Excavate to a depth of 12 inches for the drainage foundation, and
around the outside edges of the entire floor dig a trench 12 inches
wide and 18 inches deep. (This trench, filled with concrete, prevents
hog wallows from undermining the floor and keeps the rats from nesting
under it.) Fill all of this space (except the trench) to the natural
ground level with well tamped coarse gravel, crushed rock, tile culls
or brickbats. This fill forms the drainage foundation as described for
sidewalks.