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RMR system and its parameters
Short answer
The Rock Mass Rating (RMR) system (Bieniawski, 1973/1989) classifies rock mass by ratings 0–100 using five parameters: uniaxial compressive strength (0–15), RQD (3–20), joint spacing (5–20), joint condition (0–30), and groundwater (0–15), plus an orientation adjustment (0 to –12 for tunnels). It estimates stand-up time, support needs, cohesion and friction angle.
Define the RMR system of rock mass classification. Describe the different parameters of the RMR system.
Definition
The Rock Mass Rating (RMR), or Geomechanics Classification, was proposed by Bieniawski (1973, revised 1989). It classifies a rock mass by a rating from 0 to 100, obtained from five basic parameters plus an adjustment for joint orientation, to estimate the stand-up time, support requirement, cohesion and friction angle of the rock mass. It is widely used in tunnelling, slopes and foundations.
Parameters and ratings (Bieniawski 1989)
| No. | Parameter | Rating range |
|---|---|---|
| 1 | Strength of intact rock (uniaxial compressive strength or point-load index) | 0 - 15 |
| 2 | Rock Quality Designation (RQD) | 3 - 20 |
| 3 | Spacing of discontinuities | 5 - 20 |
| 4 | Condition of discontinuities (persistence, aperture, roughness, infilling, weathering) | 0 - 30 |
| 5 | Ground water condition (inflow per 10 m of tunnel, or water pressure ratio) | 0 - 15 |
| 6 | Orientation of discontinuities (adjustment) | 0 to -12 (tunnels), 0 to -25 (foundations), 0 to -60 (slopes) |
The first five are summed to give the basic RMR, and the sixth is the adjustment for the orientation of joints relative to the engineering work (very favourable to very unfavourable).
Parameter notes
- Intact strength - higher UCS gives a higher rating (>250 MPa gives 15).
- RQD - percentage of core pieces longer than 10 cm in a run; higher RQD gives a higher rating.
- Spacing - wider spacing of joints gives a higher rating (>2 m gives 20).
- Condition - tight, rough, unweathered, unfilled joints give a high rating; soft gouge, open or continuous joints give a low rating.
- Ground water - completely dry rock gets the maximum rating (15); flowing water gets zero.
- Orientation - a favourable direction needs little or no correction; unfavourable joints reduce the rating.
Rock mass classes
| RMR | Class | Description |
|---|---|---|
| 81 - 100 | I | Very good rock |
| 61 - 80 | II | Good rock |
| 41 - 60 | III | Fair rock |
| 21 - 40 | IV | Poor rock |
| < 21 | V | Very poor rock |
Each class gives an average stand-up time, cohesion (kPa) and friction angle of the rock mass.
✓ Answer checked by Claude Opus 5.5 (high effort)
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