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RMR rock mass classification
Short answer
RMR (Rock Mass Rating) by Bieniawski rates rock mass from 0 to 100 using uniaxial compressive strength, RQD, spacing and condition of discontinuties, groundwater, and orientation adjustment. The condition of discontinuties rates persistence, aperture, roughnes, infilling and weathering, giving a maximum of 30 points.
Define Rock Mass Rating (RMR). Describe the Bieniawski RMR classification system, its parameters and the condition of discontinuities in it.
Rock Mass Rating (RMR), also called the Geomechanics Classification, was proposed by Bieniawski (1973, revised 1989). It gives a single rating from 0 to 100 that describes rock mass quality for tunnels, slopes and foundations, and guides support design.
Parameters and ratings
| Parameter | Range of rating |
|---|---|
| 1. Uniaxial compressive strength of intact rock | 0 to 15 |
| 2. Rock Quality Designation (RQD) | 3 to 20 |
| 3. Spacing of discontinuities | 5 to 20 |
| 4. Condition of discontinuities | 0 to 30 |
| 5. Groundwater condition | 0 to 15 |
| 6. Adjustment for orientation of discontinuities | 0 to -12 (tunnels) |
Example ratings: UCS above 250 MPa = 15, RQD 90 to 100 % = 20, spacing above 2 m = 20, completely dry = 15.
Condition of discontinuities (maximum 30)
| Factor | Best (6) | Worst (0) |
|---|---|---|
| Persistence (length) | below 1 m = 6 | above 20 m = 0 |
| Aperture (separation) | none = 6 | above 5 mm = 0 |
| Roughness | very rough = 6 | slickensided = 0 |
| Infilling | none = 6 | soft, above 5 mm = 0 |
| Weathering | unweathered = 6 | decomposed = 0 |
Rock mass classes
| Class | RMR | Description | Cohesion (kPa) | Friction angle |
|---|---|---|---|---|
| I | 81 to 100 | Very good | above 400 | above 45° |
| II | 61 to 80 | Good | 300 to 400 | 35° to 45° |
| III | 41 to 60 | Fair | 200 to 300 | 25° to 35° |
| IV | 21 to 40 | Poor | 100 to 200 | 15° to 25° |
| V | below 20 | Very poor | below 100 | below 15° |
Use: the class gives stand-up time (e.g. 20 years for 15 m span in class I, 30 minutes for 1 m in class V), shear strength parameters and recommended excavation and support.
✓ Answer checked by Claude Opus 5.5 (high effort)
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