Looks like a great project and I can't wait to see how everything turns out. I don't know if it's available in your area, but #78 or #8 is much easier to work with in situations like that. I still use a lot of #57, especially for drainage where I want water to flow, but for backfilling or bedding stuff I love #78 or #8. It flows into crevices and self compacts very easily. A bonus is that it's easier to shovel too.