EE WAS USING A JACKHAMMER TO REMOVE A PLUG OF HARDENED CLINKER IN THE DOGLEG OF A SILO. PORTION OF THE MATERIAL PLUG CAME LOOSE AND FELL ONTO THE END OF THE JACKHAMMER WHICH WAS POSITIONED INS IDE THE CHUTE OPENING. THE FALLING MATERIAL CAUSED THE JACKHAMMER TO PIVOT UPWARD, STRIKING EE BELOW HIS JAW.