Although it is an old patent from 1972, I think it is still interesting, At least for brainstorming, especially in these days we are stuck with 1950s technology in mechanized tunneling.

Method And Apparatus For Tunneling By Melting

The abtract of the patent reads:

"A machine and method for drilling bore holes and tunnels by melting in which a housing is provided for supporting a heat source and a heated end portion and in which the necessary melting heat is delivered to the walls of the end portion at a rate sufficient to melt rock and during operation of which the molten material may be disposed adjacent the boring zone in cracks in the rock and as a vitreous wall lining of the tunnel so formed. The heat source can be electrical or nuclear but for deep drilling is preferably a nuclear reactor. "

Nice weekend reading? Link redirects into a pdf file on this site, which is a scan of the US Patent filing.

Patent doc: Method And Apparatus For Tunneling By Melting