I agree with you Charlotte about the initial animation being a bit off, so while I was refining it I thought I'd make some more improvements and add some touches -
Improvements and Revisions:
Rocks no longer shift before they fall
Some rocks remain but are displaced so that the hole doesn't look too neat
Rising dust effect added
Pebble animation refined
Here are some stills showing how the dust looks when rendered:
I haven't updated the progress sheet in a while but considering that the rest of my scenes involve this hole, it means that the sets are modelled and all that's left to do is to animate the astronauts.