Hi, some trenches, and how I build them:

The base is pastic sheet (1mm) with foam (styrodur) on top (3 mm), the trenches cut out with a sharp knife. The craters are rubber seals, fixed with glue and soft tissue. You get them in a lot of diameters, and you need a lot of them.

Then I applied bird sand on the surface and into the trenches - very fine one into the trenches, a rougher one with little stones in it for the surface.

 Mud colour to your liking..

... and darker colour for the craters, a little drybrushing, green, and thats it. Cheap and easy.

And in addition my first fort, "L'Ouvrage de Froideterre" (http://fortiffsere.fr/verdun/index_fichiers/Page4551.htm)

And finally everything together:

Liking it

Wow! that is remarkably effective terrain. It is so good it deserves a mottled mat underneath it. Good little artillery pummelled wood as well.