Rock Mass Rating (RMR)
Rock Mass Rating (RMR) System for Geotechnical Classification · Also known as: RMR, Bieniawski Classification, RMR89
The Rock Mass Rating (RMR) system, developed by Zbigniew Bieniawski starting in 1973, is an empirical classification that characterizes rock mass quality and estimates mining and civil engineering behavior. RMR combines five measurable geotechnical parameters into a single index ranging from 0 to 100, where higher values indicate stronger, more stable rock masses. It is the most widely used rock classification system worldwide for underground mining design.
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When to use it
Use RMR for underground mining design, tunnel feasibility studies, and stope layout optimization. Assume you have access to representative core from the excavation depth. Works best in granitic and metamorphic rocks. Less reliable in very weak or highly altered rock where the GSI (Geological Strength Index) is preferred.
Strengths & limitations
- Simple, repeatable scoring system; different engineers typically get similar RMR values from the same core
- Combines multiple geotechnical factors in one index; easy to compare rock masses
- Extensive empirical database of thousands of underground excavations worldwide enables confident design
- Direct link to support requirements and excavation stability predictions
- Extensible: can be modified for specific rock types or deposit-specific calibration
- Empirical based on rock types similar to where it was developed; less reliable for unusual or very weak rock
- Does not explicitly account for excavation geometry or size; large excavations may behave differently than predicted
- Joint orientation adjustment is subjective; depends on excavation direction which may be flexible
- RQD can be misleading in rocks with many small fractures; breaks RQD > fractures RMR significantly but actual stability may be low
- Does not capture anisotropy (strength variation with direction) well; foliated rocks may behave unexpectedly
Frequently asked
What RMR value is safe for an unsupported underground opening?
RMR>60 (Classes I-II) generally permits unsupported or lightly supported openings if excavation height is <5 m. Lower RMR requires progressively more support. RMR<30 (Class V) requires extensive support (bolts every 1 m, shotcrete) and likely cannot remain unsupported.
How do I estimate RMR without core samples?
Surface mapping can estimate GSI (Geological Strength Index), which correlates to RMR. Alternatively, outcrop core can be collected, though weathering may alter results. Underground core is preferred for design-level estimates.
Should I adjust RMR for very large excavations (e.g., mine pillars)?
Yes, implicitly. The RMR support recommendations are empirically based on typical excavation sizes. For very large openings, consider size effects: pillar stability equations should be applied separately, not based on RMR alone.
What if rock mass has multiple joint sets with different orientations?
RMR assumes a dominant joint set. If three or more sets exist, identify the most critical (weakest) set for your excavation orientation and rate that. Stereographic analysis or 3D discontinuity models are more rigorous.
How frequently should I update RMR as mining progresses?
At least annually, especially in early mine life or when mining a new zone. Rock mass parameters change with depth (stress increases, oxidation decreases weathering). Compare actual excavation performance to predictions and recalibrate if deviations are systematic.
Sources
- Bieniawski, Z. T. (1989). Engineering rock mass classifications. John Wiley & Sons. ISBN: 978-0-471-60437-4
- Hoek, E., Marinos, P., & Benissi, M. (1998). Applicability of the Geological Strength Index (GSI) classification for very weak and sheared rock masses. Bulletin of Engineering Geology and the Environment, 57(2), 151-160. DOI: 10.1007/s100640050031 ↗
How to cite this page
ScholarGate. (2026, June 3). Rock Mass Rating (RMR) System for Geotechnical Classification. ScholarGate. https://scholargate.app/en/mining-engineering/rock-mass-rating
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.
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