We used 1″ flex Styrofoam wrapped in black plastic as a vapor barrier between the bags and the earth. We also had a lot of back-filling and tamping/packing the dirt down.
You can look at a book called Earthbag Building by Kaki Hunter, which was what we generally followed. With the exception of a lot of “staged” photos, and techniques which might have worked for her, but not for our real world application. i.e. “Barbed wire doesn’t come out straight and easy.”
Comment 1: We lined the hole with 4 mil black plastic… used black 6″ flex irrigation tubing which exited into a deeper trench and away from the footprint. We wrapped the tubing with erosion fabric and added drain rock on top then tamped it down.