Rock-mass classification
Q-system for rock-mass classification
The Q-system characterizes rock-mass quality with six parameters and links Q and excavation dimensions to empirical support guidance.
Core relationship
Q = (RQD/Jn) · (Jr/Ja) · (Jw/SRF)
Inputs & units
- RQD [%]
- Joint-set number Jn
- Joint roughness Jr
- Joint alteration Ja
- Joint water reduction Jw
- Stress Reduction Factor SRF
Assumptions
- Parameter ratings follow the current NGI handbook and the mapped ground domain.
- The Excavation Support Ratio is chosen for the excavation purpose when support charts are used.
- The observational and empirical nature of the system is retained in interpretation.
Limitations
- Parameter selection involves judgement and must remain traceable to mapping evidence.
- Q is not interchangeable with RMR or GSI through a universal correlation.
- Support-chart use does not replace analysis of stress, groundwater, geometry, construction and failure modes.
Synthetic worked example
Synthetic audit: preserve the six ratings, photos or logs supporting them, the computed Q value and the chosen ESR. A reviewer should be able to change one rating and reproduce the effect.
Engineering use boundary
This page is educational and supports preliminary assessment. It does not replace site investigation, competent engineering judgement, final design, independent review or permitting requirements.