homer, since you have what sounds to be a stable foundation on sound footers.......the way i was tought to put in a basement was to start by digging out one corner at a time, 10` deap for an 8` basement, completely remove all the dirt for 3` on each side of the corner being sure to not disturb the soil that the footers are resting on past the 3` mark...we installed 12"x24" x24"footers with steel protruding to tie into the next footer pour and stem walls using a 7bag mix... form and pour the stem wall next....after at least 10 days grout between the pour and the existing foundation using semi-metalic grout.......first corner supported......do the remaining 3 corners....then using the same procedure move down the wall in 10` spacing leaving the existing footing bearing on undisturbed soil and install footers-n-piers......now the fun starts......you`ve effectively got the structure supported at the corners and every 10`......fill in between your piers with footers (tied to the pier footings with steel) and pour walls ....don`t forget to grout the gap!...on all but one opening.....excavate a ramp into the one unpoured openings......install a temp. beam to bear the weight of the floor joists over your access hole and start moving dirt.....as you near the "rock supports" you mention you`ll need to transfer that weight to ground......preferably via a footing....this can be done straight down, (post in your new basement) or by transfering the weight to your new footings via a beam?.......the rest should be pretty straight forward.........tod
I haven't done this either, but when we had our home built we could only afford a 1/2 basement. Once the hole was dug I saw that the entire space was dug out and it was deeper than normal. Probably had a 9.5' or 10' ceiling in that basement.
When I asked the builder about it he responded that he had to go that deep because the ground wasn't stable enough for a foundation above that spot.
Not sure what your soil conditions are but you never know what you are going to get into when you do something of this magnatude.