I hope to build a pole barn some day but the land isn't really level there. Their might be a 12"-18" drop over 40'.

What I was thinking was sinking in all the posts then filling in to grade. But how would I keep the dirt from washing out from under the concrete slab other than fitting concrete blocks between the posts and filling them with cement.

sinking the posts, using form boards and pure some concrete in making a mini retaining wall leaving the outside form board on and using it as a lower band board ?

Doing this would allow me to fill in the floor as I can also allowing nature to help settle the fill until its enough to where I can pure a slab.

Ideas ?