Past bad experiences have lead me to making digging a cistern a high priority with setting up a new fort (before the first winter). I've always done this through channelling one level at a time from the top, but I'm wondering whether instead I can use controlled cave-ins? Since I need to do a bit of micro-management anyway to stop my idiot miners making a (bloody) hash of it, I might as well go the full dwarf and do it in a marginally faster, but over-engineered and dangerous way. It's time I add controlled cave-ins to my arsenal of tricks anyway, but I haven't managed to do it just yet and was wondering if any of you can give me some advice. For instance, whether the system I describe below should work (if it does, I'm screwing up somehow), how many z-levels at a time I can collapse this way, how many layers below it will take out, to what extent this makes something simple be dangerous and counter-productive, etc.
As I understand the wiki, what I'd need to do to is have alternate layers like this (h=channel, . = untouched rock):
z = n
hhhhh
h...h
h.... <- arm holding up plug
h...h
hhhhh
and z=n-1 channelled out entirely. If I then have a miner channel out the arm (not standing on the plug), the plug should drop, and after the dust settles, I'll have a 5x5x2 pit, right? Since I find channelling out areas larger than say 7x7 across multiple z-levels to be slow and frustrating, this could be a real time-saver. And, of course, help for cutting through aquifers, cave-in traps, magma pistons, etc.
Am I missing something? Also, is there something stopping me from making the plug two or more z-levels deep?
When I tried this I had a staircase in one corner to allow the miners access to the various z-levels. The plug didn't collapse, even though staircases aren't supposed to support terrain diagonally. I was going to investigate further but then real-life intervened and I haven't touched save in a couple of weeks.