Must be a heap of information out there somewhere - the French were masters at fortifications, Marshal de Vauban was the greatest military engineer of the 17th century - and for a few after that as well. Here are some pages from greatwardifferent that may be of interest:
I am trying without much luck , to find information on French world war one pillboxes and trench bunkers and fortifications in particular. All I find on the net is German and British bunkers and pillboxes.
Not quite certain what your looking for these books contain information on trenches fortifcations and emplacements that may be relevant the best place to look is probarbly CARL look in obsolete manuals...