Q-System
Q-System (NGI Index) for Rock Mass Classification · Also known as: Q Index, Norwegian Geotechnical Institute Classification, Barton System
The Q-System (NGI Index), introduced by Nick Barton and colleagues at the Norwegian Geotechnical Institute in 1974, is an alternative rock mass classification to RMR. It combines six parameters into a dimensionless index Q ranging from 0.001 to 1000, where higher Q values indicate better rock quality. The Q-System is particularly valued for tunnel and underground excavation design due to its explicit consideration of joint roughness and groundwater effects.
Read the full method
Sign in with a free account to read this section.
Method map
The neighbourhood of related methods — select a node to explore.
When to use it
Use Q-System for tunnel design and stability assessment, especially where groundwater and joint quality are significant. Preferred over RMR when joint roughness variation is large or groundwater conditions are critical. Works well in weak rock where RMR may be less reliable.
Strengths & limitations
- Multiplicative form reflects engineering reality: one poor parameter makes the entire system poor
- Explicit groundwater term (J_w) makes it superior to RMR in wet environments
- Extensive international database of tunnel case studies correlated to Q
- Dimensionless index allows comparison across rock types
- Good for very weak rock (Q<1) where RMR may be insensitive
- More complex than RMR; requires careful parameter assessment
- Multiplicative form can be unintuitive; small changes in one parameter dramatically shift Q
- SRF (stress reduction factor) is difficult to estimate without stress measurement
- J_a (joint alteration) classification requires experience; different engineers may score differently
- Q does not explicitly address excavation size; large excavations may require larger support than Q alone predicts
Frequently asked
How do I interpret a very high Q value (e.g., Q=100)?
Q=100 indicates excellent rock: minimal support needed, low deformation risk. However, even high-Q rock can fail if excavation is very large or stress is high. Use Q as one input; combine with stress analysis and excavation geometry considerations.
What is the relationship between Q and RMR?
Approximate correlation exists: RMR ≈ 9 ln(Q) + 44. However, the correlation varies by rock type. Q and RMR use different parameters, so they can disagree. Both should be calculated and compared for confidence.
How do I handle fractured rock with many small, tight joints?
High RQD (many core pieces) but small J_n (many sets). The Q formula balances these: RQD/J_n may be moderate. This is realistic: fragmented rock with tight joints has moderate stability, not as good as coarse-jointed strong rock.
Can I use Q for mine pillar design?
Q is primarily for opening stability and support design, not pillar design. Modified versions (Q') and modifications to account for confinement have been proposed. For pillar design, use RMR, GSI, or specialized pillar design formulas.
How sensitive is Q to small changes in joint characteristics?
Very sensitive: J_r and J_a are on a scale of roughly 0.5 to 4, so changing from 'slightly rough' (Jr=1.5) to 'smooth' (Jr=1.0) reduces Q by 33%. Small errors in parameter assessment can shift Q significantly. Report Q with uncertainty ranges.
Sources
- Barton, N., Lien, R., & Lunde, J. (1974). Engineering classification of rock masses for the design of tunnel support. Rock Mechanics, 6(4), 189-236. DOI: 10.1007/BF01239496 ↗
- Barton, N. (2002). Some new Q-value correlations to assist in site characterisation and tunnel design. International Journal of Rock Mechanics and Mining Sciences, 39(2), 185-216. DOI: 10.1016/S1365-1609(02)00011-4 ↗
How to cite this page
ScholarGate. (2026, June 3). Q-System (NGI Index) for Rock Mass Classification. ScholarGate. https://scholargate.app/en/mining-engineering/q-system
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
- Hoek-Brown CriterionMining Engineering↔ compare
- Rock Mass RatingMining Engineering↔ compare
- Stope LayoutMining Engineering↔ compare