Rock mass rating (RMR) and the Q system turn field observations of rock into numbers that engineers can use. They are widely used for tunnels, slopes, foundations and underground works.
What is RMR?
RMR, developed by Bieniawski, adds up points for six parameters.
- Strength of the intact rock
- RQD (Rock Quality Designation)
- Spacing of discontinuities
- Condition of discontinuities
- Groundwater condition
- Adjustment for joint orientation
| RMR | Class | Description |
|---|---|---|
| 81–100 | I | Very good rock |
| 61–80 | II | Good rock |
| 41–60 | III | Fair rock |
| 21–40 | IV | Poor rock |
| Below 21 | V | Very poor rock |
What is the Q system?
The Q system, developed by Barton and colleagues, is calculated with this formula:
Q = (RQD / Jn) × (Jr / Ja) × (Jw / SRF)
- RQD / Jn: block size
- Jr / Ja: friction between blocks
- Jw / SRF: water and stress conditions
Q values cover a very wide range, and higher means better rock. The result is commonly used with support charts for tunnels.
RMR or Q?
Many projects record both, so that one can check the other. Indian guidance on rock mass classification is given in IS 13365.
Where the data comes from
Ratings depend on careful geological mapping, core logging and laboratory tests. Because the rock changes along an alignment, sites are divided into zones with separate ratings. Ratings are also used in rock slope studies.
FAQ
What is RQD?
Rock Quality Designation is the percentage of a core run made up of intact pieces 100 mm or longer.
Is a higher RMR better?
Yes. Higher values mean better-quality rock, which generally needs less support.
Can one rating cover a whole tunnel?
No. The rock changes along the alignment, so ratings are assigned by zone and updated during excavation.
Talk to our team
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