EMPLOYEE WAS ATTEMPTING TO SEAT A 4'X 6'PIECE OF STEEL PLATING OVER A MANHOLE. THE PLATING WAS STILL PARTIALLY SUSPENDED FROM A FORKLIFT WHEN THE EE INADVERTENTLY STEPPED ON THE PLATING. THE PLATING SHIFTED CAUSING THE EE TO FALL 4' INTO THE HOLE STRIKING HIS CHEST ON THE CONCRETE WALL.