EMPLOYEES HAD REMOVED THE WASH SCREW AT THE SIDING & HAD COMPLETED THE REINSTALLATION OF THE CONVEYOR. A CO-WORKER WAS MOVING THE BACKHOE OUT OF THE WAY, WHILE EE WAS STANDING UNDER THE WASH S CREEN, WHEN A ROCK THAT WAS WEDGED IN THE TRACKS OF THE BACKHOE POPPED LOOSE & STRUCK EE UNDER HIS LEFT EYE BENEATH HIS SAFETY GLASSES.