Here is some info sent in from a pipe bursting contractor who has obviously encountered some problems in the past. It demonstrates some interesting solutions, so we thought we would pass it on to anyone who is interested.
"As we all learn, it is near impossible to crack through the old fashioned malleable cast iron items that can be encountered at times. We found several ways of dealing with these.
We sometime used a self propelled post hole digger and used that to bore down to the problem item.
Method No 1 - Call the nearest industrial gas suppliers, B.O.C had a mobile service in Brisbane, and a quick hit with a slug of liquid nitrogen will crack the iron ring item.
Method No 2 - Try the head down with an oxy torch method to cut the mongrel out and it works too but not easy. Difficult to prevent splatter getting on the mole body.
Method No 3 - We have also used a stick welder to run a series of beads across the item to make it brittle and that worked too. The Mole was able to crack it but the trick is to get the welds down around the halfway mark on both sides, otherwise the ring will still grab the mole body, not as tight but enough to be a slight worry.
I have read of fellows in Britain using acid to free a stuck mole, but I reckon that would have to be a last resort.
I hope this isn't too outdated, but if it helps someone out of a bind someday then why not.
Cheers Bob. Pipe Bursting Contractor.