Abstract: A jet boring pre-assembly for excavating an underground area susceptible to disaggregation and for control from a working gallery below the area includes a cutting head for the top of the assembly, the cutting head having separate conduits for delivery of high pressure cutting fluid to high pressure nozzles, and a low pressure flushing fluid to flushing nozzles. A dual conduit drilling rod is located in a borehole beneath the cutting head, the rod having separate conduits for delivery of the high pressure and low pressure fluids to the cutting head. A preventor box below the cutting head seals the borehole and transmits slurry of excavated material to an outlet.