I have recently found that foam insulation works as excellent groundwork. If, by any chance, you'd have some foam wall insulation lying around, then try experimenting with it. In a small scene I am making, I have achieved a wonderful clay bank in a trench simply by gluing two pieces of it together and carving out the bank with a blunt knife. also, plaster sticks to it very well. If you have any lying around, happy modelling

Spectre,
Good tip, I have used it in the past also, just never thought to post it here. Thanks.
Regards,
Robert