Shortly after touchdown the left wing began to drop and the aircraft veered off the runway. It came to rest after colliding with a runway light and a culvert. Inspection revealed that the castellated nut from the left gear torque link pivot bolt was missing. This allowed the lower torque link to rotate with the wheel and separate from the upper torque link. It was evident that a split pin which should have locked the castellated nut had not been in place for a considerable time, thus allowing the nut to work loose. However, the reason the pin was not in place could not be positively established.