EagleEyes wrote:
The soil is very poor sand and clay...
http://www.lancs.ac.uk/postgrad/alves/pic/soil.jpg

any suggestion on how to cover it without putting more soil on top of
it? I am trying to find some grass or some very resistant ground covver
plant to cover 60sqm of this soil...

Grass will grow in worse places than that. Break it up with a fork,
rake over, picking out the worst stones, then follow instructions on
packet. If the whole patch looks like that, and you find the prospect
of forking it over too daunting, it's one of the rare instances in
which I might consider the possibility of hiring a rotovator -- but if
you aren't used to them, the fork is actually much easier work. You'll
need to rake well after the machine anyway. Certainly forget about
rotovators if it's that stony all over.

--
Mike.