My trench project is going quite nicely and I already have ideas for future pieces such as more turns, gun emplacements, and command emplacements. I have moved on to coating all the sections in a slurry of Durham's Water Putty, saw dust, and a mixture of dirt and small pieces of gravel swept up from my garage floor.
Sections with dried Durham's slurry
After the Durham's had dried (a few hours), I painted with a sample pint of brown acrylic wall paint I'd found on the clearance shelf at Home Depot. In addition, I sprinkled the floor of each section with the stuff I found on my garage floor. Most of this will be covered by planking, however I thought it would give a nice look for any dirt peeking through the planks. You can see a definitive styrofoam look to the walls, but this will all be covered in shoring. I have kicked around wattle fencing, but planking will be much faster and it'll look good enough for my purposes.
Stay tuned for drybrushing, planking/shoring, and flocking.