Q-system (geotechnical engineering)

For the linguistics formalism, see Q-systems.

The Q-system for rock mass classification is developed by Barton, Lien, and Lunde et al. (1974, 1976a, 1988. It expresses the quality of the rock mass in the so-called Q-value, on which design and support recommendations for underground excavations are based.

The Q-value 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 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 for the discontinuities around the tunnel opening.

A multiplication of the three terms results in the ‘Q’ parameter, which can range between 0.00006 for an exceptionally poor rock mass to 2666 for an exceptionally good rock mass. The numerical values of the class boundaries for the different rock mass types are subdivisions of the Q range on a logarithmic scale.

Intact rock strength influences the result only when the intact rock strength is relatively low compared to the stress environment. Jr and Ja are the parameters for the discontinuity roughness and alteration of the weakest discontinuities (Barton et al., 1974) or the discontinuity most likely to allow failure to initiate (Barton, 1976a). 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.

The Q-system uses six different parameters to assess the rock mass quality. The parameters are:

• Rock Quality Designation RQD
• Joint set number J_n
• Roughness of the most unfavorable joint or discontinuity J_r
• Degree of alteration of filling along the weakest joint J_a
• Water inflow J_w
• Stress Reduction Factor SRF

The Q-factor can then be calculated as:

$Q={\frac {RQD}{J_{n}}}{\frac {J_{r}}{J_{a}}}{\frac {J_{w}}{SRF}}$

References

• Barton, N.R. (2000). TBM Tunnelling in Jointed and Faulted Rock. Taylor & Francis. p. 184. ISBN 978-9058093417.
• Barton, N. 2002. "Some new Q-value correlations to assist in site characterization and tunnel design", Int. J. Rock Mech. & Min. Sci. Vol. 39/2:185-216.
• Barton, N. 2006. Rock Quality, Seismic Velocity, Attenuation and Anisotropy. Taylor & Francis, UK & Netherlands, 729 p.
• Barton, N. & Grimstad, E. 1994. "The Q-system following twenty years of application in NMT support selection", 43rd Geomechanic Colloquy, Salzburg. Felsbau, 6/94. pp. 428–436.

• Barton, N. Lien, R. & Lunde, J. 1974. "Engineering classification of rock masses for the design of tunnel support", Rock Mechanics. 6:4:189-236.
• Barton, N. Lien, R. & Lunde, J. 1977. "Estimation of support requirements for underground excavations", Proc. of 16th Symp. on Design Methods in Rock Mechanics, Minnesota, 1975. pp. 163–177. ASCE, NY. Discussion pp. 234–241.
• Grimstad, E. & Barton, N. 1993. Updating the Q-system for NMT. Proc. of the International Symposium on Sprayed Concrete - Modern Use of Wet Mix Sprayed Concrete for Underground Support, Fagernes, 1993, (Eds. Kompen, Opsahl and Berg) Norwegian Concrete Association, Oslo.

Further reading

• Bieniawski, Z.T. "Engineering Rock Mass Classifications", John Wiley and Sons, New York, 1989
• Hack, H R G K (1998). Slope stability probability classification SSPC, 2nd edition, ITC publication no 43, Enschede, Netherlands, ISBN 90-6164-154-3 (258).
• 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

• Rock Structure Rating
• Hoek-Brown failure criterion