On the basis of an evaluation of a large number of case
histories of underground excavations, Barton, Lien and Lunde
(1974) of the Norwegian Geotechnical Institute (NGI) proposed
a Tunnelling Quality Index (Q) for the determination of rock
mass characteristics and tunnel support requirements.
The numerical value of the index Q varies on a
logarithmic scale from 0.001 to a maximum of 1,000.