Oh, here is what you need to do:
Disconnect the section of rock you want to drop on all sides, above, below, and adjacent directly but not diagonally. Leave one tile for the final cut. Your dwarf will be safe if the final cut is made diagonally and there are no horizontal gaps for the dust to reach him on.
Example of a small collapse (assuming the the example here that you can get to each level from the NW):
top level
######## .####### .#######
######## #.###### #.#vvv##
######## to .##...## to .##...v#
######## #.#...## #.v...v#
######## ##....## ##v...v#
######## ######## ###vvv##
######## ######## ########
mid levels
######## .#######
###^^^## #.#vvv##
##^###^# to ##v###v#
##^###^# ##v###v#
##^###^# ##v###v#
###^^^## ###vvv##
######## ########
Note the top corner wont start with an up ramp on the topmost level- dont add one- just dig down
bottom level (if we have a spare rock layer to remove)
######## .#######
###^^^## #.#^^^##
##^###^# to ##^...^#
##^###^# ##^...^#
##^###^# ##^...^#
###^^^## ###^^^##
######## ########
else bottom level if we are digging through to empty space - dig out channels one at a time - dont trap your miners
######## .####### .#######
###^^^## #.#^^^## #.# ##
##^###^# to ##^###^# to ## ### #
##^###^# ##^###^# ## ### #
##^###^# ##^### # ## ### #
###^^^## ###^^ ## ### ##
######## ######## ########
finally, back to the top and dig out that last tile
.####### .#######
#@# ## #@#***##
.##... # to .#*****#
#. ... # #******#
## ... # ##*****#
### ## ###***##
######## ########
See also
http://mkv25.net/dfma/movie-1527-nomechanicscollapse(but I can do it without the constructions these days)
I've used this a few times in 2010 to get a nice straight stairwell down through underground lakes to the lava area...
--
Cave ins will destroy floors and anything constructed. Solid natural rock will remain unharmed. I'm not sure whether the falling rock will fall as separate tiles or retain its shape. IIRC solid constructed walls can also take a collapse impact
####
|
V
#
# ####
#### to ####
vs
####
|
V
####
# #
#### to ####