EMPLOYEE WAS HEATING A BRACKET THAT HOLDS THE DUST BOX ON A 320 CAT EXCAVATOR WITH A DRILL ATTACHMENT. AFTER HEATING THE BRACKET, EE WAS ATTEMPTING TO STRAIGHTEN THE BRACKET USING A LONG PIECE OF PIPE TO APPLY PRESSURE TO THE BRACKET. EE WAS STANDING IN A MUDDY AREA THAT HAD LOOSE MATERIAL. THE PIPE SLIPPED FROM THE BRACKET & EE SLIPPED, FALLING ON HIS LT. LOWER LEG BREAKING IT.