There's a lot of backfilling to be done. In addition to filling the gap between the pool shell and the newly built retaining walls, there's also the rear retaining wall that can now be safely filled in. The graphic below shows the key areas in red:
The gap between the pool shell and retaining wall / earth must be filled with crusher dust (fine crushed rock). This is the same material that was used under the pool, but unfortunately this time we don't have a skid-steer to help move it around.
A precarious route
In-fact getting it from the front of the house to the gap in the wall turned out to be the most challenging part, requiring the construction of a ramp down the existing garden stairs and some extremely hair-raising wheel barrow trips.
Once at the site, the fill on the downhill side of the pool had to be shoveled over the wall. I gave some thought to building a dirt conveyor, but in the end decided to stick with the manual route.
One more wall
The rear retaining wall sits between the pool and the house and was completed at the same time as the pool retaining walls were core filled. It also requires backfilling. This wall will eventually have a deck on the upper side and glass fence atop, as depicted above. Before either of those can be built, the area excavated for the wall and footing must be backfilled.
Backfilling involves shovelling the excavated dirt (as well as the dirt from the wall collapse) - which now resides on the pool deck - back over the wall. To prevent excessive hydrostatic pressure on the wall, adequate drainage must also be provided. A slotted agdrain at the base of the wall and gravel immediately behind the wall ensure that no significant water build-up is possible.
Beyond the installation of some landscape fabric to keep the drainage channel dirt free, it's a pretty low tech process. Various plumbing pipes reside behind this wall, so care must be taken to ensure they're well supported, but beyond that, it's just a whole lot of shoveling and manual compacting.