2.1.1 Barton’s Q-system ( Tunneling Index, Rock quality index, Norwegian Geotechnical Institute)
The Q-system for rock mass classification is developed by Barton, Lien, and Lunde. It expresses the quality of the rock mass in the so-called Q-value, on which are based design and support recommendations for underground excavations.
The rock quality index (Q) is evaluated as
Q = [(RQD/Jn)*(Jr/Ja)*(Jw/SRF)]
Where,
RQD = rock quality designation
Jn = joint set number
Jr = joint roughness number
Ja = joint alteration number
Jw = joint water reduction number
SRF =stress reduction factor.
Based on the value of Q the rock mass can be described as ― exceptionally good (Q = 400 to 1000) to―exceptionally poor (Q = 0.001to0.01).
The first term RQD (Rock Quality Designation) divided by Jn (joint set number) is related to the size of the intact rock blocks in the rock mass.
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.
A multiplication of the three terms results in the Q parameter, which can range between 0.001 for an exceptionally poor to 1000 for an exceptionally good rock mass. The numerical values of the class boundaries for the different rock mass qualities are subdivisions of the Q range on a logarithmic scale.
The Q-value determines the quality of the rock mass, but the support of an underground excavation is based not only on the Q-value but is also determined by the different terms in the above equation. This leads to a very extensive list of classes for support recommendations.
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