TWO EMPLOYEES PLANNING TO ADVANCE UTILITIES IN A SINGLE HEADING, SHUT THE MAIN VALVE TO THE AIRLINE GOING INTO HEADING. THEY FAILED TO BLEED THE AIRLINE BEFORE REMOVING CLAMP. WHEN EE #1 REMOVED CLAMP, HEADER BURST FORWARD CAUSING EE #2 TO FALL BACKWARDS; FRACTURING ONE BACK RIB & A FEW VERTABRAE TIPS. ASSESSMENT OF INJURIES IS ONGOING.