A MECHANIC FROM THE ROBINS SHOP WAS AT A SURFACE LOCATION (MONTOUR QUARRY) TO SERVICE A DUMP TRUCK. THE MECHANIC HAD THE BED RAISED PARTIALLY TO HAVE ACCESS TO THE BRAKE CYLINDERS. WHILE DETER MINING WHERE THE AIR LEAK WAS, SOME MATERIAL WHICH HAD BEEN FROZEN IN THE BOX RELEASED AND SLID DOWN. THE MECHANIC (WHO WAS DIRECTLY BEHIND TAILGATE) PULLED AWAY AS THE MATERIAL HIT THE TAILG