I still think the easiest way to do piles of rubble are to cut up the sprues from your various models (Never throw this useful plastic away) into various lengths. This I usually due with either my sprue cutters or even some tiun snips. Then I mix that along with a healthy dose of heavier grade or a mixture of sand/flock and white glue until I have a nice thick mess. I usually mix this actually in the aluminum pie tins used for craters below. I then turn it out onto a precut bit of mdf or other handy base material, throw in some odds and end pieces of recognizable bits, such as doors, armor, legs, guns, lights etc. I kind of move things around while the rubble goo is still wet until I like the look of it then I let it all dry. A bit of painting and dry brushing later and you have a nice pile of rubble. I will try and post up some pics soon, never thought to take any of this. Hope this helps.