Check this out G. We core stuff all the time at work and hire guys that do it for a living. We all use the same type of drills and rigs. Hope this link works..
As far as expanding grout, haven't ever seen it. Not saying it doesn't exist just haven't seen it. No matter what you use, anything that will be able to build enough pressure to crack a large rock or boulder will have a ton of kinetic energy that will be released when the fracture / crack occurs. Something to consider.
What about something like a partner saw or cutoff saw? Space too small?
Was just joking about the dynamite...
https://www.google.com/imgres?imgurl...act=mrc&uact=8