Here is a photo of Joy, who has just finished packing the 6 (or so) sections on the left side of the arch with castable. We screwed in the plywood boards and then packed that 4 " or 5 " section. Then we screwed in another plywood board and packed it with more castable.
It started to bulge out a bit so we screwed in the vertical piece to hold it and also put in clay to stop any leaking out in between the plywood.
Hope that helps,