I have used this technique for several years in both model RR HO scale and 1/35 scale dioramas, both about the same exept the scale.
Plaster is too brittle, try acrlyics, home repair items are for the most part acrylic based, i.e. Polycrylic water sealant for water based dioramas and caulking for ground work.
1.) Build up an area using styrofoam, florists foam, or construction foam, carve the initial patterns and elevations.
2.) Mix the caulking from the tube with water and acrylic hobby paint to get a basic color, then brush the mix onto the foam, while drying add sand, soil, or any other ground material. This will speed up drying, the mix should not crack or shrink. And should look like pancake batter. Acrylic paints added to the base color of the caulk which I found tan works will enhance the overall color effects.
3.) Continue process until the whole base is covered with about 1/4 inch of the ground material. You can then paint the whole thing for shadowing or other colors blended in.
4.) Practice with small dioramas which are actually more effective than a giant one, even my HO RR is a series of small scenes connected, as I mentioned the two hobbies are actually using the same techniques. I don't have any military dios larger than about 12"x15" and I pack a lot of detail into that small area but they are not crowded. Hopefully a Dragon kit turned into a Mortar crew along side a Tamiya M-21 Mortar Carrier will come out in FSM. We'll see.
5.) Acrylic "Poly" wood sealant works great for water scenes as well and out of the can in one step, no mixing, no bad odor, no heating on the stove and water clean-up! Again, experiment in small water dios, i.e. mud puddles, small streams or my favorite, simulating iced over streams with baking soda snow.
Watch for photos here as my digital Nikon D-40 with macro lense will get to work.
More later, and if you have a moment, I have many posts to MRR as well as FSM with vast amounts of experience from 30 out of 58 years of models and such.
Thanks,
johncpo