hello,

I want to build a fake rock outdoors (example this erratic block:
http://gsc.nrcan.gc.ca/landscapes/ph...et/img0049.jpg )
the size is 6 x 8 m and about 5 m high. it will be hollow and will be
accessible as a kind of cave.
I admit I have never worked with concrete. (I'm rather a wood
specialist)

I imagined to build a frame of iron and then spray the concrete on
both sides for a total thickness of about 15 to 20 cm.

I do not know if I need to build foundations?
is there a problem of humidity, venting?
how hard is it to do a nice finish of fake rock?
is only 15 cm thick enough?
How do I estimate my time for the production of such a piece?
where do I find a basic builder vocabulary ?
any good source book for me to read this winter?

I am looking for general advice, errors from beginner to avoid tricks,
corporations and others.

see you,
gregory