i just put up a 40x70x14 pole barn and my problem is across the width of the barn the the ground drops off about 15 inches so the bottom of the barn siding is 3 inches above the ground on one side and 15 inches on the other side can i just backfill with sand inside to get up to grade and then top it with gravel the skirt boards are to the ground all the way around the perimeter so i think that will contain the sand