 For the linguistics formalism, see Qsystems.
The Qsystem 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 socalled Qvalue, on which are based design and support recommendations for underground excavations.
The Qvalue is determined with

 Q=\frac{RQD}{J_n} * \frac{J_r}{J_a} * \frac{J_w}{SRF}
The first term RQD (Rock Quality Designation) divided by J_{n} (joint set number) is related to the size of the intact rock blocks in the rock mass. The second term J_{r} (joint roughness number) divided by J_{a} (joint alteration number) is related to the shear strength along the discontinuity planes and the third term J_{w} (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 0.00006 for an exceptionally poor to 2666 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 Qvalue determines the quality of the rock mass, but the support of an underground excavation is based not only on the Qvalue but is also determined by the different terms in the above equation. This leads to a very extensive list of classes for support recommendations.
References
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Further reading
 Bieniawski, Z.T. "Engineering Rock Mass Classifications", John Wiley and Sons, New York, 1989

 Pantelidis, L (2009). "Rock slope stability assessment through rock mass classification systems", International Journal of Rock Mechanics and Mining Sciences, 46(2), (315 325).
See also
ko:Q
