down a foot further, and reaches a
higher clay content layer, so is LESS stable!

The background of the biblical reference is that sand is excellent
when it is below a stable surface layer, but an absolute disaster
when the surface is loose and the area is prone to flash floods.
I.e. roughly the difference between the UK and the near east,
though it does apply here on sand dunes near the sea.

The same thing applies to sowing fields with salt. This poisons
them indefinitely when all of the precipitation evaporates or is
transpired, but has only a little and very temporary effect when
most of it percolates down to ground water. Even in East Anglia,
there is 10" of rain per annum that percolates down, and that can
remove a LOT of salt.


Regards,
Nick Maclaren.