It is a steel tube of 4 m ( 13 ft ) in diameter bored with high - technology machinery .
Diameter is inside a hard - rock type tunnel of 6 m ( 20 ft ) in diameter .
It is a steel tube of 4 m ( 13 ft ) in diameter , which is inside a hard - rock type tunnel of 6 m ( 20 ft ) in diameter , bored with high - technology machinery .