I've done a lot of collapsing in my time, - the link in my signature contains my first major collapse experiemnts.
However, the method I'm using a new method in my new fort, and it works very nicely.
The Idea is to designate:
- all 'inner cells' with 'Up/down'-stairs.
- all cells on the top z-layer with down stairs.
- all cells on the buttom z-layer with up stairs
- all cells on the sides with normal 'mine'.
The nice part is that this can all be designated at once and the miners will dig it out at their leasure. (The only price is possibly lowered FPS due to the full access to everywhere due to the stairs, but this is only temporary.)
When it's all done you have remove one 'up stair', and build a support in it's place.
You then channel out all the side cells, - beware of the corners. It's best to designate all corners to be channeled first, and then designate the rest of the sides for channeling.
With that done you remove all lower upstairs. Remember not to have anyone stranded inside the now floating blob of stairs.
Link the support to a lever, - pull and go for a cup of coffee while the computer calculates the result (seriously, - a big collapse can take an hour for me to calculate).
The result is NOTHING!
All the stairs collapse into dust and you will have a huge empty space to work with.
(I hope I'll soon be able to show my new fort where this was used extensively.)