The Q-system for rock mass classification is developed by Barton, Lien, and Lunde.
[1][2][3][4][5][6][7][8][9] It expresses the quality of the rock mass in the so-called Q-value, on which design are based and support recommendations for underground excavations.
The second term Jr (joint roughness number) divided by Ja (joint alteration number) is related to the shear strength along the discontinuity planes and the third term Jw (joint water parameter) divided by SRF (stress reduction factor) is related to the stress environment on the intact rock blocks and discontinuities around the underground excavation.
The numerical values of the class boundaries for the different rock mass qualities are subdivisions of the Q range on a logarithmic scale.
This leads to a very extensive list of classes for support recommendations.