we dug the dog pens about 3 feet deep. Then, we put down a layer of blue stone and then laid a cinder block floor with the holes in the blocks facing up. On top of that we put about 8 inches of washed pea gravel.which filled the holes in the cinder block and left about 6 inches of pea gravel.
How clever and so very logical. I'll remember that one for future reference. That process could apply to many things besides dog pens.