EMPLOYEE WAS ON THE DREDGE TIGHTENING A BOLT ON THE STONE BOX USING A CLOSED-END BOX WRENCH AND A CHEATER PIPE. HE STATES HE TIGHTENED A BOLT ONE STROKE AND THEN WHEN HE WENT TO TIGHTEN IT AGAIN THE PIPE SLIPPED AND HE FELL BACKWARDS, HITTING A PIECE OF 6"" CHANNEL IRON WITH HIS LOWER BACK AND RIGHT HIP.